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Regional in situ validation of satellite altimeters: Calibration and cross-calibration results at the Corsican sites
Abstract
The in situ validation of the satellite altimeter sea surface heights is generally performed either at a few local points directly flown over by the satellites or using the global tide gauge network. A regional in situ calibration method was developed by NOVELTIS in order to monitor the altimeter data quality in a perimeter of several hundred kilometres around a given in situ calibration site. The primary advantage of this technique is its applicability not only for missions flying over dedicated sites but also for missions on interleaved or non repetitive orbits. This article presents the altimeter bias estimates obtained with this method at the Corsican calibration site, for the Jason-1 mission on its nominal and interleaved orbits as well as for the Jason-2 and Envisat missions. The various regional bias estimates (8.2 cm and 7.4 cm for Jason-1 respectively on the nominal and interleaved orbits in Senetosa, 16.4 cm for Jason-2 in Senetosa and 47.0 cm for Envisat in Ajaccio, with an accuracy between 2.5 cm and 4 cm depending on the mission) are compared with the results obtained by the other in situ calibration teams. This comparison demonstrates the coherency at the centimetre level, the stability and the generic character of the method, which would also be of benefit to the new and future altimeter missions such as Cryosat-2, SARAL/AltiKa, Sentinel-3, Jason-3, Jason-CS.
... After that, there are many methods to accomplish the Cal/Val of altimeters, such as the DGB, tide gauges, transponder, moored oceanographic instruments, etc.; the tide gauges are the most commonly used method [5,7,9,11]. In this method, the absolute datum of the tide gauge should be determined using the PGSs plus leveling, kinematic GNSS measurements, or DGBs [12]. ...
... In this method, the absolute datum of the tide gauge should be determined using the PGSs plus leveling, kinematic GNSS measurements, or DGBs [12]. Moreover, the tide gauge is often placed on land/an island, while the altimeter signals are contaminated in such an area [11]. There is a distance of about 15~30 km from the tide gauge location to the non-contaminated altimeter footprints. ...
... There is a distance of about 15~30 km from the tide gauge location to the non-contaminated altimeter footprints. The mean sea surface (MSS) and tidal corrections must be taken into consideration [11,12]. The MSS gradient can reach up to 10 cm/km in some places, which is the key correction element for altimeter calibration [1]. ...
In order to accomplish the calibration and validation (Cal/Val) of altimeters, the Wanshan calibration site (WSCS) has been used as a calibration site for satellite altimeters since its completion in August 2019. In this paper, we introduced the WSCS and the dedicated equipment including permanent GNSS reference stations (PGSs), acoustic tide gauges (ATGs), and dedicated GNSS buoys (DGB), etc. placed on Zhi’wan, Wai’ling’ding, Dan’gan, and Miao’Wan islands of the WSCS. The PGSs data of Zhi’wan and Wai’ling’ding islands were processed and analyzed using the GAMIT/GLOBK (Version 10.7) and Hector (Version 1.9) software to define the datum for Cal/Val of altimeters in WSCS. The DGB was used to transfer the datum from the PGSs to the ATGs of Zhi’wan, Wai’ling’ding, and Dan’gan islands. Separately, the tidal and mean sea surface (MSS) corrections are needed in the Cal/Val of altimeters. We evaluated the global/regional tide models of FES2014, HAMTIDE12, DTU16, NAO99jb, GOT4.10, and EOT20 using the three in situ tide gauge data of WSCS and Hong Kong tide gauge data (No. B329) derived from the Global Sea Level Observing System. The HAMTIDE12 tide model was chosen to be the most accurate one to maintain the tidal difference between the locations of the ATGs and the altimeter footprints. To establish the sea surface connections between the ATGs and the altimeter footprints, a GPS towing body and a highly accurate ship-based SSH measurement system (HASMS) were used to measure the sea surface of this area in 2018 and 2022, respectively. The global/regional mean sea surface (MSS) models of DTU 2021, EGM 2008 (mean dynamic topography minus by CLS_MDT_2018), and CLS2015 were accurately evaluated using the in situ measured data and HY-2A altimeter, and the CLS2015 MSS model was used for Cal/Val of altimeters in WSCS. The data collected by the equipment of WSCS, related auxiliary models mentioned above, and the sea level data of the hydrological station placed on Dan’gan island were used to accomplish the Cal/Val of HY-2B, HY-2C, Jason-3, and Sentinel-3A (S3A) altimeters. The bias of HY-2B (Pass No. 375) was −16.7 ± 45.2 mm, with a drift of 0.5 mm/year. The HY-2C biases were −18.9 ± 48.0 mm with drifts of 0.0 mm/year and −5.6 ± 49.3 mm with −0.3 mm/year drifts for Pass No. 170 and 185, respectively. The Jason-3 bias was −4.1 ± 78.7 mm for Pass No. 153 and −25.8 ± 85.5 mm for Pass No. 012 after it has changed its orbits since April 2022, respectively. The biases of S3A were determined to be −16.5 ± 46.3 mm with a drift of −0.6 mm/year and −9.8 ± 30.1 mm with a drift of 0.5 mm/year for Pass No. 260 and 309, respectively. The calibration results show that the WSCS can commercialize the satellite altimeter calibration. We also discussed the calibration potential for a wide swath satellite altimeter of WSCS.
... In this present study region, the altimeter track 155 was passing at a distance of about 50 km and for track 116 at about 18 km away from the coastal buoy, which were the closest points from altimeter to the buoy for the tracks 155 and 116 respectively. All the comparisons were carried out at this closest point to obtain best results (Caballero et al., 2011;M Cancet et al., 2013;Cavaleri, 2005;Cheng, Andersen, &Knudsen, 2012;Fabien Durand, Shankar, Birol, & Shenoi, 2008;Feng, Tsimplis, Yelland, & Quartly, 2014;Krogstad & Barstow, 1999;Kushnir, Cardone, Greenwood, & Cane, 1997;Mercier et al., 2010;Passaro et al., 2014;Pierre Queffeulou, 2004;Richard D Ray & Beckley, 2012;Stewart, Shum, Tapley, & Ji, 1996;S. Vignudelli et al., 2005). ...
... The presence of outliers in the altimeter data were either due to low-pressure systems, land contamination, etc. (Abdalla, Janssen, & Bidlot, 2010), (M Cancet et al., 2013), (Lemoine et al., 2010), (Niedzielski & Kosek, 2011), (Lutjeharms, Boebel, & Rossby, 2003), (Pierre Queffeulou, 2004), (Reguero et al., 2012a), (Stewart et al., 1996),(S. Vignudelli et al., 2005). ...
... Because here the location of the buoy at about 1.3 km away from the coast indicates that it is coastally located and the closest altimeter -buoy distance for the track 116 was at least 18 km away from the coast indicates that the wave information obtained was still a deep water wave. (M Cancet et al., 2013) also suggested that it would be interesting to evaluate the biases from different re-trackers dedicated to coastal areas. Also, it suggests that buoy data matches well with closest 116 track than offshore track 155. ...
Most of the world's population lives close to the coast. These are the transition areas between land and sea. These are the vital regions for many offshore and marine activities, imports and exports, fishing, navigation, recreation etc. throughout history, humans have attempted to slow or alter the dynamic coastal zone. India has Coastline of about 7,517 km comprises States of Gujarat, Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Orissa and West Bengal. India, with most diverse ecosystem, high productivity and thickly populated over coastal region has gained its very own importance. Major and important cities of India were developed along the coastal regions. The Cities in the Coastline of India are notable for trading like heavy shipments through Ships from foreign and inside the coastal cities of India. A proper management of the coastal and near-shore zones is vital in the economic development of a nation. These Indian coastal regions have been subjected to several geomorphological changes under climate change scenario. Despite all of this Indian coasts are under threat due to multiple stresses like global climate change, human intervention. These stresses are driving vulnerabilities like sea-level-rise, coastal erosion, frequent extreme events, salt water encroachment etc. So, there is a need for the proper management of these coastal regions. The coastline has subjected to changes due to the action of waves, tides, currents and winds. Therefore, it is essential to better understand the physical processes, which are responsible for these changes. These coastal processes are complex and dynamic in nature as it involves to study and monitor especially the ocean surface waves and sea level variations (or tides) in the coastal regions. Remote sensing offers an excellent opportunity to study these ocean parameters using satellite radar altimeter. Preceding Jason-2, the altimeter data were too sparse and of poor quality to be of real use to study in coastal areas. Advanced instruments in Jason-2 enables us to study and monitor the coastal areas. In addition to this, Recently India in collaboration with French space agency launched an altimeter SARAL AltiKa, a first of kind to operate at high frequency with study of coastal areas as one of the main objectives. The main goal of the study is to observe, obtain retrieve and validate the wave height and sea level data from these two operationally reliable series of altimeters Jason-2 and SARAL AltiKa and to demonstrate the usefulness of high frequency data during extreme event cases like cyclone. In the present study, the Jason-2 PISTACH coastal products were observed and analyzed to identify the best re-tracker available to study and monitor coastal regions. A new processing method was applied to the re-trackers to obtain optimum results in the limited areas of the coastal region. The performance of these processed re-tracker was validated with In-situ observations. Also high frequency data from SARAL AltiKa was analyzed to observe and validate in the proximity of the coast. The validation was carried out in two methods. One at closest point analysis and at along the track. This study also presents an application of high frequency Jason-2 data for the study of extreme event cases like cyclone. A new way of approach (or) method was used for comparison of altimeter data with In-situ observations. The chapter wise contents of the thesis are briefly described below.
This thesis consists of six chapters. Chapter 1 is an introductory chapter describing the importance of coastal regions and the importance of Ocean surface waves and sea level anomalies/ variations over coastal regions. It is followed by a brief introduction, history and development of satellite radar altimetry towards coastal regions and a brief description of Jason-2 and SARAL AltiKa altimeters. This chapter is completed with a description of the scope of the problem and specific objectives of the present study followed by the structure and outline of the thesis. Chapter 2 examines and identifies the best re-tracker available in Jason-2 PISTACH coastal products to obtain, retrieve and validate ocean wave parameters in the proximity to the coast. Chapter 3 contains information about SARAL AltiKa and to observe retrieve and validate SARAL along track wave data at different coastal regions along the east coast of India. SARAL validation is carried out at the point closest to the buoy and at all the points along the track close to the coast. Chapter 4 comprises of Sea level measurements from both missions to quality control, calibrate and validate individual altimeter measurements. Chapter 5 presents an application of high frequency satellite altimetry data to study wind and wave parameters during extreme event case. Chapter 6 is concluding chapter that summarizes the results of the study and opens several new scopes for further studies.
... In this present study region, the altimeter track 155 was passing at a distance of about 50 km and for track 116 at about 18 km away from the coastal buoy, which were the closest points from altimeter to the buoy for the tracks 155 and 116 respectively. All the comparisons were carried out at this closest point to obtain best results [3,9,13,15,16,[27][28][29][30][31][32][33][34][35]. ...
... tained after data editing and applying the valid retracking flags to obtain SWH data for all the re-trackers between 10 -50 km away from the coast[12,13,15,21,28,[34][35][36][37][38]. The Jason-2 ground track 155 is an ocean to land ascending track at a distance of about 50 km away from the buoy while the Jason-2 ground track 116 is a land to ocean descending track at a distance of about 18 km away from the buoy. ...
... In addition to these, some outliers still exist. The presence of outliers in the altimeter data were either due to lowpressure systems, land contamination, etc.[9,10,13,28,33,[40][41][42][43].Also this time series comparison plots shows that all the oceanographic signals captured by altimeter at 50 km for track 155 and 18 km away from the coast for track 116 were also depicted in the coastal buoy data. The scatter plots for the Jason-2 ground tracks that there is an acceptable agreement between altimeter and buoy collocated pairs of standard OCE3 and RED3 re-trackers. ...
... In this present study region, the altimeter track 155 was passing at a distance of about 50 km and for track 116 at about 18 km away from the coastal buoy, which were the closest points from altimeter to the buoy for the tracks 155 and 116 respectively. All the comparisons were carried out at this closest point to obtain best results [3,9,13,15,16,[27][28][29][30][31][32][33][34][35]. ...
... tained after data editing and applying the valid retracking flags to obtain SWH data for all the re-trackers between 10 -50 km away from the coast[12,13,15,21,28,[34][35][36][37][38]. The Jason-2 ground track 155 is an ocean to land ascending track at a distance of about 50 km away from the buoy while the Jason-2 ground track 116 is a land to ocean descending track at a distance of about 18 km away from the buoy. ...
... In addition to these, some outliers still exist. The presence of outliers in the altimeter data were either due to lowpressure systems, land contamination, etc.[9,10,13,28,33,[40][41][42][43].Also this time series comparison plots shows that all the oceanographic signals captured by altimeter at 50 km for track 155 and 18 km away from the coast for track 116 were also depicted in the coastal buoy data. The scatter plots for the Jason-2 ground tracks 155 and 116 along with basic statistics presented in figure 5 show that there is an acceptable agreement between altimeter and buoy collocated pairs of standard OCE3 and RED3 re-trackers. ...
... Satellite altimeters need to be calibrated or validated to deal with the drift or bias associated with the measurements. Long-term time series of sea surface height (SSH) measurements from altimeters can be calibrated and validated against independent measurements at dedicated sites that have been set up specifi cally for this purpose (Haines et al., 2003;Mertikas et al., 2011Mertikas et al., , 2018Cancet et al., 2013). The calibration and validation (Cal/Val) of altimeters needs to be done over the long-term and should be repeated at diff erent places worldwide (Fu and Haines, 2013). ...
... Using a GPS buoy, the altimeter bias can be estimated using a direct absolute altimeter calibration from direct altimeter overfl ights. While this method does not require a geoid or tidal model Cancet et al., 2013), the GPS buoy should be moored right at the altimeter footprint for at least two hours and should not be operated in harsh sea conditions. When using the tide gauge, the equipment cannot be placed right at the footprint of the altimeters. ...
... For the Wanshan area, this means that the tidal diff erences need be taken into consideration for the altimeter Cal/ Val. The tidal signal from the tide gauge time series can be obtained naturally through a harmonic analysis (Cancet et al., 2013). In this study however, the time measured by tide gauge was not enough, and so the accuracy of the harmonic analysis was low and was not suffi cient to satisfy the accuracy of the altimeter calibrations. ...
We present preliminary calibration results for Jason-3 and Sentinel-3A altimeters that we set up in the Wanshan Islands in Guandong Province, China. Two campaigns were carried out in 2018, from March 8 to April 16 and from November 3 to December 11, 2018. Three GPS reference stations and tide gauges were established on the islands of Zhiwan, Dangan, and Wailingding during the campaigns. The in-situ sea surface height (SSH) of the altimeter footprint was determined using the tide gauge. The tidal and geoid connection between the tide gauge locations and the altimeter footprints were computed with the NAO.99Jb tidal prediction system and the EGM 2008 geoid, respectively. The data of the tide gauges were defi ned using the GPS buoy and GPS reference stations during the campaigns. The results show that the waveform of the altimeters was slightly contaminated by the small islands. The bias associated with Jason-3 and Sentinel-3A amounted to approximately +20.7±49.7 mm and +30.1±39.4 mm, respectively, which agreed well with the results from other in-situ calibration fi elds. This indicates that the Wanshan area was very suitable as an in-situ calibration/validation fi eld. The wet zenith delay (WZD) determined from the Microwave Radiometer (MWR) and the GPS measurements diff ered from each other for the Jason-3 and Sentinel-3A by −6.6±7.4 mm and 0±6.8 mm, respectively.
... As a first step, Paris Observatory (OBSPM) and NOVELTIS computed the absolute SSH bias estimates for the Sentinel-3A and Sentinel-3B (tandem phase) missions on track 741 in Senetosa and Ajaccio, using their own slightly different methods described in Bonnefond et al. [40] and Cancet et al. [42]) respectively. The OBSPM and NOVELTIS methods are very close and give same results on equivalent subsets of data. ...
... These were for S3A track 741 with Jason-2 track 085, and for Sentinel-3A track 044 with Jason-2 track 222 and with Envisat track 887. The offshore bias was estimated using the regional calibration technique developed by NOVELTIS [42] (and is shown in Figure 26a).The method uses the mean SSH profile determined along prior altimeter tracks to accurately transfer the reference datum offshore. In this paper, we only present the regional SSH bias estimates computed in Senetosa (indeed, the analysis of the results in Ajaccio has shown some discrepancies between the high-frequency signals (especially the ocean tides) that are observed by the tide gauge and the signals that are provided by the models, which require further investigation). ...
... (a) Generic diagram of the regional calibration method (image taken from[42]). (b) Configuration in Corsica for the Sentinel-3A mission. ...
The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges.
... Given that most of the in situ calibration sites (Harvest, Senetosa, Bass Strait and Gavdos) were specifically designed to be located under the TOPEX/Jason orbit, the classical absolute calibration technique can only be used for satellites using this orbit. The regional cal/val technique developed by Noveltis [19,20] aims to assess the altimeter range bias both on satellite passes flying over the calibration site and on satellite passes located several hundreds of kilometres away ( Figure 9). In particular, it enables the monitoring of altimetry missions that do not fly directly over the calibration sites, such as SARAL/AltiKa. ...
... In Harvest and Bass Strait, the number of selected crossover points is rather small in comparison to the number of available points. Large variability is observed in the bias estimates at those surrounding crossover points and they are discarded from the computation (see [19,20] for details). Some previous work on the Envisat mission in Bass Strait [24] showed that using a more recent tidal model with higher resolution (FES2014 global tidal model, [25]) enables us to drastically reduce the variability at some of those points and to reintegrate them in the computation in this specific region. ...
... The tracks can belong to different altimeter missions. The in situ high resolution mean surface is used to link the tide gauge (TG) measurements to the altimeter data, and the comparison is done at the point C, located on this surface (adapted from [20]). ...
The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.
... Direct absolute altimeter calibration, estimating the Jason-2 and AltiKa/Saral biases, was made from direct overflights using GPS buoys. This method does not require any modelling of geoid and tidal error [2,3]. The crossover point between Jason-2 and Saral North of Ibiza (around 40 nm) and West of Mallorca island was found to be optimal for our purposes as it allows measurements at a one-day time-lag and a similar configuration of buoys for each satellite pass. ...
... The GNSS measurements from the five buoys are first smoothed using a 5 minutes averaging window and then interpolated at the exact location and time of the altimeter pass, taking into account the elevation of the antenna for each buoy obtained during the pre-calibration phase achieved in the Ibiza harbor. The corresponding sea level is then compared the altimetry measurement to determine the absolute bias for each the altimeter following [1][2][3][4]. ...
... The differences are generally lower than 0.1 to 0.2 m (especially for Saral), except close to the Algerian coast for Jason-2 where it reaches 0.4 m. They remain lower than 0.5 m as expected for the Mediterranean Sea (see for instance [3]) and are less than 0.1 m around the crossover point. A part of the biases estimated using the data collected during this campaign can come from the differences in the corrections applied to altimetry and GNSS data. ...
This study presents the preliminary results of the 2013 Ibiza calibration campaign of Jason-2 and Saral altimeters. It took place from 14 to 16 September 2013 and was composed of two phases: the calibration of the GNSS buoys to estimate the antenna height of each of them and absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was at ∼ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and Saral nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and Saral respectively. Absolute biases of 0.1 and -4 cm were found for Jason-2 and Saral respectively.
... Besides, it is necessary for combining both spatially and temporally sea surface height (SSH) from different altimeters. The absolute calibration of the altimeter missions is used to compare the altimetry-derived SSH with independent measurements and consists in determining the bias between the SSH measured by an altimeter and an external ground truth (e.g., Cancet et al. 2013). Global Positioning System (GPS) receiver on buoys were early used for precise sea level estimates (Rocken et al. 1990) and calibration of Topex/Poseidon altimeter measurements (Born et al. 1994). ...
... A direct absolute altimeter calibration estimating the Jason-2 and SARAL biases was performed from direct overflights using GNSS buoys. This method does not require any modeling of geoid and tidal error Cancet et al. 2013). The crossover point between Jason-2 and SARAL North of Ibiza (around 40 nm) and West of Mallorca island was found to be optimal for our purposes since it allows measurements at a one-day time-lag and a similar configuration of buoys for each satellite pass ( Figure 1). ...
... averaging window and then interpolated at the exact location and time of the altimeter pass, taking into account the elevation of the antenna for each buoy obtained during the pre-calibration phase achieved in the Ibiza harbor. The corresponding sea level is then compared to the altimetry measurement to determine the absolute bias for each the altimeter following (e.g., M enard et al. 1994;Martinez Benjamin et al. 2004;Bonnefond et al. 2013;Cancet et al. 2013). ...
... Introduction Accurate monitoring of sea-level variations using satellite altimetry requires a precise determination of the error budget of the altimeter measurements. The absolute calibration of the altimeter missions is used to compare the altimetry-derived sea surface height (SSH) with independent measurements and consists in determining the bias between the SSH measured by an altimeter and an external ground truth (e.g., Cancet et al. 2013). Local absolute calibrations are performed under the altimeter groundtrack using tide gauge records or GNSS measurements either during a dedicated campaign (Bonnefond et al. 2003;Martinez Benjamin et al. 2004) or at permanent calibration facilities or Calibration/Validation (Cal/Val) sites (Bonnefond et al. 2010;Haines et al. 2010;Mertikas et al. 2010;Watson et al. 2011). ...
... Direct absolute altimeter calibration, estimating the Jason-2 and Saral biases, was made from direct overflights using GPS buoys. This method does not require any modeling of geoid and tidal error Cancet et al. 2013). The crossover point between Jason-2 and Saral North of Ibiza (around 40 nm) and West of Mallorca island was found to be optimal for our purposes as it allows measurements at a one-day time-lag and a similar configuration of buoys for each satellite pass. ...
... The GNSS measurements from the five buoys are first smoothed using a 5 minutes averaging window and then interpolated at the exact location and time of the altimeter pass, taking into account the elevation of the antenna for each buoy obtained during the pre-calibration phase achieved in the Ibiza harbor. The corresponding sea level is then compared to the altimetry measurement to determine the absolute bias for each the altimeter following (e.g., Ménard et al. 1994;Martinez Benjamin et al. 2004;Bonnefond et al. 2013;Cancet et al. 2013). ...
This study presents the results of the 2013 Ibiza (Western Mediterranean) calibration campaign of Jason-2 and SARAL altimeters. It took place from 14 to 16 September 2013 and was composed of two phases: the calibration of the GNSS (Global Navigation Satellite System) buoys to estimate the antenna height of each of them and absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was at ˜ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and SARAL nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and SARAL respectively. The GNSS data were processed using precise point positioning technique. The biases found are of (−0.1 ± 0.9) and (−3.1 ± 1.5) cm for Jason-2 and SARAL respectively.
... On-orbit calibration of the altimeter missions can improve the measurement accuracy of the altimeter sea surface height (SSH) and allow calibration of the SSH bias, which may increase over time. Altimeter SSH calibration is used to compare the SSH measured by altimetry with an independent measurement from the ground truth (Cancet et al., 2013). The two most used calibration methodologies for altimeter SSH calibration are the tide gauge and the GPS buoy. ...
... Altimetry SSH calibrations are usually performed under the altimeter ground track using a tide gauge or GPS measurements at permanent calibration sites (Bonnefond et al., 2003a(Bonnefond et al., , b, 2015Watson et al., 2003Watson et al., , 2011Watson et al., , 2016Haines et al., 2010Haines et al., , 2016Mertikas et al., 2010Mertikas et al., , 2016a. The GPS buoy method can give a direct sea level measurement at the altimeter nadir point in the same reference frame as the altimeter SSH, which does not require any modeling of the geoid or tidal error (Cancet et al., 2013). The GPS buoy method that was used in this study is therefore a purely geometric relationship and only relies on periodic GPS buoy deployment. ...
GPS buoy methodology is one of the main calibration methodologies for altimeter sea surface height calibration. This study introduces the results of the Qinglan calibration campaign for the HY-2A and Jason-2 altimeters. It took place in two time slices; one was from August to September 2014, and the other was in July 2015. One GPS buoy and two GPS reference stations were used in this campaign. The GPS data were processed using the real-time kinematic (RTK) technique. The final error budget estimate when measuring the sea surface height (SSH) with a GPS buoy was better than 3.5 cm. Using the GPS buoy, the altimeter bias estimate was about -2.3 cm for the Jason-2 Geophysical Data Record (GDR) Version ‘D’ and from -53.5 cm to -75.6 cm for the HY-2A Interim Geophysical Data Record (IGDR). The bias estimates for Jason-2 GDR-D are similar to the estimates from dedicated calibration sites such as the Harvest Platform, the Crete Site and the Bass Strait site. The bias estimates for HY-2A IGDR agree well with the results from the Crete calibration site. The results for the HY-2A altimeter bias estimated by the GPS buoy were verified by cross-calibration, and they agreed well with the results from the global analysis method.
... These results were then compared with the data from the nearest TG. In the baseline length of 19 km, the satellite altimeters can avoid contaminating by the land, which distorts the waveforms (Cancet et al., 2013). Moreover, the CSRS-PPP tool was also used to derive the SSH and then compared with the TRACK solutions. ...
A dedicated GPS buoy is designed for calibration and validation (Cal/Val) of satellite altimeters since 2014. In order to evaluate the accuracy of the sea surface height (SSH) measured by the GPS buoy, twelve campaigns have been done within China sea area between 2014 and 2021. In six of these campaigns, two static Global Navigation Satellite System stations were installed at distances of <1 km and 19 km from the buoy to assess how the baseline length influenced the derived SSH from the buoy solutions. The GPS buoy data was processed using the GAMIT/GLOBK software+TRACK module and CSRS-PPP tool to achieve the SSH. The SSH was compared with conventionally tide gauge (TG) data to evaluate the accuracy of the buoy with the standard deviation of the height element. The results showed that the difference in the standard deviation of the SSH from the buoy and the TG was less than 16 mm. The SSHs processed with different ephemeris (Ultra-Rapid, Rapid, Final) were not significantly different. When the baseline length was 19 km, the SSH solution of the GPS buoy performed well, with standard bias of less than 26 mm between the heights measured by the buoy and TG, meaning that the buoy could be used for Cal/Val of altimeters. The bias between the Canadian Spatial Reference System-precise point positioning tool and the TRACK varied a lot, and some of them were over 130 mm. This deemed too high to be useful for Cal/Val of satellite altimeters. Moreover, the GPS buoy solutions processed by GAMIT/GLOBK software+TRACK module were used for in-orbit Cal/Val of HY-2B/C satellites in ten campaigns. The SSH and significant wave height of the altimeters showed good agreements with the GPS buoy solutions.
... Together with the data expansion, new technologies emerged with highly accurate measurements combined with several assessment studies. Some of them, including the intercomparison of multiple satellite missions and cross-calibration, are [21,23,25,[52][53][54][55][56][57]. Sepulveda et al. [58] and Queffeulou and Croizé-Fillon [59] found that altimeter estimates of Hs are in close agreement with buoys, with standard deviations of the order of 0.3 m. ...
This paper investigates the spatial and temporal variability of significant wave height (Hs) and wind speed (U10) using altimeter data from the Australian Ocean Data Network (AODN) and buoy data from the National Data Buoy Center (NDBC). The main goal is to evaluate spatial and temporal criteria for collocating altimeter data to fixed-point positions and to provide practical guidance on altimeter collocation in deep waters. The results show that a temporal criterion of 30 min and a spatial criterion between 25 km and 50 km produce the best results for altimeter collocation, in close agreement with buoy data. Applying a 25 km criterion leads to slightly better error metrics but at the cost of fewer matchups, whereas using 50 km augments the resulting collocated dataset while keeping the differences to buoy measurements very low. Furthermore, the study demonstrates that using the single closest altimeter record to the buoy position leads to worse results compared to the collocation method based on temporal and spatial averaging. The final validation of altimeter data against buoy observations shows an RMSD of 0.21 m, scatter index of 0.09, and correlation coefficient of 0.98 for Hs, confirming the optimal choice of temporal and spatial criteria employed and the high quality of the calibrated AODN altimeter dataset.
... For that reason, the calibration of satellite altimeters has a long history there. This particularly applies to some selected tide gauge sites at the Mediterranean locations, like Ibiza (Martinez-Benjamin et al., 2004;Frappart et al., 2015) or Corsica (Bonnefond et al., 2003(Bonnefond et al., , 2021Cancet et al., 2013). Recently, advanced learning methods (such as deep-learning networks) have been used to improve calibrations of altimeter data (Yang et al., 2021). ...
Employed for over a century, the traditional way of monitoring sea level variability by tide gauges – in combination with modern observational techniques like satellite altimetry – is an inevitable ingredient in sea level studies over the climate scales and in coastal seas. The development of the instrumentation, remote data acquisition, processing, and archiving in the last decades has allowed the extension of the applications to a variety of users and coastal hazard managers. The Mediterranean and Black seas are examples of such a transition – while having a long tradition of sea level observations with several records spanning over a century, the number of modern tide gauge stations is growing rapidly, with data available both in real time and as a research product at different time resolutions. As no comprehensive survey of the tide gauge networks has been carried out recently in these basins, the aim of this paper is to map the existing coastal sea level monitoring infrastructures and the respective data availability. The survey encompasses a description of major monitoring networks in the Mediterranean and Black seas and their characteristics, including the type of sea level sensors, measuring resolutions, data availability, and existence of ancillary measurements, altogether collecting information about 240 presently operational tide gauge stations. The availability of the Mediterranean and Black seas sea level data in the global and European sea level repositories has been also screened and classified following their sampling interval and level of quality check, pointing to the necessity of harmonization of the data available with different metadata and series in different repositories. Finally, an assessment of the networks' capabilities for their use in different sea level applications has been done, with recommendations that might mitigate the bottlenecks and ensure further development of the networks in a coordinated way, a critical need in the era of human-induced climate changes and sea level rise.
... ∆DAC, reflecting the effects of the wind and atmospheric pressure on the sea surface, is a combination of the inverted barometer correction for periods longer than 20 days and pressure and wind-induced fluctuations for periods shorter than 20 days [44], [45]. As shown in Fig.5, the DAC at the Qianliyan site may reach values up to -60 centimeters at winter and 20 centimeters at summer and shows significantly seasonal variations. ...
Calibration and validation (Cal/Val) of the sea surface height as measured by satellite radar altimeters is essential to understand altimeter biases, observation trends, and instrument aging. It also supports the long-term stability of the produced climate change records of sea level as determined by altimetry. In this article, we report the calibration of HY-2B and Jason-2/3 using the established research infrastructure and data sharing initiative introduced by the Altimetry Calibration Cooperation Plan (ACCP) of China. Currently, three ACCP calibration sites encompass the Wanshan Islands, and two national oceanic sites are located along the China coastline. For each Cal/Val site, the components of the facilities—the geodetic, sea level, and Global Navigation Satellite System (GNSS) infrastructure—and the followed monitoring procedures and calibration methods are described. The HY-2B performance was primarily evaluated using about two years data, which indicated a mean bias of −0.2 ± 4.2 cm. Confidence in the results is strong, because the HY-2B biases were cross compared and confirmed by all the three independent sites and the three satellite ground tracks. Compared with its predecessor HY-2A, HY-2B shows very stable observations with no linear drift at present. In addition, Jason-2 and Jason-3 were mainly assessed using Qianliyan site. Our results indicate that the Jason-3 sea-surface height bias is approximately 2–3 cm smaller than that of Jason-2 and that the long-term stability of Jason-2/3 shows no significant trend, which in good agreement with the international dedicated sites. The instrument noises of Jason-2/3 and HY-2B were estimated based on the ACCP sites. The results show that the instrument noise in the previous literature is underestimated. This was also consolidated by the result from wavenumber spectrum and global crossover point analysis. The code and Wanshan data used in these Cal/Val experiments are publicly available to facilitate further work in this domain (
https://github.com/GenericAltimetryTools/CalAlti
).
... For that reason, calibration of satellite altimeters has a long history there. This particularly applies to some selected tide gauge sites in the Mediterranean locations, like Ibiza (Martinez-Benjamin 1120et al., 2004Frappart et al., 2015) or Corsica (Bonnefond et al., 2003(Bonnefond et al., , 2021Cancet et al., 2013). Recently, advanced learning methods (such as deep learning networks) have been used to improve calibrations of altimeter data (Yang et al., 2021). ...
Spanning over a century, a traditional way to monitor sea level variability by tide gauges is-in combination with modern observational techniques like satellite altimetry-an inevitable ingredient in sea level studies over the climate scales and in coastal seas. The development of the instrumentation, remote data acquisition, processing and archiving in last decades allowed for extending the applications towards a variety of users and coastal hazard managers. The Mediterranean and Black 50 seas are an example for such a transition-while having a long tradition for sea level observations with several records spanning over a century, the number of modern tide gauge stations are growing rapidly, with data available both in real-time and as a research product at different time resolutions. As no comprehensive survey of the tide gauge networks has been carried out recently in these basins, the aim of this paper is to map the existing coastal sea level monitoring infrastructures and the respective data availability. The survey encompasses description of major monitoring networks in the Mediterranean and Black 55 seas and their characteristics, including the type of sea level sensors, measuring resolutions, data availability and existence of ancillary measurements, altogether collecting information about 236 presently operational tide gauge stations. The availability of the Mediterranean and Black seas sea level data in the global and European sea level repositories has been also screened and classified following their sampling interval and level of quality-check, pointing to the necessity of harmonization of the data available with different metadata and series at different repositories. Finally, an assessment of the networks' capabilities 60 for their usage in different sea level applications has been done, with recommendations that might mitigate the bottlenecks and assure further development of the networks in a coordinated way, being that more necessary in the era of the human-induced climate changes and the sea level rise.
... The magnitude of these effects depends not only on local conditions (for example, the shape of the coast, bathymetry), but also on the distance over which the SSH correction is transferred. These differential effects (tidal models and atmospheric pressure) are not used in this experiment because the estimated impact, even using high-resolution models, is at the level of a few millimeters over the considered area (Cancet et al. 2013). Thus, the corrections for ocean tide, inverted barometer and the high frequency wind and pressure response are not applied in this study. ...
The geodetic Corsica site was set up in 1998 in order to perform altimeter calibration of the TOPEX/Poseidon (T/P) mission and subsequently, Jason-1, OSTM/Jason-2, Jason-3 and more recently Sentinel-6 Michael Freilich (launched on November, 21 2020). The aim of the present study held in June 2015 is to validate a recently developed GNSS-based sea level instrument (called CalNaGeo) that is designed with the intention to map Sea Surface Heights (SSH) over large areas. This has been undertaken using the well-defined geodetic infrastructure deployed at Senetosa Cape, and involved the estimation of the stability of the waterline (and thus the instantaneous separation of a GNSS antenna from water level) as a function of the velocity at which the instrument is towed. The results show a largely linear relationship which is approximately 1 mm/(m/s) up to a maximum practical towing speed of ∼10 knots (∼5 m/s). By comparing to the existing “geoid” map, it is also demonstrated that CalNaGeo can measure a sea surface slope with a precision better than 1 mm/km (∼2.5% of the physical slope). Different processing techniques are used and compared including GNSS Precise Point Positioning (PPP, where the goal is to extend SSH mapping far from coastal GNSS reference stations) showing an agreement at the 1-2 cm level.
... They require simultaneous in situ and altimetry measurements in the same terrestrial 2.5. RESULTS 2 67 reference frame at the exact same location or comparison point (e.g., Cancet et al. (2013)). The absolute altimeter bias (Bi as al t i met er ) is estimated as follows (Ménard et al. 1994): ...
Intertidal flats provide essential services including protection against storm surges and coastal flooding.These environments are characterized by a continuous redistribution of sediments and topographic changes.They are under increasing pressure due to anthropogenic activities and sea level rise. The continuous monitoring of their topography is fundamental for hydrodynamic and morphodynamic modeling of coastal systems. Intertidal flats are among the most logistically challenging coastal landforms for ground-based and airborne-based topography monitoring. Spaceborne-based monitoring is the only viable and the mostcost-effective approach capable of providing regularly intertidal topography maps. Recent developments in radar (altimetry and Synthetic Aperture Radar (SAR)) and optical technologies, bring new types of data and enhanced capacities for monitoring intertidal environments. This PhD dissertation investigates the use of spaceborne-based methods for monitoring the topography in intertidal areas using radar altimetry, SAR, and multispectral satellite observations. The main objective is to explore, develop, or enhance methods dedicated for intertidal topography mapping. Being a part of the preparation phase of the future SWOT (Surface Water and Ocean Topography) radar altimetry mission, first mission to operate in wide-swath SAR interferometrymode, this PhD dissertation also investigates the capability of SWOT to map intertidal topography. Methodologies developed or/and used in this thesis were applied to two intertidal environments located on the French Coast: The Arcachon Bay and the Bay of Veys. We showed, for the first time, that recent advancements in technologies enabled satellite radar altimetry to extract intertidal topography profiles along the altimeters ground tracks. Second, we introduced an improved and quasi-automatic approach for the use of the waterline method (most common method for intertidal topography mapping) to derive intertidalDigital Elevation Models (DEMs). The changes include faster, more efficient and quasi-automatic detection and post-processing of waterlines. We also brought to light the ability of SWOT to generate highly accurate intertidal DEMs using the waterline method. The methodologies used here allow the generation of intertidal topography measurements regularly, that with careful usage, can be used for detecting topographic changes in intertidal environments. We showed that the two study areas eroded during 2016-2018, losing 1,12 × 106 m3 and 0,70 × 106 m3 for the Arcachon Bay and the Bay of Veys respectively. Updated DEMs provide useful and needed information for several scientific applications (e.g., sediment balance, hydrodynamic modelling), but also for authorities and stakeholders for coastal management and implementation of ecosystem protectionpolicies.
... Up to now, there have been four absolute, permanent and historic calibration sites all over the world. Corsica, which was established in 1998, is running by the French space agency CNES in Corsica, France (Bonnefond et al., 2003a(Bonnefond et al., , b, 2010(Bonnefond et al., , 2015(Bonnefond et al., , 2018Cancet et al., 2013). It supports continuous monitoring of Jason-2 and Jason-3 (formerly T/P and Jason-1), and Sentinel-3A and SARAL/ALtiKa (formerly ERS and Envisat). ...
Satellite altimeter needs to be calibrated to evaluate the accuracy of sea surface height data. The dedicated altimeter calibration field needs to establish a special calibration strategy and needs to evaluate its calibration ability. This paper describes absolute calibration of HY-2B altimeter SSH using the GPS calibration method at the newly Wanshan calibration site, located in the Wanshan Islands, China. There are two HY-2B altimeter passes across the Wanshan calibration site. Pass No. 362 is descending and the ground track passes the east of Dan’gan Island. Pass No. 375 is ascending and crosses the Zhiwan Island. The GPS data processing strategy of Wanshan calibration site was established and the accuracy of GPS calibration method of Wanshan calibration site was evaluated. Meanwhile, the processing strategies of the HY-2B altimeter for the Wanshan calibration site were established, and a dedicated geoid model data were used to benefit the calibration accuracy. The time-averaged HY-2B altimeter bias was approximately 2.12 cm with a standard deviation of 2.08 cm. The performance of the HY-2B correction microwave radiometer was also evaluated in terms of the wet troposphere path delay and showed a mean difference −0.2 cm with a 1.4 cm standard deviation with respect to the in situ GPS radiosonde.
... However, some elements such as instrument drift will require a much longer period that could extend to the mission lifetime (e.g. Cancet et al., 2013). ...
Given the considerable range of applications within the European Union Copernicus system, sustained satellite altimetry missions are required to address operational, science and societal needs. This article describes the Copernicus Sentinel-6 mission that is designed to provide precision sea level, sea surface height, significant wave height, inland water heights and other products tailored to operational services in the ocean, climate, atmospheric and land Copernicus Services. Sentinel-6 provides enhanced continuity to the very stable time series of mean sea level measurements and ocean sea state started in 1992 by the TOPEX/Poseidon mission and follow-on Jason-1, Jason-2 and Jason-3 satellite missions. The mission is implemented through a unique international partnership with contributions from NASA, NOAA, ESA, EUMETSAT, and the European Union (EU). It includes two satellites that will fly sequentially (separated in time by 5 years). The first satellite, named Sentinel-6 Michael Freilich, launched from Vandenburg Air Force Base, USA on 21st November 2020. The satellite and payload elements are explained including required performance and their operation. The main payload is the Poseidon-4 dual frequency (C/Ku-band) nadir-pointing radar altimeter that uses an innovative interleaved mode. This enables radar data processing on two parallel chains the first provides synthetic aperture radar (SAR) processing in Ku-band to improve the received altimeter echoes through better along-track sampling and reduced measurement noise; the second provides a Low Resolution Mode that is fully backward-compatible with the historical reference altimetry measurements, allowing a complete inter-calibration between the state-of-the-art data and the historical record. A three-channel Advanced Microwave Radiometer for Climate (AMRC) provides measurements of atmospheric water vapour to mitigate degradation of the radar altimeter measurements. The main data products are explained and preliminary in-orbit Poseidon-4 altimeter data performance data are presented that demonstrate the altimeter to be performing within expectations.
... The magnitude of these effects depends not only on local conditions (for example, the shape of the coast, bathymetry), but also on the distance over which the SSH is transferred. Tidal models and atmospheric pressure are not used in Corsica because the estimated impact, even using high-resolution models, is at the level of a few millimeters over the considered area (Cancet et al., 2013). Thus, the corrections for ocean tide, inverted barometer and the high frequency wind and pressure response are not applied to the altimetry measurements in equation (2) nor to the in situ ones because the signal is supposed to be identical. ...
Initially developed for monitoring the performance of TOPEX/Poseidon and follow-on Jason legacy satellite altimeters, the Corsica geodetic facilities that are located both at Senetosa Cape and near Ajaccio have been developed to calibrate successive satellite altimeters in an absolute sense. Since 1998, the successful calibration process used to calibrate most of the oceanographic satellite altimeter missions has been regularly updated in terms of in situ instruments, geodetic measurements and methodologies. In this study, we present an assessment of the long-term stability of the in situ instruments in terms of sea level monitoring that include a careful monitoring of the geodetic datum. Based on this 20-yr series of sea level measurements, we present a review of the derived absolute Sea Surface Height (SSH) biases for the following altimetric missions based on the most recent reprocessing of their data set: TOPEX/Poseidon and Jason-1/2/3, Envisat and ERS-2, CryoSat-2, SARAL/AltiKa and Sentinel-3A&B. For the longest time series the standard error of the absolute SSH biases is now at a few millimeters level which is fundamental to maintain the high level of confidence that scientists have in the global mean sea level rise.
... The absolute bias of each mission and its change are monitored at dedicated Calibration/Validation (cal/val) sites by direct comparison of the altimetric data with in-situ data (Christensen et al. 1994;Haines et al. 2003;Watson et al. 2011;Bonnefond et al. 2018). Specific calibrations scenarios, as in Crétaux et al. (2013) who perform GPS surveys from a boat cruising along the satellite tracks, and relative calibration approaches as in Cancet et al. (2013), are also used. The relative biases between the missions is the difference of the absolute calibration biases and can be determined independently by crossover analysis. ...
By convention the absolute bias in sea surface height (SSH) is the difference between the altimeter and the in-situ reference SSH heights above the Earth ellipsoid. Both the absolute and the relative bias of the CryoSat-2 and Sentinel-3A missions are derived in this study at four stations along the German coasts.
Firstly, the coastal data processed in Delay Doppler altimeter (DDA) mode, also called SAR mode (SARM), are shown to be less noisy than data in pseudo-low resolution mode (PLRM), which is comparable to the conventional low-resolution mode (LRM). The best agreement with in-situ data is reached by the SARM data retracked with the SAMOSA+ coastal retracker (hereafter SAR/SAMOSA+) from the ESA GPOD SARvatore service.
Secondly the absolute bias and its standard deviation are computed for each mission and product type. The mean mission absolute bias depends on location and altimeter product. Both the absolute and relative biases are small and the standard deviation is smaller than 4 cm and larger than the bias. Departures between absolute biases evaluated at different stations are possibly related to geoid inaccuracy in the coastal zone. Finally, the smaller standard deviation of the bias time series confirms that SAR altimetry is more accurate than PLRM, the minimum standard deviation is 2 cm.
... Comparisons between altimetry-based and in situ SSH from tide gauge were performed. They require simultaneous in situ and altimetry measurements in the same terrestrial reference frame at the exact same location or comparison point (e.g., Cancet et al. [36]). The absolute altimeter bias (Bias altimeter ) is estimated as follows [37]: ...
Radar altimetry was initially designed to measure the marine geoid. Thanks to the improvement in the orbit determination from the meter to the centimeter level, this technique has been providing accurate measurements of the sea surface topography over the open ocean since the launch of Topex/Poseidon in 1992. In spite of a decrease in the performance over land and coastal areas, it is now commonly used over these surfaces. This study presents a semi-automatic method that allows us to discriminate between acquisitions performed at high tides and low tides. The performances of four radar altimetry missions (ERS-2, ENVISAT, SARAL, and CryoSat-2) were analyzed for the retrieval of sea surface height and, for the very first time, of the intertidal zone topography in a coastal lagoon. The study area is the Arcachon Bay located in the Bay of Biscay. The sea level variability of the Arcachon Bay is characterized by a standard deviation of 1.05 m for the records used in this study (2001–2017). Sea surface heights are very well retrieved for SARAL (R~0.99 and RMSE < 0.23 m) and CryoSat-2 (R > 0.93 and RMSE < 0.42 m) missions but also for ENVISAT (R > 0.82 but with a higher RMSE >0.92 m). For the topography of the intertidal zone, very good estimates were also obtained using SARAL (R~0.71) and CryoSat-2 (R~0.79) with RMSE lower than 0.44 m for both missions.
... In making the cross-validation of altimetry versus tide gauge, we have removed the mean sea surface along the CryoSat-2 passes using the DTU15 mean sea surface model. This is a necessary operation because CryoSat-2 is a satellite with a long repeat cycle (369 days) and hence different relative passes in the same repeat cycle sample differently the mean sea surface (see Fenoglio-Marc et al., 2015a andCancet et al., 2013). We have also removed the tidal dynamics between the tide gauge and CryoSat-2 passes using the TPXO8-ATLAS tide model because it is expected the tide to vary between tide gauge position and CryoSat-2 pass, being the region of interest a high dynamic tidal area. ...
Unlike previous altimetric missions, the CryoSat-2 altimeter features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. The scope of this study is to quantify regionally the skills of CryoSat-2 SAR altimetry for distances to coast smaller than 10 km, during the mission lifetime and at different time scales. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid, the significant sea wave height and wind speed, all computed at 20 Hz. These have been compared to in situ and regional model data along the coasts of German Bight and West Baltic Sea during a time interval of almost six years, from July 2010 to March 2016, to investigate both instantaneous and seasonal behaviour.
From CryoSat-2 FBR (Full Bit Rate) data, a Delay-Doppler processing and waveform retracking tailored specifically to the coastal zone has been carried out, by applying a Hamming window and zero-padding, using an extended vertical swath window in order to mitigate tracker errors. Moreover, a dedicated SAMOSA-based coastal retracker (here referred to as SAMOSA+) has also been implemented.
Since one of the highest remaining uncertainties in the altimeter parameters estimated in coastal shallow waters arises from residual errors in the applied range and geophysical corrections, innovative and high resolution solutions for ocean tide model, geoid, mean sea surface and wet tropospheric correction have been selected. As CryoSat-2 SAR and LRM (Low Rate Mode) modes are not collocated in time, in order to quantify the improvement in the coastal zone with respect to pulse-limited altimetry, 20 Hz PLRM (pseudo-LRM) data from CryoSat-2 FBR were built and retracked, adopting the ALES adaptive sub-waveform approach, with a numerical Brown-based retracker, here referred to as TALES.
The cross-validation proves the good consistency between PLRM and SAR sea level anomaly in the coastal zone. The regional ocean model (BSH) shows the highest agreement with the SAR sea level anomaly, with a standard deviation of the differences (stdd) of 24 cm, whereas the corresponding value with respect to PLRM is 55 cm. Distance to coast plots show that land contamination begins to affect sea level and wave measurements at 2 km from the coast in SAR and at 3.5 km in PLRM TALES. The analysis of monthly mean time-series shows that SAR Altimetry is able to measure the sea level monthly mean in the coastal zone of the region of interest, during the entire mission, more precisely than PLRM. The cross-validation against in situ data also proves the higher accuracy of SAR SAMOSA+ compared to PLRM TALES in the coastal zone, with average SLA stdd of 4.4 cm and 8.4 cm respectively.
... The absolute in situ calibration of the altimeter missions could insure the regular and long-term control of the altimeter sea surface height (SSH) time series with independent measurements [1] . At present there are mainly four absolute calibration sites over the world, which have been providing absolute biases for multiple satellite altimeters from the T/P mission (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)) to the Jason-3 mission launched in 2016. ...
... These datasets have been used, for example, for the studies of the marine geoid, tides, currents, and presentday sea-level rise. To assess the data quality from the altimeters, generally, the sea surface height (SSH) products can be analyzed using altimeter crossover analysis, and/or the comparison with in situ observations from Global Navigation Satellite System (GNSS) buoys, and/or tidal gauge observations over oceans and large lakes (Ménard et al., 1999;Shum et al., 2003;Chambers et al., 2003;Dorandeu et al., 2004;Francis et al., 2005;Cheng et al., 2010;Haines et al., 2010;Crétaux et al., 1990;Cancet et al., 2012;Watson et al., 2012;Bonnefond et al., 2012). Thus, calibration and validation of satellite altimeter data are critical and necessary, as the results of the analysis and comparisons performed have led to and will lead to the improvement of sensor calibrations and the geophysical algorithms that are the key to producing robust science data processing system and climate data records (Lillibridge et al., 2013). ...
Since the launch of China’s first altimetry and scatterometry satellite, Haiyang-2A (HY-2A), various validation studies of HY-2A radar altimetry using preliminary data products have been conducted. Here, we present the first comprehensive result assessing HY-2A’s altimeter data quality and the altimetry system performance using an improved HY-2A Geophysical Data Record (GDR) product (Institute of Geodesy and Geophysics reprocessed GDR product version A, GDR_IGGA). The main improvements include altimeter timing and waveform retracking, and tropospheric, ionospheric, and sea-state bias (SSB) corrections, which resulted in more accurate HY-2A sea surface height observations. Jason-2 altimeter observations are used for the cross calibration of the HY-2A altimeter over the oceans between ±60° latitude bounds, primarily due to the limitation of Jason-2 coverage. The statistical results from single- and dual-satellite altimeter crossover analysis demonstrated that HY-2A fulfills its mission requirements. We uncovered a mean relative bias of −0.21 cm (with respect to Jason-2), and a standard deviation of 6.98 cm from dual-satellite crossover analysis. In addition, we present the results of a detailed analysis on variance reduction studies for the various geophysical corrections from HY-2A and compared them with Jason-2. The wave-number spectra of HY-2A and Jason-2 sea-level anomalies show similar spectral content, verifying the performance of HY-2A altimetry to be similar to Jason-2. Open issues and the remaining HY-2A data problems have been identified, allowing prospective future studies for further improvement of its accuracy.
By convention the absolute bias in sea surface height (SSH) is the difference between the altimeter and the in-situ reference SSH heights above the Earth ellipsoid. Both the absolute and the relative bias of the CryoSat-2 and Sentinel-3A missions are derived in this study at four stations along the German coasts.
Sentinel-3A is scheduled for launch in Oct. 2015, with Sentinel-3B to follow 18 months later. Together these missions are to take oceanographic remote-sensing into a new operational realm. To achieve this a large number of processing, calibration and validation tasks have to be applied to their data in order to assess for quality, absolute bias, short-term changes and long-term drifts. ESA has funded the Sentinel-3 Mission Performance Centre (S3MPC) to carry out this evaluation on behalf of ESA and EUMETSAT. The S3MPC is run by a consortium led by ACRI [1] and this paper describes the work on the calibration/validation (cal/val) of the Surface Topography Mission (STM), which is coordinated by CLS and PML.
The geodetic Corsica site was set up in 1998 in order to perform altimeter calibration of the TOPEX/Poseidon (T/P) mission and subsequently, Jason-1 and OSTM/Jason-2. The scope of the site was widened in 2005 in order to undertake the calibration of the Envisat mission and most recently of SARAL/AltiKa. Here we present the first results from the latter mission using both indirect and direct calibration/validation approaches. The indirect approach utilizes a coastal tide gauge and, as a consequence, the altimeter derived sea surface height (SSH) needs to be corrected for the geoid slope. The direct approach utilizes a novel GPS-based system deployed offshore under the satellite ground track that permits a direct comparison with the altimeter derived SSH. The advantages and disadvantages of both systems (GPS-based and tide gauges) and methods (direct or indirect) will be described and discussed. Our results for O/IGD-R data show a very good consistency for these three kinds of products: their derived absolute SSH biases are consistent within 17 mm and their associated standard deviation ranges from 31 to 35 mm. The AltiKa absolute SSH bias derived from GPS-zodiac measurement using the direct method is −54 ±10 mm based on the first 13 cycles.
We describe results from two decades of monitoring vertical seafloor motion at the Harvest oil platform, NASA’s prime verification site for the TOPEX/Poseidon and Jason series of reference altimeter missions. Using continuous GPS observations, we refine estimates of the platform subsidence—due most likely to fluid withdrawal linked to oil production—and describe the impact on estimates of stability for the altimeter measurement systems. The cumulative seafloor subsidence over 20 yrs is approximately 10 cm, but the rate does not appear constant. The apparent non-linear nature of the vertical motion, coupled with long-period GPS errors, implies that the quality of the seafloor motion estimates is not uniform over the 20-yr period. For the Jason-1 era (2002–2009), competing estimates for the subsidence show agreement to better than 1 mm yr−1. Longer durations of data are needed before the seafloor motion estimates for the Jason-2 era (2008–present) can approach this level of accuracy.
An absolute calibration of the TOPEX/Poseidon (T/P) and Jason-1 altimeters has been undertaken during the dedicated calibration phase of the Jason-1 mission, in Bass Strait, Australia. The present study incorporates several improvements to the earlier calibration methodology used for Bass Strait, namely the use of GPS buoys and the determination of absolute bias in a purely geometrical sense, without the necessity of estimating a marine geoid. This article focuses on technical issues surrounding the GPS buoy methodology for use in altimeter calibration studies. We present absolute bias estimates computed solely from the GPS buoy deployments and derive formal uncertainty estimates for bias calculation from a single overflight at the 40-45 mm level. Estimates of the absolute bias derived from the GPS buoys is -10 ± 19 mm for T/P and + 147 ± 21 mm for Jason-1 (MOE orbit) and + 131 ± 21 mm for Jason-1 (GPS orbit). Considering the estimated error budget, our bias values are equivalent to other determinations from the dedicated NASA and CNES calibration sites.
Unlike the past International Terrestrial Reference Frame (ITRF) versions where global long-term solutions were combined, the ITRF2005 uses as input data time series (weekly from satellite techniques and 24-h session-wise from Very Long Baseline Interferometry) of station positions and daily Earth Orientation Parameters (EOPs). The advantage of using time series of station positions is that it allows to monitor station non-linear motion and discontinuities and to examine the temporal behavior of the frame physical parameters, namely the origin and the scale. The ITRF2005 origin is defined in such a way that it has zero translations and translation rates with respect to the Earth center of mass, averaged by the Satellite Laser Ranging (SLR) time series spanning 13 years of observations. Its scale is defined by nullifying the scale and its rate with respect to the Very Long Baseline Interferometry (VLBI) time series spanning 26 years of observations. The ITRF2005 orientation (at epoch 2000.0) and its rate are aligned to the ITRF2000 using 70 stations of high geodetic quality. The estimated level of consistency of the ITRF2005 origin (at epoch 2000.0) and its rate with respect to the ITRF2000 is respectively 0.1, 0.8, 5.8 mm and 0.2, 0.1, 1.8 mm/yr along the X, Y and Z-axis. We estimate the formal errors on these components to be 0.3 mm and 0.3 mm/yr. We believe that this low level of agreement between the two frame origins is most probably due to the poor SLR network geometry and its degradation over time. The ITRF2005 combination involving 84 co-location sites revealed a scale inconsistency of 1 ppb (6.3 mm at the equator), at epoch 2000.0, and 0.08 ppb/yr between the SLR and VLBI long-term solutions as obtained by the stacking of their respective time series. Possible causes of this inconsistency may include the poor SLR and VLBI networks and their co-locations, local tie uncertainties, systematic effects and possible inconsistent model corrections used in the data analysis of both techniques. For the first time of the ITRF history, the ITRF2005 rigorous combination provides self-consistent series of EOPs, including Polar Motion from VLBI and satellite techniques and Universal Time and Length of Day from VLBI only. A velocity field of 152 sites with an error less than 1.5 mm/yr is used to estimate absolute rotation poles of 15 tectonic plates that are consistent with the ITRF2005 frame. This new absolute plate motion model supersedes and significantly improves that of the ITRF2000 which involved six major tectonic plates.
This article describes an “absolute” calibration of Jason-1 (J-1) altimeter sea surface height bias using a method developed for TOPEX/Poseidon (T/P) bias determination reported previously. The method makes use of U.K. tide gauges equipped with Global Positioning System (GPS) receivers to measure sea surface heights at the same time, and in the same geocentric reference frame, as Jason-1 altimetric heights recorded in the nearby ocean. The main time-dependent components of the observed altimeter-minus-gauge height-difference time series are due to the slightly different ocean tides at the gauge and in the ocean. The main harmonic coefficients of the tide differences are calculated from analysis of the copious TOPEX data set and then applied to the determination of T, P, and J-1 bias in turn. Datum connections between the tide gauge and altimetric sea surface heights are made by means of precise, local geoid differences from the EGG97 model. By these means, we have estimated Jason-1 altimeter bias determined from Geophysical Data Record (GDR) data for cycles 1–61 to be 12.9 cm, with an accuracy estimated to be approximately 3 cm on the basis of our earlier work. This J-1 bias value is in close agreement with those determined by other groups, which provides a further confirmation of the validity of our method and of its potential for application in other parts of the world where suitable tide gauge, GPS, and geoid information exist.
[1] A global simulation of the ocean response to atmospheric wind and pressure forcing has been run during the Topex/Poseidon (T/P) period (1992–2002), using a new hydrodynamic finite element (FE) model, MOG2D-G. Model outputs are compared to in situ observations with tide gauge data (TG) and bottom pressure gauge data (BPR), and also with T/P altimetric cross over points (noted CO). Intercomparisons were performed over the 1993–1999 period. The model correction reduces the sea level variance by more than 50% at TG locations, and by more than 15% at T/P CO, when compared to the classical inverse barometer correction (IB). The model impact differs between high and low latitudes: in the very energetic high latitudes areas, MOG2D-G is efficient in reducing the variance, while at low latitudes, the results are similar to the IB static response. In shallow water, the model shows an oceanic response very different from the IB response. In conclusion, MOG2D-G models the high frequency (HF) atmospheric forced variability of the global ocean with unprecedented accuracy.
We present calibration results from Jason-1 (2001-) and TOPEX/POSEIDON (1992-) overflights of a California offshore oil platform (Harvest). Data from Harvest indicate that current Jason-1 sea-surface height (SSH) measurements are high by 138 ± 18 mm. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from the overflights. In orbit for over 10 years, the T/P measurement system is well calibrated, and the SSH bias is statistically indistinguishable from zero. Also reviewed are over 10 years of geodetic results from the Harvest experiment.
The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.
This paper presents the improvements made on the calibration methodology conducted at the Gavdos calibration/validation facility along with the latest altimeter calibration results for Jason-1 and Jason-2 satellite missions. Calibration results are presented, for the first time, for both ascending and descending passes of Jason satellites. The altimeter bias for Jason-2 has been estimated to be +173 ± 4 mm for Pass No. 109 and +171 ± 5 mm for Pass No. 018 over cycles 1–79. In tandem mission, the difference between Jason-1 and Jason-2 has been determined to be 72mm (Pass No. 109) and 68 mm (pass No. 018) and over cycles 2–20.
Updated absolute calibration results from Bass Strait, Australia, are presented for the TOPEX/Poseidon (T/P) and Jason-1 altimeter missions. Data from an oceanographic mooring array and coastal tide gauge have been used in addition to the previously described episodic GPS buoy deployments. The results represent a significant improvement in absolute bias estimates for the Bass Strait site. The extended methodology has allowed comparison between the altimeter and in situ data on a cycle-by-cycle basis over the duration of the dedicated calibration phase (formation flight period) of the Jason-1 mission. In addition, it has allowed absolute bias results to be extended to include all cycles since the T/P launch, and all Jason-1 data up to cycle 60. Updated estimates and formal 1-sigma uncertainties of the absolute bias computed throughout the formation flight period are 0 ± 14 mm for T/P and +152 + 13 mm for Jason-1 (for the GDR POE orbits). When JPL GPS orbits are used for cycles 1 to 60, the Jason-1 bias estimate is 131 mm, virtually identical to the NASA estimate from the Harvest Platform off California calculated with the GPS orbits and not significantly different to the CNES estimate from Corsica. The inference of geographically correlated errors in the GDR POE orbits (estimated to be approximately 17 mm at Bass Strait) highlights the importance of maintaining globally distributed verification sites and makes it clear that further work is required to improve our understanding of the Jason-1 instrument and algorithm behavior.
ITRF2008 is a refined version of the International Terrestrial Reference Frame based on reprocessed solutions of the four
space geodetic techniques: VLBI, SLR, GPS and DORIS, spanning 29, 26, 12.5 and 16years of observations, respectively. The
input data used in its elaboration are time series (weekly from satellite techniques and 24-h session-wise from VLBI) of station
positions and daily Earth Orientation Parameters (EOPs). The ITRF2008 origin is defined in such a way that it has zero translations
and translation rates with respect to the mean Earth center of mass, averaged by the SLR time series. Its scale is defined
by nullifying the scale factor and its rate with respect to the mean of VLBI and SLR long-term solutions as obtained by stacking
their respective time series. The scale agreement between these two technique solutions is estimated to be 1.05 ± 0.13 ppb
at epoch 2005.0 and 0.049 ± 0.010ppb/yr. The ITRF2008 orientation (at epoch 2005.0) and its rate are aligned to the ITRF2005
using 179 stations of high geodetic quality. An estimate of the origin components from ITRF2008 to ITRF2005 (both origins
are defined by SLR) indicates differences at epoch 2005.0, namely: −0.5, −0.9 and −4.7mm along X, Y and Z-axis, respectively. The translation rate differences between the two frames are zero for Y and Z, while we observe an X-translation rate of 0.3mm/yr. The estimated formal errors of these parameters are 0.2mm and 0.2mm/yr, respectively. The
high level of origin agreement between ITRF2008 and ITRF2005 is an indication of an imprecise ITRF2000 origin that exhibits
a Z-translation drift of 1.8mm/yr with respect to ITRF2005. An evaluation of the ITRF2008 origin accuracy based on the level
of its agreement with ITRF2005 is believed to be at the level of 1cm over the time-span of the SLR observations. Considering
the level of scale consistency between VLBI and SLR, the ITRF2008 scale accuracy is evaluated to be at the level of 1.2ppb
(8mm at the equator) over the common time-span of the observations of both techniques. Although the performance of the ITRF2008
is demonstrated to be higher than ITRF2005, future ITRF improvement resides in improving the consistency between local ties
in co-location sites and space geodesy estimates.
KeywordsReference systems–Reference frames–ITRF–Earth rotation
During the 1990s, a large number of new tidal atlases were developed, primarily to provide accurate tidal corrections for
satellite altimetry applications. During this decade, the French tidal group (FTG), led by C. Le Provost, produced a series
of finite element solutions (FES) tidal atlases, among which FES2004 is the latest release, computed from the tidal hydrodynamic
equations and data assimilation. The aim of this paper is to review the state of the art of tidal modelling and the progress
achieved during this past decade. The first sections summarise the general FTG approach to modelling the global tides. In
the following sections, we introduce the FES2004 tidal atlas and validate the model against in situ and satellite data. We
demonstrate the higher accuracy of the FES2004 release compared to earlier FES tidal atlases, and we recommend its use in
tidal applications. The final section focuses on the new dissipation term added to the equations, which aims to account for
the conversion of barotropic energy into internal tidal energy. There is a huge improvement in the hydrodynamic tidal solution
and energy budget obtained when this term is taken into account.
The determination of global and regional mean sea level variations with accuracies better than 1 mm/year is an important yet challenging problem, the resolution of which is central to the current debate on climate change and its impact on the environment. To address this, highly accurate time series from both satellite altimetry and tide gauges are needed. In both cases, the desired accuracy represents a significant challenge for the geodetic community. From the perspective of space borne altimetry, systematic errors from the orbit, reference frame and altimeter systems are all important limiting factors and must be minimized in order to derive data products of greatest geophysical value. Indeed, the objective for the overall accuracy of future altimeter systems is 1-cm (RMS) along with a stability of 1 mm/year. From the terrestrial perspective, estimating the vertical velocity of tide gauge sites to sufficient accuracy is also one of the most important and challenging problems in modern geodesy. Essential to reaching these goals in the measurement of mean sea level variation are ultra-precise validation and calibration techniques, including in situ absolute calibration experiments. Most of the present calibration experiments are on or near the coast, reinforcing the need for developing such techniques to unify the altimetric error budget for both open-ocean and local (coastal) conditions.
The altimeter range should be corrected for tropospheric path delays due to atmospheric pressure at sea level and atmospheric humidity. Over open ocean, these corrections are performed with enough accuracy using the microwave radiometer for the wet path delay and meteorological model analyses for the dry path delay. In coastal areas, specific studies are needed to assess the quality of the standard products and to propose specific processing if necessary. For the wet tropospheric correction, new promising approaches are presented based on optimal combination of radiometer, meteorological model, GNSS and land information. For the dry tropospheric correction, an assessment of the accuracy of the model-based estimation is provided.
In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites' ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of -0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.
The Corsica site has been established in 1996 to perform altimeter calibration on TOPEX/Poseidon and then on its successors Jason-1 and Jason-2. The first chosen location was under the #85 ground track that overflight the Senetosa Cape. In 2005, it was decided to develop another location close to Ajaccio, to be able to perform the calibration of Envisat and in a next future of SARAL/AltiKa that will flight over the same ground tracks. Equipped with various instruments (tide gauges, permanent GPS, GPS buoy, weather station…) the Corsica calibration site is able to quantify the altimeter Sea Surface Height bias but also to give an input on the origin of this bias (range, corrections, orbits, …). Due to the size of Corsica (not a tiny island), the altimeter measurement system (range and corrections) can be contaminated by land. The aim of this paper is to evaluate this land contamination by using GPS measurements from a fixed receiver on land and from another receiver onboard a life buoy. Concerning the altimeter land contamination, we have quantify that this effect can reach 8 mm/km and then affects the Sea Surface Height bias values already published in the framework of the Corsica calibration site by 5–8 mm for TOPEX and Jason missions. On the other hand, the radiometer measurements (wet troposphere correction) are also sensitive to land and we have been able to quantify the level of improvement of a dedicated coastal algorithm that reconciles our results with those coming from other calibration sites. Finally, we have also shown that the standard deviation of the GPS buoy sea level measurements is highly correlated (∼87%) with the Significant Wave Height derived from the altimeters and can be used to validate such parameter.
Updated absolute bias estimates are presented from the Bass Strait calibration site (Australia) for the TOPEX/Poseidon (T/P), Jason-1 and the Ocean Surface Topography Mission (OSTM/Jason-2) altimeter missions. Results from the TOPEX side A and side B data show biases insignificantly different from zero when assessed against our error budget (−15 ± 20 mm, and −6 ± 18 mm, respectively). Jason-1 shows a considerably higher absolute bias of +93 ± 15 mm, indicating that the observed sea surface is higher (or the range shorter), than truth. For OSTM/Jason-2, the absolute bias is further increased to +172 ± 18 mm (determined from T/GDR data, cycles 001–079). Enhancements made to the Jason-1 and OSTM/Jason-2 microwave radiometer derived products for correcting path delays induced by the wet troposphere are shown to benefit the bias estimate at the Bass Strait site through the reduction of land contamination. We note small shifts to bias estimates when using the enhanced products, changing the biases by +11 and +3 mm for Jason-1 and OSTM/Jason-2, respectively. The significant, and as yet poorly understood, absolute biases observed for both Jason series altimeters reinforces the continued need for further investigation of the measurement systems and ongoing monitoring via in situ calibration sites.
TOPEX measurements of sea level variability have been compared to tide gauge measurements from 40 sites and to dynamic topography measurements computed from temperatures recorded at 23 Tropical Ocean-Global Atmosphere (TOGA)-Tropical Atmosphere-Ocean (TAO) buoys in the eastern Pacific and mean temperature-salinity profiles. Buoy data in the western Pacific were not used because of large long-term slopes in the data that appear to be due to interannual salinity variations. The relative drift between TOPEX and the two different in situ sets of data agree within 1 mmyr-1, with a weighted average of -2.6 mmyr-1 and an estimated uncertainty of 1.5 mmyr-1, if values from an internal calibration of the TOPEX altimeter are applied. The consistency of the two relative drifts suggests that the slope is due at least in part to a drift in the TOPEX measurement. A substantial portion of this drift may be due to a drift in the TOPEX microwave radiometer (TMR), since comparisons with three independent external measurements indicate a drift in sea level due to the TMR measurement of about -2 mmyr-1.
A method is described for using tide gauge sea levels to monitor time-dependent drift in satellite altimetric measurements of sea surface height. The method depends on a careful assessment of the quality of the tide gauge measurements available for this application and also takes into account the degree of independence between the altimeter minus tide gauge differences in order to construct an optimal drift estimate and an accurate error estimate for it. The method is applied to the TOPEX altimeter measurements, and a recently discovered algorithm error, which resulted in a slow drift in the TOPEX sea surface heights, is exploited to evaluate the success of the tide gauge drift estimation. It is important to note that the tide gauge analysis was done without any prior knowledge of this error. The result is that the tide gauge analysis reproduces the drift due to the algorithm error to within 6 mm rms, which is comparable to the 5-6-mm internal estimate of the uncertainty of the drift analysis. The analysis is then made using the TOPEX data that have been corrected for the algorithm error and shows that the satellite heights are stable to better than 10 mm over the nearly 4 years of data available, although a drift on the order of 2 mm yr-1 remains, the source of which is unknown. This inferred stability is more than adequate for the majority of applications, although questions still exist for more demanding applications, such as the calculation of global sea level change. Limitations of the tide gauge analysis are discussed, along with potential improvements that might be possible in the future.
We present a 17-year calibration record of precise (Jason-class) spaceborne altimetry from a California offshore oil platform (Harvest). Our analyses indicate that the sea-surface-height (SSH) biases for all three TOPEX/Poseidon (1992–2005) measurement systems are statistically indistinguishable from zero at the 15 mm level. In contrast, the SSH bias estimates for the newer Jason-1 mission (2001–present) and the Ocean Surface Topography Mission (2008–present) are significantly positive. In orbit for over eight years, the Jason-1 measurement system yields SSH biased by +94 ± 15 mm. Its successor, OSTM/Jason-2, produces SSH measurements biased by +178 ± 16 mm.
The Italian Lampedusa island in the Mediterranean Sea was chosen by the Centre National d'Etudes Spatiales (CNES) to support the verification of altimeters on board the TOPEX/POSEIDON satellite. The orbit was phased in such a way that a descending pass overflies a small islet called Lampione, 20 km west of Lampedusa. During the TOPEX/POSEIDON verification phase, until mid-December 1992, the satellite passed nine times over Lampedusa. The Lampedusa site was equipped with all the required instrumentation to provide independent and accurate knowledge of the altimeter system components: sea level, orbit, and geophysical corrections. In addition, a reprocessing of the altimeter data was performed to better adjust the waveform parameters estimation especially when quasi-specular echoes due to flat sea conditions have disturbed the on-board estimation. Three different local orbits computed by expert groups (Centre d'Etudes et de Recherches Geodynamiques et Astronomiques, Delft University of Technology, and Jet Propulsion Laboratory), plus CNES and NASA global orbits were considered. The bias of the CNES altimeter (SSALT) was then calculated, based on six overflights, giving a mean value of 0.7 cm. A SSALT bias also was calculated at Harvest, the NASA calibration site, using six overflights. The deduced bias is 1.3 cm. Integrating both site results gives a mean SSALT bias of 1 cm with an uncertainty of 2.4 cm. The NASA altimeter (ALT) bias computed at Lampedusa is -18.5 cm with an uncertainty of 3.4 cm, but this result must be carefully considered because it was obtained with only two overflights. Considering the Lampedusa SSALT calibration results and those derived from statistical repeat track analysis giving an estimation of the relative bias between SSALT and ALT (e.g., Le Traon et al., this issue) a bias of -14.8 cm with an uncertainty of 2.6 cm was found for the ALT altimeter.
The Jason-1 altimeter satellite and its follow-on mission Jason-2/OSTM were launched in December 2001 and June 2008, respectively, to provide the scientific community with a high-accuracy continuous record of observations of the ocean surface topography. Both missions carry on board three state-of-the-art tracking systems (DORIS, GPS, SLR) to meet the requirement of better-than-1.5 cm radial accuracy for the operational orbit included in the geophysical data record (GDR) product.This article outlines the common set of models and processing techniques applied to both Jason reprocessed and operational orbits included in version C of the GDR, referred to as GDR-C standards for precision orbit determination (POD), and describes the systematic components of the radial error budget that are of most interest for the altimeter data analysts. The nonsystematic component of the error budget, quantified by intercomparison of orbits using similar models or with reduced dependency on the dynamic models, is generally at or below 7 mm RMS (root-mean-square). In particular, the average daily RMS of the radial difference between the JPL and CNES reduced-dynamic orbits on Jason-2 is below 6 mm. Concerning the dynamic models employed, the principal contributors to residual systematic differences appear to be the time varying gravity and solar radiation pressure, resulting in geographically correlated periodic signals that have amplitudes at the few-mm level. Concerning the drifts of the orbits along the North/South direction, all solutions agree to better than the 1 mm/year level.
Absolute calibration of sea level measurements collected from space-borne radar altimeters is usually performed with respect to collocated sea level in situ records from tide gauges or GPS buoys (Ménard et al. 199415.
Ménard , Y. , Jeansou , E. and Vincent , P. 1994 . Calibration of the TOPEX-Poseidon altimeters at Lampedusa: Additional results at Harves . J. Geophys Res. , 99 ( C12 ) : 24487 – 24504 . http://dx.doi.org/10.1029%2F94JC01300 [CrossRef], [Web of Science ®], [CSA]View all references; Haines et al. 19969.
Haines , B. J. , Christensen , E. J. , Norman , R. A. , Parke , M. E. , Born , G. H. and Gill , S. K. 1996 . Altimeter calibration and geophysical monitoring from collocated measurements at the Harvest oil platform . EOS Trans. Suppl. , 77 ( 22 ) : W16 View all references; Bonnefond et al. 2003; Haines et al. 200310.
Haines , B. J. , Dong , D. , Born , G. H. and Gill , S. K. 2003 . The Harvest experiment: Monitoring Jason-1 and TOPEX/Poseidon from a California offshore platform . Mar. Geod. , 26 : 239 – 259 . [Taylor & Francis Online]View all references; Schum et al. 200318.
Schum , C. K. , Yi , Y. , Cheng , K. , Kuo , C. , Braun , A. , Calmant , S. and Chambers , D. 2003 . Calibration of Jason-1 Altimeter over Lake Erie . Mar. Geod. , 26 : 335 – 354 . [Taylor & Francis Online]View all references; Watson et al. 200321.
Watson , C. , Coleman , R. , White , N. , Church , J. and Govind , R. 2003 . Absolute calibration of TOPEX/ Poseidon and Jason-1 using GPS buoys in Bass Strait, Australia . Mar. Geod. , 26 : 285 – 304 . [Taylor & Francis Online]View all references; Watson et al. 200422.
Watson , C. , White , N. , Coleman , R. , Church , J. , Morgan , P. and Govind , R. 2004 . TOPEX/Poseidon and Jason-1: Absolute calibration in Bass Strait, Australia . Mar. Geod. , 27 : 107 – 131 . http://dx.doi.org/10.1080%2F01490410490465373 [Taylor & Francis Online]View all references). Such a method allows regular and long-term control of altimetric systems with independent records. However, this approach is based on a single, geographically dependent point. In order to obtain more significant and accurate bias and drift estimates, there is a strong interest in multiplying the number of calibration opportunities. This article describes a method, called the “offshore method” that was developed to extend the single-point approach to a wider regional scale. The principle is to compare altimeter and tide gauge sea level data not only at the point of closest approach of an overflying pass, but also at distant points along adjacent satellite passes. However, connecting sea level satellite measurements with more distant in situ data requires a more accurate determination of the geoid and mean ocean dynamic topography slopes, and also of the ocean dynamical changes. In this demonstration experiment, 10 years of averaged TOPEX/Poseidon mean sea level profiles are used to precisely determine the geoid and the mean ocean circulation slope. The Mog2d barotropic ocean model (Carerre et Lyard 20033.
Carrère , L. and Lyard , F. 2003 . Modelling the barotropic response of the global ocean to atmospheric wind and pressure forcing-comparisons with observations . GRL , 30 ( 6 ) : 1275 [CrossRef], [Web of Science ®]View all references) is used to improve our estimate of the ocean dynamics term. The method is first validated with Jason-1 data, off Corsica, where the dedicated calibration site of Senetosa provides independent reference data. The method is then applied to TOPEX/Poseidon on its new orbit and to Geosat Follow On. The results demonstrate that it is feasible to make altimeter calibrations a few tens to hundreds of kilometers away from a dedicated site, as long as accurate mean sea level altimeter profiles can be used to ensure the connection with reference tide gauges.
Altimetry has become a powerful tool to understand the dynamics of the deep-sea ocean circulation. Despite the technical problems encountered in the coastal zone by this observational technique, resulting in large data gaps in those areas, solutions already exist to mitigate this issue and to allow the retrieval of coastal information from existing altimetric data. Using some of these solutions, we have reprocessed a new set of 14.5 years of the TOPEX/Poseidon and Jason-1 satellite altimeter data over the Northwestern Mediterranean Sea, leading to a significant increase in the quantity of available data near coastlines. Time series of geostrophic surface velocity anomalies have been computed from the along-track altimeter sea level anomalies. In this paper, we evaluate the ability of these altimeter-derived currents to capture the main surface circulation features and the associated seasonal variability in the area of interest.
Goddard Ocean Tide model GOT99.2 is a new solution for the amplitudes and phases of the global oceanic tides, based on over six years of sea-surface height measurements by the TOPEX/POSEIDON satellite altimeter. Comparison with deep-ocean tide-gauge measurements show that this new tidal solution is an improvement over previous global models, with accuracies for the main semidiurnal lunar constituent M2 now below 1.5 cm (deep water only). The new solution benefits from use of prior hydrodynamic models, several in shallow and inland seas as well as the global finite-element model FES94.1. This report describes some of the data processing details involved in handling the altimetry, and it provides a comprehensive set of global cotidal charts of the resulting solutions. Various derived tidal charts are also provided, including tidal loading deformation charts, tidal gravimetric charts, and tidal current velocity (or transport) charts. Finally, low-degree spherical harmonic coefficients are computed by numerical quadrature and are tabulated for the major short-period tides; these are useful for a variety of geodetic and geophysical purposes, especially in combination with similar estimates from satellite laser ranging.
The use of a spar buoy equipped with a Global Positioning System (GPS) antenna to calibrate the height measurement of the TOPEX radar altimeter is described. In order to determine the height of the GPS antenna phase center above the ocean surface, the buoy was also equipped with instrumentation to measure the instantaneous location of the waterline, and tilt of the bouy from vertical. The experiment was conducted off the California coast near the Texaco offshore oil platform, Harvest, during cycle 34 of the TOPEX/POSEIDON observational period. GPS solutions were computed for the bouy position using two different software packages, K&RS and GIPSY-OASIS II. These solutions were combined with estimates of the waterline location on the bouy to yield the height of the ocean surface. The ocean surface height in an absolute coordinate system combined with knowledge of the spacecraft height from tracking data provides a computed altimeter range measurement. By comparing this computed value to the actual altimeter measurement, the altimeter bias can be calibrated. The altimeter height bias obtained with the buoy using K&RS was -14.6 +/- 4 cm, while with GIPSY-OASIS II it was -13.1 +/- 4 cm. These are 0.1 cm and 1.6 cm different from the -14.7 +/- 4 cm result obtained for this flight overflight with the tide gauge instruments located on Platform Harvest.
Envisat RA2/MWR ocean data validation and cross-calibration activities. Yearly Report 2010, available on the AVISO website http://www.aviso.oceanobs.com
- A Ollivier
- Y Faugère
Ollivier, A., Faugère, Y., Envisat RA2/MWR ocean data validation and cross-calibration activities. Yearly Report 2010, available on the AVISO website http://www.aviso.oceanobs.com/fileadmin/docu-ments/calval/validation_report/EN/annual_report_en_2010.pdf. Philipps, S., Ablain, M., Jason-2 Validation and Cross-Calibration Activities, Annual Report 2010, available on the AVISO website http://www.aviso.oceanobs.com/fileadmin/documents/calval/valida-tion_report/J2/annual_report_j2_2010.pdf.
Ascending and descending passes for the determination of the altimeter bias of Jason satellites using the Gavdos facility. Marine Geod Monitoring the stability of satellite altimeters with tide gauges
- S P Mertikas
- A Daskalakis
- I N Tziavos
Mertikas, S. P., Daskalakis, A., Tziavos, I. N., et al., Ascending and descending passes for the determination of the altimeter bias of Jason satellites using the Gavdos facility. Marine Geod. 34, 3-4, 261-276, 2011. Mitchum, G.T. Monitoring the stability of satellite altimeters with tide gauges. J. Atmos. Ocean Technol. 15, 721–730, 1998. Obligis, E., Desportes, C., Eymard, L., et al. Tropospheric Corrections for Coastal Altimetry, in: Vignudelli, S. et al. (Eds.), Coastal Altimetry. Springer-Verlag, Berlin Heidelberg, http://dx.doi.org/10.1007/978-3-642-12796-0_6, 2011.
Jason-1 Validation and Cross-Calibration Activities
- G Valladeau
- S Philipps
- M Ablain
Valladeau, G., Philipps, S., Ablain, M., Jason-1 Validation and Cross-Calibration Activities, Annual Report 2010, available on the AVISO website http://www.aviso.oceanobs.com/fileadmin/documents/calval/ validation_report/J1/annual_report_j1_2010.pdf.
COMAPI: New regional tide atlases and high frequency dynamical atmospheric correction. Ocean Surface Topography Science Team meeting Modelling the barotropic response of the global ocean to atmospheric wind and pressure forcing – comparisons with observations
- M Cancet
- M Lux
- C Pé
Cancet, M., Lux, M., Pé, C., et al., COMAPI: New regional tide atlases and high frequency dynamical atmospheric correction. Ocean Surface Topography Science Team meeting, Lisbon, Portugal, 2010. Carrère, L., Lyard, F., Modelling the barotropic response of the global ocean to atmospheric wind and pressure forcing – comparisons with observations. Geophys. Res. Lett. 30 (6), 1275, pp.
Modelling the global ocean tides: modern insights from FES2004. Ocean Dyn Calibration of the TOPEX/ POSEIDON altimeters at Lampedusa : additional results at Harvest
- F Lyard
- F Lefevre
- T Letellier
Lyard, F., Lefevre, F., Letellier, T., et al. Modelling the global ocean tides: modern insights from FES2004. Ocean Dyn. 56, 394–415, 2006. Mé, Y., Jeansou, E., Vincent, P., Calibration of the TOPEX/ POSEIDON altimeters at Lampedusa : additional results at Harvest. J. Geophys. Res., 99(C12), 24487–24504, 1994.
Report of the 2011 OSTST meeting, available on the University of Washington website http
- J Willis
Willis, J., Report of the 2011 OSTST meeting, available on the University of Washington website http://depts.washington.edu/uwconf/ostst2011/ OSTST_11_Meeting_Report_Final.pdf.
SLOOP: Toward a new methodology for handling S1 and S2 tidal waves in DAC for altimetry products. Ocean Surface Topography Science Team meeting
- J Lamouroux
- M Lux
- Lyard
, 615–629, 2004. Lamouroux, J., Lux, M., Lyard, et al., SLOOP: Toward a new methodology for handling S1 and S2 tidal waves in DAC for altimetry products. Ocean Surface Topography Science Team meeting, Lisbon, Portugal, 2010.