Article

Sea level rise for India since the start of tide gauge records

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Abstract

The global mean sea level (GMSL) changes derived from modelling do not match actual measurements of sea level and should not be trusted. Compilations of individual tide gauges of sufficient quality and length provide much more reliable information. The present work is a contribution towards a better understanding of the observed of sea levels in India and its relation to worldwide observations. The latest average relative rate of rise of worldwide sea levels from a compilation of 170 stations with more than 60 years of data returns an average relative rate of rise +0.25 mm/year. The individual rates of rise are about constant in between subsequent updates suggesting the absence of any acceleration. Observation in key sites suggests a similarly stable pattern. Along the coastline of India, the average rate of rise of sea level is +1.06 mm/year computed by considering the 11 longest tide gauges of average length 51 years. Shorter records may overrate the sea level rate of rise because of the local phasing of the quasi-60-year oscillation. In the longest records, the rates of rise are decreasing since 1955. The lack of any GPS monitoring of the vertical position of the tide gauge does not permit the determination of the absolute rates of rise.

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... Mörner [1] studied the tide gauges of Mumbai and Visakhapatnam, found on opposed sites of the Deccan Plateau, both characterized by the same four-parted segments, which they [1] interpreted as individual records of eustatic changes. More recently, [2][3][4], referred to interruptions in the tide gauge recording, artifacts by crustal movement, or changes in the tide gauge location, as different segments in a tide gauge record. This is what is discussed in the present paper. ...
... While it is extremely important to have long records, care must be taken with segmented records, which should not be used to compute rates of rise or accelerations. Many works have reported sea level records that are not a single measurement, but a composition of different records [1][2][3][4][5][6]14,15]. The combination of different tide gauge segments mostly produces very confusing results [1,14], as only very rarely can two segments be combined satisfactorily. ...
... While the issue of different sea and land contributions moving from one tide gauge location to the other, or other biases to the tide gauge results originating from malfunctioning of the instrument or measurement errors, could not be addressed, there was the opportunity to verify if the alignments of the different segments satisfied some conditions, such as breakpoint alignments assuming a known pattern-for example, linear-across the segments, and extrapolate the values at the interface from both sides [2,3,21]. Regarding breakpoints detection in oceanic environmental variables, the reader is also referred to [22]. ...
Article
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Records of measurements of sea levels from tide gauges are often “segmented”, i.e., obtained by composing segments originating from the same or different instruments, in the same or different locations, or suffering from other biases that prevent the coupling. A technique is proposed, based on data mining, the application of break-point alignment techniques, and similarity with other segmented and non-segmented records for the same water basin, to quality flag the segmented records. This prevents the inference of incorrect trends for the rate of rise and the acceleration of the sea levels for these segmented records. The technique is applied to the four long-term trend tide gauges of the Indian Ocean, Aden, Karachi, Mumbai, and Fremantle, with three of them segmented.
... The sea levels in India, including Mumbai, and in Karachi, Pakistan, have been recently analysed and discussed in Parker and Ollier (2015) and in Parker (2016). In both cases, it was shown that the latest positive trends in the PSMSL RLR data are only the result of arbitrary alignments, and alternative and more legitimate alignments reveal very stable sea-level conditions. ...
... Contrary to what is claimed in PSMSL (2016), there is no reason to expect that Aden may have a strong correlation to Mumbai and Karachi. In addition, these other two tide gauges also suffered of arbitrary corrections Parker and Ollier (2015) and Parker (2016 1878-1936; 1937-1994; 2005-2011. Notice especially that the data 1878-1936 are closed by the December month, and the data 1937-1994 start with the January month, so there is virtually no time gap, yet there is a 677-mm sudden difference between the measurements collected before and after the so-called breakpoint. ...
... The latest RLR data are not consistent with past works such as Unnikrishnan (2007), Unnikrishnan and Shankar (2007), Douglas (1991), Pirazzoli (1986), and with the recent work Parker and Ollier (2015). ...
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The sea level records since the start of the twentieth century show oscillations with many periodicities up to multi-decadal. The sea level may then change because of local factors such as subsidence or uplift, and global factors such as mass addition and thermal expansion of the oceans.
... Additionally, the choice of statistical and dynamical forecast model or defining of an initial forecast system are required. Several kinds of studies have been carried out related to different aspects of the Indian coastal environment using remote sensing techniques (Parker and Ollier 2015;Deepa et al. 2021). There are few organizations of India such as the Institute for Ocean Management at Anna University, the Indian National Centre for Ocean Information Services, the National Institute of Oceanography, the State Remote Sensing Centers of Gujarat and the National Centre for Earth Science Studies, etc., operationally using satellite data to collect the Indian coastal zone information. ...
... Specifically, data from PORT BLAIR has a major gap from Jan 2002 to Sep 2010. Parker and Ollier (2015) suggested that the linear fitting for a short time window may have underestimated or overestimated the rate of relative sea-level rise while for a full-time window showed a similar rate of relative sea-level rise that is already connected to the sinking of the tide gauges. Hence here these two sites (NAGAPATTINAM and PORT BLAIR) indicate much larger relative rates of rising with the trend of 8.0 mm/ year and 178.1 mm/year respectively and apparent acceleration of − 18.8 and − 53.6 mm/year 2 . ...
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The study investigates sea-level measurements observed from 12 tide gauge sites over the Indian coastal area during the last two decades. Initially, the rise of sea-level (slope) and acceleration is estimated by fitting the linear and parabolic equations in the recorded data from tide gauge measurements. The estimated results show the abrupt change of relative sea-level measurements at the distinct site depends upon the tide gauge site locations. To accurately analyse the regional coastal sea level pattern, the tide gauge time series data are decomposed in sine and cosine functions at different frequencies followed by the spectral analysis. Analysis of the results confirm that the tide gauges peaks do not occur at the fixed period at each tide gauge site, instead they repeat with varying time periods. The spectrum peaks width fluctuate at distinct sites and the general pattern of frequency spectrum does not follow a unique model. Such type of characteristic variation with the time is possibly because of the effective variables, which affects the steadiness of sea-level changes. The study concludes that the experimental results from the Indian coastal region must be included during the comparison of global data sets and other contemporary oceanic models.
... eustasy vs tectonics and compaction), whilst the studies summarized inFigure 1[21] primarily concerns the horizontal (lateral) changes (i.e. the dynamic changes). The absences of a rapid rise in sea level and any tendency of a recent acceleration have also been noted by others (e.g.[36] [37]). ...
... +0.55 ± 0.10 mm/yr, the revised satellite altimetry values of [15]. +0.25 ± 0.19 mm/yr, the mean of 170 PSMSL tide gauge stations having a length of more than 60 years [27]. ±0.0 mm/yr, the value obtained from many global test sites [11][12][13]28]; the Maldives, Bangladesh, Goa in the Indian Ocean, Tuvalu, Vanuatu, Kiribati, Majuro, Fiji in the Pacific, Surinam-Guyana in NE South America, Venice in the Mediterranean. ...
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Sea level changes is a key issue in the global warming scenario. It has been widely claimed that sea is rising as a function of the late 20 th 's warming pulse. Global tide gauge data sets may vary between +1.7 mm/yr to +0.25 mm/yr depending upon the choice of stations. At numerous individual sites, available tide gauges show variability around a stable zero level. Coastal morphology is a sharp tool in defining ongoing changes in sea level. A general stability has been defined in sites like the Maldives, Goa, Bangladesh and Fiji. In contrast to all those observations, satellite altimetry claim there is a global mean rise in sea level of about 3.0 mm/yr. In this paper, it is claimed that the satellite altimetry values have been " manipulated ". In this situation, it is recommended that we return to the observational facts, which provides global sea level records varying between ±0.0 and +1.0 mm/yr; i.e. values that pose no problems in coastal protection.
... The PSMSL database includes 170 stations with a length of >60 years. The mean of those stations is a vague sea level rise of 0.25 ±0.19 mm (Parker and Ollier, 2015;Parker, 2016). ...
... The PSMSL database includes 170 stations with a length of >60 years. The mean of those stations is a vague sea level rise of 0.25 ±0.19 mm (Parker and Ollier, 2015;Parker, 2016). ...
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"New Dawn of Truth" is the theme of the London Conference on Climate Change: Science and Geoethics. Day-1 is devoted to observational facts on a predominant solar forcing of chi mate change on Planet Earth. Day-2 covers the small to negligible effects from the increase in atmospheric CO2 content, and the disastrous effect of the general fixation of a co2-driven global warming. The conference ends with an open multifaceted debate with mutual respect in the centre.
... The PSMSL database includes 170 stations with a length of >60 years. The mean of those stations is a vague sea level rise of 0.25 ±0.19 mm (Parker and Ollier, 2015;Parker, 2016). ...
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... The PSMSL database includes 170 stations with a length of >60 years. The mean of those stations is a vague sea level rise of 0.25 ±0.19 mm (Parker and Ollier, 2015;Parker, 2016). ...
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The measurement of mean sea-level change from satellite altimetry requires extreme stability of the altimeter measurement system. In particular, the orbit and reference frame within which the altimeter measurements are situated, as well as the associated altimeter corrections, must be stable and accurate enough to permit robust mean sea level (MSL) estimates over an extended time period. The terrestrial reference frame is linked inseparably to the measurement of global mean sea level estimates from satellite altimetry and provides the context for the interpretation of the causes of current mean sea level trends. In an effort to adhere to cross mission consistency, we have generated the full time series of orbits for both TOPEX/Poseidon (TP) and Jason-1 through reduced dynamic methods based on the GGM02C GRACE derived gravity field within a consistent well defined ITRF2005 terrestrial reference frame. The recent release of the entire revised Jason-1 Geophysical Data Record (GDRB), and recalibration of the TOPEX microwave radiometer correction also require the further re-examination of TP/Jason-1 consistency issues. Here we present an assessment of these recent improvements to the accuracy of the TP/Jason-1 sea surface height time series, and evaluate the subsequent impact on global and regional mean sea level estimates.
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Power spectra of global surface temperature (GST) records reveal major periodicities at about 9.1, 10-11, 19-22 and 59-62 years. The Coupled Model Intercomparison Project 5 (CMIP5) general circulation models (GCMs), to be used in the IPCC (2013), are analyzed and found not able to reconstruct this variability. From 2000 to 2013.5 a GST plateau is observed while the GCMs predicted a warming rate of about 2 K/century. In contrast, the hypothesis that the climate is regulated by specific natural oscillations more accurately fits the GST records at multiple time scales. The climate sensitivity to CO2 doubling should be reduced by half, e.g. from the IPCC-2007 2.0-4.5 K range to 1.0-2.3 K with 1.5 C median. Also modern paleoclimatic temperature reconstructions yield the same conclusion. The observed natural oscillations could be driven by astronomical forcings. Herein I propose a semi empirical climate model made of six specific astronomical oscillations as constructors of the natural climate variability spanning from the decadal to the millennial scales plus a 50% attenuated radiative warming component deduced from the GCM mean simulation as a measure of the anthropogenic and volcano contributions to climatic changes. The semi empirical model reconstructs the 1850-2013 GST patterns significantly better than any CMIP5 GCM simulation. The model projects a possible 2000-2100 average warming ranging from about 0.3 C to 1.8 C that is significantly below the original CMIP5 GCM ensemble mean range (1 K to 4 K).
Article
The sea level change is a crucial indicator of our climate. The spatial sampling offered by satellite altimetry and its continuity during the last 18 years are major assets to provide an improved vision of the sea level changes. In this paper, we analyze the University of Colorado database of sea level time series to determine the trends for 18 large ocean regions by means of the automatic trend extraction approach in the framework of the singular spectrum analysis technique. Our global sea level trend estimate of 3.19 mm/year for the period from 1993 to 2010 is comparable with the 3.20-mm/year sea level rise since 1993 calculated by AVISO Altimetry. However, the trends from the different ocean regions show dissimilar patterns. The major contributions to the global sea level rise during 1993–2010 are from the Indian Ocean (3.78 ± 0.08 mm/year).
Article
The spatial sampling offered by TOPEX and Jason series of satellite radar altimeters and its continuity during the last twenty years are major assets to provide an improved vision of the global mean sea level (GMSL). The objective of this paper is to examine the recent GMSL variations (1993–2012) and to investigate the correlation between the GMSL and ENSO (El Niño-southern oscillation) episodes. For this purpose, a mean sea level anomalies time series, obtained from TOPEX, Jason-1 and Jason-2 measurements, is used to determine the trend of GMSL changes by using a simplified form of an unobserved components model (namely UCM). Then, to investigate the impact of the ENSO phenomenon on the GMSL changes, we considered the sea surface temperature anomalies (SSTA) index over the Niño3 region (5N–5S 150W–90W). Cross wavelet transform and wavelet coherence analysis are performed to expose common power between the GMSL changes and the SSTA index and their relative phase in the time–frequency space. The results indicate that there are in the estimated GMSL's trend a number of fluctuations over short periods that are least partly related to the El Niño/La Niña episodes. Cross wavelet transform and wavelet coherence analysis indicate that a significant correlation between GMSL and ENSO occurred during 1997–1998, 2006–2007, 2009–2010 El Niño events and 2007–2008 and 2010–2011 La Niña ones. All these areas show in-phase relationship, suggesting that GMSL and SSTA index vary synchronously.
Article
The paper presents the sea level rises (SLR) computed for the United States tide gauges with more than 100 years of recording. It is shown that the monthly sea levels oscillate about an almost linear longer-term trend with important multidecadal periodicities. The SLR time history is computed by linear fitting of 20, 30 and 60 years of data up to a given time (SLR20, SLR30 and SLR60) and is compared to the value obtained by considering all the data. It is shown that SLR60 has smaller oscillations, while SLR20 and SLR30 have much larger and frequent fluctuations. While SLR60 may oscillate ±10–30 % about the latest longer-term value, SLR30 may fluctuate ±50–100 % and SLR20 ± 100–200 %. The values obtained by considering all the data with a minimum of 60 years (SLRA) also fluctuate ±5–15 % about the latest longer-term value. This indicates the need to use the time history of SLR60 or SLRA when the record is longer than 60 years to assess the accelerating trend. For all the stations, the sea levels regularly oscillate about the linear longer-term trend, and if acceleration has to be computed, this is eventually negative, that is, the SLR is reducing.
Article
It is shown in the short comment that the sea levels are oscillating about a longer-term trend and that the sea level rise (SLR) computed with time windows of 20, 30 or 60 years also oscillates, with the amplitude of these latter oscillations reducing as the time window increases. The use of only two values of the SLR distribution is misleading to infer conclusions about the accelerating behaviour. In particular, the comparison of the 30-year SLR 1950–1979 with the 30-year SLR 1980–2009 for the tide gauges along the Atlantic coast of North America north of Cape Hatteras to infer an accelerating behaviour is particularly wrong because the 30-year time window is a too short interval to appreciate the longer-term sea level trend cleared of the multi-decadal oscillations, and the two values from the SLR distribution are computed, respectively, at the times of a valley and a peak for the 60-year Atlantic Ocean multi-decadal oscillation. By using a 60-year time window or all the data since opening when more than 60 years of recording are available and by analysing the SLR time history, the only conclusion that can be inferred from the analysis of the tide gauges along the North American Atlantic coast is that the sea levels are oscillating without too much of a positive acceleration along their longer-term trend.
Article
A proper coastal management requires an accurate estimation of sea level trends locally and globally. It is claimed that the sea levels are rising following an exponential growth since the 1990s, and because of that coastal communities are facing huge challenges. Many local governments throughout Australia, including those on the coast, have responded to the various warnings about changes in climate and increases in sea levels by undertaking detailed climate change risk management exercises. These exercises, which use projections passed on by the relevant state bodies, are expensive, but still a fraction of the cost of the capital works that they recommend. Several councils have complained to an Australian Productivity Commission report on climate change adaptation they do not have the money for the capital works required. It is shown here that the exponential growth claim is not supported by any measurement of enough length and quality when properly analysed. The tide gauge results do not support the exponential growth theory. The projections by the relevant state bodies should therefore be revised by considering the measurements and not the models to compute the future sea level rises for the next 30 years following the same trend experienced over the last 30 years.
Article
The government of Australia is supporting the statement that sea levels are rising faster than ever before as a result of increased carbon dioxide emissions. Consequent to this, low-lying coastal areas, where the majority of Australians are concentrated, have been declared at risk of sea level inundations. Maps with 0.5, 0.8 and 1.1 m sea level rise have been proposed for Sydney, the major Australian city. However, long term tide gauges, recording sea levels worldwide, as well as along the coastline of Australia, and within the bay of Sydney, do not show any sign of accelerating sea level rises at present time.
Article
Long-term change of the global sea level resulting from climate change has become an issue of great societal interest. The advent of the technology of satellite altimetry has modernized the study of sea level on both global and regional scales. In combination with in situ observations of the ocean density and space observations of Earth’s gravity variations, satellite altimetry has become an essential component of a global observing system for monitoring and understanding sea level change. The challenge of making sea level measurements with sufficient accuracy to discern long-term trends and allow the patterns of natural variability to be distinguished from those linked to anthropogenic forcing rests largely on the long-term efforts of altimeter calibration and validation. The issues of long-term calibration for the various components of the altimeter measurement system are reviewed in the paper. The topics include radar altimetry, the effects of tropospheric water vapor, orbit determination, gravity field, tide gauges, and the terrestrial reference frame. The necessity for maintaining a complete calibration effort and the challenges of sustaining it into the future are discussed.
Article
Published values for the long-term, global mean sea level rise determined from tide gauge records exhibit considerable scatter, from about 1 mm to 3 mm/yr. This disparity is not attributable to instrument error; long-term trends computed at adjacent sites often agree to within a few tenths of a millimeter per year. Instead, the differing estimates of global sea level rise appear to be in large part due to authors' using data from gauges located at convergent tectonic plate boundaries, where changes of land elevation give fictitious sea level trends. In addition, virtually all gauges undergo subsidence or uplift due to postglacial rebound (PGR) from the last deglaciation at a rate comparable to or greater than the secular rise of sea level. Modeling PGR by the ICE-3G model of Tushingham and Peltier (1991) and avoiding tide gauge records in areas of converging tectonic plates produces a highly consistent set of long sea level records. The value for mean sea level rise obtained from a global set of 21 such stations in nine oceanic regions with an average record length of 76 years during the period 1880-1980 is 1.8 mm/yr {plus minus} 0.1. This result provides confidence that carefully selected long tide gauge records measure the same underlying trend of sea level and that many old tide gauge records are of very high quality.
Article
Space geodesy is entrusted with the establishment and maintenance of reference frames that are widely used by the scientific and other user communities. Over the past decade, the burden of this task was primarily carried by the services of the International Association of Geodesy (IAG), led by IERS--the International Earth Rotation and Reference Systems Service. The new IAG initiative, the Global Geodetic Observing System--GGOS, places the utmost importance on the development, maintenance and wide distribution of an International Terrestrial Reference Frame (ITRF) of high accuracy and stability. At present, the goal is the definition of the origin accurate to 1 mm or better (at epoch) and a temporal stability on the order of 0.1 mm/y, with similar numbers for the scale and orientation components. The stability, integrity and applicability of the ITRF are directly related to how accurately we can account for mass redistribution during the analysis and reduction process of the data used for its development. Long wavelength variations of the gravity field driven by these mass redistributions produce geometric effects that are manifested as changes in the origin and orientation between the instantaneous and the mean reference frame. An uneven distribution of the stations that realize the ITRF on the globe generates biases and distortions in the combined product due to the dissimilarity of the combined networks and the de facto lopsided overlap of the combined networks. The poor geometry of the constituent networks results in increased correlations between the similarity transformation parameters, and they thus lead to biased and unstable results. The currently existing networks do not support high accuracy products and it is widely accepted that they are urgently in need of serious modernization and resource redistribution. Using simulations of geodetic data that we expect to collect with the future geodetic networks (SLR and VLBI), we provide preliminary options for the design of the complementary networks that will ensure the desired accuracy in the origin, scale and orientation definition of the ITRF.
Article
Estimates of the L1, L2 and LC (ionosphere-free linear combination) phase centers for the BLK IIA and BLK IIF GPS satellite antennas have been determined. These results indicate that the L1, L2, and LC phase centers have z-axis (toward the earth) offsets that differ significantly from the currently assumed offset values. These cali- brations used the BLK II antennas as receiving, rather than transmitting antennas. The BLK IIA &F antennas were pointed at the zenith from the roof of the Boeing facility in Seal Beach, CA and were connected to standard dual-frequency, geodetic-quality GPS receivers. By computing baseline vectors from a nearby reference antenna, first to a standard geodetic-quality GPS antenna, and then to the BLK II antennas, the phase centers of the BLK II antennas relative to the standard antenna could be deter- mined. This calibration procedure is essentially identical to that used by NGS numer- ous times on previous antenna calibrations. The primary difference for this calibration was to limit the data used to satellites above 60z elevation due to the highly directive antenna beam and to include only those periods when at least two satellites were in view. Graphs of the phase residuals versus elevation from these solutions show greater azimuthal variations for L1 than L2. These effects may be unique for these particular antennas and may depend on the tuning done to produce the desired amplitude re- sponse. The relation of satellite antenna offsets to position and timing measurements and importance of accurate antenna calibration will also be discussed.
Article
An effective technique for real-time differential orbit determination of two low Earth orbiters with GPS bias fixing is formulated. With this technique, only moderate- quality GPS orbit and clock states (e.g., as available in real-time from the NASA Global Differential GPS System with 10-20 cm accuracy) are needed to seed the process. The onboard, real-time orbital states of user satellites (few meters in accuracy) are used for orbit initialization and integration. An extended Kalman filter is constructed for the estimation of the differential orbit between the two satellites as well as a reference orbit, together with their associating dynamics parameters. The technique assumes that the two satellites are separated by a moderately long baseline (hundreds of km or less), and that they are of roughly similar shape. The differential dynamics, therefore, can be tightly constrained, strengthening the orbit determination. Without explicit differencing of GPS data, double-differenced phase biases are formed by a special transformation matrix. Integer- valued fixing of these biases is then performed, greatly improving the orbit estimation. A 9-day demonstration with the two GRACE spacecraft (with baselines of ~200 km) indicates that ~80% of the double-differenced phase biases can be successfully fixed, and the differential orbit can be determined to ~7 mm 1D RMS as compared to direct measurements of the micron-precision, onboard K- band ranging sub-system.
Article
Estimates of regional patterns of global sea level change are obtained from a 1° horizontal resolution general circulation model constrained by least squares to about 100 million ocean observations and many more meteorological estimates during the period 1993-2004. The data include not only altimetric variability, but most of the modern hydrography, Argo float profiles, sea surface temperature, and other observations. Spatial-mean trends in altimetric data are explicitly suppressed to isolate global average long-term changes required by the in situ data alone. On large scales, some regions display strong signals although few individual points have statistically significant trends. In the regional patterns, thermal, salinity, and mass redistribution contributions are all important, showing that regional sea level change is tied directly to the general circulation. Contributions below about 900 m are significant, but not dominant, and are expected to grow with time as the abyssal ocean shifts. Estimates made here produce a global mean of about 1.6 mm yr1, or about 60% of the pure altimetric estimate, of which about 70% is from the addition of freshwater. Interannual global variations may be dominated by the freshwater changes rather than by heating changes. The widely quoted altimetric global average values may well be correct, but the accuracies being inferred in the literature are not testable by existing in situ observations. Useful estimation of the global averages is extremely difficult given the realities of space-time sampling and model approximations. Systematic errors are likely to dominate most estimates of global average change: published values and error bars should be used very cautiously.
Article
Climate models (http://climatecommission.govspace.gov.au/files/2011/05/4108-CC-Science-Update-PRINT-CHANGES.pdf, 2011; http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_synthesis_report.htm, 2011; Rahmstorf, 2007, 2010) calculate that temperatures are increasing globally and sea level rises are increasing due to anthropogenic carbon dioxide emissions. More recent predictions (http://climatecommission.govspace.gov.au/files/2011/05/4108-CC-Science-Update-PRINT-CHANGES.pdf, 2011; Rahmstorf, 2007, 2010) have forecasted that sea level rises by 2100 will be higher than the 2007 projections by the Intergovernmental Panel on Climate Change (http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_synthesis_report.htm, 2011), with projected sea level rises increasing from 18–59cm to 100cm. In this brief communication, the predictions of Rahmstorf (2007) are validated against the experimental evidence over a 20year period. The University of Colorado Sea Level satellite monitoring shows that the rate of rise of the sea level is not only well below the values computed in http://climatecommission.govspace.gov.au/files/2011/05/4108-CC-Science-Update-PRINT-CHANGES.pdf (2011) and Rahmstorf (2007, 2010), but actually reducing rather than increasing (http://sealevel.colorado.edu/, 2011b; 10,11). These results suggest that sea level predictions based solely on the presumed temperature evolution may fail to accurately predict the long term sea levels at the end of the century.
Article
The U.S./French Jason-1 satellite is carrying a state-of-the-art GPS receiver to support precise orbit determination (POD) requirements. The performance of the Jason-1 “BlackJack” GPS receiver was strongly reflected in early POD results from the mission, enabling radial accuracies of 1–2 cm soon after the satellite's 2001 launch. We have made further advances in the GPS-based POD for Jason-1, most notably in describing the phase center variations of the on-board GPS antenna. We have also adopted new geopotential models from the Gravity Recovery and Climate Experiment (GRACE). The new strategies have enabled us to better exploit the unique contributions of the BlackJack GPS tracking data in the POD process. Results of both internal and external (e.g., laser ranging) comparisons indicate that orbit accuracies of 1 cm (radial RMS) are being achieved for Jason-1 using GPS data alone.
Article
Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1-m-high concrete pillar, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7±0.8 mm when the minimum elevation angle was increased from 10° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation-angle-dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar-antenna configurations identical or similar to the one used for this study at the Westford WFRD GPS site.
Article
The Geodetic Reference Antenna in Space (GRASP) is a micro satellite mission concept dedicated to the enhancement of all the space geodetic techniques, promising revolutionary improvements to the definition of the TRF, its densification, and accessibility. GRASP collocates GPS, SLR, VLBI, and DORIS sensors on a supremely calibrated and modelable spacecraft, offering an innovative space-based approach to a heretofore intractable problem: establishing precise and stable ties between the key geodetic techniques used to define and disseminate the TRF. GRASP also offers a solution to another difficult problem, namely, the consistent calibration of the myriad antennas used to transmit and receive the ubiquitous signals of the present and future Global Navigation Satellite Systems (GNSS). The resulting improvement in GNSS signal modeling will benefit all precision applications of these systems, which are the cornerstone of many Earth science missions. This paper will describe the GRASP mission concept, and the simulations analyses carried out to quantify the science benefits of this mission.
Article
Future requirements for the Terrestrial Reference Frame (TRF) are likely to be about an order of magnitude more accurate than currently being achieved. These requirements arise from the need for ensuring that the temporal changes we observe in the Earth system, such as global mean sea level, sea surface topography changes, crustal deformation and gravity changes due to mass transport, are real geophysical signals and not artifacts of the measurement system. Considerable investment in the improvement and deployment of the ground components of the geodetic network is going to be required to achieve this, but the space components may also need to be considered. In particular, the definition of the origin for the TRF is critically dependent on satellite laser ranging (SLR) to a few large, high-altitude geodetic satellites. The size of these satellites leads to significant uncertainty in the calculation of the center of mass offset correction, which is now the largest source of uncertainty in the determination of the Earth's gravitational coefficient (GM). Their high altitude makes it difficult for some lower power ranging systems to acquire data, particularly during daylight. This high altitude was originally necessary to ensure that the errors in the available models for the Earth's gravity field were greatly attenuated, but, with the advent of gravity models from the Gravity Recover and Climate Experiment (GRACE), the longer wavelength gravity model errors have been reduced by orders of magnitude. As a result, it is likely that geodetic satellites at a lower altitude (but still well above any significant drag effect) can provide a stronger and more accurate contribution to the TRF origin determination. Being lower, the dynamical tie to the Earth's center of mass will be stronger, and a shorter orbital period will provide more ranging opportunities. These lower altitude satellites can also be made smaller, supporting more accurate calculations of the center of mass offset correction and leading to increased accuracy in the determination of GM and the reference frame scale. This paper presents the results of a preliminary analysis to test the benefit to the reference frame determination from adding new geodetic SLR satellites.
Article
Uncertainties in the phase-center variations (PCV) of the GPS transmitter antennas are among the limiting sources of error in GPS-based global geodesy. We have used data from the BlackJack GPS receivers onboard the twin GRACE satellites to develop new estimates of GPS satellite antenna PCV. The estimates are expressed as tracking observable (distance) corrections mapped in two dimensions (nadir angle and azimuth). We have developed maps for both ionosphere-free carrier phase (LC) and pseudorange (PC). The GRACE tandem mission offers a number of substantial advantages for developing GPS PCV maps. The scale (mean height) of our GRACE orbit solutions is well determined at the cm level from dynamical constraints, and there is no troposphere signal to confound interpretation of the measurements. The multipath environment is also very favorable. We discuss our strategy for determining the GPS satellite PCV estimates from these data, and describe evaluations of the estimates using independent GPS data from both the TOPEX/POSEIDON (T/P; 1992--2005) and Jason-1 (2001 - ) missions. A heretofore unexplained 5--6 cm offset in the solved-for position of the T/P receiver antenna is reduced to less than 1 cm by applying the GRACE-based GPS PCV maps. The corresponding offset for Jason-1 is similarly decreased. Equally important, a spurious long-term (4-yr) drift in the daily estimated Jason-1 offsets is significantly reduced. These results hint at the potential benefits of these new GPS antenna PCV maps for wide-ranging geodetic applications wherein scale and long-term stability are important.
Article
The precise point whose position is being measured when a GPS baseline is determined is generally assumed to be the phase center of the GPS antenna. However, the phase center of a GPS antenna is neither a physical point nor a stable point. For any given GPS antenna, the phase center will change with the changing direction of the signal from a satellite. Ideally, most of this phase center variation depends on satellite elevation. Azimuthal effects are only introduced by the local environment around each individual antenna site. These phase center variations affect the antenna offsets that are needed to connect GPS measurements to physical monuments. Ignoring these phase center variations can lead to serious (up to 10 cm) vertical errors. This article will describe the procedure by which the National Geodetic Survey is calibrating GPS antennas and how this information may be obtained and used to avoid problems from these antenna variations. © 1999 John Wiley & Sons, Inc.
Article
A method for the estimation of the phase center variations of GPS satellite antennas using global GPS data is presented. First estimations have shown an encouraging repeatability from day to day and between satellites of the same block. Thus, two different satellite antenna patterns for Block II/IIA and for Block IIR with a range of about 4 cm and an accuracy of less than 1 mm could be found. The present approach allows the creation of a consistent set of receiver and satellite antenna patterns and phase center offsets. Thereby, it is possible to switch from relative to absolute phase center variations without a scale problem in global networks. This changeover has an influence on troposphere parameters, reduces systematic effects due to uncorrect antenna modeling and should diminish the elevation dependence of GPS results.
Article
The Global Positioning System is a constellation of 24–28 satellites, which can be used to define a global terrestrial reference frame. Daily offsets between a GPS defined frame and ITRF2000 have been estimated using more than a decade of GPS observations from 1990–2001. A linear fit to the full span of data shows agreement between the two frames at the level of –1ppb and –0.1ppb/year for scale, 5mm and 0mm/year for the X component of center of mass, –2mm and –3mm/year for the Y component, and 4mm and 6mm/year for the Z component. GPS is a viable tool for defining the global reference frame either alone, or in combination with other geodetic techniques.
Article
In the last 5000 years, global mean sea level has been dominated by the redistribution of water masses over the globe. In the last 300 years, sea level has been oscillation close to the present with peak rates in the period 1890–1930. Between 1930 and 1950, sea fell. The late 20th century lack any sign of acceleration. Satellite altimetry indicates virtually no changes in the last decade. Therefore, observationally based predictions of future sea level in the year 2100 will give a value of +10±10 cm (or +5±15 cm), by this discarding model outputs by IPCC as well as global loading models. This implies that there is no fear of any massive future flooding as claimed in most global warming scenarios.
Article
Mean-sea-level data from coastal tide gauges in the north Indian Ocean were used to show that low-frequency variability is consistent among the stations in the basin. Statistically significant trends obtained from records longer than 40 years yielded sea-level-rise estimates between 1.06–1.75 mm yr− 1, with a regional average of 1.29 mm yr− 1, when corrected for global isostatic adjustment (GIA) using model data. These estimates are consistent with the 1–2 mm yr− 1 global sea-level-rise estimates reported by the Intergovernmental Panel on Climate Change.
Article
Over the past years, satellite altimetry has produced several significant improvements in our scientific understanding of the oceans. However, several results related to global or regional sea level changes still too often rely on the assumption that orbit errors coming from station coordinates adoption can be neglected in the total error budget. The goal of this paper is to specifically study this general assumption and to assess its limitation. In a first step, in the case of the TOPEX/Poseidon satellite, we first characterized orbital errors coming from the adoption of a specific Terrestrial Reference Frame using a Monte-Carlo-based simulation method using DORIS data. In a second step, we analyzed the effect of these systematic orbital errors on the mean sea level derived from altimeter data. From these results, we derived linear transfer functions that can be used for several purposes like error budget estimation in altimetry or local tie specifications for the implementation of new tracking stations. These simulations show that the main source of errors comes from current imprecision in the Z-axis realization of the frame. A 10 mm error in the TZ realization can create a −1.2 mm of systematic errors in the derived mean sea level due to the North–South asymmetric distribution of the oceans all over the world. Significant sea level rise could erroneously be attributed to a possible warming of the biosphere while they just come from systematic errors in the Terrestrial Reference Frame used to generate the satellite operational orbits. Finally, we assessed the accuracy of present Terrestrial Reference Frame realizations and derived a realistic error budget for this specific source of error. For the ITRF97 realization, a current precision of 3.0 mm in sea level and 0.37 mm/yr in sea level rise was obtained. These precisions should gradually improve with future Terrestrial Reference Frame realizations.