added an update
The International DORIS Service (IDS) was created in 2003 under the umbrella of the International Association of Geodesy (IAG) to foster scientific research related to the French DORIS tracking system and to deliver scientific products, mostly related to the International Earth rotation and Reference systems Service (IERS). We first present some general background related to the DORIS system (current and planned satellites, current tracking network and expected evolution) and to the general IDS organization (from Data Centers, Analysis Centers and Combination Center). Then, we discuss some of the steps recently taken to prepare the IDS submission to ITRF2013 (combined weekly time series based on individual solutions from several Analysis Centers). In particular, recent results obtained from the Analysis Centers and the Combination Center show that improvements can still be made when updating physical models of some DORIS satellites, such as Envisat, Cryosat-2 or Jason-2. The DORIS contribution to ITRF2013 should also benefit from the larger number of ground observations collected by the last generation of DGXX receivers (first instrument being onboard Jason-2 satellite). In particular for polar motion, sub-milliarcsecond accuracy seems now to be achievable. Weekly station positioning internal consistency also seems to be improved with a larger DORIS constellation.
For the preparation of ITRF2008, the IDS processed data from 1993 to 2008, including data from TOPEX/Poseidon, the SPOT satellites and Envisat in the weekly solutions. Since the development of ITRF2008, the IDS has been engaged in a number of efforts to try and improve the reference frame solutions. These efforts include (i) assessing the contribution of the new DORIS satellites, Jason-2 and Cryosat2 (2008-2011), (ii) individually analyzing the DORIS satellite contributions to geocenter and scale, and (iii) improving orbit dynamics (atmospheric loading effects, satellite surface force modeling…). We report on the preliminary results from these research activities, review the status of the IDS combination which is now routinely generated from the contributions of the IDS analysis centers, and discuss the prospects for continued improvement in the DORIS contribution to the next international reference frame.
This research focuses on the investigation of the deterministic and stochastic parts of the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) weekly time series aligned to the newest release of ITRF2014. A set of 90 stations was divided into three groups depending on when the data was collected at an individual station: from 1993 to 2003, from 2003 to 2010 and the latest observations. To reliably describe the DORIS time series, we employed a mathematical model that included the long-term nonlinear signal, linear trend, seasonal oscillations and a stochastic part, all being estimated with Maximum Likelihood Estimation (MLE). We proved that the values of the parameters delivered for DORIS data are strictly correlated with the time when observations were collected. The quality of the most recent data has significantly improved comparing to the previous observations. For a few sites, the non-linearities change between different groups of observations. A clear shift in phase in annual signal was noticed for different stations operating at the same site but classified to different groups, revealing an impact of additional satellites and an improvement in the quality of observations with time. The amplitudes of draconitic oscillation are much higher for stations situated in equatorial area than they are for high latitudes. They were also greater for observations collected in the 90s than they were for the 2000s and for recent times. This can be clearly observed for all components. The noise level and its type changed significantly. Among several tested models, the power-law process may be chosen as the preferred one for most of the DORIS data. Moreover, the preferred noise model has changed through the years from an autoregressive process to pure power-law noise with few stations characterised by a positive spectral index. Also, the standard deviation of noise, meaning a dominant noise, decreased when the oldest observations are compared to the newest. We conclude, that (by omitting some outliers) it is possible to determine the velocity from the DORIS-derived time series with a reliability of c.a. 0.5 mm/yr. For the latest observations, the medians of the velocity errors were equal to 0.3, 0.3 and 0.4 mm/yr, respectively, for the North, East and Up components. In the best cases, a velocity uncertainty of DORIS sites of 0.1 mm/yr is achievable when the appropriate coloured noise model is taken into consideration. This finding is really important under the assumption of 0.1 mm/yr which is going to be achieved within the Global Geodetic Observing System.
While accuracy of tracking station coordinates is of key importance for Precise Orbit Determination (POD) for altimeter satellites, reliability and operationality are also of great concern. In particular, while recent ITRF realizations should be the most accurate at the time of their computation, they cannot be directly used by the POD groups for operational consideration for several reasons such as new stations appearing in the network or new discontinuities affecting station coordinates. For POD purposes, we computed a new DORIS terrestrial frame called DPOD2008 derived from ITRF2008 (as previously done by DPOD2005 with regards to ITRF2005). In a first step, we will present the method used to validate the past ITRF2008 using more recent DORIS data and to derive new station positions and velocities, when needed. In particular, discontinuities in DORIS station positions and/or velocities are discussed. To derive new DORIS station coordinates, we used recent DORIS weekly time series of coordinates, recent GPS relevant time series at co-located sites and also dedicated GPS campaigns performed by IGN when installing new DORIS beacons. DPOD2008 also contains additional metadata that are useful when processing DORIS data, for example, periods during which DORIS data should not be used or at least for which data should be downweighted. In several cases, a physical explanation can be found for such temporary antenna instability. We then demonstrate improvements seen when using different reference frames, such as the original ITRF2008 solution, for precise orbit determination of altimeter satellites TOPEX/Poseidon and Jason-2 over selected periods spanning 1993–2013.
The International DORIS Service (IDS) submitted input data for the most recent realization of the International Terrestrial Reference System (ITRS), the International Terrestrial Reference Frame 2014 (ITRF2014). As one of the ITRS Combination Centers, DGFI-TUM is in charge to analyze and assess the quality of the submitted data and to compute a combined global TRF solution (called DTRF2014) using observations of the four geodetic space techniques GNSS, VLBI, SLR and DORIS. The combination methodology used at DGFI-TUM is based on the combination of datum-free normal equations. Together with station coordinates and velocities, terrestrial pole coordinates are estimated in one common adjustment. The paper presents the analysis results of the most recent DORIS submission IDS-d09 and evaluates its quality w.r.t. the DTRF2008 (IDS-only) solution. In the most recent version of the analysis, we introduce in total 55 station discontinuities and reduce 15 stations due to a too short time span or too few observations. Time series of weekly IDS solutions are computed and validated w.r.t. DTRF2008. The transformation parameter time series and the station residuals are discussed in detail. Especially the scale parameter time series shows a significant improvement compared to the DTRF2008 input data. The scatter of the x- and y-translation is significantly reduced to 5.7 mm and 7.1 mm compared to 6.6 mm and 8.1 mm for the DTRF2008 (IDS-only) solution. The z-translation time series still shows a high correlation with solar activity. 10% of all station residuals are significantly affected by spectral peaks at draconitic period harmonics of the altimetry satellites Jason and TOPEX/Poseidon and up to 48% of all station residual time series contain significantly determined frequencies with a 14 day period. The multi-year IDS solution is validated w.r.t. DTRF2008 and the consistently estimated terrestrial pole coordinates are analyzed and compared to IERS 08 C04. The x-pole spectra comprises prominent peaks at various draconitic frequencies.
In a geodetic radio frequency observing system the phase center offsets and phase center variations of ground antennae are a fundamental component of mathematical models of the system observables. In this paper we describe work aimed at improving the DORIS Starec ground antenna phase center definition model. Seven antennas were analyzed in the Compact Antenna Test Range (CATR), a dedicated CNES facility. With respect to the manufacturer specified phase center offset, the measured antennae varied between -6. mm and +4. mm due to manufacturing variations. To solve this problem, discussions were held with the manufacturer, leading to an improvement of the manufacturing process. This work results in a reduction in the scatter to ±1. mm. The phase center position has been kept unchanged and associated phase law has been updated and provided to users of the International DORIS Service (IDS). This phase law is applicable to all Starec antennas (before and after manufacturing process consolidation) and is azimuth independent. An error budget taking into account these updated characteristics has been established for the antenna alone: ±2. mm on the horizontal plane and ±3. mm on the up component, maximum error values for antennas named type C (Saunier et al., 2016) produced with consolidated manufacturing process. Finally the impact of this updated characterization on positioning results has been analyzed and shows a scale offset only of the order of +12. mm for the Terrestrial Reference Frame.
This research focuses on the investigation of the deterministic and stochastic parts of the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) weekly coordinate time series from the IDS contribution to the ITRF2014A set of 90 stations was divided into three groups depending on when the data was collected at an individual station. To reliably describe the DORIS time series, we employed a mathematical model that included the long-term nonlinear signal, linear trend, seasonal oscillations (these three sum up to produce the Polynomial Trend Model) and a stochastic part, all being resolved with Maximum Likelihood Estimation (MLE). We proved that the values of the parameters delivered for DORIS data are strictly correlated with the time span of the observations, meaning that the most recent data are the most reliable ones. Not only did the seasonal amplitudes decrease over the years, but also, and most importantly, the noise level and its type changed significantly. We examined five different noise models to be applied to the stochastic part of the DORIS time series: a pure white noise (WN), a pure power-law noise (PL), a combination of white and power-law noise (WNPL), an autoregressive process of first order (AR(1)) and a Generalized Gauss Markov model (GGM). From our study it arises that the PL process may be chosen as the preferred one for most of the DORIS data. Moreover, the preferred noise model has changed through the years from AR(1) to pure PL with few stations characterized by a positive spectral index.
This paper focuses on the investigation of the deterministic and stochastic parts of the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) weekly time series aligned to the newest release of ITRF2014. A set of 90 stations was divided into three groups depending on when the data was collected at an individual station. To reliably describe the DORIS time series, we employed a mathematical model that included the long-term nonlinear signal, linear trend, seasonal oscillations and a stochastic part, all being estimated with Maximum Likelihood Estimation (MLE). We proved that the values of the parameters delivered for DORIS data are strictly correlated with the time span of the observations. The quality of the most recent data have significantly improved. Not only did the seasonal amplitudes decrease over the years, but also, and most importantly, the noise level and its type changed significantly. Among several tested models, the power-law process may be chosen as the preferred one for most of the DORIS data. Moreover, the preferred noise model has changed through the years from an autoregressive process to pure power-law noise with few stations characterised by a positive spectral index. For the latest observations, the medians of the velocity errors were equal to 0.3, 0.3 and 0.4 mm/yr, respectively, for the North, East and Up components. In the best cases, a velocity uncertainty of DORIS sites of 0.1 mm/yr is achievable when the appropriate coloured noise model is taken into consideration.
Analysis of DORIS stations positions time series from IDS combined solution over time period 2000-2012 has revealed discontinuities linked not with geophysical phenomenon, but correlated with beacons frequency shifts. This result was the consequence of assuming the nominal frequency in the measurement model, and not accounting for beacon frequency shifts during an arc or over time. This affected the DORIS data analysis for the software at the ESA, GSC, LCA, and GAU analysis centers. The GIPSY-OASIS analysis centers (IGN, INA) were unaffected since they model DORIS as a difference in two phase measurements and they estimate ground frequency drift for the station clock. This mismodelling affected the DORIS contribution to ITRF2008. Since late 2011, the software at the affected ACs has been modified and these frequency shifts are now taken into account. We computed mean stations positions and velocities over the time period 1993-2008 with the original (uncorrected) and new SINEX files to estimate impact of this missmodeling on the IDS contribution to ITRF2008. Finally, as we know the exact number of stations affected, the amplitude of the effect and when, we can predict the effect on the TRF scale. After presentation of these results, we will also give the status of the main guidelines of the DORIS contribution to ITRF2013.
For the preparation of ITRF2008, the IDS processed data from 1993 to 2008, including data from TOPEX/Poseidon, the SPOT satellites and Envisat in the weekly solutions. Since the development of ITRF2008, the IDS has been engaged in a number of efforts to try and improve the reference frame solutions. These efforts include assessing the contribution of the new DORIS satellites, Jason-2 and Cryosat2, in terms of (1) geocenter and scale solutions of the IDS combination; (2) stations positions, and individually analyzing the DORIS satellite contributions to geocenter and scale. This analysis has revealed that when Jason-2 is included the Tz geocenter component better centered. To understand this result, IDS conducted several single satellite studies which have shown that (i) The Tz geocenter component is centered much better with Jason-2 and this benefits the IDS combination ; (2) The Tx and Ty geocenter components exhibit a 120-day oscillation with Jason-2, indicative of problems associated with the radiation force model for Jason2; (3) The Tz phenomenon is the consequence of the new DGXX 7-channel DORIS receiver on board of Jason-2 (and latter missions), which can track up to seven beacons simultaneously. The study on the stations positions confirmed that stations of high latitude much benefit of the polar orbit of Cryosat2.
After recalling the principle of the Jason-1 data corrective model in relation to the South Atlantic Anomaly (SAA) developed by Lemoine and Capdeville (2006), we present a model update which takes into account the orbit changes and the recent DORIS data. We propose also here a method to the International DORIS Service (IDS) Analysis Centers (ACs) in their contribution to the ITRF2014 for adding DORIS Jason-1 data into their solutions. When the Jason-1 satellite is added to the multi-satellite solution (orbit of inclination of 66° complements the polar-orbiting satellites), the stability of the geocenter Z-translation is improved (standard deviation of 11.5 mm against 16.5 mm). In a second part we take advantage of a high-energy particles dosimeter (CARMEN) on-board Jason-2 to improve the corrective model of Jason-1. We completed a correlation study showing that the CARMEN >87 MeV integrated proton flux map averaged over the period 2009–2011 is the energy band of the CARMEN maps which are the most coherent with the one obtained from Jason-1 DORIS measurements. The model based on the Jason-1 map and the one based on the CARMEN map are then compared in terms of orbit determination and station position estimation. We derive and validate a SPOT-5 data corrective model. We determine the SAA grid at the altitude of SPOT-5 from the frequency time derivative of the on-board frequency offsets and estimated the model parameters. We demonstrate the impact of the SPOT-5 data corrective model on the Precise Orbit Determination and the station position estimation from the weekly solutions, based on two individual Analysis Centers solutions, GOP (Geodetic Observatory Pecny) and GRG (Groupe de Recherche de Géodésie Spatiale). The SPOT-5 data corrective model significantly improves the Precise Orbit Determination (reduction of 1.4% in 2013 of RMS of the fit, reduction of 25% in normal direction of arc overlap RMS) and the overall statistics of the station position estimation (reduction of 2% of repeatability RMS, reduction of 3–7% of geocenter variations). Moreover, the application of the data corrective model strongly reduces the individual station bias in the North component for all the SAA-affected stations and the height bias for the most affected SAA stations. The East bias is, however, not reduced by this data corrective model.
In the context of the 2014 realization of the International Terrestrial Reference Frame (ITRF2014), the International DORIS Service (IDS) has delivered to the IERS a set of 1140 weekly SINEX files including station coordinates and Earth orientation parameters, covering the time period from 1993.0 to 2015.0. From this set of weekly SINEX files, the IDS Combination Center estimated a cumulative DORIS position and velocity solution to obtain mean horizontal and vertical motion of 160 stations at 71 DORIS sites. The main objective of this study is to validate the velocities of the DORIS sites by comparison with external models or time series. Horizontal velocities are compared with two recent global plate models (GEODVEL 2010 and NNR-MORVEL56). Prior to the comparisons, DORIS horizontal velocities were corrected for Global Isostatic Adjustment (GIA) from the ICE-6G (VM5a) model. For more than half of the sites, the DORIS horizontal velocities differ from the global plate models by less than 2-3 mm/yr. For five of the sites (Arequipa, Dionysos/Gavdos, Manila, Santiago) with horizontal velocity differences wrt these models larger than 10 mm/yr, comparisons with GNSS estimates show the veracity of the DORIS motions. Vertical motions from the DORIS cumulative solution are compared with the vertical velocities derived from the latest GPS cumulative solution over the time span 1995.0-2014.0 from the University of La Rochelle (ULR6) solution at 31 co-located DORIS-GPS sites. These two sets of vertical velocities show a correlation coefficient of 0.83. Vertical differences are larger than 2 mm/yr at 23 percent of the sites. At Thule the disagreement is explained by fine-tuned DORIS discontinuities in line with the mass variations of outlet glaciers. Furthermore, the time evolution of the vertical time series from the DORIS station in Thule show similar trends to the GRACE equivalent water height.
In preparation of the 2014 realization of the International Terrestrial Reference Frame (ITRF2014), the International DORIS Service delivered to the International Earth Rotation and Reference Systems Service a set of 1140 weekly solution files including station coordinates and Earth orientation parameters, covering the time period from 1993.0 to 2015.0. The data come from eleven DORIS satellites: TOPEX/Poseidon, SPOT2, SPOT3, SPOT4, SPOT5, Envisat, Jason-1, Jason-2, Cryosat-2, Saral and HY-2A. In their processing, the six analysis centers which contributed to the DORIS combined solution used the latest time variable gravity models and estimated DORIS ground beacon frequency variations. Furthermore, all the analysis centers but one excepted included in their processing phase center variations for ground antennas. The main objective of this study is to present the combination process and to analyze the impact of the new modeling on the performance of the new combined solution. Comparisons with the IDS contribution to ITRF2008 show that (i) the application of the DORIS ground phase center variations in the data processing shifts the combined scale upward by nearly 7-11. mm and (ii) thanks to estimation of DORIS ground beacon frequency variations, the new combined solution no longer shows any scale discontinuity in early 2002 and does not present unexplained vertical discontinuities in any station position time series. However, analysis of the new series with respect to ITRF2008 exhibits a scale increase late 2011 which is not yet explained. A new DORIS Terrestrial Reference Frame was computed to evaluate the intrinsic quality of the new combined solution. That evaluation shows that the addition of data from the new missions equipped with the latest generation of DORIS receiver (Jason-2, Cryosat-2, HY-2A, Saral), results in an internal position consistency of 10. mm or better after mid-2008.