Contributions of the French Institut Géographique National (IGN) to the International DORIS Service

DORIS is one of the four geodetic space techniques participating to the Global Geodetic Observing System (GGOS). Since the early development of this system, the Institut Géographique National played a specific and active role. Within, the International DORIS Service (IDS), IGN is in a particular position. While it is responsible for the installation and the maintenance of the DORIS ground tracking stations, it also handles one of the two IDS data center delivering DORIS data and products and has been an IDS Analysis Center for years, providing all possible IDS products, in particular the latest ignwd08 time series in preparation of ITRF2008. This paper explains the different aspects of the IGN contribution to IDS from an historical point of view, presents current activities and scientific results and provides a perspective for future activities. Recent DORIS results show a 10 mm precision or better when more than four DORIS satellites are available. Comparisons between recent DORIS solutions (ign07d02 and ign09d02) and past ITRF realizations show that errors are shared between the DORIS and the ITRF realizations. Some problems related to DORIS data processing are also discussed and possible ways to solve them in the future are discussed. In particular, we can now reject the tropospheric origin of the problem detected in the Envisat data after the software upload of October 12, 2004. A few applications in geodesy (terrestrial reference frame, Earth’s polar motion) and geophysics are also discussed as a natural extension of these service-type activities.     

DORIS geodesy: A dynamic determination of geocentre location

Geoscience Australia contributed a multi-satellite, multi-year weekly time series to the International DORIS Service combined submission for the construction of International Terrestrial Reference Frame 2008 (ITRF2008). This contributing solution was extended to a study of the capability of DORIS to dynamically estimate the variation in the geocentre location. Two solutions, comprising different constraint configurations of the tracking network, were undertaken. The respective DORIS satellite orbit solutions (SPOT-2, SPOT-4, SPOT-5 and Envisat) were verified and validated by comparison with those produced at the Goddard Space Flight Center (GSFC), DORIS Analysis Centre, for computational consistency and standards. In addition, in the case of Envisat, the trajectories from the GA determined SLR and DORIS orbits were compared. The results for weekly dynamic geocentre estimates from the two constraint configurations were benchmarked against the geometric geocentre estimates from the IDS-2 combined solution. This established that DORIS is capable of determining the dynamic geocentre variation by estimating the degree one spherical harmonic coefficients of the Earth’s gravity potential. It was established that constrained configurations produced similar results for the geocentre location and consequently similar annual amplitudes. For the minimally constrained configuration Greenbelt–Kitab, the mean of the uncertainties of the geocentre location were 2.3, 2.3 and 7.6 mm and RMS of the mean uncertainties were 1.9, 1.2 and 3.5 mm for the X, Y and Z components, respectively. For GA_IDS-2_Datum constrained configuration, the mean of the uncertainties of the geocentre location were 1.7, 1.7 and 6.2 mm and RMS of the mean uncertainties were 0.9, 0.7 and 2.9 mm for the X, Y and Z components, respectively. The mean of the differences of the two DORIS dynamic geocentre solutions with respect to the IDS-2 combination were 1.6, 4.0 and 5.1 mm with an RMS of the mean 21.2, 14.0 and 31.5 mm for the Greenbelt–Kitab configuration and 4.1, 3.9 and 4.3 mm with an RMS 8.1, 9.0 and 28.6 mm for the GA_IDS-2_Datum constraint configuration. The annual amplitudes for each component were estimated to be 5.3, 10.8 and 11.0 mm for the Greenbelt–Kitab configuration and 5.3, 9.3 and 9.4 mm for the GA_IDS-2_Datum constraint configuration. The two DORIS determined dynamic geocentre solutions were compared to the SLR determined dynamic solution (which was determined from the same process of the GA contribution to the ITRF2008 ILRS combination) gave mean differences of 3.3, −4.7 and 2.5 mm with an RMS of 20.7, 17.5 and 28.0 mm for the X, Y and Z components, respectively for the Greenbelt–Kitab configuration and 1.1, −5.4 and 4.4 mm with an RMS of 9.7, 13.3 and 24.9 mm for the GA_IDS-2_Datum configuration. The larger variability is reflected in the respective amplitudes. As a comparison, the annual amplitudes of the SLR determined dynamic geocentre are 0.9, 1.0 and 6.8 mm in the X, Y and Z components. The results from this study indicate that there is potential to achieve precise dynamically determined geocentre from DORIS.     

An inter-comparison of zenith tropospheric delays derived from DORIS and GPS data

Doppler Orbitography Radiopositioning Integrated by Satellite (DORIS) and Global Positioning System (GPS) techniques are similarly affected by propagation delays in the neutral atmosphere (troposphere) and hence make use of similar data processing strategies for reducing this effect. We compare Zenith Tropospheric Delays (ZTDs) estimated from 52 DORIS and GPS station pairs co-located at 35 sites over the 2005–2008 period. We find an overall systematic negative mean bias of −4 mm and a median bias of −2 mm, with a large site-to-site scatter and especially stronger biases over South America, potentially linked to remaining problems related to the South Atlantic Anomaly (SAA) in the current DORIS data processing. The standard deviation of ZTD differences is in the range 4–12 mm over the globe (8 mm on average), with larger values located in the southern hemisphere. The spatial variability of differences is consistent with previous work but remains largely unexplained. DORIS is shown to be much less sensitive to instrumental changes than GPS (only the switch from Alcatel to Starec antenna at Toulouse is detected as an offset of −4 mm in the ZTD time series). On the opposite, discontinuities and spurious annual signals are found in the GPS ZTD solutions. A discontinuity of +5 mm is found on 5 November 2006, linked to the switch from relative to absolute GPS antenna models used in the data processing. The use of modified GPS antennas (e.g. at GODE) or improved antenna models is shown to reduce the spurious annual signal (e.g. from 5 mm to 2 mm at METS). Overall, the agreement between both techniques is good, though DORIS shows a significantly larger random scatter. The high stability and good spatial and temporal coverage make DORIS a potential candidate technique for meteorology and climate studies as long as reasonable time averaging can be applied (e.g. differences are reduced from 8.6 to 2.4 mm with 5-day averages) and no real-time application is considered. This technique could be considered as a potential contributor to Global Geodetic Observing System (GGOS) for climatology.     

Impact of orbit modeling on DORIS station position and Earth rotation estimates

The high precision of estimated station coordinates and Earth rotation parameters (ERP) obtained from satellite geodetic techniques is based on the precise determination of the satellite orbit. This paper focuses on the analysis of the impact of different orbit parameterizations on the accuracy of station coordinates and the ERPs derived from DORIS observations. In a series of experiments the DORIS data from the complete year 2011 were processed with different orbit model settings. First, the impact of precise modeling of the non-conservative forces on geodetic parameters was compared with results obtained with an empirical-stochastic modeling approach. Second, the temporal spacing of drag scaling parameters was tested. Third, the impact of estimating once-per-revolution harmonic accelerations in cross-track direction was analyzed. And fourth, two different approaches for solar radiation pressure (SRP) handling were compared, namely adjusting SRP scaling parameter or fixing it on pre-defined values.     

Status of the T2L2/Jason2 Experiment

The T2L2 (Time Transfer by Laser Link) project, developed by CNES and OCA will permit the synchronization of remote ultra stable clocks and the determination of their performances over intercontinental distances. The principle of the experiment derives from Satellite Laser Ranging (SLR) technology with dedicated space equipment. T2L2 was accepted in 2005 to be on board the Jason2 altimetry satellite. The payload consists of both event timer and photo detection modules. The system uses the ultra-stable quartz oscillator of DORIS as on-board reference clock on one hand, and the Laser Reflector Array, making T2L2 a real two-way time transfer system on the other hand. The expected time stability of the T2L2 instrument (detection and timing), referenced by the DORIS oscillator and including all internal error sources should be at the level of 10–12 ps at 1 s and <1 ps at 1000 s. The metrological specifications of T2L2 should permit to maintain a precision of 1 to a few ps when measuring the phase of a clock during around 1000 seconds.     

Improving DORIS geocenter time series using an empirical rescaling of solar radiation pressure models

Even if Satellite Laser Ranging (SLR) remains the fundamental technique for geocenter monitoring, DORIS can also determine this geophysical parameter. Gobinddass et al. (2009) found that part of the systematic errors at 118 days and 1 year can be significantly reduced by rescaling the current solar radiation pressure models using satellite-dependent empirical models. Here we extend this study to all DORIS satellites and propose a complete set of empirical solar radiation parameter coefficients. A specific problem related to SPOT-5 solar panel realignment is also detected and explained. New DORIS geocenter solutions now show a much better agreement in amplitude with independent SLR solutions and with recent geophysical models. Finally, the impact of this refined DORIS data strategy is discussed in terms of Z-geocenter monitoring as well as for other geodetic products (altitude of high latitude station such as Thule in Greenland) and Precise Orbit Determination. After reprocessing the full 1993.0-2008.0 DORIS data set, we confirm that the proposed strategy allows a significant reduction of systematic errors at periods of 118 days and 1 year (up to 20 mm), especially for the most recent data after 2002.5, when more DORIS satellites are available for geodetic purposes.     

IDS contribution to ITRF2008

For the first time, the International DORIS Service (IDS) has produced a technique level combination based on the contributions of seven analysis centers (ACs), including the European Space Operations Center (ESOC), Geodetic Observatory Pecny (GOP), Geoscience Australia (GAU), the NASA Goddard Space Flight Center (GSFC), the Institut Géographique National (IGN), the Institute of Astronomy, Russian Academy of Sciences (INASAN, named as INA), and CNES/CLS (named as LCA). The ACs used five different software packages to process the DORIS data from 1992 to 2008, including NAPEOS (ESA), Bernese (GOP), GEODYN (GAU, GSC), GIPSY/OASIS (INA), and GINS (LCA). The data from seven DORIS satellites, TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5, Envisat and Jason-1 were processed and all the analysis centers produced weekly SINEX files in either variance–covariance or normal equation format. The processing by the analysis centers used the latest GRACE-derived gravity models, forward modelling of atmospheric gravity, updates to the radiation pressure modelling to improve the DORIS geocenter solutions, denser parameterization of empirically determined drag coefficients to improve station and EOP solutions, especially near the solar maximum in 2001–2002, updated troposphere mapping functions, and an ITRF2005-derived station set for orbit determination, DPOD2005. The CATREF software was used to process the weekly AC solutions, and produce three iterations of an IDS global weekly combination. Between the development of the initial solution IDS-1, and the final solution, IDS-3, the ACs improved their analysis strategies and submitted updated solutions to eliminate troposphere-derived biases in the solution scale, to reduce drag-related degradations in station positioning, and to refine the estimation strategy to improve the combination geocenter solution. An analysis of the frequency content of the individual AC geocenter and scale solutions was used as the basis to define the scale and geocenter of the IDS-3 combination. The final IDS-3 combination has an internal position consistency (WRMS) that is 15 to 20 mm before 2002 and 8 to 10 mm after 2002, when 4 or 5 satellites contribute to the weekly solutions. The final IDS-3 combination includes solutions for 130 DORIS stations on 67 different sites of which 35 have occupations over 16 years (1993.0–2009.0). The EOPs from the IDS-3 combination were compared with the IERS 05 C04 time series and the RMS agreement was 0.24 mas and 0.35 mas for the X and Y components of polar motion. The comparison to ITRF2005 in station position shows an agreement of 6 to 8 mm RMS in horizontal and 10.3 mm in height. The RMS comparison to ITRF2005 in station velocity is at 1.8 mm/year on the East component, to 1.2 mm/year in North component and 1.6 mm/year in height.     

Impact of ITRS 2014 realizations on altimeter satellite precise orbit determination

This paper evaluates orbit accuracy and systematic error for altimeter satellite precise orbit determination on TOPEX, Jason-1, Jason-2 and Jason-3 by comparing the use of four SLR/DORIS station complements from the International Terrestrial Reference System (ITRS) 2014 realizations with those based on ITRF2008. The new Terrestrial Reference Frame 2014 (TRF2014) station complements include ITRS realizations from the Institut National de l’Information Géographique et Forestière (IGN) ITRF2014, the Jet Propulsion Laboratory (JPL) JTRF2014, the Deutsche Geodätisches Forschungsinstitut (DGFI) DTRF2014, and the DORIS extension to ITRF2014 for Precise Orbit Determination, DPOD2014. The largest source of error stems from ITRF2008 station position extrapolation past the 2009 solution end time. The TRF2014 SLR/DORIS complement impact on the ITRF2008 orbit is only 1–2 mm RMS radial difference between 1992–2009, and increases after 2009, up to 5 mm RMS radial difference in 2016. Residual analysis shows that station position extrapolation error past the solution span becomes evident even after two years, and will contribute to about 3–4 mm radial orbit error after seven years. Crossover data show the DTRF2014 orbits are the most accurate for the TOPEX and Jason-2 test periods, and the JTRF2014 orbits for the Jason-1 period. However for the 2016 Jason-3 test period only the DPOD2014-based orbits show a strong and statistically significant margin of improvement. The positive results with DTRF2014 suggest the new approach to correct station positions or normal equations for non-tidal loading before combination is beneficial. We did not find any compelling POD advantage in using non-linear over linear station velocity models in our SLR & DORIS orbit tests on the Jason satellites. The JTRF2014 proof-of-concept ITRS realization demonstrates the need for improved SLR+DORIS orbit centering when compared to the Ries (2013) CM annual model. Orbit centering error is seen as an annual radial signal of 0.4 mm amplitude with the CM model. The unmodeled CM signals show roughly a 1.8 mm peak-to-peak annual variation in the orbit radial component. We find the TRF network stability pertinent to POD can be defined only by examination of the orbit-specific tracking network time series. Drift stability between the ITRF2008 and the other TRF2014-based orbits is very high, the relative mean radial drift error over water is no larger than 0.04 mm/year over 1993–2015. Analyses also show TRF induced orbit error meets current altimeter rate accuracy goals for global and regional sea level estimation.     

Demonstration of the use of the Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS) measurements to validate GPS ionospheric imaging

There is a lack of independent ionospheric data that can be used to validate GPS imaging results at mid latitudes over severe storm times. Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS), a global network of dual-frequency ground to satellite observations, provides this missing data and here is employed as verification to show the accuracy of the ionospheric GPS images in terms of the total electron content (TEC). In this paper, the large-scale ionospheric structures that appeared during the strong geomagnetic storm of 20 November 2003 are reconstructed with a GPS tomographic algorithm, known as MIDAS, and validated with DORIS TEC measurements. The main trough shown in an extreme equatorward position in the ionospheric imaging over mainland Europe is confirmed by DORIS satellite measurements. Throughout the disturbed day, the variations of relative slant TECs between DORIS data and MIDAS results agree quite well, with the average of the mean differences about 2 TECu. We conclude that as a valuable supplement to GPS data, DORIS ionospheric measurements can be used to analyse TEC variations with a relatively high resolution, ∼10 s in time and tens of kilometres in space. This will be very helpful for identification of some highly dynamic structures in the ionosphere found at mid-latitudes, such as the main trough, TID (Travelling Ionospheric Disturbances) and SED (Storm Enhanced Density), and could be used as a valuable auxiliary data source in ionospheric imaging.     

Vertical crustal motion derived from satellite altimetry and tide gauges,and comparisons with DORIS measurements

A somewhat unorthodox method for determining vertical crustal motion at a tide-gauge location is to difference the sea level time series with an equivalent time series determined from satellite altimetry. To the extent that both instruments measure an identical ocean signal, the difference will be dominated by vertical land motion at the gauge. We revisit this technique by analyzing sea level signals at 28 tide gauges that are colocated with DORIS geodetic stations. Comparisons of altimeter-gauge vertical rates with DORIS rates yield a median difference of 1.8 mm yr⁻¹ and a weighted root-mean-square difference of 2.7 mm yr⁻¹. The latter suggests that our uncertainty estimates, which are primarily based on an assumed AR(1) noise process in all time series, underestimates the true errors. Several sources of additional error are discussed, including possible scale errors in the terrestrial reference frame to which altimeter-gauge rates are mostly insensitive. One of our stations, Malè, Maldives, which has been the subject of some uninformed arguments about sea-level rise, is found to have almost no vertical motion, and thus is vulnerable to rising sea levels.     

The crustal dynamics data information system: A resource to support scientific analysis using space geodesy

Since 1982, the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by the National Aeronautics and Space Administration (NASA) as well as national and international programs. The CDDIS provides easy, timely, and reliable access to a variety of data sets, products, and information about these data. These measurements, obtained from a global network of nearly 650 instruments at more than 400 distinct sites, include DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), GNSS (Global Navigation Satellite System), SLR and LLR (Satellite and Lunar Laser Ranging), and VLBI (Very Long Baseline Interferometry). The CDDIS data system and its archive have become increasingly important to many national and international science communities, particularly several of the operational services within the International Association of Geodesy (IAG) and its observing system the Global Geodetic Observing System (GGOS), including the International DORIS Service (IDS), the International GNSS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International Earth rotation and Reference frame Service (IERS). Investigations resulting from the data and products available through the CDDIS support research in many aspects of Earth system science and global change. Each month, the CDDIS archives more than one million data and derived product files totaling over 90 Gbytes in volume. In turn, the global user community downloads nearly 1.2 Tbytes (over 10.5 million files) of data and products from the CDDIS each month. The requirements of analysts have evolved since the start of the CDDIS; the specialized nature of the system accommodates the enhancements required to support diverse data sets and user needs. This paper discusses the CDDIS, including background information about the system and its user communities, archive contents, available metadata, and future plans.