DORIS processing at the European Space Operations Centre

This paper gives an overview of the DORIS related activities at the Navigation Support Office of the European Space Operations Centre. The DORIS activities were started in 2002 because of the launch of the Envisat satellite where ESOC is responsible for the validation of the Envisat Precise Orbits and a brief overview of the key Envisat activities at ESOC is given. Typical orbit comparison RMS values between the CNES POE (GDR-C) and the ESOC POD solution is 6.5, 18.8 and 23.1 mm in radial-, along- and cross-track direction. In the framework of the generation of the ITRF2008 ESOC participated in the reprocessing of all three space geodetic techniques; DORIS, SLR, and GPS. Here the main results of our DORIS reprocessing, in the framework of the International DORIS Service (IDS), are given. The WRMS of the weekly ESOC solution (esawd03) for the 2004–2009 period compared to the IDS-1 combined solution is of the order of 12 mm. Based on the long time series of homogeneously processed data a closer look is taken at the estimated solar radiation pressure parameters of the different satellites used in this DORIS analysis. The main aim being the stabilization of the Z-component of the geocentre estimates. We conclude that the ESOC participation to the IDS ITRF2008 contribution has been beneficial for both ESOC and the IDS. ESOC has profited significantly from the very open and direct communications and comparisons that took place within the IDS during the reprocessing campaign.     

ERS-1 and ERS-2 operational and precise orbit determination

This paper presents the European Space Operations Centre's orbit determination and prediction systems for the ERS-1 mission. The routine operational orbit determination and prediction subsystem is discussed briefly, and statistics of the accuracy compared to the requirements are given. The precise orbit determination subsystem is then described, and the accuracy of its results are compared to those of the operational orbit system and to the D-PAF preliminary orbit solutions. Some geophysical results from the altimeter data, processed in these orbit determinations, are also presented. The ESOC/OAD ‘ERS-1 Orbit Report’ is introduced as a document providing this information on a monthly basis. Finally, this paper describes how the experience gained with the precise orbit determination will be exploited to further improve the accuracy of the routine system that will be used for ERS-2, and provides an estimate of this accuracy.     

Generating precise and homogeneous orbits for Jason-1 and Jason-2

Driven by the GMES (Global Monitoring for Environment and Security) and GGOS (Global Geodetic Observing System) initiatives the user community has a strong demand for high-quality altimetry products. In order to derive such high-quality altimetry products, precise orbits for the altimetry satellites are a necessity. With the launch of the TOPEX/Poseidon mission in 1992 a still on-going time series of high-accuracy altimetry measurements of ocean topography started, continued by the altimetry missions Jason-1 in 2001 and Jason-2/OSTM in 2008. This paper contributes to the on-going orbit reprocessing carried out by several groups and presents the efforts of the Navigation Support Office at ESA/ESOC using its NAPEOS software for the generation of precise and homogeneous orbits referring to the same reference frame for the altimetry satellites Jason-1 and Jason-2. Data of all three tracking instruments on-board the satellites (beside the altimeter), i.e. GPS, DORIS, and SLR measurements, were used in a combined data analysis. About 7 years of Jason-1 data and more than 1 year of Jason-2 data were processed. Our processing strategy is close to the GDR-C standards. However, we estimated slightly different scaling factors for the solar radiation pressure model of 0.96 and 0.98 for Jason-1 and Jason-2, respectively. We used 30 s sampled GPS data and introduced 30 s satellite clocks stemming from ESOC’s reprocessing of the combined GPS/GLONASS IGS solution. We present the orbit determination results, focusing on the benefits of adding GPS data to the solution. The fully combined solution was found to give the best orbit results. We reach a post-fit RMS of the GPS phase observation residuals of 6 mm for Jason-1 and 7 mm for Jason-2. The DORIS post-fit residuals clearly benefit from using GPS data in addition, as the DORIS data editing improves. The DORIS observation RMS for the fully combined solution is with 3.5 mm and 3.4 mm, respectively, 0.3 mm better than for the DORIS-SLR solution. Our orbit solution agrees well with external solutions from other analysis centers, as CNES, LCA, and JPL. The orbit differences between our fully combined orbits and the CNES GDR-C orbits are of about 0.8 cm for Jason-1 and at 0.9 cm for Jason-2 in the radial direction. In the cross-track component we observe a clear improvement when adding GPS data to the POD process. The 3D-RMS of the orbit differences reveals a good orbit consistency at 2.7 cm and 2.9 cm for Jason-1 and Jason-2. Our resulting orbit series for both Jason satellites refer to the ITRF2005 reference frame and are provided in sp3 file format on our ftp server.     

The GLONASS-M satellite yaw-attitude model

The proper modeling of the satellites’ yaw-attitude is a prerequisite for high-precision Global Navigation Satellite System (GNSS) positioning and poses a particular challenge during periods when the satellite orbital planes are partially eclipsed. Whereas a lot of effort has been put in to examine the yaw-attitude control of GPS satellites that are in eclipsing orbits, hardly anything is known about the yaw-attitude behavior of eclipsing GLONASS-M satellites. However, systematic variations of the carrier phase observation residuals in the vicinity of the orbit’s noon and midnight points of up to ±27 cm indicate significant attitude-related modeling issues. In order to explore the GLONASS-M attitude laws during eclipse seasons, we have studied the evolution of the horizontal satellite antenna offset estimates during orbit noon and orbit midnight using a technique that we refer to as “reverse kinematic precise point positioning”. In this approach, we keep all relevant global geodetic parameters fixed and estimate the satellite clock and antenna phase center positions epoch-by-epoch using 30-second observation and clock data from a global multi-GNSS ground station network. The estimated horizontal antenna phase center offsets implicitly provide the spacecraft’s yaw-attitude. The insights gained from studying the yaw angle behavior have led to the development of the very first yaw-attitude model for eclipsing GLONASS-M satellites. The derived yaw-attitude model proves to be much better than the nominal yaw-attitude model commonly being used by today’s GLONASS-capable GNSS software packages as it reduces the observation residuals of eclipsing satellites down to the normal level of non-eclipsing satellites and thereby prevents a multitude of measurements from being incorrectly identified as outliers. It facilitates continuous satellite clock estimation during eclipse and improves in particular the results of kinematic precise point positioning of ground-based receivers.     

Improvement of the SLR Borowiec station position in the global network ITRF91

This paper describes briefly two station coordinates solutions, the first one computed at Space Research Centre (SRC) using tracking data from Lageos satellite, and the second one computed at European Space Operations Centre (ESOC) using tracking data from Lageos 1, ERS-1 and TOPEX/Poseidon in a multi-arc solution. In particular the solution computed for the Borowiec station in ITRF91 system is described extensively. The Borowiec station position was estimated simultaneousely, considering or not the existence of range biases, with other geophysical parameters such as: daily polar motion parameters x_p, y_p, ocean tide coefficients, earth gravitational constant GM, etc.     

TOPEX/Poseidon precise orbit determination using GPS double difference phase observables

Data from the experimental onboard GPS receiver were used to accurately compute the orbit of TOPEX/Poseidon. This represents a unique opportunity to intercompare with two other classical tracking techniques (SLR and DORIS). A review of the methodology used is given together with current results.     

High precision orbits for the international GPS service for geodynamics (IGS)

The objectives of the IGS include the regular computation of precise orbits for the GPS satellites, an essential prerequisite for geodetic, geophysical and navigation applications of GPS relying on accuracies at the level of metres to centimetres. This involves the deployment of advanced GPS receivers at a considerable number of globally distributed ground points, including several ESA tracking sites; the largely automated daily retrieval and processing of large amounts of remotely generated data; and regular independent comparisons of solutions computed by the various participating analysis centres, showing for example differences of typically 15 cm in solutions for all orbits in the GPS constellation. The paper reviews the IGS activities being carried out at ESOC in the context of the overall service.