J.M. Dow

Routine operational and high-precision orbit determination of envisat

Abstract ESAs Earth observation satellite Envisat was successfully launched on 1 March 2002 by an Ariane-5 launcher, and ESOC immediately took over the task of determining and predicting the orbit using S-band tracking data, and optimising the manoeuvre sequence to bring the spacecraft into an orbit accurately phased with ERS-2. On-board, Envisat carries, among others, a radar altimeter, a DORIS instrument and a laser retro-reflector array (SLR). Data from these instruments are being used at ESOC for high-precision orbit determination, for verification of the routine orbit determination and for cross-comparison with orbits computed on-board by the DORIS navigator and with those delivered with the Envisat products. This paper presents the first consolidated results obtained for Envisat routine and high-precision orbit determination. All orbit determination and control activities were performed with the software package NAPEOS, which was developed in-house.

Precise orbit determination of GLONASS satellites at the European space agency

Abstract As an active Analysis Centre of the International GPS Service (IGS) the European Space Operations Centre (ESOC) joined the IGEX program for precise orbit determination of the GLONASS satellite constellation since its inception in 1998. This paper describes the orbit determination processing strategy, the specific GLONASS modelling issues implemented and a discussion of the processing results.

High precision altimetry from the Envisat mission

The paper will outline the orbit determination and associated activities planned at ESOC in the scope of flight dynamics support of the ESA Envisat mission. As a result of nearly a decade of experience with POD of ERS and TOPEX/Poseidon, Envisat will be able to build on a base of very accurate models. Of particular interest is the processing of altimetry to determine more accurate mean and dynamic sea surface topography models. The nearly complete coverage by precise DORIS tracking in zones of major interest like the Pacific Ocean will make it possible to compute more precise orbits where simultaneous tracking and altimetry were not available for the ERS spacecraft. To support the orbital operations (determination and control) of this satellite, ESOC is developing the NAvigation Package for Earth Observation Satellites (NAPEOS). The paper will conclude with a description of some salient aspects of this software package.

GPS orbit processing in support of low earth orbiter precise orbit determination

Abstract There are currently an increasing number of LEO missions incorporating dual frequency GPS receivers for Satellite to Satellite Tracking. The majority of LEO precise orbit determination (POD) strategies rely on high quality GPS orbits and clocks such as those supplied by the IGS Final product. The availability of these products may not satisfy operational requirements due to their ten day latency. This paper studies the effect on the accuracy of the LEO POD of different kinds of GPS products as produced by ESOC, an IGS Analysis Centre. GPS products have been used for generating LEO POD results that differ in latency, frequency, and in being estimated or predicted. These results are then compared as part of the IGS LEO Pilot project to determine the improvement and degradation that can be expected by using the different GPS products. Additionally this paper will discuss an approach for the simultaneous solution of LEO and GPS POD. A simultaneous solution approach has the potential of providing improvements both in the GPS and in the LEO products, or to use fewer ground stations for generating the IGS products. In addition, it can reduce the latency that appears in the generation of LEO results based on separate GPS products.

Relative and absolute navigation in earth orbit

Abstract Three experiments involving simultaneous collection of GPS data by spacecraft took place between November 1996 and October 1997, with the goal of demonstrating the feasibility of using GPS for relative navigation of spacecraft. The experiments took place within ESAs ATV (Automated Transfer Vehicle) Rendezvous Pre-development (ARP) program. This program aims to validate rendezvous technologies that will be used in ATV for its proximity operations around the International Space Station. The spacecraft involved were the Space Shuttle and the retrievable Astrospas platform for the first flight demonstration (on Shuttle flight STS-80), and the Shuttle and the Russian MIR space station during the STS-84 and -86 flights. GPS receivers were installed in the spacecraft involved and GPS data were collected for several intervals during the rendezvous and separation phases. ESOC Flight Dynamics was entrusted with the challenge of providing the most accurate trajectories that could be computed using the on-board collected GPS data in combination with ground collected data. The paper will present the strategy used to obtain the best estimated trajectories, the problems found during the analysis of the data and the results obtained, including comparisons with trajectories obtained using other tracking systems or algorithms.

ERS-1 and ERS-2 tandem mission: Orbit determination, prediction and maintenance

Abstract After a short summary of the successful orbit operations during the ERS-2 Launch and Early Orbit Phase, the transition to and implementation of the tandem mission for the two satellites is described, including a comparison of the required and achieved ground track spacing. The new implementation of the routine operational and precise orbit determination and prediction systems is presented briefly, and an estimate of the current orbit prediction and determination accuracy is given. These statistics include tracking data residuals, comparison between routine and precise orbits, comparisons with the D-PAF preliminary orbit solutions, and routine orbit prediction accuracy. Some geophysical results from the altimeter data, obtained in the precise orbit determination, are presented, with special attention to the calibration of the ERS-2 instrument. This paper also introduces the information presented on the Near-Earth Navigation and Geodesy Sections WWW pages on the Internet, where information about the routine and precise orbit determination for ERS-1 and ERS-2 is updated on a daily to weekly basis. Finally, some first results of the ERS-2 PRARE data processing at ESOC are presented.

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

Abstract This paper presents the European Space Operations Centres 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.

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

Abstract 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

Abstract 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)

Abstract 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.