IAG Special Study Group 2.162


IAG Section II


Remko Scharroo (Chair)

Delft Institute for Earth-Oriented Space Research,

Kluyverweg 1, 2629 HS, Delft, The Netherlands,

e-mail: remko@deos.tudelft.nl



The IAG Special Study Group 2.162 “Precise Orbits Using Multiple Space Techniques”' got a reincarnation in 2000, after having a bit of a dormant state. During 2000 members of the Special Study Group have been developing and improving techniques to better determine the orbits of satellites, mainly of low-earth orbiters that carry remote sensing equipment that require high precision orbits.

These activities are focusing particularly on current and upcoming satellite missions that require precise orbit determination and have more than one tracking data type available, such as SLR, DORIS, radar altimeter, GPS and/or GLONASS. To be launched in 2001 are Envisat and Jason-1, Cryosat in 2003. \ers1, \ers2, TOPEX/Poseidon and GFO are used currently.

Orbit determination of GPS and GLONASS satellites using its microwave instruments in connection with SLR shows still some complications with the definition of the phase and optical centres.


1 Introduction and background

Modern satellites that require precise positioning are equipped with several independent tracking devices. The ERS satellites were the first to combine Satellite Laser Ranging (SLR) and Doppler tracking with the Precise Range And Range-rate Equipment (PRARE) for precise orbit determination in support of the radar altimeter (RA). It was soon shown that the RA itself proves an important tracking device. Interferometric Synthetic Aperture Radiometry (InSAR) has recently developed to become another demanding consumer of precise satellite orbits.

TOPEX/POSEIDON (T/P) carries, apart from its RA, four independent tracking systems including SLR, Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS), Global Positioning System (GPS), and the Tracking and Data Relay Satellite System (TDRSS). For the first time, the force model errors, especially gravity, have been reduced to a point where a comparison of the various satellite tracking systems at or near their noise level is possible.

Results, as expected, show that each system has its own strengths and weaknesses. Therefore, recent precise orbit determination improvements for \ers2 and T/P have been obtained using a combination of multiple tracking techniques. With PRARE on ERS-1 and GPS on Geosat Follow-On (GFO) on the limb, orbits for these satellites will likely remain to be based partly on altimeter tracking data.

The next generation of altimeter satellites (Jason-1, Envisat and Cryosat) will also be equipped with several tracking systems to support their altimeter, either DORIS or GPS in combination with SLR. There are great expectations for achieving orbits with sub-centimeter precision with a latency of about a month. Operational near real-time orbit determination is rapidly gaining interest and precision. With the approach of DIODE real-time orbits will be at hand.

In the future navigation and tracking satellites (GPS, GLONASS, and TDRSS) will start demanding higher precision orbit determination, because they are and will be used as reference for Low Earth Orbiters (LEOs) in high- low satellite-to-satellite tracking configurations (cf. IAG Subcommission on Precise Orbit Determination for Low Earth Orbiting Satellites). Some of these navigation satellites are equipped with more than one tracking system. An important aspect is also to assess the respective tracking station coordinate solutions and evaluate misfits between the solutions.

GRACE will provide precise satellite-to-satellite tracking in a low-low configuration. Since precise orbit information for this satellite is so important, it will be wise to combine this tracking data type with e.g. the readings of the accelerometers. This is a joint research topic with SSG 2.193 (chaired by Pieter Visser)

The focus of this study group will be to further evaluate and characterize the various tracking systems, develop and assess new tracking techniques, and apply the products to improve the state-of-the-art in precision orbit determination.


2 SSG 2.162 members

The IAG Special Study Group 2.162 consists of 22 members, including the chair and 1 corresponding member. The names and affiliation of the members is listed below:

Chair:Remko Scharroo (TU Delft, The Netherlands)

Members:Boudewijn Ambrosius (TU Delft, The Netherlands), Per-Helge Andersen (FFI, Norway), Jean-Paul Berthias (CNES, France), Willy Bertiger (JPL, USA), John Dow (ESA, Germany), Ramesh Govind Coleman (AUSLIG, Australia), Bruce Haines (JPL, USA), Jaroslav Klokocnik (Czech Academy of Sciences, Czech Republic), Scott Luthcke (GSFC, USA), Franz-Heinrich Massmann (GFZ, Germany), Francois Nouel (CNES, France), Erricos Pavlis (UMD, USA), John Ries (UT/CSR, USA), Markus Rothacher (AIUB, Switzerland), Ernst Schrama (TU Delft, The Netherlands), Ladislav Senhal (Czech Academy of Sciences, Czech Republic), C.K. Shum (OSU, USA), Tim Springer (AIUB, Switzerland), Mike Watkins (JPL, USA), René Zandbergen (ESOC, Germany), Shengyuan Zhu (GFZ, Germany)

Corresponding member: Pieter Visser (TU Delft, The Netherlands)

The members have all been active in satellite orbit determination and have contributed to the improvement of orbit precision in various ways: by the development of accurate measurement models and techniques to combine various types of tracking data, or by the comparison of orbits based on different tracking data. Results of these activities have been presented at various international conferences and symposia like those of EGS, AGU, and IAG, and satellite-specific symposia and workshops like the T/P Science Working Team meetings and the ERS-Envisat Symposium.


3 Specific results and outlook

Most efforts currently focus at the improvement of LEO satellites with remote-sensing instrumentation that requires highly accurate orbit determination, such as ERS1, ERS2, T/P, and Envisat. Ground breaking work has been conducted in the past combining SLR and altimeter data for the orbit determination of ERS1 and ERS2, in the absence of a more precise microwave tracking instrument. When PRARE became available, the combination of the three tracking data types became an important issue. However, the intricacies of the differences between the various orbits computed with the different tracking instruments are not yet understood.

The GLONASS and GPS satellites also obtain more attention. Attempts have been made to combine GPS/GLONASS and SLR tracking data for the orbit determination. Here, it was found that the position and the phase centers and the optical centre of the laser reflector array are often less known than expected. More research is to be expected in this field.

Operational, near-real time orbit determination appears feasible now using only SLR and altimeter tracking data. Accuracies of near realtime ERS2 orbits are in the neighbourhood of 10 cm in radial direction.

DORIS is getting more attention as it matures into the most effective microwave tracking instrument ever developed. The next generation will provide real-time orbit information. Next studies will have to identify to which extend DORIS and SLR tracking data are compatible or complementary. The same holds for these tracking types with respect to GPS. Unfortunately, the later issue is still under-researched.

During the next phase, the study group will be extended with experts in the area of GLONASS and GPS orbit determination. A proper database of all relevant papers and presentations will be gathered and be available through the web page of the special study group.



 Berthias, J.-P., P. Cauquil, C. Jayles, D. Laurichesse, and S. Nordine (2000), Real-time on-board precise orbit determination with doris, Paper presented at the European Geophysical Society XXV General Assembly, Nice, France, 25-29 April 2000.

Lemoine, F. G., N. P. Zelensky, D. D. Rowlands, G. C. Marr, S. B. Luthcke, and D. S. Chinn (2000), Precise orbit determination for the geosat follow-on spacecraft, Paper presented at the European Geophysical Society XXV General Assembly, Nice, France, 25-29 April 2000.

Massmann, F.-H., J. C. Raimundo, C. Reigber, C. Falck, F. Flechtner, and A. Scherbatschenko (2000), The prare system onboard ers-2: Status and results, in “Proceedings of the ERS-ENVISAT Symposium, Gothenburg, Sweden, 16-20 October 2000, Eur. Space Agency Spec. Publ., ESA SP-461”.

Scharroo, R., P. N. A. M. Visser, and N. R. Peacock (2000), ERS orbit determination and gravity field model tailoring: Recent developments, poster presented at the European Geophysical Society XXV General Assembly, Nice, France, 25-29 April 2000.

Vincent, P., B. Duesmann, and R. Scharroo (2000), The validation of envisat orbits, in “Proceedings of the ERS-ENVISAT Symposium, Gothenburg, Sweden, 16-20 October 2000, Eur.\ Space Agency Spec.\ Publ., ESA SP-461”.



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