IAG
Special Study Group 2.162
PRECISE
ORBITS USING MULTIPLE SPACE TECHNIQUES
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
Abstract
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.
References
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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