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Ocean Surface Topography from Space
SCIENCE
Contribution of space techniques to Jason-1 altimeter calibration
Figure 1

Authors:


P. Exertier
OCA/CERGA, France
P. Bonnefond
OCA/CERGA, France
Y. Ménard
CNES, France
E. Jeansou
NOVELTIS, France

CORRESPONDING AUTHOR:
Pierre Exertier
Observatoire de la Côte d'Azur
Départ. CERGA
av. Copernic
06130 Grasse - France
Pierre.Exertier@obs-azur.fr



Abstract:

With uncertainties currently running at around a few millimetres per year, it is hard for
space geodesy to identify signatures of oceanographic phenomena of interest and, at the same
time, drift and/or inherent errors (biases) in a complete space system, i.e., including orbit
models and measuring instruments. The large amount of effort already expended on the TOPEX/POSEIDON
mission shows the difficulty of conducting long-term altimetry at the millimetre-per-year level.
In this respect, radar altimeter calibration and orbit validation work undertaken for altimetry
projects is extremely important.

Introduction

With uncertainties currently running at around a few millimetres per year, it is hard for
space geodesy to identify signatures of oceanographic phenomena of interest and, at the same
time, drift and/or inherent errors (biases) in a complete space system, i.e., including orbit
models and measuring instruments. The large amount of effort already expended on the TOPEX/POSEIDON
mission shows the difficulty of conducting long-term altimetry at the millimetre-per-year level.
In this respect, radar altimeter calibration and orbit validation work undertaken for altimetry
projects is extremely important.

The calibration experiment

The role of laser ranging systems during calibration campaigns is to achieve centimetre
accuracy locally for the altimetry satellite orbit [Bonnefond et al., 1995]. For radar
altimeter calibration to be successful, campaigns must be performed if possible under
the satellite track in zones where the slope of the geoid is smooth, away from the coast
(on an island or offshore platform) and in a region where the logistics involved are not
prohibitively expensive.

Absolute calibration of radar altimeters based on the SLR technique has been used previously
for numerous missions such as Seasat in Bermuda (1978), for ERS-1 in Venice (1991), for
POSEIDON in Lampedusa (1993), and for TOPEX on the Harvest platform (CA) [Francis, 1992;
Mitchum, 1994; Christensen et al., 1994; Ménard et al., 1994]. These "conventional" experiments
proved very troublesome to implement, given the remoteness of the sites and the complex
combination of satellite-based, geodetic surveying and tide gauge techniques.

With these aims and future space oceanography missions such as Jason-1 in mind, we have
set up a semi-permanent site in Corsica with the lowest possible installation and monitoring
costs [Bonnefond et al., 1997]. In addition, the French space agency CNES, the French
mapping and survey agency IGN, and OCA-CERGA have developed a new SLR concept from the
1990s called the French Transportable Laser Ranging Station (FTLRS). The idea behind this
project was to build a very small SLR station (telescope 13 centimetres in diameter, weighing
300 kilograms) that is easily transportable and can be installed, for example, in oceanic
zones on islands and oil drilling platforms [Nicolas et al., 2000]. The main objective is
to play a leading role in space oceanography in the early 2000s, via satellite tracking,
centimetre calibration of radar altimeters, positioning, and geodynamics.

The ultra-mobile FTLRS system, which is under technological development at OCA-CERGA, France,
in order to achieve centimetre accuracy, will be deployed in Corsica in 2001 first for the
Jason-1 validation phase. For the reasons explained above, this kind of experiment is planned
to continue if possible for several years to detect any drift in the spaceborne instruments.

Conclusion

The quality of space geodesy and Earth science research depends to a large extent on our
ability to obtain numerous, accurate measurements covering a wide spatial and temporal
spectrum.

All calibration experiments thus far have attempted to decorrelate the drift of altimeters
and associated devices (e.g., water vapor radiometer) from the long-term variability of the
mean sea level, at a level of one millimetre per year at least. The many absolute
calibration sites worldwide and the range of calibration techniques now used are of
great value for monitoring satellite altimetry performance at this level of accuracy,
because each site and each technique brings its own systematic errors. Further, these
concepts, which aim to improve the permanent vertical positioning of tide gauge sites
used for altimetry, using GPS or DORIS [e.g., Cazenave et al., 1999], also have a major
role to play in establishing a uniform and identical vertical reference frame for
oceanography missions, stable to within one millimetre per year.

References

Bonnefond P., P. Exertier, P. Schaeffer, S. Bruinsma, F. Barlier, 1995: Satellite altimetry from a short-arc orbit technique: Application to the Mediterranean. J. of Geophys. Res., 100(C12), 25365-25382.

Bonnefond P., P. Exertier, Y. Ménard, E. Jeansou, G. Manzella, S. Sparnocchia, F. Barlier, 1997: Calibration of Radar altimeters and validation of orbit determination in the Corsica-Capraia area. in Proceedings of the 3rd ERS Symposium, Florence, Italy; Vol. 3, 1525-1528.

Cazenave A., K. Dominh, L. Soudarin, F. Ponchaud, Ch. Le Provost, 1999: Sea level changes from TOPEX/POSEIDON altimetry and tide gauges, and vertical crustal motions from DORIS. Geophys. Res. Lett., 26(14), 2077-2080.

Christensen E.J. et al., 1994: Calibration of TOPEX/POSEIDON at Platform Harvest. J. of Geophys. Res., 99(C12), 24265-24485.

Francis C.R., 1992: The height calibration of the ERS-1 radar altimeter, in Proceed. of the first ERS-1 Symp. - Space at the Service of our Environment, ESA Spec. Pub., ESA SP-359(1), 381-393.

Ménard Y., et al., 1994: Calibration of the TOPEX/POSEIDON altimeters at Lampedusa: Additional results at Harvest. J. Geophys. Res., 99(C12), 24487-24504.

Mitchum G.T., 1994: Comparison of TOPEX sea surface heights and tide gauge sea levels. J. Geophys. Res., 99(C12), 24541-24553.

Nicolas J., F. Pierron, M. Kasser, P. Exertier, P. Bonnefond, F. Barlier, J. Haase, 2000: French Transportable Laser Ranging Station: Scientific objectives, technical features, and performance. Applied Optics, 39(3), 402-410.

Tapley B.D., B.E. Schutz, R.J. Eanes, J.C. Ries, M.M. Watkins, 1993: Lageos laser ranging contributions to geodynamics, geodesy, and orbital dynamics, in Contributions of Space Geodesy to Geodynamics: Earth Dynamics. Geodyn. Ser., 24, 147-174.


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