Aviso Newsletter 4
Barely a year after the launch of TOPEX/POSEIDON, the oceanographic community was producing excellent work from Geophysical Data Records. "No special knowledge in areas of […] orbit mechanics was needed", said Cheney et al. in “TOPEX/POSEIDON, the 2-cm solution”, JGR vol. 99, # C12, page 24, 562. The authors were certainly not saying there was nothing left to achieve in orbitography. They were simply stressing the fact that the high-quality results from the Precise Orbit Determination (POD) people at NASA and CNES were enhancing the initial specifications. What’s happening now, three years after the launch, at the start of the T/P Follow-On mission?
The initial analyses showed that the dynamic orbits supplied by the PODs have comparable error spectra. At the same time, GPS experiments and analyses of the data showed geographical areas where the orbital errors appeared to be correlated. Also, the closer the initial dynamic orbits were to reality, the better the processing using the reduced dynamics technique. In the long term, however, laser and DORIS remained the sole nominal systems for calculating T/P trajectories.
These realizations led to a lot of work by U.S. and French orbitography specialists. First, there have been two particularly important developments. We have a new gravity model, JGM3, to which GPS has made an unmistakeable contribution. Similarly, an impressive number of ocean tide models are now available for the NASA and CNES PODs. Second, DORIS is undeniably essential as a tracking system for T/P, in terms of continuity and availability. An adapted reduced dynamics approach, based on the way DORIS works, has been developed and has led to the production of the ELFE orbit. Both POD orbits should provide higher and comparable quality than previous orbits, but will have different error spectra, offering users two statistically independent samples. In any case, that is our aim.
To maintain continuity, all the T/P cycles have been reprocessed and are available both as dynamic orbits from the NASA POD and as ELFE reduced dynamic orbits from the CNES POD. This is a lot of work and will call for a corresponding contribution from the oceanographic user community. The payoff we hope for in this exciting adventure is centimetre-by-centimetre improvement.
F. Nouel (CNES, France)
Editor : Patrick Vincent
Editorial board : Frédérique Blanc
Authors and other contributors : N. Ayoub, J. Barckicke, B. Barotto, S. Barstow, J.-P. Berthias, E. Blayo, P. Brasseur, N. J. Bravo, C. Brossier, P. De Mey, D. B. Enfield, L.L. Fu, J. E. Harris, G. Larnicol, P. Lasnier, P.-Y. Le Traon, S. Loeul, Y. Ménard, J.-F. Minster, F. Nouel, G. Pihos, R. D. Smith, J. Verron, R. Zaharia.
English adaptation : N. Greenwood
Design : D. Ducros