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110 users receiving Aviso CD-Roms

Sophie Coutin-Faye, Patrick Vincent (Cnes, France)

Aviso operations started in October 1992 with the first Interim Geophysical Data Records-Merged (IGDR-Ms). These were generated from:
- Topex IGDRs sent by our American colleagues at the Jet Propulsion Laboratory (JPL) Po-Daac,
- Poseidon IGDRs from the French Doris-Poseidon ground segment,
- semi-precise orbit ephemeris from the Doris Orbitography Group at Cnes, considerably better than the logistic orbit initially planned for the IGDR-Ms.

Since then, more cycles have been produced and ten were delivered in time to be used by the scientists at the JVT in Pasadena at the end of February 1993. CD-ROM turned out to be practical, reliable, and easy to use by PIs and co-Is. Some 110 people now receive them. IGDR-M production for the current cycles will continue through July 1993, when the final processing systems will be implemented. Production of GDR-Ms will then commence, both for the new cycles and to reprocess cycles since the launch. We expect to finish all the reprocessing by September 1993, one year after the launch. Production will then flatten out.

Throughout this early stage we have tried to give you as much information as possible about the data. First, when production began we released:

- Aviso CD-Rom User Manual: Merged Topex/Poseidon Products, ref. AVI-NT-02-100-CN,
- Aviso Handbook: Merged Topex/Poseidon Products, ref. AVI-NT-02-101-CN.

We also sent out a set of test data to prepare users for the actual results. Then, each time a cycle was sent out, we sent information on the data quality and warnings of any remaining errors. Three Calval bulletins were sent at regular intervals describing the results of statistical analyses and quality control over the scientific quality.

Topex/Poseidon merged products

The first four IGDR-M cycles were distributed as version 0 data, then as version 1 so that we could quickly implement modifications requested by the French and American ground segments after expert analysis of the first 40 days on the final orbit. It also gave us an opportunity to include an orbit with 10 to 15 cm accuracy on the radial component, right from the Verification phase. These enhancements and the correction of residual errors-implemented as soon as they were detected-were documented in the notes sent with the data.

The JVT meeting at JPL in Pasadena (February 22-25, 1993) confirmed the usefulness of merged Topex/Poseidon products. Comparisons between the instruments are easier, as is the scientific use of both data sets. The crossover points file, also distributed on CD-Rom, was changed from cycle 5 onward to eliminate as many spurious points-such as ice-as possible. The file was particularly useful for estimating the relative bias between the two altimeters, and often came in handy for detecting errors in the IGDR-Ms.

For the Observational phase, the GDR-Ms and crossover points files, with two cycles on each CD-Rom, are an extension of the Verification phase products. They integrate all the recommendations from the February 1993 JVT meeting.

To meet one of the user community's wishes, we will put on the CD-Roms a simplified file of the Nasa and Cnes precise orbit ephemeris used to build up the GDR-Ms, plus a readout program. You will be able to use the data from several satellites together, for example at the orbit crossover points. The file description and the contents of the GDR-M products and crossover points will be given in the manuals quoted above. These will be updated before the first GDR-M CD-Roms are distributed.


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