In accordance with TOPEX/ POSEIDON Science Working Team (SWT) recommendations, accepted by the Joint Steering Group (JSG), AVISO has agreed to update the current products by producing a homogeneous version-C series of GDR-M data, to cover the entire mission.

Ever more ambitious scientific goals

Since the start of the TOPEX/ POSEIDON (T/P) verification phase, AVISO and PO DAAC have been dis-tributing merged Geophysical Data Records (GDR-Ms). These bring to-gether and harmonise the data from various instruments. Meetings of the Joint Verification Team (JVT) and Science Working Team (SWT) have shown this approach to be useful. The information in the products dis-tributed has made it easier to evaluate performance of the complete T/P al-timeter mission, from instruments through to orbit, including the instrumental, environmental and geophys-ical corrections needed for scientists to use the results. In particular, the use of merged data from the two al-timeters has been shown to be both feasible and worthwhile.

At the end of the verification phase, an early set of enhanced algorithms was used to create a set of GDR-M products whose quality exceeded the performance requirements initially set to meet the scientific goals. Four years of GDR-Ms (1992-1996) provided the science community with the first opportunity to reliably quantify some of the features of ocean dynamics, and even to make new discoveries.

During the same period, the project teams and various scientific teams continued systematically validating the measurements and monitoring the long-term performance of all T/P sub-systems. At the end of the nominal mission, three years after the satellite launch, the consolidated validation results pointed to various possible enhancements in ground processing. These would make it possible to follow ever-more ambitious scientific goals. They included enhanced instrument data processing, calculation of instrument bias and temporal drift revealed by methodical long-term monitoring, phenomenal progress in orbit calculations thanks to Doris, successful modelling of ocean tides, use of new surfaces such as the geoid or the mean surface of the ocean, and much more. The T/P community at last has access to sea level variations on the order of one millimetre a year.

Producing the new version-C merged data

Following all the SWT recommendations regarding enhancements to GDR-M contents and format, the T/P JSG decided in 1996 that AVISO and PO DAAC would produce a new generation of merged data to cover the entire mission. To achieve this, AVISO was to simultaneously: include the enhancements in routine data processing from cycle 133, chosen by the SWT, and reprocess the data up to cycle 132. In practice, this meant creating a version C of the AVISO GDR-Ms. They were to be distributed as follows:

1) From cycle 133, and for all later T/P data, each user receives monthly CD-ROMs containing three GDR-M cycles produced directly as version-C data.

2) For cycles 1 to 132, each user will receive a complete set of CD-ROMs containing the version-C GDR-Ms, in several deliveries as they become available, to replace version-B.

Point 1 concerns routine operations, while point 2 means reprocessing 1320 days of data. After receiving cycles 1 to 132, each user will thus be sure of having fully homogeneous version-C data for the entire T/P mission, covering both the nominal operations and extended operations phases.

Producing version-C GDR-Ms meant setting up and simultaneously oper-ating two processing systems, one for reprocessing, the other for routine operations. It called for close coordination between the TOPEX Ground Segment (TGS), the DORIS-POSEIDON Control Centre (CTDP) and the AVISO and PO.DAAC centres, to ensure the greatest possible consistency when changes occurred under the responsibility of either entity.

Routine processing has resumed, and the data from cycle 133 are on your desktop or on their way. By the end of 1997, you will have a complete data set for cycles 1 to 132. New insights are doubtless on their way.

For those of you who do not wish to wait a few more months to have a homogeneous series from cycle 1, remember that a dedicated data set is available over the Internet (please contact us!). These data will let you do your own updating of your version -B GDR-Ms for cycles 1 to 132.

May we also remind you of three reference works that should help you understand the contents of version-C GDR-Ms (format changes; list and details of enhancements since version-B, etc.):

• AVISO, 1996, "AVISO CD-ROM User Manual for Merged TOPEX/POSEIDON products", AVI-NT-02-100-CN, ed. 3.0.
  
• AVISO, 1996, "AVISO User Handbook: Merged TOPEX/POSEIDON products (GDR-Ms)", AVI-NT-02-101-CN, ed. 3.0.
  
• AVISO, 1996, "AVISO User Handbook: TOPEX/POSEIDON Update product", AVI-NT-02-102-CN, ed.1.0.

We have crossed a new threshold in the mission, but the work of AVISO is still ongoing. In this newsletter, Pierre-Yves Le Traon (CLS) discusses new version-C high-level products (corrected sea surface heights, sea level anomalies) built up from T/P data, as well as ERS-1 and ERS-2 data. AVISO is as relevant as ever, continuing to do routine long-term monitoring of all T/P subsystems and distributing the data. We will also continue our many interactions with you, to learn ever more about your needs as altimetry data users, whether for scientific or operational applications.