Towards an operational global coastal altimetry product: AltiCAP (ALTimetry Innovative Coastal Approach Product)

Although spatial altimetry was originally developed for measurements on the open sea, it can be used in a coastal context, even though this type of use is far more complex. Part of the problem lies in the land contamination of radar observation in the last few kilometers from the coast. In the last few dozen kilometers approaching the coastline, this problem is further aggravated by the poor quality of some of the geophysical corrections applied. In view of the large number of altimetry applications, the international community and space agencies have been trying to resolve this issue for more than 15 years. Following extensive work and studies, a number of processing algorithms have been developed and a few experimental products have proposed.

In order to move towards the routine use of sea level altimetry measurements along the coastal strip on a global scale, we have conducted a Round Robin study aimed at comparing 25 algorithms used to calculate sea level anomalies (SLA) from altimetry in low resolution mode (LRM) and targeting the ocean region between 0 and 200 km from the coast. For each one, a significant number of diagnostics was carried out at global and regional levels. The most immediate outcome of this study is the new global AltiCAP altimetry product available on the AVISO+ portal since January 2024.

Note that the processing solution which was adopted is a compromise between:

• The capability of each algorithm (correction or parameter) to provide the best SLA dataset over the entire strip between 0 and 200 km from the coast (and not necessarily in the most coastal zone) in order to guarantee continuity with the open ocean.

• The availability of the correction or parameter on several altimetry missions.

• A guarantee of product continuity in the future.

Integrated with the Jason-3 mission in the first instance, this high resolution (20Hz) altimetry product will rapidly include other missions in order to extend the use of the product in time and space, and ensure a long-term implementation. It is provided with different case studies in the form of Python notebooks. Its distribution will also be supported by training workshops for the international community (virtual schools).