Since the middle of the eighties 11 satellite radar altimeter missions have been launched. Most of these missions have different data formats and processing standards. Since then the correction models applied to the measured ranges have further evolved constantly. As long as single mission studies are performed, these differences can be neglected. However, when comparing or combining different radar altimeter data for e.g. geodetic or climate studies, a harmonization is necessary. In addition, for several studies (e.g. close to the coast) it is necessary to select a set of correction models best fitting the particular application.
In order to integrate, process, update, and harmonize data from different radar altimeter missions the Altimeter Database and Processing System (ADS) has been developed at GFZ. Currently it holds sea surface heigths, winds and wave heights from the following missions for the denoted periods of time:
|Geos-3||04/1975 to 11/1978|
|Geosat||03/1985 to 10/1989|
|ERS-1||08/1991 to 06/1996|
|Topex/Poseidon||09/1992 to 10/2005|
|ERS-2||05/1995 to 06/2011 (North Atlantic only since 05/2003)|
|GFO-1||01/2000 to 09/2008|
|Jason-1||01/2002 to 06/2013|
|Envisat||10/2002 to 04/2012|
|ICESat||02/2003 to 10/2009|
|Jason-2||07/2008 to ongoing|
|Cryosat-2||01/2010 to ongoing|
|HY-2A||10/2011 to ongoing|
|Saral/Altika||03/2013 to ongoing|
While the data originates mainly from oceanic areas there is also data from inland waters available.
In addition to the radar altimeter data the ADS holds state of the art instrumental, orbital, and environmental correction models.
The structure of ADS allows for integrating upcoming data corrections and new correction models in a timely manner. The parallel storage of different correction models for the most disturbing effects allows for testing their influence on derived quantities, such as mean sea levels and crossover statistics.
In order to produce long and consistent time series the altimeter group at GFZ has been working on the harmonization of the correction models of different missions. Wherever possible the same correction models were provided consistently for all missions. This includes corrections for all tidal effects, dry troposphere and inverse barometer. Other correction models, like the ionospheric or wet tropospheric correction, rely on the satellite hardware. In this case the most accurate correction available is used for each mission. Systematic differences between the different correction models were investigated. Other correction models including, e.g. sea state bias and orbits, are mission dependent and have been recalculated in a consistent manner.
For the inter-calibration of data from different missions two different approaches are taken. Global biases are calculated by multi-mission cross-over point analysis. The other approach is to reference the time series to temporal mean values common for two (or more) altimeter missions.
The ADSC web interface allows external users to retrieve and process data and to derive many statistical quantities.