Calibration of the altimeter data is crucial to the understanding, analysis and ultimately the proper representation of the scientific results extracted from it. The errors definedin the next section, rendering the altimeter data itself suspect, make it necessary to adoptother methods of independent data analysis as a means of verification of the measurement accuracy. These methods include:
Sea & Island based Platform Sites
Currently two sites are used in conjunction with TOPEX/Poseidon monitoring.One on an oil platform off the coast of southern California and the other on Lampedusaisland in the Mediterranean Sea. The oil platform site incorporates SLR tracking from mainland sites, GPS measurements of the SLR-platform distance, and tide gauge measurements using several different instruments attached to the platform. Also included are local environmental measurements.
Tide Gauge Data Sites
Currently, there are approximately 70 tide gauge sites located in a global network (primarily in the Pacific Ocean). The referencing of tide gauge data is employed in the following manner: The mean from each time series is removed and the differences in the measured sea level variations between each tide gauge and the neighboring altimeter data averaged over each TOPEX/Poseidon repeat cycle is computed. One problem with this procedure is that the land which the tide gauges are attached to moves and the amount may be of the same magnitude as that of the change in mean sea level. A possible rectification of this anomaly is the incorporation of GPS positioning on all tide gauge sites.
Onboard Altimeter Calibration
There is an onboard internal instrument calibration loop which can be used to monitor the long-term behavior of the altimeter. The disadvantage of this technique is that only the altimeter electronics are monitored with no correlation to the supporting environmental corrections being obtained.
Error Sources in Altimeter Measurements