During the last year of the Geosat mission (late-1988 onward), both the coverage and accuracy of the altimeter data rapidly degraded compared to the first 3.5 years of observations. These data should therefore be used with some caution. As described below, both problems were related to the approach of solar maximum in mid-1989.
Geosat maintained nadir pointing of the altimeter antenna by means of a gravity gradient stabilization system. This method gave outstanding mechanical reliability, but allowed excursions off-nadir of 1 degree or more. Because the beam width of the altimeter was only 2 degrees, the nadir footprint was not always fully illuminated, making it difficult for the onboard tracker to lock-on to the return pulse during transitions from land to water. This resulted in permanent losses of data as shown by the sample coverage map below. (Coverage maps for each cycle of the GM and ERM can be found in subdirectory cyc_maps).
The problem of data loss existed during both the GM and ERM, but became increasingly severe as the mission progressed because of added drag and solar radiation pressure associated with the approach of solar maximum. The figure below shows the number of records for each day of the mission.
Orbit accuracy is also related to the solar cycle. When the sun is more active, the increased drag and solar radiation forces on the satellite become more difficult to model. At the same time, the quality of the Doppler tracking data is degraded by the ionosphere.
Global crossover differences, shown in the figure below, can be interpreted as a measure of orbit precision. (A crossover is defined as the intersection of the satellite ground track with itself. At this location, the two crossing passes provide independent sea level measurements at the same place but at different times). During the first 3.5 years of the Geosat mission, the crossover differences are about 13 cm for both 23-day (GM) and 17-day (ERM) cycles. This is consistent with 10-cm orbit precision, taking sea level variability into account. But near the end of 1988 the values rise to more than 30 cm and then fluctuate wildly, indicating that the orbit is not well-determined.
Nov 19-25, 1987 - Geosat carried two momentum wheels to reduce off-nadir pointing. Only one wheel was used during the first 2.5 years, but mechanical degradation was noted, and the second wheel was activated on Nov 18, 1987. During spin-up, spacecraft attitude was disturbed by as much as 7 degrees, and altimeter data collection was reduced for several days thereafter.
Aug 18-25, 1988 - Less than half of the normal number of observations were collected during this time when one of Geosat's batteries was reconditioned.
Mar 16-22, 1989 - Severe magnetic storms associated with solar disturbances resulted in a total loss of Geosat data during this time.
Oct 4, 1989 -Collection of global data ended when the altimeter amplifier was shut down by an automatic protection circuit. Although by-passing this circuit enabled the amplifier to be powered back up in mid-October, failure of both onboard tape recorders limited subsequent data collection to the western North Atlantic using direct broadcast to the Johns Hopkins APL ground station.
Jan 4, 1990 - Degradation in the output power of the Geosat altimeter resulted in termination of the mission on this date.