Spot-5 and South Atlantic Anomaly

The SAA is based on the geometry of the Van Allen radiation belts. The Van Allen radiation belts are symmetric with the Earth's magnetic axis, which is tilted with respect to the Earth's rotational axis by an angle of ~11 degrees. Because of this tilt, the inner Van Allen belt is closest to the Earth's surface over the south Atlantic ocean, and farthest from the Earth's surface over the north Pacific ocean.

Fig 2. Height offset for chosen South American stations, behavior from 2003.0 to 2010.5

Fig 3. Daily behavior of the estimated frequency offset for Master beacons. SPOT-5 and Envisat

The analysis of single-satellite solutions disclosed a SPOT-5 specific abnormality, related with the well known South Atlantic magnetic Anomaly (SAA). The effect of the SSA on the Jason-1 DORIS observation has been well known, but the significant effect on the SPOT-5 observations is an original discovery of GOP analyses center. The estimated station height and other related parameters, derived from SPOT-5 single-satellite solution, are significantly biased. Fig. 2 shows the differences between the weekly estimated station coordinates, using the SPOT-5 single-satellite solution and the combined solution GOP31. The highest bias was found in the station height, where the differences between both solutions reached the highest values for South America stations Cachoeira Paulista (-205 mm), Arequipa (-74 mm) and Santiago (-68 mm). In addition, the bias is getting higher during the time, as documented by Fig. 2. The horizontal positions show the largest absolute differences (from 30 to 45 mm) for the same stations and also for Kourou.

Corresponding differences were similarly detected also in comparison between estimated ZTD (Zenithal Troposphere delay) and GNSS ZTD and confirmed by analyses of the estimated frequency offset. Figure 3 shows the daily behavior of the estimated oscillator frequency offset, displayed for the master beacons (equipped with atomic clock). The behavior for SPOT-5 (affected by SAA) and Envisat (not significantly affected) is completely different. In the case of SPOT-5, frequency is strongly rising during flight through the SAA area. This part of the daily arc corresponds to the observation time span of the station Kourou (KRVB).