4.10 SAA and Scheduling System Issues

Changes in the WFPC2 observation scheduling system were introduced early in 1999 primarily in order to increase the scheduling efficiency of HST observations starting with Cycle 8.

First, the South Atlantic Anomaly (SAA) contours used to limit WFPC2 observations were modified slightly. The SAA is a region where irregularities in the Earth's magnetic field cause very high cosmic ray rates. WFPC2 imaging is generally not scheduled near the SAA, so as to avoid excessive cosmic ray hits which degrade images by obliterating data in numerous pixels. These adverse effects are usually minimized by operating each instrument only when HST is outside a designated "SAA avoidance contour." (WFPC2 observations of time-critical phenomena can be taken inside the SAA avoidance contour, if necessary.) Biretta and Baggett (1998) analyzed available WFPC2 data, together with data from Air Force satellites flying in similar orbits, and redefined the WFPC2 SAA avoidance contour. This resulted in a 3% increase in designated SAA-free orbits, which allows better scheduling efficiency, and negatively impacts less than 0.1% of WFPC2 science observations. The current (post-1999) contour is given by the M26 area in Figure 4.11.

Figure 4.11: SAA Avoidance Contours.

Second, WFPC2 visits are limited to a maximum length of 5 orbits. Very long visits (up to an earlier maximum of 8 orbits) have very limited opportunities for scheduling, reduce the efficiency of telescope use, and can cause long delays in execution, with long GO wait times. Shorter visits improve the scheduling opportunities for large proposals. One possible drawback is the lower pointing repeatability across visits; this is significant only for programs with special dithering requirements.

A third change since Cycle 8 is that an extra minute of overhead was added to each orbit, which allows splitting an orbit in the Phase II proposal into two separate spacecraft alignments. This one-minute "efficiency adjustment" allows much more efficient scheduling of HST orbits impacted by the SAA.