TGS Mode: What are the scheduling implications?

Use of TGS mode has two main effects on scheduling.

1. For multi-orbit visits, guide star reacquisitions (which took 5 minutes in three gyro operation) will not be possible, so the full guide star acquisition (6 minutes) will need to be repeated.
2. The use of the Fixed Head Star Trackers (FHSTs) as part of the two gyro acquisition scenario dramatically reduces the times when targets are schedulable. The graph below presents results based on models run using a 5 degree by 5 degree grid of targets, a 30 minute exposure time, no scheduling constraints (e.g., no orientations or timing requirements), and no guide star availability considerations.

For Two Gyro mode, an individual target is observable, on average, for about 150 days per year, with that time clumped together. In the three gyro mode, the worst-case target is available for 240 days of the year.

To see how the entire sky is affected by use of TGS mode, please see the short (~2 minutes)
Narrated Movie. This will give you insight into the scheduling situation when in TGS mode.

The reduced schedulability may affect programs that involve:

• a large number of orbits at one pointing (mosaics)
• long-term (i.e., months) timing links between visits
• specific orientations