The spacecraft is generally aligned to solar north, although it oscillates around slightly. The actual S/C roll value is determined from the star tracker and is stored in the ATT database file. There is also a one degree offset between the spacecraft and the SXT CCD-north. The orientation of the offset is shown in Fig. 1.
Figure 1: Roll Angle Offset for the SXT
The sign convention of the roll value which is returned by GET_ROLL is shown in Fig. 2.
Figure 2: Sign Convention for the SXT Roll Angle
Figs. 1 and 2 can be displayed on the screen by using the IDL routine HELP_ROLL.
Some sample calls to GET_ROLL are (it only accesses the date and time in index):
IDL> roll = get_roll(index)
IDL> print, get_roll('23-jun-93 1:00')
IDL> print, get_roll('23-jun-93 1:00', /predict)
The /predict option tells it to not access the ATT database but to use the
predicted roll based on seasonal variations. This value can be very incorrect
for certain times. The /notsxt option will return the S/C roll value
without the SXT offset, and /sxt_offset will just return the fixed
offset value between S/C north and SXT north. The routine SXT_CEN calls
GET_ROLL automatically and you can get the roll using the keyword roll
with SXT_CEN.