BACK-UP MODE OPERATION
EXOSAT is a 3-axis stabilised satellite relying primariIy on a 3-axis + skew gyro pack for attitude control. Gyro drift is automatically compensated on-board by comparing the gyro outputs with the star tracker and Fine Sun Sensor (FSS) outputs, directly or via the OBC programs ROLLER and SMC.
In the event of a double gyro failure, 3-axis attitude measurement via the gyro pack is no longer possible. Provision was therefore made within the AOCE microprocessor to synthesise the gyro data from the star tracker and FSS readings. This is the so-called "Back-up Mode" which will allow EXOSAT operations to continue should a further gyro fail.
This mode was implemented late in the design phase of the satellite and has the following drawbacks.
- the star tracker data in particular is noisy in comparison with the gyro data, which wiII inevitably result in increased propane consumption and/or decreased pointing stability. A trade-off between the two can be made by adjusting the control loop gains which are stored in the microprocessor RAM.
- slew manoeuvres pose severe operational problems because of the size of the star tracker FOV (3° square). This restricts the magnitude of a slew to about 2° since it is not possible to switch from one star to another during the slew. Large manoeuvres would therefore be accomplished as a series of short steps, known as "star-hopping", which is very timeconsuming and expensive in propane usage.
Because of the problems associated with the 'star hopping' technique for slewing,
consideration has been given to using the remaining healthy gyros. In this
respect, it is fortunate that the failed gyro is the X-gyro which can always be
replaced by the FSS (with the restriction that the sun azimuth angle remains in
the range 30° to 150°). Work is in progress to define the procedures for
utilising the two remaining gyros (after a further single failure) instead
of the star tracker to control the Y and Z axes during the slew. This function
will be carried out by an OBC program which reads the relevant gyro outputs and
interfaces with the AOCE via the microprocessor RAM to induce the required
thruster firings. Reductions in slewing time, propane consumption and operational
complexity can be achieved by replacing the original 'star hopping' technique
with this procedure.
- a degradation in pointing stability from ± 2" to ± 10".