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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.
Work on the definition of the OBC program is now almost complete and coding has begun. Testing of the program and drafting of the supporting operational procedures should be finished shortly. The program is expected to occupy more than one application program slot in the OBC.

To summarise, if and when the Back-up Mode becomes operational, the impact will be as follows:

- a degradation in pointing stability from ± 2" to ± 10".

- an increase in overall propane consumption of about 25%.

- a restriction of the solar aspect angle to 30° - 150° (constrained in any case to 90° less equal beta symbol less equal 130° for nearly all observations).

- a time penalty during manoeuvres (~1 hr).

- continuous use of the FSS implies a roll about the spacecraft X-axis during observations of up to 1°/day.

- a limitation in space available in the OBC for payload programs during manoeuvres.

P. Prior
Spacecraft Operations Group

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