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.
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° 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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