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I was reading recently about the near-loss of the Solar and Heliospheric Observatory (SOHO) spacecraft back in 1998, in this Wikipedia article. This article mentions that

Only one gyro remained operational after this recovery, and on December 21 that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7 kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.

What is this gyroless operations mode? What does it depend on to determine the spacecraft's attitude? Are there other spacecraft that use it?

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SOHO as well as most spacecraft use star trackers for attitude determination, aided by Sun sensors for recovery and fault protection operations. Gyroless operation is normal. In fact it is common to turn them off when you don't need them to preserve lifetime. Gyros can only tell you your roll rates and how your attitude is changing over time. They can't tell you your attitude by themselves. They are used to improve your attitude knowledge over short time scales.

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The issue, and it's resolution are listed in detail on the NASA web-site. The web-site links to the exact maneuver appear to be broken.

However this paper sums up the gyro-less operation as follows

On December 21 1998, the third and last gyro was lost, which put SOHO into ESR mode. With new solutions to:

  • reduce the orbit perturbation of the ESR mode (by manual yaw braking from the ground),
  • accurately measure the roll rate (wheels spun-up so that there is a net momentum on pitch, which combined with the roll rate creates a yaw disturbance torque),
  • patch the Attitude Control Software to fly SOHO without gyro control,
  • implement new or updated procedures, including the one to recover from ESR (now done without gyroscope control),

SOHO spacecraft was put back into normal mode on February 1, 1999. Since then it is the first three-axis-stabilized ESA spacecraft to be operated without a gyro.

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