  A fault detection and isolation method for satellite rate gyros is proposed
based on using the satellite-to-satellite measurements such as relative
position beside orbit parameters of the primary satellite. By finding a
constant of motion, it is shown that the dynamic states in a relative motion
are restricted in such a way that the angular velocity vector of primary
satellite lies on a quadratic surface. This constant of motion is then used to
detect the gyroscope faults and estimate the corresponding scale factor or bias
values of the rate gyros of the primary satellite. The proposed algorithm works
even in time variant fault situations as well, and does not impose any
additional subsystems to formation flying satellites. Monte-Carlo simulations
are used to ensure that the algorithm retains its performance in the presence
of uncertainties. In presence of only measurement noise, the isolation process
performs well by selecting a proper threshold. However, the isolation
performance degrades as the scale factor approaches unity or bias approaches
zero. Finally, the effect of orbital perturbations on isolation process is
investigated by including the effect of zonal harmonics as well as drag and
without loss of generality, it is shown that the perturbation effects are
negligible.
