Anewapproachtogyrocalibrationispresented,wherethespacecraftdynamicsequation,attitudemeasurements, and the gyro outputs are used in a pseudolinear Kalman e lter that estimates the calibration parameters. Also an algorithm is presented for calibrating a skewed quadruplet rather than the customary triad gyro set aligned along the body coordinate axes. In particular, a new misalignment error model is derived for this case. The new calibration algorithm is applied to the EOS-AQUA satellite gyros. The effectiveness of the new algorithm is demonstrated through simulations. YRO calibration, as well as calibration of other instruments, includes two stages. During the e rst stage, the instrument er- ror parameters are estimated, and in the second stage, those errors are continuously removed from the gyro readings. In the classical approach to gyro calibration, the gyro outputs are used to maintain or compute body orientation rather than used as measurements in the context of e ltering. In inertial navigation, for example, 1 gyro errors cause erroneous computation of velocity and position, and then when the latter are compared to measured velocity and posi- tion, a great portion of the computed velocity and position errors can be determined. The latter errors are then fed into a Kalman e lter (KF) that uses the inertial navigation system error model to infer the gyro errors. Similarly, when applying the classical approach to spacecraft(SC) attitude determination, the gyro outputs are used to compute theattitude, andthentheattitude measurements 2;3 areused to determine the attitude errors, which again using a KF indicates what the gyro errors are. Several ways of treating gyro calibration have been presented in the literature. 4i6 In the approach adopted in this work, the gyro outputs are used as angular rate measurements and are compared to estimated angular rates. However,thisapproachrequires the knowl- edge of the angular rate. In the past, 7 the estimated angular rate was computed in a rather simplistic way, assuming basically that the rate was constant. In the present work, the estimated angular rate is derived using a KF whose input can be any kind of attitude mea- surement; therefore, the angular rate experienced by the SC can be continuously changing, and yet a good estimate of the rate, neces- sary for calibration, can be obtained. The calibration algorithm presented in this work was derived for a set ofquadrupletgyros. This required the derivation of anewerror model,particularly forthe gyro misalignments.Thenewcalibration algorithmwas applied to thegyro package ofthe EOS-AQUA satel- lite. The latter consists of four gyros, which are given the task of measuring the three components of the SC angular velocity vector resolved in the body Cartesian coordinates.