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Random gyro drift, which is the uncorrectable drift that occurs in gyros, represents the major error input of inertial navigation systems that are required to operate over long time intervals. This random gyro drift must be taken into account whenever performing an error analysis or optimization study on such a system; otherwise, the results of the analysis or study would be meaningless. Random gyro drift can be taken into account by assuming the drift to be describable as a random process. Two mathematical models for random gyro drift, based on this assumption, are discussed and evaluated. The evaluation of these two models has as its basis actual gyro drift rate test data. Normally, such data are readily available. The first phase of the evaluation consists of a comparison of the model's description of the gyro drift process to statistics computed directly from test data. The second phase of the evaluation consists of a comparison of the rms navigation errors predicted using the model to actual rms navigation errors.