This paper presents a method for accurate on-orbit calibration of rate-integrating threeaxis gyroscopes. On-orbit calibration is required for operationally responsive space systems in order to deal with rapidly changing technology and support needs. However, a tradeoff usually occurs between accuracy and speed of convergence of the to-be-determined calibration parameters. In order to achieve a fully autonomous and accurate calibration approach, a two-step process is proposed. The first step determines an initial estimate of the gyro calibration parameters using an angular velocity estimate determined from a star tracker. This angular velocity estimate is provided without the need to identify stars, thereby bypassing the associated adverse issues that typically arise in star tracker attitude determination systems. The second step uses the initial gyro calibration estimates in a generalized multiple-model adaptive estimation approach, which combines outputs from three filters to determine estimates that are better than each filter alone. Simulation results are shown to assess the performance of the proposed on-orbit calibration approach.