Chip-scale inertial sensor and inertial measurement unit

A chip-scale inertial sensor for detecting a rate of rotation of the inertial sensor about an axis, the inertial sensor comprising: an optical vibratory gyroscope 204 and an optical sagnac gyroscope 206. The gyroscopes detect a first and second rate of rotation respectively about the axis and a microcontroller determines a corrected first rate of rotation based on the inputs from the two gyroscopes. The optical vibratory gyroscope and the optical Sagnac gyroscope may receive light from the same light source 202. Determining the corrected first rate of rotation may comprise comparing the first and second rates of rotation, wherein the difference between the main signal (corresponding to the first rate of rotation of the optical vibratory gyroscope) and supplementary signal (corresponding to the second rate of rotation of the optical sagnac gyroscope) is calculated to determine the short and long term errors of the optical vibratory gyroscope and, correcting the main signal to remove the short and long term errors.