An inertial navigation system includes a first inertial measurement unit with at least a first sensor and a second inertial measurement unit with at least a second sensor corresponding in type to the first sensor. The first inertial measurement unit is rotatably mounted relative to the second inertial measurement unit, The inertial navigation system further include a controller arranged to: acquire a first set of measurements simultaneously from both the first inertial measurement unit and the second inertial measurement unit; rotate the first inertial measurement unit relative to the second inertial measurement unit; acquire a second set of measurements simultaneously from both the first inertial measurement unit and the second inertial measurement unit; and calculate from the first set of measurements and the second set of measurements at least one error characteristic of the first sensor and/or the second sensor.