A scheme for a three-axes orientation measurement device that provides low sensitivity of orientation measurement errors to gyro errors is analyzed. The low-sensitivity property of the device is provided at the expense of continuous, one-directional rotation of the gyro unit. Gyro unit rotation enables measurement of spatial orientation when all gyros but one fail. This results from the fact that three components of the moving vehicle full-angle speed vector modulate three different parameters of the output signal of each gyro. The results obtained are significant for the development of small-sized strapdown inertial navigation systems that have high accuracy and reliability and employ low-cost optical gyros-laser, fiber optic, and integrated optic gyros.