Mitigation of drift effects in secondary inertial measurements of an isolated detector assembly

A kinetic energy kill vehicle (KV), comprising:

• a frame;
• divert and attitude control thrusters mounted to the frame to alter the trajectory of the KV;
• a main IMU coupled to measure 6-axis rigid body motion of the KV, said 6-axis rigid body motion including linear accelerations in x, y and z directions and angular rates of change of roll, pitch and yaw about the respective x, y and z axes;
• a compliant isolator mounted to the frame;
• a seeker for capturing images of scene, said seeker including an isolated sub-assembly comprising a telescope and a detector mounted on said compliant isolator, said isolator and isolated sub-assembly having a resonant frequency between 5 Hz and 5 kHz to couple low-frequency rigid body motion of the KV below the resonant frequency to the seeker and to partially isolate said sub-assembly from high-frequency attitude noise above the resonant frequency, said isolation inducing a pseudo-motion in yaw and pitch between the detector assembly and the frame;
• at least two secondary rate sensors that exhibit a time-varying drift in their measurements, said secondary rate sensors coupled to output secondary measurements of the angular rate of change in yaw and pitch of the isolated sub-assembly, said secondary measurements comprising a rigid body motion component, a time-varying drift component and a pseudo-motion component;
• and
• a computer that uses the main IMU's yaw and pitch measurements as a reference to remove the rigid body motion component from the secondary measurements of yaw and pitch to generate yaw and pitch difference signals, implements an estimator that processes the difference signals to generates estimates of the time-varying drift component for yaw and pitch, and subtracts the estimates from the secondary measurements of yaw and pitch to produce corrected secondary measurements of yaw and pitch that include the rigid body motion component and pseudo-motion component, said computer processing measurements of the 6-axis rigid body motion and the corrected measurements of the angular rate of change in yaw and pitch of the isolated sub-assembly to estimate a 6-axis position in x, y and z and attitude in roll, pitch and yaw of the isolated sub-assembly.

A method of measuring 6-axis position and attitude of a detector assembly on an imaging vehicle, said vehicle comprising a propulsion system mounted to a frame and a main IMU coupled to measure 6-axis rigid body motion of the vehicle's frame, said 6-axis rigid body motion including linear accelerations in orthogonal x, y and z directions and angular rates of change of roll, pitch and yaw about the respective x, y and z axes, said detector assembly including an isolated sub-assembly comprising a telescope and a detector, said method comprising:

• mounting the isolated sub-assembly on a compliant isolator to provide a resonant frequency between 5 Hz and 5 kHz for yaw and pitch, said compliant isolator coupling low-frequency rigid body motion of the vehicle below the resonant frequency to the isolated sub-assembly and isolating the isolated sub-assembly from high-frequency attitude noise above the resonant frequency;
• using at least two secondary rate sensors that exhibit a time-varying drift to measure the angular rate of change in yaw and pitch of the isolated sub-assembly, said secondary measurements comprising a rigid body motion component, a time-varying drift component and a pseudo-motion component;
• using the main IMU's yaw and pitch measurements as a reference to remove the rigid body motion component from the secondary measurements of yaw and pitch to generate yaw and pitch difference signals;
• processing the difference signals to generate estimates of the time-varying drift component for yaw and pitch;
• subtracting the estimates from the secondary measurements of yaw and pitch to produce corrected secondary measurements of yaw and pitch that include the rigid body motion component and pseudo-motion component;
• and
• processing measurements of the 6-axis rigid body motion and the corrected measurements of the angular rate of change in yaw and pitch of the isolated sub-assembly to estimate a 6-axis position in x, y and z and attitude in roll, pitch and yaw of the isolated sub-assembly.