An IMU (Inertial Measurement Unit) is a sensor that measures body-frame acceleration and angular rate measurements. IMUs are often integrated into Inertial Navigation Systems (INSs) used to track spacecraft position.
IMU sensors can experience both static and dynamic errors that could impede performance.
Attitude
As the spacecraft rotates in orbit, its attitude must be determined. To do this, an observer looks out into the sky for some reference object and compares its location relative to that of Earth, which fills most of their sensor field of view.
Accuracy in sensors relies upon their ability to differentiate targets accurately, as well as on how many observations are made. Sensors also contain noises which must be eliminated or otherwise disregarded if accurate information is to be gained from them.
IMUs are integrated into Inertial Navigation Systems (INS), which use IMU data to calculate vehicle attitude, angular rate and linear velocity in relation to global references. INSs form the core of many commercial and military vehicles such as crewed aircraft, missiles, ships, submarines and satellites as well as autonomous robots such as UAVs or industrial robots.
Velocity
Spacecraft velocity can be measured with an IMU sensor that measures its angular displacement and displacement in three-dimensional space.
An IMU data is used by spacecraft’s onboard navigation systems to assess their current state vector in real-time and translate steering commands into control surface, engine gimbal and reaction control system thruster fire command signals for steering control surface control surface engine gimbal reaction control system thruster fire commands signals. Shuttle flights require three IMUs as redundancies.
As part of their role during flight, IMUs must detect and compensate for uncompensated gyroscope drift through a process called covariance filtering. Our EMCORE(r)-HawkeyeTM series MEMS IMU product offers exceptional in-orbit bias stability of less than 0.02 deg/h (1 s), as well as angle random walk (ARW) performance of about 0.006 deg/h1/2 which represents one of the best publicly reported performances for commercial aerospace MEMS IMUs in this application to date.
Direction
An IMU on a spacecraft measures the angular rates and linear velocity in three axes – pitch, roll and yaw) in order to provide essential data for navigation purposes and generate control signals to help steer its craft where it must go.
On long missions, astronauts often need a space sextant to stay aware of their position.
IMUs are also essential components of Inertial Navigation Systems (INS), which combine raw output from accelerometers and gyroscopes to calculate attitude, linear velocity and position relative to a global reference frame. Event triggering using IMUs provides more precise event triggering than using timers – increasing mission reliability and success while decreasing trajectory dispersions.
Speed
IMU gyroscopes can measure acceleration, providing linear displacement data. This can then be combined with their output of angular velocity for spacecraft speed measurement.
IMU inertial sensors contain inaccuracies in their measurements that must be corrected with a Kalman filter, often as part of firmware updates.
This complex algorithm utilizes sensor outputs to provide more accurate state estimates than direct calculations of spacecraft positions, but requires much more computational power compared to direct calculations of position. Therefore, this technique should only be attempted on large satellites where GPS receivers provide backup. It would not be wise to try using such techniques on smaller satellites without GPS as backup.