Aerospace Control Systems

Spacecraft GNC Systems Engineering




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What is the effect of thermal stability on attitude determination performance? PDF Print E-mail

The most obvious effect of thermal variations (or thermal gradients) on attitude determination performance is thermo-mechanical misalignment between the attitude sensors, inertial measurement unit (IMU), and payload. The misalignment varies with temperature and can be thermo-elastic, hysteretic, or random due to creep. Hysteretic misalignment was found on IceSat. Heater input in response to periodic external thermal input can resulted in sub-harmonic thermal and misalignment variations on the Aqua spacecraft. See Sections 8.3 and 8.6 of the survey paper on attitude sensor and gyro calibration for a more detailed discussion of these examples.

Large relative misalignment between attitude sensors causes large residuals in an attitude determination filter. The misalignment can interfere with a measurement edit test to detect bad measurements, and can cause the estimated attitude to wander.Varying relative misalignment between attitude sensors and the gyros, induced by temperature variations, should be estimated in real-time on board a spacecraft. The relative alignment between the attitude sensors, gyros, and the payload will also vary and, depending on the payload, can be much harder to estimate and compensate.

In addition to misalignment, thermal variations can affect the performance of star trackers and IMUs. High-performance IMUs and star trackers are calibrated and thermally controlled to perform to high accuracy, but within a narrow range of temperature and temperature gradient. Unless the attitude determination system design engineer pays close attention to the thermal environment, thermal sensitivity may dominate the stochastic (random) error of the sensors, thus precluding optimal performance and possibly not meeting attitude determination accuracy requirements.

The RADICAL attitude determination/calibration filter can estimate relative misalignments and gyro calibration parameters. It can also be made to estimate payload misalignment in systems where payload measurements can be used as attitude measurements. However the payload misalignment is estimated, the system has to be designed so that the payload misalignment does not change significantly from the time it is calibrated through the period in which the payload performs its primary mission.