Extending Attitude Estimation Capability for Agile Precision Target Tracking NanoSats

Abstract

Though NanoSats are becoming increasingly capable of featuring active payloads, and acquiring inertial data at arcsec level precision; the payload operational duty-cycle largely remains limited by its data downlink capabilities. NanoSat laser communication terminal (LCT) promises 1 Gbps data downlink capabilities. However, LEO to ground free space optical data transmission requires precision attitude knowledge under agile ground target tracking conditions. Under such stringent operational conditions, the calibration of gyro-stellar misalignment and scale factors is a well-established practice for traditional satellites. Nevertheless, the impact of such non-idealities in the context of MEMS gyro-stellar based NanoSat precision pose estimation is unclear. Furthermore, the gyro-stellar calibration performance that can be achieved with NanoSat ADCS system requires further investigation. Under agile operational conditions, slew rate induced star tracker drop-out is common for NanoSats, which limits the operational envelope of agile precision tracking missions. To increase our understanding of attitude knowledge estimation capability of today's NanoSats, this thesis work is extended at Hyperion Technologies. In collaboration with Hyperion Technologies, gyro-stellar sensors are characterised; and CubeCAT LCT is used to laydown the reference payload and mission requirements of an agile precision terrestrial target tracking NanoSat mission. Based on state steady noise PSD properties, and attitude knowledge requirements set by the LCT, gyro-stellar configurations are selected, and characterised. A rigid body attitude simulator is developed, considering the characterised sensor suite, to generate representative nominal and non-ideal sensor outputs under four different mission phases: 1.) Inertial pointing with no angular rate, 2.) Non-harmonic sinusoidal manoeuvres 3.) Agile precision ground target tracking and 4.) Inertial target tracking with non-zero angular rate. Reaction-wheel time delay and torque limits are evaluated to analyse the feasibility of attitude pointing requirements under agile target tracking conditions. Due to favourable properties of robustness to large initialisation errors, fast convergence, and preservation of attitude quaternion unity norm constraint; UnScented QUaternion Estimator (USQUE) variant of UKF based filter is synthesised for attitude estimation. The USQUE filter is further extended to facilitate the calibration of gyro-stellar misalignment and scale factors. It is demonstrated that the presence of gyro-stellar misalignment and scale factor, and star tracker occultation under agile slew rates significantly deteriorates the attitude estimation performance of USQUE based attitude estimator. Star tracker misalignments and dropouts are observed to have a significantly larger impact on the attitude knowledge estimation performance, when compared against MEMS rate-gyro scale factor and misalignment. Degraded MEMS rate gyro sensors are observed to have little impact on the attitude knowledge estimation performance of 6/7 state USQUE filter. However, it has a considerable impact on the convergence performance of star tracker misalignment calibration. More persistent calibration manoeuvres are observed to improve the star tracker misalignment parameter estimation performance. Gyro misalignment and scale factor calibration objectives were not met. A root-cause analysis showed that the signal distortions introduced by such non-idealities are below the gyro-stellar noise floor. Calibration filter synthesised has the potential to improve autonomy, reduce commissioning and ground calibration times, and enhance NanoSat pose estimation performance.