Attitude Determination through Image Registration Model and Test-case for Novel Attitude System in Low Earth Orbit

Abstract

[Abstract] The mass and cost requirements of high-accuracy satellite pointing systems often inhibit the potential application of smaller and more affordable satellites. There is therefore an increasing need to develop high accuracy attitude systems that do not breach small satellite mass and cost constraints. This paper proposes a novel method of attitude determination using imagery from two canted, Earth pointing, push-broom sensors. The effects of attitude on inter-imager shifts are modelled, with model inversions proving the techniques potential, given an adequate registration scheme, for determining attitude over 3-axes. Simulated results are presented alongside real data from the Surrey Satellite Technology Ltd (SSTL) Disaster Monitoring Constellation (DMC), confirming the viability of using Earth observational cameras to measure attitude, rotation rates or onboard vibrations to a high degree of accuracy. Since the technique is capable of working with conventional onboard imaging sensors, the implementation costs and additional payload mass of such a system are deemed negligible. I. Introduction OR Earth observation the quality of imagery is dependant upon the stability of the platform and accuracy of attitude telemetry. Small rotations of the satellite will result in large displacements for imager ground projections, while changes in rate of rotation will stretch or shorten pixels of pushbroom scanners. High accuracy attitude systems however, incur mass and cost constraints that limit the scope of imaging applications for small or more inexpensive satellites. A need therefore exists for low mass low cost attitude systems capable of obtaining high accuracy attitude telemetry, especially during the period of image capture, onboard small satellites. The Surrey Space Centre, at the University of Surrey, has been studying the effects of attitude perturbations on pushbroom-imagery using the Surrey Satellite Technology Ltd (SSTL) Disaster Monitoring Constellation (DMC) 1 . This paper details the development of such research and proposes a novel method of attitude determination using imagery from two canted, Earth pointing, push-broom sensors viewing the same area of the Earth surface. Through registration of Earth features and analysis of perspective or timing based distortions, attitude or rotation rates can be determined over 3-axes. The technique is applicable to existing satellites with an appropriately angled sensor pair, such as onboard the DMC multi-spectral imager. Although much work has been done on image-based spacecraft pointing it is often related to rendezvous monitoring and flyby missions 2,3 . The potential for onboard calibration of satellite imagery in Low Earth Orbit (LEO) has been recognised by the European Space Agency (ESA) however. In order to reduce the effects of rate induced distortions within imagery, ESA have proposed SmartScan 4 , a novel imaging system that is designed to use additional imaging sensors to detect and compensate for motion about the focal plane of the imagers. Although currently untested in space, such studies help to highlight the potential of using Earth-pointing cameras to determine attitude. This paper models the effect of attitude and rates on registration shifts, as measured between images from angularly displaced pushbroom imagery. A model inversion methodology is presented allowing extraction of attitude from imagery. Simulated results are presented, proving the potential of the technique, given a suitable sub-pixel level registration scheme, to separate attitude over 3 axes. Attitude estimates extracted from DMC imagery, captured during yaw and pitch rate manoeuvres, is compared to onboard telemetry, confirming the