Stephen Mackin

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

[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

Low-cost mid-wave IR microsatellite imager concept based on uncooled technology

A new class of low-cost mid-wave infrared (MWIR) Earth observation (EO) data will become available with the flight of miniature MWIR EO instruments in micro-satellite constellations. Due to the frequent ground repeat times inherent in constellations, this data set would provide a unique alternative for those wishing to analyse trends or rapidly detect anomalous changes in the MWIR characteristics of the Earths surface (e.g. fire detection) or atmosphere. To date, the MWIR imagers have been based on highly responsive cooled detector technology, which traditionally has been the only real option for collecting useful data in this waveband from space. However, state-of-the art microbolometers, adapted from their original design for operation in the LWIR, are thought to be a potential alternative for low-cost MWIR constellations. Following the laboratory evaluation of a modified microbolometer arrays in the MWIR, a low-mass instrument concept was designed and evaluated for a variety of candidate MWIR mission areas. If implemented, the imager concept would complement a larger imaging suite (visible, near IR, and long-wave IR) on a sub-100kg Surrey Space Technology Ltd. (SSTL) micro-satellite and open up several new potential mission areas for the SSTL-engineered Disaster Monitoring Constellation.

Low-cost microsatellite UV instrument suite for monitoring ozone and volcanic sulphur dioxide

The potential of microsatellite instrumentation is analysed in the context of volcanic plume monitoring. In October 1998 the Nyamuragira volcano (Dem. Rep. Congo, Africa) erupted releasing a significant amount of sulphur dioxide (SO2). The Ozone Mapping Detector (OMAD) instrument on-board the FASAT-Bravo microsatellite observed this event as an anomaly in solar backscattered UV. The event was similarly detected by NASAs Total Ozone Mapping Spectrometer (TOMS) with higher wavelength resolution. The response of the two instruments was analysed using instrument models and the MODTRAN radiative transfer code. Good quantitative agreement was observed despite the inherent instrumental differences. Volcanic plume events have been observed before by large space-based instruments, however this is the first such detection by a small, low-cost instrument operating on a microsatellite. Using OMAD as a basis, a new miniaturised UV spectrometer is proposed with the aim of monitoring volcanic-SO2 plumes in the UV region between 305-315 nm, with additional channels at 340 nm and 360 nm for potential aerosol retrievals and processing purposes. This new instrument will use a Silicon Carbide (SiC) detector-array, due to its solar-visible-blind response and its high detectivity. This instrument concept, if flown in a constellation of microsatellites, can augment and complement current missions.

Cloud detection and height estimation through registration of Disaster Monitoring Constellation imagery

For the purpose of detecting clouds over large areas it is necessary to use satellite imagery. Although a variety of techniques for cloud detection and cloud height estimation exist, they often make assumptions concerning the radiometry and spectral coverage available to the sensor payload. This paper explores the use of registration shifts observed between dual-bank single-band image pairs from the DMC multi-spectral imager to detect and estimate the height of clouds. The Disaster Monitoring Constellation (DMC) comprises a network of five disaster-monitoring micro-satellites that have been built by Surrey Satellite Technology Ltd (SSTL). Each DMC satellite has a multi-spectral imager (MSI) consisting of 2 banks of 3 channels pairs. The proposed technique uses a narrow angle between imagers to discern altitude and is comparable to stereo imaging but able to distinguish absolute cloud height without reference to the ground surface, using satellite telemetry. Simulations have shown that with a sub-degree angle between imagers and appropriate sub-pixel level registration scheme, vertical accuracies in the order of a few hundred metres maybe extracted. Preliminary results using phase correlation registration to ensure sub-pixel accuracies between DMC imagery have helped confirm the viability of the technique and will be presented alongside simulations.

Comparison of Atmospheric Ozone Measurements Between NASA’s Total Ozone Mapping Spectrometer (TOMS) and the FASAT-BRAVO Ozone Mapping Detector (OMAD)

The Ozone Layer Monitoring Experiment (OLME) on board the FASAT-Bravo microsatellite, launched in July 1998, observed backscattered UV to retrieve total atmospheric ozone concentrations using two instruments: the Ozone Ultraviolet Backscatter Imager (OUBI) and Ozone Mapping Detector (OMAD). Initial results from this experiment have shown good qualitative agreement with NASA’s Total Ozone Mapping Spectrometer (TOMS) cite ch09:bib01. Recent studies on OMAD and TOMS data found quantitative agreement in the radiances and indicated the detection of the volcanic eruption plume of Nyamuragira volcano (due to its sulphur dioxide content)[2].