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Low-cost earth sensors (ES) currently do not exist on the market due to the lack of a suitable sensing concept. Their availability would allow earth sensors to be used in new scenarios and to improve system reliability by providing a low-cost back-up sensor which could be used for applications where neither milli-degree accuracy nor operation at high angular rates are required, but where low cost is essential and lower (about 1 – 5°) accuracy is acceptable.
The objectives of this project are to demonstrate the feasibility and reliability of using low-cost detectors to sense the atomic oxygen airglow of the atmosphere from space, and estimate the performance of a low-cost earth sensor that uses this concept. A breadboard has been designed, manufactured and tested to confirm that this concept is viable. An array of 32x32 & 128x128 SPADs (Single Photon Avalanche Diodes) has been designed and fabricated to serve as a possible detector for this Earth Sensor.
The key issues to be investigated were:
The proposed system is expected to have a recurring cost a factor of 3 lower than currently available Earth Sensors, to be more compact, and draw less power.
Cross-section of GEO Instrument
The Earth appearance at 762 nm was modelled over different seasons. This model will allow the optics and the detector to be sized. Algorithms were developed to determine the Earth vector from expected images. Computation power was estimated.
A 128x128 SPAD detector array was be designed to be radiation tolerant. It was be fabricated at a CMOS foundry. Its performance as an imaging system was characterized (dark count rate, sensitivity, etc.), as will the effect of TID (Gamma) and 11 MeV and 60 MeV protons. A control circuit for the SPAD array was designed and fabricated.
A preliminary mechanical and electrical design of the ES was developed. The optics and filter were designed, and the feasibility of the filter manufacturing was investigated. A mass power and volume budget was be established.
A laboratory breadboard was built to: display simulated images of the Earth as seen from GEO, image those Earth picture with one or more candidate detectors, and test the performance of the Earth Centre finding algorithms as a function of temperature and Earth appearance conditions. A development plan has been written.
The project is completed.