This Optics Express article tackles this issue and describes a prototype implementation for such a sensor, based on existing APS CMOS technology. This technology allows for the application of the Electronic Rolling Shutter (ERS) concept for the acquisition of sun images; with ERS one does not acquire a single full image at rather low frequency like one would with a global shutter, but instead a high-frequency (in the kHz range, depending on the CMOS size) sequence of single image lines. Each image line contains sufficient information to derive the sun direction vector within some limited accuracy, but hundreds or thousands of these measurements within a one second timeframe lead to a dramatic improvement if simply averaged. The authors propose to do more than just averaging; instead a prediction / update filtering procedure can be used.
In order not to limit the proposed sensor’s utilization to high-accuracy sun observation missions but to make it attractive also for “standard” spacecraft designers looking for a reasonably good sun sensor for “safe mode” or failure detection applications, the sensor must offer a wide field-of-view (FOV). In the “safe mode” operation of a satellite, in which all non-essential systems are shut down in order to protect them while the cause of a malfunction is investigated, it is in fact important to be able to localize the object to which the satellite orientation is referenced to even if the satellite orientation has moved far away with respect to the optimal state, and a wide FOV is essential for this purpose To achieve this, the authors have developed an image mask which sub-divides the single FOV into several sub-FOVs, avoiding in this way the reduction of the measurement resolution while increasing the overall FOV. The dual use of the sun sensor makes it possible to save space and weight on board of the saltellite.
Both major new design approaches, i.e. ERS and sub-masking, together lead to an unprecedented accuracy (1,1" / 1 sigma) and output frequency (1 kHz) for a sun sensor opening, if space-qualified and flight-proven, new application areas for these devices. Real hardware tests with the implemented prototype, realized with the help of a sun simulation collimator, validate its current design and suggest that a technology readiness level (TRL) of 4-5 has already been reached.
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