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Spotlight on Optics


  • April 2014

Optics InfoBase > Spotlight on Optics > An implementation method based on ERS imaging mode for sun sensor with 1 kHz update rate and 1″ precision level

An implementation method based on ERS imaging mode for sun sensor with 1 kHz update rate and 1″ precision level

Published in Optics Express, Vol. 21 Issue 26, pp.32524-32533 (2013)
by Minsong Wei, Fei Xing, and Zheng You

Source article Abstract | Full Text: XHTML | Full Text: PDF

Spotlight summary: Imagine a spacecraft whose primary mission is to observe the sun from an arbitrary orbit or trajectory while maintaining a long-term highly-stable orientation (two recent examples for this type of mission are the Solar & Heliospheric Observatory SOHO and the Solar Orbiter SOLO, the latter being currently under development by the European Space Agency). But which orientation to choose for the spacecraft –inertially (e.g. star) fixed, earth-fixed, third-body-fixed or sun-fixed? The most logical choice would of course be a sun-fixed orientation because it provides a direct reference to the object of interest. As a matter of fact, however, this is not an easy task today because the necessary high-precision sun sensors, i.e. sensors capable to detect the sun direction vector relative to the spacecraft to within the accuracy of an arc-second or even less, are not available.

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.

--Jens Levenhagen

Technical Division: Optical Design and Instrumentation
ToC Category: Instrumentation, Measurement, and Metrology
OCIS Codes: (040.1880) Detectors : Detection
(120.4640) Instrumentation, measurement, and metrology : Optical instruments
(120.6085) Instrumentation, measurement, and metrology : Space instrumentation

Posted on April 01, 2014

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