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The objective of the contract is to develop and test an elegant breadboard of an Active Pixel Sensor (APS) based star sensor, and to analyse the competitiveness of these sensors against existing CCD based star sensors.
The development phase has been recently harmonised with the @BUS phase C/D, which include the realization of a Proto Flight Model (PFM) and a Flight Model (FM).
The advantages presented by the replacement of CCD with APS are:
The contract will have as starting point the experience gained in the frame of the Miniaturised Star Tracker for Harsh Environment APS based star tracker, in the frame of the Bepi Colombo technological study (photo below).
The star sensor characteristics goals are:
The introduction of the APS detector in the star sensor, combined with a request for robustness of the sensor operation in high radiation environment (as in case of solar flares), force to significantly change the way to acquire images and image processing, with respect to current star trackers based on CCD. This innovation has been already started in the frame of the Miniaturised Star Tracker for Harsh Environment and will continue in the frame of the current contract. In addition, design solutions must be always aimed to realise a product competitive in terms of cost, performance and robustness with respect to the currently available CCD based star sensors.
The capability to achieve a good signal to noise ratio, when APS electro-optical characteristics are considered, is also of fundamental importance, to guarantee adequate star availability for sensor operations.
The project will allow developing a new generation of APS based star sensors that will be lighter, smaller and cheaper than the state of the art CCD based star sensors.
This improvement will be obtained without penalising the sensor performance and functionalities, but just taking full advantage of the APS characteristics. The APS is a fully digital device, so that the entire analogue electronics needed to operate the CCD, to acquire the analogue output signal and to perform the Analogue to Digital conversion, can be thus eliminated in the new sensor configuration. The power converter results also simplified, as a reduced number of secondary voltages is needed, while CCD requires several voltages to operate.
The new algorithms under development will allow the star sensor to operate in hostile radiation environment, where currently available star sensors suffers for limitation due to a large number of false targets produced by the space radiations. In particular, CCD?s suffer for charge transfer efficiency when subjected to proton environment. This leads to a significant blurring of the image during the readout. This effect is not present in the APS, where charge must not be transferred toward the output amplifier during readout, but pixel readout is achieved via direct access (similar to RAM access).
The successful completion of the development, will allow to realise star trackers attractive from a commercial point of view, and with characteristics of robustness even better than state of the art CCD based star tracker
The sensor architecture was subjected to trade off analysis in the first part of the contract.
The star tracker will use the second generation of APS, providing higher quantum efficiency and reduced Fixed Pattern Noise with respect to the APS used in the Bepi Colombo development study.
Due to compactness achievable with the replacement of the CDD with the APS, all the electronics needed to operate the sensor (detector, microprocessor, DC/DC converter) is included in a single box. All the analogue electronics needed to operate CCD driving, video signal amplifier and AD conversion can be in fact completely eliminated in this sensor configuration.
This configuration minimises the costs, mass and size of the sensor.
The sensor block diagram for this case is reported in the following.
The above described configuration is based on a well consolidated architecture implemented in the CCD based star tracker currently available in GA. This allows taking full advantage in mass, cost and dimensions reduction, due the insertion of the APS, without introducing development risks, as the new electronics is directly derived from a configuration that was already space qualified and flown.
To maintain the APS at the temperature needed to suppress dark current and dark current non uniformity, a Peltier Cooler is used. The thermal control of the APS is performed by the STR S/W. The current flowing in the Peltier is modulated on the basis of the APS temperature, monitored using the temperature sensor integrated in the APS chip.
A detailed model of the dark current and current non uniformity is under completion, to accurately predict the APS behaviour with the temperature and radiation.
The development of a new Peltier from a European company, and the development of a 1553 B ASIC, allows for a fully ITAR free version even with the 1553 B data I/F.
The project plan was initially divided in two phases: phase 1 was devoted to the trade offs execution and architectural design, and to assess competitiveness of the APS based star tracker with respect to the CCD based start tracker.
Phase 2 was devoted to the detailed design of the Elegant Breadboard of the APS based star sensor, the manufacturing and testing of the B/B. In parallel, the design of the flight configuration, with the definition of the electronics parts to be used in the flight configuration is also performed, to demonstrate the feasibility of the adopted design at flight standard.
After harmonisation with phase C/D, the model philosophy has been readdressed, so that an Optical Model (OM), including the main structure and optics will be realised to have advanced characterisation of the opto-mechanical design. An Electrical Model (EM) of the digital electronics will be also realised, to verify also integration with the S/W. The EM will allow also to acquire images from the OM. The EQM testing will be completed within June 2006. PFM and FM, will be delivered within November 2007.
The contract started on December 2003. Trade off analysis has been performed, in cooperation with JPT, with the aim to find the optimum configuration from the sensor point of view as well as from the system / Spacecraft point of view.
All design key aspects have been defined, and a conceptual review was held on September 2004. The competitiveness analysis against CCD based star tracker has been carried out.
Phase1 has been completed in December 2004.
Phase 2 has been started in March 2005. The detailed design of the STR structure and electronics is in progress. The phase C/D kick off was held on July 2005. The Equipment Suitability Review, aimed to freeze all the key design aspects, was held on November 2005.
A successfully Preliminary Design Review for phase C/D has been held on July 2006.
SW requirement review successfully held on April 2006. S/W design review is scheduled for October 2006. The S/W to be used in this new development will be largely based on the one produced in the frame of the Miniaturised Star Tracker for Harsh Environment.
The Electrical Model has been already manufactured and it is currently under testing at GA facilities.