PAGE CONTENTS
Objectives
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:
- Simplification of proximity electronics
- Totally digital electronics
- Simplification of supply voltages and relevant DC/DC converter
- Possible improvement in radiation survivability (especially protons)
- Reduced size and mass
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:
- Mass : 2.6 Kg (including baffle for 25 deg sun rejection angle)
- Power consumption: 6.1 W at +10 °C; 8.8 W at 40 °C.
- Functions:
-Perform autonomous attitude determination in lost in space conditions, and in presence of large number (> 800) of false stars produced by space radiation
-Perform attitude tracking after initialisation - Accuracy on attitude:
-3 arcsec (1 sigma) pitch/yaw
-27 arcsec (1 sigma) around bore sight - Power I/F: 100 V unregulated
- Data I/F: MIL 1553 ?B
- ITAR free design
Challenges
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.
Plan
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.
Current Status
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.