The objective of the activity is to develop an Active Pixel Sensor (APS) based star sensor, compliant with the Alphabus/Large Platform Development. The pre-development activities are important to reduce the schedule and technical risk of the overall large platform development. The purpose of this activity is to analyse the economic interest, and to develop and test an elegant breadboard of an APS based star sensor.
The identified advantages of APS with regard to CCD technology are:
- Random access pixel readout,
- Simplification of proximity electronics,
- Totally digital optical head and communication to processing electronics,
- Simplification of supply voltages and hence unit power consumption,
- Possible improvement in radiation survivability,
- Reduced mass.
The specific objectives of this activity are:
Should the second of these objectives be demonstrated, this activity has the further objective:
- To design, manufacture and test an elegant breadboard APS based STR for Alphabus and demonstrate that it is capable of meeting all the technical and commercial requirements placed on it.
The key issues of Phase 1 are the following:
- To show feasible design solutions for an APS based star tracker with available APS technologies,
- To perform a competitiveness analysis that shows the technical and commercial gain from APS- versus CCD-based star tracker systems, taken in mind risk assessments with respect to schedule, cost and technology.
The key issues of Phase 2 are the following:
- To design and implement the APS based star tracker in the Alphabus environment in close cooperation with the Joint Project Team and the agencies,
- To manufacture and test an elegant breadboard of an APS based star tracker.
These pre-development activities in the field of the star tracker technology are necessary to reduce the schedule and technical risk of the overall large platform development.
The key features of APS detectors are: a true xy-address random access, a single supply (+5V or + 3.3V only), the high on-chip functional integration level including the AD-converter and the inherent radiation robustness compared to CCD's.
Therefore, a star tracker system takes benefit from the replacement of the CCD detector by the Active Pixel Sensors. This can be shown in reduction of the unit mass budget from 3.5kg (for state of the art star trackers) down to less than 1.5kg for APS based star trackers. The APS inherent radiation robustness against gamma radiation allows the design of long life and robust star tracker systems. Also due to the drastic reductions in the parts count, the unit reliability can be improved. This qualifies the APS based star tracker for the challenging requirements that are specified for large long life geo-telecom platforms. The commercial aspect for the introduction of the APS technology is the reduction of the star tracker unit cost and finally to keep the competitiveness on the international marked on system and sub-system level.
The key technical parameters of the APS based star tracker are the low mass budget with <1.5kg in a single box design, the low power consumption with <5W and the attitude quaternion accuracy of <9arcsec (3sigma, EoL) at a 10Hz update rate. These data define a new level of compactness compared to state-of-the-art CCD based star trackers for the geo-telecom marked (up to 15 years life time).
The improvements in system compactness and performance are realized with the replacement of the CCD detector by the APS detector technology. The mass and envelope benefit is based on the very high functional integration of the APS detectors. The whole analogue read-out and sampling electronics including the AD-conversion is placed on the detector chip. This saves PCB area compared to CCD based systems. The parts count for an APS based star tracker is reduced accordingly. Along with that, the unit costs are reduced and the system reliability is increased.
|The block diagram (left) shows a minimal configuration for a star tracker system that takes full benefit from the APS detector technology. The APS detector is part of the processor memory environment due to its compatible interfaces.|
The compactness of the electronics as well as the low power consumption results in a feasible smart single box design. The single box contains all the processing electronics that are necessary to provide attitude quaternions as output information. There is sufficient space (two side cavities) for the implementation of the customer specific data (MIL-1553 or RS422) and primary power interf
The project plan is structured in Phase 1 and Phase 2 activities. During Phase 1 a detailed technical trade-off shall show the feasibility of APS based star tracker designs. In case of promising results from the Phase 1 activities the Joint Project Team (Alcatel/Astrium/GPA) and the agencies (ESA/CNES) decide to proceed with Phase 2. In this phase an elegant breadboard of an APS based star tracker shall be manufactured and tested.
The star tracker EM test campaign was continued from January till March 2006. All tests as agreed in the test plan could be performed successfully. The APS star tracker imaging system was subjected to an mechanical environmental test in order to derive valuable information for the EQM re-design activities. On the complete APS star tracker EM model tests like functional/performance, stray light sensitivity, sun exclusion angle and direct sun viewing, detector characterization, etc. were performed and documented in the test reports. During this test campaign lots of real sky viewing experiments were carried out. Based on the findings from the laboratory and the real sky experiments the APS star tracker S/W was continuously improved.
The results of these project activities were used as input for the APS star tracker qualification kicked off in 2006. The project final data package was established till May 2006 and submitted to the ESA technical officer for review. The final presentation of the project results was held on 4th July 2006 at ESTEC.