The objective of the project is to establish the Phase A documentation to enable the opportunity to flight an APS Star Tracker as Technology Demonstration Payload (TPD) on AlphaSat.
This requires amongst others the refining of the Payload Technical Specification and the Concept Definition, the agreement with ESA of the interfaces to the spacecraft, the preparation of a TDP development, verification and validation approach, the defining of the Phase B tasks (including necessary technology predevelopments) and the schedule for Phase B and Phase C/D, and the provision of necessary cost estimates.
The high level mission objective of the test flight is to demonstrate in orbit the performance, robustness and lifetime of the APS star tracker and thereby complete the testing of this new type of star tracker and enable its use on other spacecraft.
In particular, the following will be demonstrated:
- Launch survivability,
- Autonomous and aided operation under nominal conditions,
- Acquisition probability and acquisition time,
- Operation with non-stellar objects in the field-of-view,
- Tracking robustness to radiation effects and main engine burn,
- Long term behaviour of several new technologies in the real space environment,
- Ability of the STR to change in orbit the software and to modify sensor operation and calibration parameters via the TM/TC interface.
The key issues to be addressed in the Alphasat TDP Feasibility Study are:
- To establish the TDP Mission Description Document,
- To complete the Design Definition File (incl. the technical budgets),
- To establish and agree the Interface Control Document,
- To propose the TDP development, verification and validation approach,
- To define the technology predevelopments to be carried out in Phase B,
- To define the Phase B project plan and the schedule for Phase B and C/D,
- To estimate the TDP costs.
Under the ARTES-8 pre-development programme a contract has been let to Jena-Optronik GmbH for the development of an APS based star tracker with the purpose of cost and mass reduction. The results of the pre-development are convincing and have the potential to be even more robust to radiation effects than other state of the art designs.
It is known that tests of star trackers on ground can never be complete because sky images generated electrically and optically for stimulation of the star tracker are not able to reflect fully the real situation in orbit. At present time the testing is performed using high-accurate single star simulators, electrically simulated images and night sky testing, but each of those has restrictions.
Especially, the verification of lifetime, stray light performance, sky coverage and radiation robustness by simulation is not possible or is faulty and requires in orbit testing to ensure full confidence. A test flight continues to remain a pre-requisite for completion of testing of a new type of star tracker and is often a pre-requisite to be accepted as main bus equipment.
The APS Star Tracker (also labeled as ASTRO-APS, and belonging to the star tracker ASTRO series product family of the Jena-Optronik GmbH) provides autonomously high accurate 3-axis attitude information with regard to the J2000 inertial reference frame and 3-axis angular rate measurements of the spacecraft. The APS Star Tracker is accomplished as a compact single box on which a 26 deg baffle is mounted for suppressing the stray light.
High quality images of the celestial sphere are generated in the field-of-view of the autonomous Star Tracker by means of an well-corrected optics and an APS (Active Pixel Sensor) detector. The digital signal processing performed in hardware and software carries out the star acquisition and attitude tracking. An on-board Guide Star Catalogue (GSC) together with dedicated algorithms facilitates the star identification, that is prerequisite to the subsequent attitude determination as part of the attitude tracking.
The APS Star Tracker is characterised by the following basic features:
|Sensor Field of View||20deg|
|Limiting Star Magnitude||5.8mi|
|EoL-Condition||15 years geo-orbit @ TAPS-Chip = +10ºC|
|Sky Coverage||100 percent for Â³ 9 stars at EoL|
|Sun Exclusion||Angle 26 deg|
|Axis Attitude Accuracy||6/45 arcsec (3s) for the xy/z axes|
|Angular Rate||up to 3 deg/sec|
|Temperature Range||-30 & +60ºC|
|Dimension||12 x 12 x 22.5 cm, incl. baffle|
< 1.9 kg, incl.: ba
The current Phase A Alphasat TDP Feasibility Study was started in January 2006 and will be completed at the end of July 2006 with a Preliminary Requirement Review.
A payload review and an accommodation review will be performed in March and May 2006, respectively. Afterwards it is planned to start Phase B in September 2006, which will be concluded in December 2007. The Phase C/D will begin in the April 2006 and is expected to finish at the beginning of 2008 with the delivery of the APS Star Tracker Protoflight that will be used as TDP on AlphaSat.
In course of the Phase A study the following documentation has been prepared:
- Star Tracker Mission Description Document,
- Star Tracker Design Definition File (that includes documents as Detailed and Top Level Design Reports, IDC, SW ICD, Declared Material and Process Lists, Product Tree),
- Compliance Matrix,
- Payload / AlphaSat Interface Specification,
- Preliminary Schedule for Phase B and C/D,
- Synthesis of Programmatic Aspects.
The work currently in progress concerns the TDP cost estimation, the Engineering Plan for Phase B, and the In-Flight Verification Activities Plan