The objective of the activity was to derive an optimized star tracker design for Telecommunication satellites based on the Faint Star detector. Main development focused on optimizing the use of the Faint Star sensor and the Star Tracker software in order to host the Star Tracker Software in the avionics on-board computer.
The main challenges of the project were the following:
- Development of a low-impact star tracker software architecture (including associated implementation guidelines), that eases integration of the star tracker run-time library (STRLib) in on-board computers, thus providing an attractive option for the telecom primes.
- Meetings with the major European telecom primes revealed the need for a redundant star tracker interface, covering both power and communication. The actual implementation of the redundant interface into the current compact Faint Star camera head unit design turned out to be challenging.
The developed FaintStar-based star tracker provides a unique combination of features that are difficult to match by any star tracker currently available on the market:
- High accuracy – comparable with the top range of the star trackers on the market.
- Radiation robustness in a class adequate for telecom missions, equivalent to 15 years in GEO.
- A compact, low-mass, low-power optical head allowing accommodation near thermal gradient sensitive payloads.
- Easy hosting of star tracker software on the avionics computer, providing optimum reliability as well as mass and cost savings.
- Few components making the design suitable for cost-effective, large-scale production, targeting large satellite constellations in the telecoms market.
In fact, the accuracy and reliability offered by this star tracker is unmatched by any other star tracker with a price tag that is attractive for large constellations.
Competing star trackers – with equivalent accuracy and robustness characteristics – primarily rely on integrated thermoelectric Peltier coolers, resulting in a significant increase in power consumption and a negative impact on boresight thermal stability, instrument reliability, and cost.
The dedicated FaintStar sensor is the key contributor to the high performance and robustness offered by this star tracker.
Integration of this high-end image sensor into a very compact and rigid structural design is – in combination with a 3rd generation optical system – part of the secret behind this unique star tracker.
The key design drivers for the optical head have been:
- A compact, boresight symmetrical design with all Titanium focal plane assembly for maximum thermal stability
- A CTE-matched sensor PCB layer stack-up providing excellent thermal stability
- An aspherical optical design for minimized baffle envelope
- Straylight attenuation designed into the optical system for optimized compatibility with mission specific and compact single stage baffles
The star tracker software library is to a large extent based on the Terma heritage star trackers flying on several US and ESA missions. The high level of robustness has been further enhanced using the experience from the long term Cryosat-2 mission. Building on this, the current study has contributed to the development of a user-friendly star tracker software architecture with associated implementation guidelines, making the deployment on future avionics computers an attractive option with predictable resource requirements.
The overall system architecture consists of the following entities:
- Customer furnished on-board computer (OBC) (part of the spacecraft bus) with modest amount of spare resources, a SpaceWire interface, and software SpaceWire interface drivers
- Terma star tracker optical head
- Terma star tracker library (STRLib)
- Small supervisor application (developed by customer or Terma) equivalent to the Terma STRLib example supervisor
This architecture eliminates the need for a dedicated computer for processing of the image stream from the optical head, thus reducing total mass and power consumption.
As a proof-of-concept, the STRLib was integrated standalone on a customer-furnished spacecraft computer for interfacing to the optical head. The customer subsequently integrated STRLib in a space and time partitioned safety-critical real-time operating system, thereby demonstrating the ease with which the combination of flight-proven STRLib and high accuracy optical head may be integrated with the avionics platform.
The project has been executed according to the following schedule:
- Kick-Off: September 2017
- Baseline Design Review: May 2018
- Mid-Term Review/Test Readiness Review: December 2019
- Test Review Board/Final Review: March 2021