The Miniature Star Tracker for the European Student Earth Orbiter

Paper number

IAC-05-E2.1.08

Author

Mr. Nicolas Schaeffer, SUPAERO, Spain

Year

2005

Abstract

The Supaero Star Tracker is aimed at developing, testing and integrating the first student-built autonomous star tracker. The project is part of a bigger endeavour, the SSETI (Student Space Education and Technology Initiative) Association, which brings together students from all over Europe to design a series of microsatellites with the support of the European Space Agency. The star tracker project builds on the CIMI’s (Design of Integrated Imaging Matrices) expertise in CMOS sensors to design an FPGA-controlled camera that communicates with the AOCS CPU-based star identification algorithms. The project is managed with spiral development in mind: a first level prototype used existing lab hardware to demonstrate the combination of the sensor, the FPGA and the PC and achieved “first light in July 2004. Development of a fully integrated miniature camera was achieved in February 2005: PCB design, component selection (electronics and optics), software development were all done by the student team. The major milestone was achieved in May when the system was able to take a nightsky picture, transmit it to the PC where the algorithms were able to derive an attitude estimate from the stars’ positions. This achievement was instrumental in transfering project management from the founding team to a new team that will lead the project towards flight article development and testing.
The camera’s electronics are divided into two PCBs, one for the digital and one for the analog components, preventing digital noise from contaminating the analog readout from the sensor. The digital board manages all the interfaces with the spacecraft, including the power supply and the data link. The camera accepts the standart unregulated 28V supply from the main power bus and converts it then to the various regulated tensions required by the hardware. The three different voltage settings required by the FPGA, coupled to the needed decoupling of analog and digital power supplies translates into seven different voltage conversion units. Both linear and buck converters were used to find a compromise between voltage stability and power consumption. The FPGA is an off-the-shelf, low-cost, flash-configurable Spartan-3 from Xilinx. Its multiple-write capability allows easy debugging and fast implementation at low cost of code revisions. An on-board digital oscilloscope helps tracking errors using hardware-in-the-loop, thus cutting down on simulation time. The sensor is a research prototype developed by the imaging sensor laboratory of Supaero, Toulouse. It features a 512 by 512-pixel CMOS matrix with a 30 microns pitch. This oversized sensor is particularly well-suited for the space environment and has successfully passed a radiation test at EADS Astrium’s facilities.
The algorithms were developed with the valuable input of the star tracker expert at ESA, Stephen Airey. Based on star brightness and shape recognition, they derive the spacecraft’s attitude in the inertial reference frame by applying the QUEST algorithm. The software starts in acquisition mode, taking a full picture and extracting from it the stars’ position. It then matches star patterns with an on-board catalog. Upon successful recognition, it switches to the tracking mode, taking only small windows around several stars and using a Kalman filter to increase the attitude precision. The development code is written is Java, allowing increased efficiency and debugging speed compared to direct C coding. Once the software is deemed fully functional, it will be an easy step translating it to C.
As an integral part of a European project, the SST team has to report on a regular basis to a young graduate trainee, acting as project manager and based in ESA’s European Science and Technology Centre in The Netherlands. This oversight helps structure the project and coordinate the information flow between the various teams involved. In February 2005, team leaders gathered at ESTEC for a workshop where system-wide decisions were made. It gave the teams the opportunity to flesh out configuration issues and to agree upon the mass and power budgets for the various subsystem, including the Star Tracker.

Abstract document

IAC-05-E2.1.08.pdf

Manuscript document

IAC-05-E2.1.08.pdf (🔒 authorized access only).

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