Testing the testers
Tom Shelley reports on a system that validates testing systems for satellites
EADS Astrium has developed an automatic test equipment system for testing spacecraft test equipment.
Without some means of validating test equipment, testing becomes meaningless because malfunctioning test systems are unable to reveal malfunctioning equipment. Since testing of complex systems is normally automated, because of the large number of tests that have to be run, it makes sense to automate the test equipment testing the testers.
The solution, according to Clive Catley of EADS Astrium, has been to develop an Electric Ground Support Equipment (EGSE) validation system or EVS.
The first target application is an ESA probe called Gaia, a billion pixel digital camera capable of measuring the equivalent of an object the size of a human hair at a distance of 1000km. Scheduled to be launched in 2012, it is to be placed 1.5m km beyond earth orbit and produce a star map of the more than 1 billion stars in our galaxy and by measuring their precise positions, identify hundreds of thousands of extra solar planets.
Typical for one-off science mission spacecraft, a "FlatSat" of the probe is first built with a set of engineering model electronic units, identical to those to be used in the actual satellite, in order to validate electrical interfaces, on board software and closed loop control systems. The platform consists of: a computer, an electrical interface, a power control unit, attitude and orbit control gyroscopes, star trackers sun sensors, a propulsion system and a radio frequency sub system. In addition, there are instruments, instrument support units, and devices to process all the data they produce. To support ground testing of the spacecraft, specialised test equipment typically includes: a solar array simulator, umbilical power and monitoring, a battery simulator, a radio frequency front end, an avionics front end, a spacecraft simulator, payload front ends and a checkout system. In practice, this means racks and racks of complicated equipment.
To take just one item, the avionics front end supplies interfaces to the spacecraft attitude control units, and in tandem with the spacecraft simulator, is capable of simulating the units themselves. To accomplish this, it has more than 600 I/O interfaces including: analog, pulse generation, pulse acquisition, RS422, 1553, SpaceWire BUS interfaces and power simulators.
All test equipment has to be validated before use, and all electrical interfaces to the spacecraft tested before connection. Previously, there were two alternative means of doing this.
One was to procure EGSE with built in test facilities. Because of complexity, this can typically cost more than €50,000 per piece of equipment, adding hundreds of thousands of euros to the cost of a project. The alternative is to validate each piece of test equipment manually using break out boxes and standalone test equipment, which typically takes two to three days. Since EGSE is often relocated between different test facilities, this is also costly, since validation has to be repeated following any shipment from one place to another.
The new EVS, on the other hand provides multiple generic test functions for validating spacecraft test equipment using National Instruments PXI platform instrumentation hardware and the same company's LabWindows/CVI to produce the software. Catley told us that using this tool, he was able to write, "The core software in two or three months", doing some of the work in his spare time. The hardware unit, he said, is small enough to be taken, "Up to the top of a launch tower and go inside vacuum equipment." It nonetheless, "Provides the majority of the test functionality required for multiple spacecraft projects, based on standard I/O specified in spacecraft GDIR and common functions such as power projection testing and solar array simulation." It is expected to save, "Many thousands of euros in EGSE costs."
* A single, portable Electric Ground Support Equipment Validation System has been developed that can validate almost all space probe equipment, drastically reducing costs as well as saving time.
* The development depends on extensive use of National Instruments hardware modules and the company's automated graphical interface software writing environment, which allowed the core software to be written in two or three months
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