June 18, 2004  




GLAST Test Bed Complete

By Davide Castelvecchi

Peeking through the glass doors of a room in Bldg. 84, the occasional passer-by puzzles at a giant, revolving electronics contraption skewered on what looks like a cow-sized spit. The imposing apparatus, completed last month, is the Large Area Telescope (LAT) Test Bed, a hardware simulator part of the Gamma-Ray Large Area Space Telescope (GLAST) development.

Sergio Maldonado (REG) at the LAT’s test bed front-end electronics simulator (FES). (Photo by Diana Rogers)

Scheduled to launch in 2007, GLAST will scan the sky in our galaxy and elsewhere in the universe for such gamma-ray sources as black holes and supernova explosions. With its unprecedented energy range (20 MeV-300 GeV) and resolution, the four-ton probe may also discover new and unexpected sources of gamma rays.

The LAT is GLAST’s instrument, an array of 16 tower modules. Each tower module, assembled in collaboration with Italy’s INFN, will consist of a silicon strip tracker (designed by UC Santa Cruz and Japanese physicists) and a cesium iodide calorimeter (provided by the Naval Research Laboratory in Washington, D.C., in collaboration with Swedish and French physicists).

Gamma rays hitting the LAT will cause showers of electrons. Based on which silicon strips pick up the shower and on the energy absorbed by the calorimeter, the read-out electronics will reconstruct the gamma ray’s trajectory and energy.

The LAT will also include an Anti-Coincidence Detector to pick up the ricocheting particles, which indicate if an event is caused by a background particle rather than a genuine gamma ray. “The read-out electronics will have to decide whether all this data corresponds to an actual cosmic gamma-ray event, and say: 'I think I see a gamma ray. We should store this data, and send it to Earth,'” Jana Thayer (REG) explains.

Designed to check if the on-board electronics work the way they are supposed to, the test bed has two main parts. On the front end is a four-by-four array of stacks of electronics boards, which simulate the signals produced by the LAT tower modules. Each board is the same size as the base of an actual tower module, so the whole simulator gives an idea of the dimensions of the whole probe.

On the rear end—the side hidden from the gawker’s eye—is a full prototype of the LAT’s read-out electronics. Gunther Haller (REG) headed the design team. “Soon we will put fake data in—about 100 megabytes per second—and read it from the other side, to see if what we get out matches what we put in,” says Jana Thayer.

The simulator will also be useful once the real GLAST is in orbit, assisting with the LAT’s calibration and helping diagnose any problems that may come up.

The first, preliminary version of the entire system was powered up on May 10.

“I just put the last chip in,” says Gregg Thayer (REG) as, tethered to his bench by an anti-static wrist strap, he finishes assembling a Data Acquisition Board, a crucial component of the simulator. The rear end of the simulator is functionally identical to the electronics of the actual LAT, boasting over 200 custom made ASIC and programmable-logic (FPGA) chips—though not the $8,000 apiece, flight-certified versions. Radically different from your usual Pentium chip, an FPGA can be programmed, allowing for extreme flexibility during the design and development of the instrument.

Some of the boards fabricated at SLAC will be shipped to other GLAST labs around the world, but this will be the only lab with a ‘fully populated’ simulator, according to Thayer.

Both the tracker and the calorimeter are close relatives of instruments found in B
ABAR, which explains the , which explains the crucial role of high-energy physicists in the project. “The LAT is a nice, little particle physics detector—only it is going into space,” said Jana Thayer, whose previous experience includes working at Cornell’s CLEO detector.

The amount of data GLAST will handle is not comparable to B
ABAR’s terabytes, but is still impressive for a space probe. And GLAST’s circuits will embed extra redundancies, since humans will not be on hand to replace failed parts. “This is an unprecedented amount of electronics going into space,” Thayer says.


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Monday June 28, 2004 by Emily Ball