There's no place like space

SCOTTSDALE, AZ—"In space, no one can hear you scream," says the promotion for the 1979 movie Alien. Humans, though, aren't alone in finding inhospitable conditions once they leave Earth. The lack of atmosphere that prevents your voice from carrying also affects how electronics operate—and it is just one of the harsh conditions that electronics face in space. Thus, engineers must test for every conceivable environmental condition before launching a spacecraft.

Test leader Joe Irman oversees all testing of SDSTs and other equipment.

Since its beginnings as a division of Motorola, General Dynamics C4 Systems has sent communications electronics into space for more than 50 years, starting with the Redstone missile in July 1950. The division employs more than 7000 people and has its headquarters in Scottsdale.

The Scottsdale campus houses an operation devoted to space-based communications. It manufactures the Small Deep Space Transponder (SDST; www.gdds.com/space/products/transponders.html). SDSTs collect telemetry data from scientific instruments on the spacecraft and modulate the digital signals onto a carrier in the X-Band or Ka-Band (Ref. 1). The SDST, which sells for $1.5 million, first flew in the 1998 Deep Space One mission. Two SDSTs are currently on Mars, and other missions are planned into 2006 (Table 1 ).

Because each mission an SDST flies is unique, each mission has different test requirements. The customer, NASA's Jet Propulsion Lab (JPL; www.jpl.nasa.gov), specifies the tests, which take several weeks to run.

Project leader Keith Siemsen oversees development of the Small Deep Space Trasnponder.

Rolling racks

Environmental test equipment such as shock testers, vibration testers, and thermal vacuum chambers reside in different rooms. Therefore, a test rack follows its transponder around the building. Test leader Joe Orman noted, "A test rack follows its transponder so that we can run all tests with the same calibrated instruments." An in-house calibration lab keeps the test equipment within tolerance. (See "Calibrations at C4 Systems," below.)

Figure 1. Each Small Deep Space Transponder goes through a series of environmental tests. A dedicated test system measures its performance throughout the tests.

Once fully assembled, the SDST undergoes a full functional test at 25°C in the transponder lab. That provides engineers with a baseline level of performance. A test suite includes measurements on a downlink's transmitter power and frequency, uplink sensitivity, bit-error rate (BER), and subcarrier modulation levels. (A downlink is a spacecraft-to-Earth communications channel.) Table 2 provides a list of equipment in the test rack and the tests each instrument performs.

In the uplink-sensitivity test, for example, the test system must calibrate uplink carrier power. The SDST measures and digitizes its received carrier power level and sends that information back to Earth over the downlink; this tells engineers on Earth how much signal power is reaching the SDST so they can determine whether it is strong enough to maintain control of the unit. A software script in the tester processes the digitized uplink signal power, displays its level on the computer screen, and calibrates the link power to the proper levels.

Software engineer Dave Andersen developed test software in Agilent Vee (www.agilent.com/find/vee) that performs the functional tests. His software controls the test equipment, simulates control signals from Earth, and simulates digital telemetry from onboard instruments. A modular software design lets him use the same software architecture for every SDST even though the test parameters change from mission to mission. "With the automated test systems," said Andersen, "tests that used to take three hours manually now take three minutes."

Software engineer Dave Andersen writes software to control equipment that tests spacecraft communications systems.

All equipment that goes into space must first survive a launch and a stage separation. Environmental test lab manager Tom Lullo uses an Unholtz-Dickie (www.udco.com) vibration table to simulate launch conditions. He runs random-vibration tests for 1-min to 3-min periods at frequencies from 20 Hz to 2000 Hz.

Lullo says that a 1-min test simulates a normal launch but he tests the first piece for each mission for 3 min, just to be sure it will work. He also runs 3-min vibration test at double the power (+3 dB) of the amplitude of the 1-min tests. During vibration tests, the test rack measures an SDST's DC current draw, output power, phase noise, and receiver BER. A subset of the tests, which checks the most critical functions of the unit, are performed immediately before and after subjecting a transponder to each axis of vibration.

Triaxial accelerometers on the vibration table monitor the vibration during a test. Lullo also uses the vibration data to monitor the table's performance. By tracking the vibration profiles, he can detect potential problems in the vibration table before they occur.

Vibration occurs during a launch, then quickly subsides. But a spacecraft also must endure shock forces when a rocket's stages separate. (The bolts that hold stages together get blown off with explosives to force separation.) Shock tests subject an SDST to forces up to 2000 g with acceleration frequencies up to 1 kHz. To perform this test, engineers place the SDST on a shock table. A hydraulically driven rod hammers the table, thus producing the shock.

When a spacecraft such as a Mars Rover reaches its destination planet, all of its components must survive the shock and vibration of a landing. As each Rover approached the Martian surface, airbags inflated just before the vehicle detached from its parachute. The Rover then free-fell to the surface and rolled inside the airbags to a stop. To simulate the impact and roll of the landing the SDST would encounter in the Rover, General Dynamics' engineers subjected the transponder to a series of 150-Hz sine-wave bursts of five vibration cycles each.

After completing shock and vibration tests, an SDST goes into a thermal vacuum chamber that simulates conditions in space. A thermal vacuum chamber uses a mechanical pump to remove most of its air. It then uses a cryogenic pump to reduce its internal pressure to below 10^-5 Torr, measured with an ion gauge. After reaching that pressure, the chamber remains sealed for three weeks while its temperature cycles from -40°C to +60°C. The test rack runs the full suite of functional tests 24 hrs a day at the temperature extremes.

The lack of atmosphere in space not only prevents you from hearing a scream but also prevents the cooling of electronic equipment. "In space, there's no atmosphere to convectively cool electronic components," noted Siemsen. "Heat can build up, and we must test a transponder's ability to conductively transfer heat through the SDST's frame to the spacecraft's thermal heat sink."

A lack of atmosphere produces other problems, too. Under extremely low pressure and without humidity, electrical discharges caused by corona problems can damage electronic components. The thermal vacuum chambers let engineers test the SDST for arcing under reduced atmospheric pressure in zero humidity. But because the SDST must withstand humidity until the spacecraft leaves the atmosphere, the engineers also perform humidity tests at full atmospheric pressure.

Throughout the environmental tests, General Dynamics' engineers collect and analyze data. They look for trends that can reveal potential problems. After all environmental tests are complete, the engineers run a full suite of tests at 25°C. They look for differences in performance that may indicate how the SDST will perform on a mission.

When the engineers have completed their testing, the SDST is delivered to JPL for compatibility testing. Only after JPL engineers verify that the SDST is compatible with the JPL communications network is it ready for spacecraft integration. T&MW