TRW-Built TOMS Spacecraft Restored To Normal Operation; Spacecraft Already Exceeds Design Life By 25 Percent.

REDONDO BEACH, Calif.--(BUSINESS WIRE)--Jan. 25, 1999--TRW engineers, supporting their counterparts at NASA's Goddard Space Flight Center and NASA's satellite tracking networks, devised a clever solution that recovered the agency's only satellite dedicated to ozone mapping from a near-fatal emergency.

Based on detailed knowledge of the Total Ozone Mapping System (TOMS)-Earth Probe satellite, built by TRW, engineers last month came up with the idea to bypass the fuel-depleted propulsion system and use magnetic torque rods to restore the spacecraft to its normal Earth-pointing orientation.

"The dedicated efforts of an industry/government team brought back online a critical environmental tool," said Joanne Maguire, vice president and general manager, TRW Space & Laser Programs Division. "TOMS has already exceeded its two-year design life by 25 percent, and as a result of this fix, will continue its mission of supplying daily measurements of the global ozone layer to scientists around the world.

"The robust spacecraft design enabled engineers to devise a solution that required a one-byte patch to the software code," Maguire continued. "It is stunning in its simplicity, and yet solved a problem of great complexity. The solution is based on the recognition that torque rods, which balance out small disturbances in the attitude of the spacecraft, could be used to re-establish the orientation of a satellite."

The emergency occurred in mid-December and is believed to have been caused by a single event upset (SEU). SEUs occur in space when a high energy particle hits the spacecraft electronics, disrupting the spacecraft's computer operations.

In this case, TOMS went into a "safehold" mode. In the safehold mode, the spacecraft is programmed to point toward the Sun so the solar panels get power. The satellite is then stabilized by being automatically commanded to spin at three degrees per second.

When TOMS thrusters fired to begin spinning the spacecraft, unexpected residual gyroscopic forces of the spacecraft caused it to wobble. This led to the continuous firing of thrusters as it tried to compensate for the wobble, exhausting the onboard fuel. The spacecraft was rotating in the Sun-line at a rate of approximately 15 degrees per second.

So engineers modified, or "patched," the control software to allow the spacecraft to de-spin itself and then recover its operational status using the magnetic torque rods. This patch was validated first on the ground, using a table-top version of the spacecraft computer.

Because the magnetic torque rods exert such small torques on the spacecraft, the de-spin maneuver from the initial 15 degrees per second to approximately three degrees per second required three days, completing very early on New Year's Day. A subsequent one-and-a-half day maneuver brought the satellite back to its operational "science mode" status.

"This recovery could only have been accomplished by the kind of dedicated people that work our projects, both government and industry," said Michael Luther, deputy associate administrator, Office of Earth Sciences, NASA headquarters.

The satellite was launched in July 1996 from Vandenberg Air Force Base, Calif. TRW built the 650-pound satellite and integrated the TOMS-EP instrument as a part of NASA's Earth Science enterprise initiative to gather data on the global environment.

The ozone layer, at the outer reaches of the Earth's atmosphere, protects life on Earth from the deadly radiation of space. Scientists use TOMS-EP data in conjunction with the atmospheric chemistry measurements from other satellites to understand the processes that drive the global creation, destruction and distribution of the Earth's ozone layer.

In December 1997, NASA directed the boost of TOMS into a higher orbit to fill the void left by the loss of the Advanced Earth Observing Satellite (ADEOS). The higher altitude widened the coverage of the TOMS instrument and exerted less atmospheric drag on the TRW-built satellite.

The reboost orbit enabled the satellite to provide ozone measurements for three additional years, or at least until the next satellite with a Total Ozone Mapping Spectrometer is launched.

TRW Space & Electronics Group builds communications, scientific and defense spacecraft for military, civil and commercial customers, and produces, integrates and tests payloads; develops advanced space instruments; and integrates experiments into spacecraft.

It is an operating unit of TRW Inc., which provides advanced technology products and services for the automotive, space-and- defense, and information-technology markets worldwide.