TacSat-2 Provides Testbed for New Navigation Sensor

An experimental navigation sensor flying aboard the Pentagon's TacSat-2 satellite was successfully activated Dec. 27, clearing the way for several weeks of dedicated testing of the NASA-funded device.

The Inertial Stellar Compass, a low-mass, low-power instrument combining a star camera and miniaturized gyroscopes with a microprocessor, was one of two NASA-funded payloads on board the Minotaur rocket that lifted off Dec. 16 from Wallops Island, marking the first orbital launch from Virginia's eastern shore in more than a decade. The other NASA payload, Genesat-1, is a self-contained microbiology laboratory about the size of a shoebox.

Unlike the freeflying GeneSat-1, the Inertial Stellar Compass is installed on board TacSat-2, a small, experimental remote-sensing satellite built by MicroSat Systems, of Littleton, Colo., under a contract from the Air Force Research Laboratory.

Development of the Inertial Stellar Compass was funded by NASA's New Millennium space technology development program. NASA has invested about $10 million in the project, which is expected to aid the development of new classes of small spacecraft suitable for a diverse range of missions including comet and asteroid encounters and lunar landings. Larger spacecraft taking advantage of the miniaturized navigation technology, according to project officials, could see mass savings of 10 kilograms or more and power savings of 30 watts or more over conventional systems.

Linda Fuhrman, space science program manager at Cambridge, Mass.-based Draper Laboratory, said Jan. 2 that the Inertial Stellar Compass she and her colleagues built has been working flawlessly since it was first activated on the 12th day of TacSat-2's mission. "Within a second or so, we got telemetry back indicating it had powered on and proceeded to go into a self-initializing mode," Fuhrman said. TacSat-2 then passed beyond range of its live telemetry downlink. By the time the spacecraft came back around about 90 minutes later, Fuhrman said, the Inertial Stellar Compass was up and running and accurately reporting the spacecraft's position.

TacSat-2 operators were not so lucky on their first day of flight. Although TacSat-2 is operating properly now, the satellite went silent for a couple of days shortly after reaching orbit.

Air Force Research Laboratory spokesman Michael Kleiman said flight controllers received initial telemetry data from the satellite 77 minutes after launch Dec. 16, and then lost contact with the satellite until Dec. 18.

In a Jan. 3 written response to questions, Kleiman said the problem was likely caused by human error with the ground systems. While the problem has been corrected, the Air Force has not fully investigated the matter as it remains focused on checking out the satellite and preparing for its experiments, Kleiman said.

One of those experiments is the Inertial Stellar Compass. Fuhrman said the experimental payload requires nothing from its host spacecraft beyond a modest amount of power and occasional updates of its clock. Other than that, it initializes itself on its own, snaps a picture of a star field, identifies guide stars from its own preprogrammed catalog, and uses so-called lost-in-space algorithms to determine its position and the direction it is pointing.

If tests planned for the weeks ahead verify the Inertial Stellar Compass's performance under a variety of mission conditions, spacecraft designers will have at their disposal a flight-verified attitude determination system that offers significant mass and power savings. Fuhrman said the 2.9 kilogram instrument package runs on 3.6 watts of power -- less than half the mass and power of conventional systems.

A key to the Inertial Stellar Compass's power and mass savings are its microelectromechanical systems (MEMS) gyroscopes, microchip-sized devices that detect a rotational change in velocity. MEMS devices are being increasingly used in automobiles, for example, to deploy airbags or activate dynamic stability control systems.

Although MEMS gyroscopes have been flown in space in fairly simple experiments meant to show that they can function in a weightless environment, Furhman said the Inertial Stellar Compass's flight aboard TacSat-2 marks the first use of a MEMS gyroscope in a complete spacecraft attitude control system.

While TacSat-2 relies on a conventional attitude determination system, Fuhrman said the Inertial Stellar Compass is on standby to serve as a back up for the nominal one-year mission. All that would be required to press the experimental navigation sensor into service as TacSat's primary system would be to upload a few lines of software code.

The Inertial Stellar Compass has yet to be manifested on any future spacecraft missions, but Fuhrman said Draper Laboratory has been talking to scientists and engineers interested in using the device on NASA Discovery-class science spacecraft. She said the lab also has been talking to the Air Force about using the devices on future TacSats as the primary attitude determination system.

NASA struggled for several years to find flight accommodations for the Inertial Stellar Compass. The space agency originally planned to fly the instrument on board the space shuttle, but it was bumped from the manifest even before the 2003 Space Shuttle Columbia accident made it even more difficult for non-space station payloads to find a berth on the shuttle. Fuhrman said NASA looked into flying the instrument on a proposed commercial solar sail mission and one of its own Earth science satellites before finding the TacSat opportunity.

Draper Laboratory intends to spend the rest of January and February testing the Inertial Stellar Compass under increasingly challenging conditions, gauging, for example, the instrument's ability to maintain a position lock even as TacSat demonstrates its ability to quickly maneuver to image a new ground target. Fuhrman, however, considers the instrument's on-orbit performance to date as proof positive that the device works.

"We are looking for partners who might be interested in licensing the technology or transitioning it to a commercial product. But we are definitely the people to talk to at this point."

Staff writer Jeremy Singer contributed from Boston.

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