Artist's rendering of tethered picosats in orbit. Photo: The Aerospace Corp.

The Aerospace Corporation and the Defense Advanced Research Projects Agency (DARPA) on February 10 successfully concluded their pioneering mission to fly the smallest satellites ever placed into orbit.

Aerospace engineers made the decision to close out the mission when power aboard the tiny spacecraft -- which measure 4-by-3-by-1 inches and weigh less than one-half pound each -- began to diminish after completing a number of groundbreaking operations.

These began Sunday, Feb. 6, when the tethered pair of picosatellites were released from their "mother" satellite, built by Stanford University students.

The first milestone achieved during the operations period, from Feb. 6 through Feb. 10, was release from OPAL (Orbiting Picosatellite Automated Launcher), the mother ship.

Figuring in were a new four-stage launch vehicle -- the Air Force's Orbital Suborbital Program Space Launch Vehicle constructed from refurbished Minuteman 2 rocket motors and Pegasus XL motors -- and a succession of deployments from university-built satellites.

OPAL, the Stanford satellite, was released by JAWSAT, the joint Air Force Academy Weber State University Satellite, and the picosatellites were released from OPAL. The launch itself occurred on Jan. 26 from a new commercial spaceport at Vandenberg Air Force Base, a first from that site.

Distinguished Engineer Ernest Y. Robinson of The Aerospace Corporation said a major achievement, in addition to getting the picosats launched successfully and released from their mother satellite, was getting them located and tracked by the U.S. Space Command’s Space Surveillance Network, a significant challenge because they presented a diminutive profile. The insertion of thin strands of gold in the tether connecting the two satellites was a key factor in enabling them to be located by radar and tracked, Robinson said.

Another achievement was establishing communications from the 50-meter dish antenna at the SRI International ground station at Menlo Park, Calif. Achieved were uplink and downlink of data, including the transmission of commands to the picosats; receipt of data on the condition of the satellites, for example, battery and temperature data; and reports on microelectromechanical systems (MEMS) experiments. Validating an array of MEMS radio frequency switches designed by Rockwell Science Center of Thousand Oaks, Calif., was the primary goal of the mission, which was funded by the Microsystems Technology Office of DARPA.

Infrastructure established
"We've established the infrastructure which we will use and refine in validating MEMS and microsystems for integration into miniature satellites of the future," Robinson said. He said the picosasts represent a link toward nanosatellites, envisioned by Aerospace Corporation researchers as "inevitable" alternatives to many large and costly spacecraft flown today.

Picosatellites, less than one-half pound each, are shown against a coffee mug. Photo: The Aerospace Corp.

The picosatellites were designed and built by The Aerospace Corporation, El Segundo, Calif., under funding from DARPA. The Aerospace Corporation tested and integrated the satellite components and conceived the mission.

The Aerospace Corporation has been heavily involved in research into miniature satellites for a number of years and formally advanced the concept of nanosatellites at the 44th International Astronautical Federation Congress in Graz, Austria, in 1993. A paper presented at this conference described the concept of nanosatellites, slightly larger than picosats, and how to build, power and use them.

The mission closed out today was the first of a series designed to validate MEMS technology and demonstrate the capabilities of miniature satellites operating in constellations. Another picosat mission is planned for mid-June aboard the MightySat 2.1 satellite built by the Air Force Research Laboratory, and a third more complex "inspector" mission is planned for 2003.

Pioneering MEMS experiments also were conducted in space aboard the space shuttle Columbia last summer in an experiment designed by The Aerospace Corporation in collaboration with a number of universities and research organizations.

Decision to End Mission
In addressing the decision to close out the mission, Robinson said a failure to communicate with the picosats during two passes they made over the ground station on the morning of February 10 convinced the team of operators to terminate the experiment.

"Our ability to communicate became more and more faint and we were in a situation of diminishing returns," said Robinson.

Part of the problem with power was that it took nearly 24 hours after the picosats were released from their mother ship on Feb. 6 to establish communications from the ground station.

Because the tethered pair of satellites had been emitting beacon signals continually during this period, they used up a third of their power. Engineers then commanded the satellites to conserve power and issue beacon signals only on passes over the ground station. Most of the additional power was used up by exercises with the MEMS devices, the primary payload. Considerable power also was used by one picosat during a 12-hour period when a communications gap precluded transmission of commands to put its beacon signal to rest.

"We knew we were using up our power budget," Robinson said. "By Wednesday we had used two thirds of the power available (from the lithium thionyl chloride batteries). Then, because of a number of passes with no logins, we decided to close down the experiment."

Communications challenges
Among challenges the engineers faced was doubling up radio transmitting equipment used in tandem with the SRI antenna. Affixed to the antenna dish was equipment Stanford students used to communicate with OPAL, the mother satellite, as well as equipment used by Aerospace Corp. engineers to communicate with the picosats. Stanford moved its equipment to the SRI dish because it afforded more power and "pointing" capabilities than the ground station at the university they used initially. In addition, operations were interrupted on at least two occasions when JPL radio scientists used the dish antenna in efforts to detect signals from the Mars Polar Lander.

Aerospace engineers had planned to to initiate network communications among the orbiting picosats and another picosat affixed to the horn of the SRI dish antenna and perform "data hopping" exercises. But the power loss precluded this exercise, Robinson said.