PUBLIC INFORMATION OFFICE JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011 http://www.jpl.nasa.gov Contact: Mary Beth Murrill FOR IMMEDIATE RELEASE November 7, 1996
NASA's intercontinental link-up -- or "arraying" -- of giant antennas was developed to retrieve the maximum amount of data possible from NASA's Galileo spacecraft, whose planned high-speed, high-power telecommunications voice was reduced to a whisper when its main antenna failed to open four years ago.
The array began operations in time for Galileo's flyby of Jupiter's moon Callisto on Monday, Nov. 4, when Galileo came within just 1,110 kilometres of that moon at 13:34 Universal Time (23:34 EAST). It is the closest any spacecraft has ever flown to Callisto, which displays the most ancient, cratered surface of any body in the solar system.
Starting Friday, Nov. 1, the large collecting areas of these big antennas were devoted to concurrently receiving the spacecraft's faint transmissions for about four hours a day as Galileo neared Callisto and returned data from its flyby. The spacecraft's closest approach to Callisto occurred while the spacecraft was at one of its most distant points from the Earth, which made receipt of Galileo's weak signal even more difficult. The arraying technique, however, allowed more of the spacecraft's signal to be captured, thereby enabling a higher data rate.
The debut of routine arraying of the Deep Space Network antennas represents the final installment of several imaginative engineering solutions that have allowed the Galileo project team to carry out its mission despite the loss of the spacecraft's main telecommunications antenna.
"With our spacecraft software and ground receiving station improvements already in place, this new arraying capability is the icing on the cake," said Galileo Mission Director Neal Ausman at NASA's Jet Propulsion Laboratory. "The new array is critical to getting Galileo's scientific data from the Jupiter orbital tour back to Earth."
Arraying, together with data encoding in the space-to-ground communications link, increases by 10 times the quantity of raw data received from Galileo than would otherwise be possible. Changes in the way the Galileo spacecraft edits and compresses data result in an additional factor of 10. When taken together, these improvements enable Galileo to meet 70 percent of its original science goals. These software changes on the spacecraft now ensure that every bit of science and engineering telemetry from the spacecraft is crammed with as much information as possible. Consequently, while the data amount received from Galileo is comparatively small, all of it is highly valued.
Galileo's high-gain antenna was to have provided a 134-kilobit-per-second real-time data rate from Jupiter. Had no improvements been made in the Deep Space Network, only a 10-bit- per-second data rate would have been possible with Galileo's small low-gain antenna for most of the mission. These improvements, however, along with the changes made on the spacecraft, further increase the downlinked data to an effective rate of 1,000 bits per second.
"As the Earth turns relative to Galileo's position in the sky, different antennas will 'hand-off' the receipt of data from Galileo over a 12-hour period," said Leslie J. Deutsch of JPL, one of the principal innovators behind the solution for Galileo's communications problem. The array electronically links the stadium-size, 70-meter diameter dish antenna at the Deep Space Network complex in Goldstone, CA, with an identical antenna located at the Australia site, in addition to two 34- meter antennas at the Canberra complex. The California and Australia sites concurrently pick up communications with Galileo. The Parkes radio telescope joins in with the Canberra station for about ten hours each day.
"For two hours each day, a total of up to five antennas are pointing in unison to receive transmissions from Galileo," Deutsch said.
The new hardware, software and operations that make these improvements (including antenna arraying) possible for Galileo represents a major improvement in the world's deep space telecommunications system for other missions as well, said Paul Westmoreland, director of telecommunications and mission operations at JPL. The effort cost US$30.5 million.
"The data compression and encoding techniques will be especially useful for the new era of our faster, better, cheaper interplanetary missions," Westmoreland said. "This opens the way for mission developers to reduce future spacecraft and operations costs by using smaller spacecraft antennas and transmitters."
In the future, after the Galileo mission, other antennas may be arrayed when additional performance is required for spacecraft communications and radio science experiments. Among them are the 27, 25-meter diameter antennas that make up the Very Large Array of radio telescopes in Socorro, NM, and Japan's 64-meter radio telescope facility at Usuda.
"Galileo gets credit for giving arraying a huge push," said Joseph I. Statman, telecommunications specialist at JPL, who engineered the system. "This is the first time we will see routine arraying of antennas for spacecraft communications day in, day out." Several 34-meter antennas at Goldstone are to be outfitted with the equipment needed so they can operate together as an array, he added. "This represents the wave of the future because no more 70-meter antennas will be built for the Deep Space Network; only 34-meter antennas will be added to the network from now on," Statman said. When a future deep space mission requires extra telemetry performance, the Deep Space Network will have the option of supplying it through arrays of its 34-meter antennas rather than using the 70-meter dishes.
Galileo was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. The mission is managed by JPL for NASA's Office of Space Science, Washington, DC.
Return to John's Home Page