ANTENNA ARRAYING

Antenna arraying combines the signals received by multiple antennas at different locations to synthesize a single large antenna. It is commonly used to improve reception of weak signals. Arraying is beneficial in deep space communications where the signal transmitted by a spacecraft becomes very weak as it travels across vast interplanetary distances. When the signal arrives at Earth, it is spread over a large area, and the ground antenna is able to receive just a small part of the signal. Arraying allows the capture of these very weak signals and enables a higher data rate.

Arraying combines the signals received by several smaller antennas to provide the performance of a single large antenna.

The Deep Space Network used arraying for single missions in the early 1970s, and in 1977 began developing arraying capability for the entire network. Experiments with a prototype arraying system for the Voyager encounters at Jupiter and the Pioneer 11 spacecraft at Saturn told engineers a great deal about how to gain increased sensitivity. The 1980 Voyager encounters with Saturn made intensive use of arrayed configurations at the three DSN complexes at Goldstone, California; Madrid, Spain; and Canberra, Australia. By the time Voyager flew by Uranus in 1986, the DSN was combining up to four independent signals in a single processor. For Voyager's Neptune encounter, the DSN combined signals from Australia's Parkes Radio Telescope into the Canberra complex, and combined signals from the 27 antennas of the Very Large Array in New Mexico into the Goldstone array.

The Galileo mission to Jupiter employed arraying in 1996-1997 to increase the science data return during its primary mission. For Galileo, the DSN arrayed up to five antennas from three tracking facilities (Goldstone, Canberra, Parkes) and two continents. The result was a factor of 3 improvement in data return compared with that of a single 70-meter (230-foot) antenna. Arraying, plus advances in data compression and encoding techniques, helped make Galileo so successful that its mission was extended to more than a dozen years of valuable observations of Jupiter and its moons.

Other missions have benefited from arraying: for example missions that have certain operational phases that require more performance than a single antenna can provide, such as Cassini's high-value science data return during Saturn orbit. Other missions that had to relay critical science data to Earth in the shortest amount of time possible. Stardust and Deep Impact for example minimized the risk of losing data after their comet encounters through the use of arraying.

Arraying also provides flexibility in managing DSN antenna resources. An array of several 34-meter (112-foot) antennas can serve as the equivalent of the 70-meter antenna when the larger antenna is down for maintenance. Improved utilization of available resources can be obtained with arraying. A shortfall in a 34-meter antenna's performance would normally require the use of the big 70-meter. With arraying, the DSN has the option of linking 34-meter antennas incrementally to meet mission demands.

The DSN arraying capability has been instituted at the Goldstone, Canberra and Madrid complexes. This second-generation system employs the full-spectrum arraying method first used to support the Galileo mission in 1996, and can array as many as eight antennas in a complex. Full-spectrum arraying combines the signals from multiple antennas in real time at higher data rates and corrects for the signals' differing delays and Doppler shifts. Receiver processing is improved owing to higher input signal level. Tracking, navigation, and telemetry data all benefit from the use of full-spectrum arraying.