The first fully operational digital terminal for mobile satellite communications system has been successfully field-tested in an automobile by Jet Propulsion Laboratory researchers.
The prototype terminal -- designed to use satellite relay for mobile communication in remote areas, similar to cellular telephones in urban regions -- was tested during experiments in July and August 1988 near Erie, Colorado.
JPL researchers used relay mounted on tower to simulate the role of the orbiting satellite while testing two-way digital voice and data communication with the prototype mobile terminal.
It is believed to be the first time all elements of mobile satellite terminal have been tested in standard car, according to Dr. William Rafferty, manager of JPL's Communications Section, which conducts the Mobile Satellite Experiment (MSAT-X) program for NASA.
During the test, researchers used recently developed conformal rooftop phased-array antenna which is electronically steered to keep the mobile terminal locked on to the orbiting satellite. The antenna's external dimensions -- 21 inches diameter by 0.7 inch height -- make it particularly suited to commercial vehicle applications, Rafferty said.
The antenna was able to track the simulated satellite while the car traveled at speeds from 10 to 70 miles per hour while executing extreme turns and U-turns.
Voice and data were digitally transmitted at rate of 4,800 bits per second over 5-kiloHertz channel. The resulting sound quality, said Rafferty, was "typical good -- or better than you typically experience if you make call over standard government phone lines."
Future plans in JPL's MSAT-X program call for tests later this year in which prototype mobile terminal will be flown on an airplane. full-duplex voice and data link will be established between the ground and the airplane via an orbiting satellite.
A fully developed mobile satellite communications system would extend mobile telephone service to remote areas of the country which cannot be served by cellular telephone systems.
Besides such users as private drivers, cross country trucks, forestry personnel and law-enforcement agents, such system could also serve ships at sea and planes in flight.
JPL's role is strictly to develop new technologies required for mobile satellite system. NASA plans to seek cooperative agreements with the commercial operator of first-generation satellite system whereby the space agency will launch the first satellite in exchange for the right to conduct technology validation experiments using small percentage of the satellite's capacity for the first two years of operation.
Rafferty said JPL will continue work developing technologies for rooftop antennas in addition to modulation encoding and networking methods.
In this summer's tests, researchers used 1,000 foot tower between Boulder and Denver, Colorado, operated by the National Oceanic and Atmospheric Administration's (NOAA's) Wave Propagation Laboratory.
JPL contractors which developed segments of the tested system include Teledyne Ryan Inc., phased-array antenna; the University of California at Santa Barbara, 4.8K baud speech coder; and Pacific & Inc., which developed equipment to simulate the satellite relay.
MSAT-X is funded by the Communications Division of NASA's Office of Space Science and Applications.
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