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JPL researchers have successfully tested an antenna design expected to play key role in the Laboratory's Mobile Satellite Experiment (MSAT-X) program for NASA.

The newly developed antenna, designed to be mounted on the roof of passenger vehicle, uses mechanical steering to track an Earth-orbiting satellite while the vehicle is in motion. It and other JPL-developed technologies are intended to pave the way for an eventual mobile satellite communications system.

In field tests during August, researchers mounted the antenna on passenger vehicle and used it to lock on to an INMARSAT communications satellite while driving over 300 miles of roadway in variety of terrain in southern and central California.

The test is believed to be the first time steerable antenna mounted on passenger vehicle has tracked an in-orbit satellite in mobile satellite configuration.

Researchers were particularly pleased because the satellite, in geosynchronous orbit over the western Pacific Ocean, was only 15 degrees above the horizon from the test vehicle's perspective, said Dr. William Rafferty, MSAT-X program manager at JPL. The antenna is designed to cover nominal range of 20 to 60 degrees elevation.

The antenna demonstrated tracking in both closed- loop signal-seeking mode and an open-loop inertial reference mode. The antenna automatically switched between those modes as the satellite line-of-sight was intermittently blocked by terrain conditions and roadside foliage.

A mobile satellite communications system would extend mobile telephone service, comparable to that currently provided by cellular systems in urban areas, and provide data service systems to remote areas throughout the country.

The system, which would be built and operated by industry, consists of one or more satellites in geosynchronous orbit and network management centers on the ground. Each of the satellites, to be deployed beginning in the early 1990s, would serve as relay station for mobile users communicating by voice and data links.

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 conduct technology validation experiments using small percentage of the satellite's capacity for the first two years of operation. In addition, NASA/JPL-developed technologies would enter the public domain for use by commercial enterprises in other applications.

Besides vehicle drivers in remote areas, mobile satellite communications could serve potential users such as forestry personnel, ships at sea and planes in flight and is projected to become billion-dollar industry. It is expected to complement rather than compete with existing cellular technology.

The mechanically steered antenna tested in August uses commonly available parts in order to keep costs low, Rafferty noted. In coming months MSAT-X researchers expect to test more compact electronically steered antenna, in addition to other technologies under development such as vocoders to digitize human speech.

MSAT-X is funded by the Communications Division of NASA's Office of Space Science and Applications. George H. Knouse is responsible for the program at NASA Headquarters.

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