Communications and Position Fixing Experiments Using the ATS Satellites

Abstract

NASA's ATS-5, ATS-3 and ATS-1 satellites were used in a series of ranging, position fixing and communications experiments at L-band and VHF. Communications included voice, digital, teletype and facsimile transmissions. Many transponders were used in the tests. They provided communications and responded automatically to ranging interrogations. When a transponder was to be located, a central ground terminal transmitted a short signal containing the individual address of the transponder through one satellite. All of the transponders received the signal, and the one that was addressed responded through two satellites. The central ground terminal determined the ranges from the two satellites to the transponder and computed its position fix. Three to ten independent position fixes could be made each second in one channel of an operational system. The DICAR digital communications and ranging technique that has developed from these experiments uses a single signalling waveform and a single modem for communications and ranging. Experiments with the signalling parameters suggested for an operational system achieved a ranging precision better than 50 feet with a modulation frequency less than 10 kHz, an RF bandwidth less than 60 kHz, and a ranging signal duration of 30 milliseconds. An automatic transponder that responds at L-band through ATS-5 and at VHF through ATS-3 was located with a precision approaching 0.1 nmi., 1 sigma. Every factor that affects communications reliability and position fixing accuracy was measured and evaluated in the experimental program between 1968 and 1973. Automatic transponders were carried on ships in the Atlantic, Pacific and Gulf of Mexico and on the Mississippi River; on jet and propeller driven aircraft flying over the continental United States and North Atlantic to Shannon, Ireland and Thule, Greenland; on a buoy moored in deep water off Bermuda; and in a panel truck driven over country roads in upstate New York. Automatic transponders at Shannon, Ireland; Reykjavik, Iceland; Schenectady, New York; Kings Point, New York; Seattle, Washington; and Buenos Aires, Argentina were used to test a trilateration technique for real time satellite location and to measure propagation factors that affect communication reliability and ranging accuracy. Hundreds of hours of communications and more than a million range measurements have provided data on all factors that affect communications reliability and position fix accuracy. The experiments have confirmed that all the factors can be controlled and that high quality, reliable communications and useful position fixing accuracy can be provided to maritime and aeronautical users by practical means using satellites at L-band.