POSITIONING ACCURACY ESTIMATED In the Geostar System, the errors in height are those of the on-board altimeter or, in the case of surface users, those of the digital terrain map stored at the ground station. Latitude and longitude errors depend on the errors in the timing of the transceiver reply and of its arrival at the ground station. Each of these is typically about 5 nsec, leading to a combined round-trip jitter of about 7 nsec, and therefore to an error of roughly 1 m in the distance from a satellite to the transceiver. With satellites separated by at least 30° in longitude, triangulation for longitude is good. In typical conditions, longitude should be determined to a few meters by the Geostar System. For latitude, triangulation is good everywhere except close to the Equator. In midlatitudes, typical errors in latitude are calculated to be a few meters plus approximately twice the altimeter error. Despite the poorer triangulation for latitudes close to the Equator, the latitude determination is usable fairly close to that region. For total timing and altitude errors of 3 m, the latitude determination becomes useless only within about 4 km of the Equator. At 8 km from the Equator, the latitude errors are about 2 km, comparable to those of VLF/Omega, and they are below 100 m,* beyond about 160 km from the Equator. The reduction of latitude precision for aircraft close to the Equator can be overcome inexpensively, where necessary, by emplacing a relay for transceiver replies 50-150 km from the Equator to send the signals to the satellites. That restores good triangulation. Fortunately, the major populated areas which are close to the Equator (Singapore, Kuala Lumpur, Quito, and Nairobi, for example) all have high volcanic mountain peaks conveniently located for such relays. Geostar can provide high accuracy with inexpensive equipment because it is a two-way, interactive system. Most of the errors in other satellite location systems (ionospheric delay variations, imprecise knowledge of the geoid, errors in satellite position, etc.) cancel out in Geostar, because positions are measured relative to fixed-site “benchmark” transceivers. Geostar is an all-differential, closed-loop system rather than an open-loop system. The combination of high precision, low cost, low power drain and nondirectional antennas that is realized in the Geostar RDSS would not be possible if Geostar were to attempt to provide channelized voice communications. A voice channel 5-kHz wide gives a ranging precision about 3,000 times worse than that of Geostar’s single digital channel, which is 16.5-MHz wide. And, the average power required to transmit voice is several hundred times more than for equivalent information transmitted digitally. EXPERIMENTAL TESTS PERFORMED In 1983, a test for the Geostar System was put in place, consisting of a ground station, user transceivers, and satellite emulators located on mountain peaks in the Sierra [Nevada] Mountains. The conditions emulated operation in extreme Northern latitudes near the limits of satellite coverage. The system demonstrations included guiding a pedestrian to a hidden marker in a *Typical estimated errors of the U.S. proposed Global Positioning System (GPS) civil-access accuracy.
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