Propagation errors in VHF satellite-to-aircraft ranging

An airplane navigation system based on the measurement, at very-high frequency, of the range between the aircraft and a geostationary satellite is under development by NASA. An examination is made of the errors resulting from the unknown propagation characteristics of the signal through the ionosphere. These errors are found to be a function of the distance between subaircraft and subsatellite points. At short distances unfavorable geometry causes small ranging errors to be translated into large position errors. As the distance increases, the errors become smaller, until a minimum is reached at some 5000 km. At even larger distances the errors again increase due to the greater path length of the signal in the ionosphere. Completely disregarding ionospheric effects leads to position errors of some 5 km at a 5000-km distance during the midday period near the solar cycle maximum. By using good predictions of the ionospheric electron content, it may be possible to reduce such errors to 1 km on a representative day. Since the errors are proportional to the columnar electron content, they become correspondingly smaller at night and decrease by, roughly, a factor of 3 during the sunspot minimum period.