The role of time-frequency in satellite position determination systems

Many present-day radio navigation satellite systems employ the use or measurement of time or frequency to provide a position fix for the user vehicle. For example, the position fix obtained by the U. S. Navy's Navigation Satellite Systems (Transit) is based on the precise measurement of the Doppler shift from a single satellite observed by the user vehicle, plus the satellite's orbital elements and time. The time of the satellite's orbit parameter transmission is controlled by a crystal oscillator having a short-term stability of 2 parts in 10
¹¹
. Multiple geostationary satellites (36000-km altitude) have the capability to provide a near instantaneous position fix using range measurements of the two-way propagation time of an RF signal initiated at a ground station and sent to a user vehicle by one satellite. The user retransmits this signal through the satellites to the ground where a position fix is obtained. Knowledge of user altitude, in the case of an aircraft, also is required. Low-altitude satellites can also be employed to provide a position fix via two-way ranging, but with a significant delay in obtaining a position fix. Inherent to this and many other navigation concepts is the accurate measurement of frequency, time, or time interval. If range is to be described by the time required for an RF signal to propagate between the satellite and the user or ground station, time interval becomes significant. For example, it takes approximately 1 µs for a radio wave to travel 0.3 km in free space. Time interval is often measured by counting cycles or by phase comparisons using highly stable and accurate frequency sources. NASA conducted tests of these concepts employing the ATS 1 and 3 satellites, and Nimbus 3 and 4 satellites. The test results indicate that locations of ships and aircraft can be determined to within 3-5 km, one sigma, by these satellite methods, and that location accuracy is both a function of the RF employed as well as the extent of knowledge of the satellite orbit.