Satellite positioning systems (SPS), such as the Global Positioning System (GPS), permit the determination of a geographic location of an entity, such as a person, that is equipped with an SPS receiver. SPS receivers, such as those which operate in GPS or other satellite positioning systems, normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites, such as GPS, NAVSTAR, or other satellites.
These satellites typically transmit, as part of their satellite data message, timing and satellite positioning data, which is sometimes referred to as “ephemeris” data. The term “ephemeris” or “satellite ephemeris” is generally used to mean a representation, such as an equation, which specifies the positions of satellites (or a satellite) over a period of time or time of day. In addition, the satellites can transmit data to indicate a reference time, such as time-of-week (TOW) information, that allows a receiver to determine unambiguously local time.
Typically, an SPS receiver computes one or more “pseudorange” measurements, each of which represents the range between the receiver and a satellite vehicle (SV). The term “pseudorange” is generally used to point out that the range measurement may include error due to one or more factors, including, for example, the error between time as indicated by the clock of the SPS receiver and a reference time, such as the reference time associated with the more accurate atomic clock of the satellites. Thus, the SPS receiver typically uses the pseudoranges, along with timing and ephemeris data provided in the satellite signal to determine a more accurate set of navigational data, such as position, time, and/or range.
Alternatively, the SPS receiver may not determine its location directly, but rather uploads information regarding its position, such as pseudoranges, to a server computer. The server computer computes the geographical location for the receiver from the pseudoranges. While an SPS receiver can determine its position or cause a server system to do so, it is also desirable to make the location known to other SPS receivers or other types of receivers under controlled circumstances.