A satellite navigation system typically includes a plurality of satellites, the receiver, and one or more ground stations. Each of the satellites transmits a signal that contains a code and certain prescribed information useful to the receiver in determining its position. In response to a user's request, map data stored in a server placed at a map data base site are transmitted to such a navigating terminal as an on-vehicle navigation unit, mobile navigation unit, PDA (personal digital assistants), or mobile phone, through communication means. The navigating terminal includes a GPS to detect a vehicle's current position, and provides a map on a monitor thereof using both the vehicle's current position and the received map data. The satellite receivers contain proprietary circuitry and software that process the satellite signals and extract navigation information, such as the user's position, time, velocity, and attitude. In a typical navigation system, a processor is connected with a memory system having data corresponding to a geographic map image, such as a geographic information systems (GIS) database. GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite. Global positioning system (GPS) receivers use signals received from typically three or more earth-orbiting satellites to determine navigational data such as position and velocity. The velocity of the receiver is calculated from the doppler frequency shift of signals transmitted from space while the exact position of the receiver is calculated from the time shift of data due to the distance the signals must travel from the satellite. This distance is called range and the doppler shift yields range rate. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. On-board vehicle navigation systems often rely on the global positioning system (GPS) for sensing an absolute or actual position of a vehicle hosting the on-board system. Such on-board navigation systems include a mobile GPS receiver installed in the vehicle, and integrated with a host of other sensors, such as an odometer and/or gyroscope. The mobile GPS receiver senses a vehicle position based on signals received at the GPS receiver from a plurality of GPS satellites.

Satellite navigation receivers such as GPS receivers are all-weather, worldwide, continuous coverage, satellite-based radio navigation systems. Satellite navigation provides the user with estimates of the three coordinates of the user's position: x, y, and z. Satellite navigation receivers measure and analyze signals from satellites, and estimate the corresponding three coordinates of the receiver position, as well as the instantaneous receiver clock bias. These receivers measure time delays and decode messages from satellites within view of the receivers to determine the information necessary to complete position and time bias calculations. A GPS receiver inversely decodes a GPS signal received from a GPS satellite according to a PRN code, and then interprets a navigation message. A time difference between the instant a GPS signal is transmitted and the instant the GPS signal is received is multiplied by the velocity of light, whereby the distance between the GPS satellite and receiver is measured. The range from a satellite is determined by measuring the satellite signal transmission and receiving time in conjunction with a clock synchronized to the satellite's clock, and calculating the distance from a specified position of the satellite at the transmission time. The distance is analyzed in conjunction with the position of the GPS satellite which is detected from the navigation message, thus determining the position of the receiver. Generally, the quality of pseudorange measurements is affected by errors in the predicted ephemeris of the satellites, instabilities in the satellite and receiver clocks, receiver noise and RF interference. The ionospheric induced delay in the reception of the GPS signals limits the accuracy of the determination of the position of the GPS receiver and is the largest location dependent error source in the calculation of the position of the GPS receiver. The signals transmitted by the satellites can be adversely affected, for example, by atmospheric conditions which can lead to improper position determinations by the receiver.