The global positioning system (GPS) is a satellite-based radio navigation, positioning and time transfer system based on an earth-orbiting constellation of twenty-four satellites, each broadcasting its precise location and ranging information. A satellite based positioning system is used to determine a position of a receiver and typically includes satellite control facilities, a plurality of satellites, the receiver, and one or more local or regional 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. The receiver synchronizes itself to the codes of at least four satellites and uses the information in the signals from these satellites in order to perform a triangulation like procedure so as to determine its coordinates and time offset with respect to a reference. GPS provides highly accurate navigational information on a continuous global basis to an unlimited number of properly-equipped users. The global positioning system (GPS) satellite navigation system provides a worldwide position, velocity and time reference available for utilization in electronic navigation systems. Important aspects of the system are that GPS is unaffected by weather and it provides a worldwide common grid reference system that is based on an earth-fixed coordinate system. The orbits of the GPS satellites are arranged in multiple planes, in order that the signals can be received from at least four GPS satellites at any selected point on or near the earth. The global positioning system (GPS) consists of a constellation of globally-dispersed satellites with synchronized atomic clocks that transmit radio signals. Time, as maintained by each satellite, is embedded in the transmitted radio signal of each satellite. The difference between the time embedded in a satellite's radio signal and a time measured at the point of reception of the radio signal by a clock synchronized to the satellite clocks is a measure of the range of the satellite from the point of reception. Since the clocks in the system cannot be maintained in perfect synchronism, the measure of range is referred to as "pseudorange" because it includes both a satellite clock error and the clock error at the point of reception. Each satellite transmits, in addition to its clock time, its position in an earth-fixed coordinate system and its own clock error.

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. GPS signals are available worldwide at no cost and are now being routinely used to determine the location of automobiles. With a GPS device, information signals transmitted from a plurality of satellites to a GPS receiver are analyzed using known trilateration techniques in order to determine the geodetic coordinates of the receiver, wherein the geodetic coordinates are typically provided in latitude and longitude. Typically, the signals transmitted by each satellite convey three types of information: satellite trajectory data, system timing, and ranging information. When a wireless terminal can acquire the signals from three or more satellites the wireless terminal can determine its position through triangulation. A GPS receiver determines its global position based on the signals it receives from orbiting GPS satellites. A typical global positioning satellite signal comprises a high frequency carrier signal which is modulated by one or more pseudo-random number sequences (PR-codes) of lower frequency, which in turn is modulated a 50 Hz data stream which provides navigational information. The signal transmitted by each satellite is comprised of a carrier that is modulated by at least a binary pseudorandom (PRN) code, which consists of a seemingly random sequence of 1s and 0s that periodically repeat. A user, by measuring the pseudoranges to four satellites and correcting the pseudoranges for the satellite clock errors, can first of all determine his actual range to each satellite and his own clock error. The user can then determine his own position in the earth-fixed coordinate system, knowing his range to each of the four satellites and the position of each satellite in the earth-fixed coordinate system. The GPS receiver calculates the difference between the time a satellite transmits its signal and the time that the receiver receives the signal. The receiver then calculates its distance, or "pseudorange," from the satellite based on the associated time difference. Using the pseudoranges from at least four satellites, the receiver determines its global position. The GPS receiver triangulates its own position by obtaining the GPS signals from a set of satellites, typically three of four orbiting satellites. The position of the receiver is determined in the form of a geographic position--longitude and latitude--to, for most receivers, within meters of an actual location.

GPS receivers for receiving satellite-based navigation signals comprise a receiving unit for receiving navigation signals emitted by satellites, wherein said receiving unit is connected with a computer unit for processing and forwarding said signals and data for navigation purposes. The receiver is connectable via an interface with a navigation computer. From any location on or near the earth, a GPS receiver with an unobstructed view of the sky should be able to track at least four satellites thereby being able to calculate the receiver's precise latitude, longitude, and elevation. The GPS receivers contain antennas, reception equipment, and processors that are utilized for determining position and timing based on satellite ranging. The GPS receiver continuously receives signals put out from at least three GPS satellites. Every predetermined period of time the GPS receiver measures distances up to each of the three GPS satellites and a change rate of those distances, thereby generating GPS data including a current position and a traveling direction (orientation). The GPS receiver also performs filtering with respect to the GPS data thus sequentially generated, and generates positioning data to be outputted to a navigation device and the like. The global positioning system principle of operation is based on range triangulation. Because the satellite position is known accurately via ephemeris data, the user can track the satellite's transmitted signal and determine the signal propagation time. The GPS receiver device acquires spread spectrum GPS satellite signals from at least three satellites to calculate its two-I dimensional position by triangulation. Acquisition of an additional signal, resulting in signals from a total of four satellites, permits the GPS receiver device to calculate its three-dimensional position. The receiver correlates locally-generated codes with the codes received from the respective satellites to derive timing information relating the receiver time relative to the local generation of code chips to the satellite time at the times of the transmission of the corresponding code chips.

The global positioning system has become increasingly popular as a position determination or time determination mechanism for a variety of applications. Car navigation systems employing GPS (global positioning system) have been widely used. This car navigation system includes a display unit (monitor) for displaying map information and the like, a main unit for containing a computation processing unit and the like therein, a remote control unit for inputting control information into the car navigation system. Usually a car navigation system consists of a map-data storage device such as a DVD (digital versatile disk), having map data recorded thereon, a display, and a vehicle-movement detector for detecting a current position and orientation of the vehicle, including a gyro, a GPS (global positioning system) receiver, and a velocity sensor. Vehicle navigation systems typically include a display device with a video display that provides a graphical interface for the user. A main function of the video display is to depict the desired map area and route on which the user's vehicle is traveling. It is also known to give voice guidance to a driver when a vehicle approaches a branching point on the planned route. The navigation system reads map data for an area including the current position of the vehicle from the map-data storage device, draws a map image of an area around the current position of the vehicle based on the map data and superimposes a vehicle-position mark (location) on the map image. The use of map matching navigation and other similar full function navigation systems has generally provided a user with the ability to follow calculated turn-by-turn instructions as computed by the navigation system.