The global positioning system (GPS) is a satellite-based navigation system that continuously transmits timing, frequency and satellite position information to potential users. The global positioning system consists of a constellation of GPS satellites that broadcast the GPS signal, ground stations to control those satellites, and radio receivers to capture the GPS signals and extract navigation information from them. A typical global positioning satellite signal comprises a high frequency carrier signal which is modulated by one or more pseudo-random number sequences of lower frequency. GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite. Encoded on the transmissions of each satellite are messages that indicate the location of the satellite and time of transmission of the signal. The signals from several satellites allow the receiver to compute its position, velocity and time parameters through known triangulation techniques. The signals provided by the GPS can be received globally and continuously. The satellite data is often augmented by data from additional sensors in order to improve navigation system performance in situations where satellite data may be available only intermittently or may be degraded by intentional or unintentional interference. By acquiring the signal of four satellites, and by performing calculations to determine the difference between the time of transmission and time of reception by the user equipment, a user can triangulate and determine latitude, longitude, elevation, and time. 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. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). Depending on the application, the GPS navigation system may be used to determine position only, position and velocity, or a larger set of parameters.

The global positioning system (GPS) has been used in a wide variety of applications including space, air, sea and land vehicle navigation, precise positioning, time transfer, altitude referencing and surveying. Satellite positioning system receivers such as GPS receivers make precise determinations of latitude, longitude, elevation and time by using time difference of arrival and Doppler measurement techniques on precisely-timed spread spectrum signals transmitted by orbiting satellites. A GPS receiver uses multiple hardware correlating units to thereby increase a speed in synchronous acquisition for the GPS satellites. A typical GPS receiver comprises a number of subsystems, including an antenna assembly, an RF (radio frequency) assembly, and a GPS processor assembly. The antenna assembly receives the L-band GPS signal and amplifies it prior to insertion into the RF assembly. Since each satellite transmits its own unique pseudo-random noise (PRN) code, the receiver searches for a particular satellite by locally generating the corresponding PRN code sequence, processing the RF signal received by the receiver antenna, and "mixing" the PRN code sequence with the RF signal. Because the signal transmitted by each satellite uses a PRN code or a carrier frequency unique to that satellite, the receiver may separate the signals from different satellites using code division multiple access or frequency division multiple access techniques. By knowing the orbital position of four GPS satellites (ephemeris data), and the distance from itself to each of four GPS satellites, the receiver can successfully triangulate its own position. Receiver position calculations performed using greater than four satellites generally have an improved accuracy. GPS receivers commonly use broad antenna gain patterns so that all of the satellites above the horizon can be tracked. GPS systems have been developed to be extremely accurate in locating and tracking a receiver on the surface of the earth.

In the GPS tracking system, transmitters are positioned on orbiting satellites. The receiver is synchronized with the data stream by matching an identical suedo-random data stream albeit with a time offset. The time offset between the receiver's data stream and the data stream received from the satellite give the distance to that satellite via the speed of light that the radio signal traveled. The receiver generates a set of codes identical to those codes transmitted by the satellites.  Receivers typically utilize the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. Extensive systems have been deployed to use GPS capabilities for the purpose of tracking vehicle fleets or cargos. Such systems have been often referred to as "asset tracking systems" and deploy asset tracking units designed to be attached to individual vehicles. Each asset tracking unit includes a GPS receiver that is capable of receiving GPS signals from a plurality of GPS satellites and determining the unit's location based on the GPS signals. Vehicle tracking systems using global positioning system (GPS) satellite information are highly accurate. Also located in the vehicle tracking system is a communication device to transmit the GPS signal from the vehicle. These communication devices include cellular transmission systems, pager systems or radios. A base center or monitoring center receives the GPS signal from the vehicle, and processes the information to determine the position of the vehicle. With the GPS tracking system, control center will have information concerning the whereabouts of all vehicles in the fleet. This may lead to significant efficiencies in planning and managing assignments of vehicles to particular tasks.