The Car GPS tracker device navigation message is a data stream added to both the L1 and the L2 carriers as binary biphase modulation at a low rate of 50 kbps. It consists of 25 frames of 1,500 bits each, or 37,500 bits in total. This means that the transmission of the complete navigation message takes 750 seconds, or 12.5 minutes. The navigation message contains, along with other information, the coordinates of the GPS satellites as a function of time, the satellite health status, the satellite clock correction, the satellite almanac, and atmospheric data. Each satellite transmits its own navigation message with information on the other satellites, such as the approximate location and health status.
Dual-frequency receivers are the most sophisticated and most expensive receiver type. Before the activation of AS, dual-frequency receivers were capable of outputting all of the GPS Tracking Device signal components (i.e., L1 and L2 carriers, C/A-code, P-code on both L1 and L2, and the navigation message). However, after the AS activation, the P-code was encrypted to Y-code. This means that the receiver cannot output either the P-code or the L2 carrier using the traditional signal-recovering technique. To overcome this problem, GPS receiver manufacturers invented a number of techniques that do not require information of the Y-code. At the present time, most receivers use two techniques known as the Z-tracking and the cross-correlation techniques. Both techniques recover the full L2 carrier, but at a degraded signal strength. The amount of signal strength degradation is higher in the cross-correlation techniques compared with the Z-tracking technique.
Time plays a very important role in positioning with 3G navigator device . The GPS signal is controlled by accurate timing devices, the atomic satellite clocks. In addition, measuring the ranges (distances) from the receiver to the satellites is based on both the receiver and the satellite clocks. GPS is also a timing system, that is, it can be used for time synchronization. Static GPS surveying is a relative positioning technique that depends on the carrier-phase measurements. It employs two (or more) stationary receivers simultaneously tracking the same satellites. One receiver, the base receiver, is set up over a point with precisely known coordinates such as a survey monument (sometimes referred to as the known point). The other receiver, the remote receiver, is set up over a point whose coordinates are sought (sometimes referred to as the unknown point). The base receiver can support any number of remote receivers, as long as a minimum of four common satellites is visible at both the base and the remote sites.
In principle, this method is based on collecting simultaneous measurements at both the base and remote receivers for a certain period of time, which, after processing, yield the coordinates of the unknown point. The observation, or occupation, time varies from about 20 minutes to a few hours, depending on the distance between the base and the remote receivers (i.e., the baseline length), the number of visible satellites, and the satellite geometry. The measurements are usually taken at a recording interval of 15 or 20 seconds, or one sample measurement every 15 or 20 seconds.