Government policies (smart tracker GPS ) currently define a standard positioning service (SPS) based on the C/A-code observations and a precise positioning service (PPS) based on P(Y)-code observations. SPS and PPS address “classical satellite” navigation methods where one receiver observes several satellites in order to determine its geocentric position, using the broadcast ephemeris. Typically, a position is computed for every epoch of observation. The advantages of relative positioning have long been recognized as a way to satisfy the high accuracy requirements of geodesy, surveying, and other geosciences. In relative positioning, also called differential positioning, the relative location between co-observing receivers is determined. In this case many common errors cancel, or their impact is significantly reduced.
During the pioneering years of GPS tracker app , there appeared to be a clear distinction between applications in navigation and surveying. This distinction, if ever real, has rapidly disappeared. Whereas navigation solutions used to incorporate primarily pseudorange observations, surveying solutions have always been based on the millimeter-accurate carrier phase observations. Modern approaches combine both types of observables in an optimal manner. This leads to a unified GPS positioning theory for both surveying and navigation. The availability of precise, postprocessed ephemerides—even predicted precise ephemerides—allows for single-point positioning that is better than specified for SPS or even PPS. Powerful processing algorithms reduce the time required for data collection, so as to render even the distinction between static (both receivers are static) and kinematic (at least one receiver moves) techniques unnecessary.
The achievable accuracy very much depends on many factors that will be detailed throughout this book. In order to emphasize the characteristic difference between geocentric and relative position accuracy, let us simply state that geocentric position accuracy ranges from meters to decimeters, whereas the relative position accuracy is at the centimeters to millimeters level. The secrets that make Car GPS tracker device such a powerful positioning device can be readily explained. At the center is the ability to measure carrier phases to about 1/100 of a cycle, which equals about 2 mm in linear distance. The high frequencies (L1 and L2) penetrate the ionosphere relatively well. Because the time delay caused by the ionosphere is inversely proportional to the square of the frequency, carrier phase observations at both frequencies can be used to model and, thus, eliminate most ionospheric effects.
Dual-frequency observations are particularly useful when the station separation is large and when shortening the observation time is important. There has been significant progress in the design of stable clocks and their miniaturization, providing precise timing at the satellite. The GPS satellite orbits are stable because at such high satellite altitudes only the major gravitational forces affect their motion. There are no atmospheric drag effects acting on satellites. The impact of the sun and the moon on the orbits is significant but can be computed accurately. The remaining worrisome physical aspects are solar radiation pressure on the satellites, as well as the tropospheric delay and multipath effects on signal propagation. On the algorithmic side, much is gained by using linear combinations of the basic phase observables.