Tracking device has been very widely applied in several areas

GPS is a Global Positioning System based on satellite technology. The fundamental technique of GPS is to measure the ranges between the receiver and a few simultaneously observed satellites. The positions of the satellites are forecasted and broadcasted along with the GPS signal to the user. Through several known positions (of the satellites) and the measured distances between the receiver and the satellites, the position of the receiver can be determined. The position change, which can be also determined, is then the velocity of the receiver. The most important applications of the GPS are positioning and navigating. Through the developments of a few decades, GPS tracker is now even known by school children. GPS has been very widely applied in several areas, such as air, sea and land navigation, low earth orbit (LEO) satellite orbit determination, static and kinematic positioning, flight-state monitoring, as well as surveying, etc. 

electronic tracking devices has become a necessity for daily life, industry, research and education. If some one is jogging with a GPS watch and wants to know where he is located, what he needs to do is very simple; pressing a key will be enough. However, the principle of such an application is a complex one. It includes knowledge of electronics, orbital mechanics, atmosphere science, geodesy, relativity theory, mathematics, adjustment and filtering as well as software engineering. Many scientists and engineers have been devoted to making GPS theory easier to understand and its applications more precise.

Galileo is an EU Global Positioning System and GLONASS is a Russian one. The positioning and navigating principle is nearly the same compared with that of the US GPS system. The GPS theory and algorithms can be directly used for the Galileo and GLONASS systems with only a few exceptions. A global navigation satellite system of the future is a combined GNSS system by using the GPS, GLONASS and Galileo systems together. In order to describe the distance measurement using a mathematical model, coordinate and time systems, orbital motion of the satellite and GPS observations have to be discussed. The physical influences on GPS measurement such as ionospheric and tropospheric effects, etc. also have to be dealt with. Then the linearised observation equations can be formed with various methods such as data combination and differentiation as well as the equivalent technique. 

It is well-known that differential GPS positioning results depend on the accuracy of the reference station(s). However, it is not quite clear how strong this dependency is, or in the other words, how accurate the reference coordinates should be determined for use in kinematic differential positioning. During AGMASCO data processing, it was noticed that the accuracy of the reference coordinates is very important. A bias in the reference station coordinates will cause not only a bias in the kinematic flight path, but also a significant linear trend. Such a liner trend depends on the flight direction and the location of the reference receiver(s). Therefore, in precise kinematic positioning, the coordinates of the static reference station should be carefully determined by, for example, connecting these stations to the nearby IGS stations. A detailed study of the relationship between the accuracy of the reference station coordinate and the quality of kinematic and static positioning has been carried out by Jensen (1999).

More GPS tracking solutions at http://www.jimilab.com/ .