GPS (Global Positioning System) was an artificial satellite system being composed of 24 artificial satellites
covering entire earth. The system can guarantee that at any moment, any point on earth, and at least four
satellites can be observed contemporaneously, to ensure that the satellites can be collected to the latitude,
longitude and height of the observation points, in order to achieve navigation, positioning and timing
capabilities. GPS global positioning system was consisted of three parts: the space part - GPS constellation;
ground control part - the ground monitoring system; users of the equipment - GPS signal receivers. GPS
positioning technology with the advantages of all-weather, high coverage, high precision, high efficiency and
low cost, has been more wide range of application in all types of geodetic control network to strengthen reform
and establishment, and in highway engineering survey and large-scale structures of measuring deformation
measurement.
GPS positioning including absolute positioning and relative positioning:
Absolute positioning methods: absolute positioning also called single-point position, it refers to directly
determine the coordinates of observation stations with the coordinate system whose center was the Earth's
center as coordinate origin. The principle was to take distance between consumer receiver air wire and GPS
artificial satellite observation amounts as basis, and according to the known instantaneous satellite coordinates,
to determine the user receiver antenna at the corresponding point coordinates. The microtime satellite bell and
the receiver bell were difficult to keep strict synchronism, so the distance between the actual observation points
and the satellite contains the distance affected by bell difference; this distance was called "pseudo-range ". The
satellite bell error can be corrected by bell difference parameters of navigation message, but the receiver clock
error can not know in advance, so it ought to go to find the solution together as a unknown with the
three-dimensional coordinates of observation points in the data processing, so a Real-time observation point to
solving four unknowns using at least four satellites which were observed at the same time.
The relative positioning was used two GPS receivers placed in the two ends of the baseline, in parallel, the
same GPS satellite observations, to determine the relative position of a baseline in the coordinate system or
baseline vector. Because in two or more simultaneous observation points observing the same satellite can
effectively eliminate or weaken the satellite orbit errors, satellite clock error, the impact of poor receiver bell.
At present, China's crustal movement monitoring was static relative positioning using this method, the accuracy
of10-8~10-9.