Differential Global Positioning System (DGPS)
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Still, these interferences equally effect neighboring points; thus, the relative positioning (the position of the points to each other) is far more precise than the absolute position (in the WGS 84). Here, “neighboring” means distances to about 100 km. The functional principle of DGPS is to quantify the deviation of the measured position to the actual coordinates on the earth's surface.
For this reason, a GPS positioning to a point with known absolute coordinates is conducted simultaneously to the current GPS positioning. From the difference of the known (correct) coordinates and those obtained from the measuring to the reference point, the deviation can be calculated and used for the correction of other points.
The accuracy gained through DGPS depends on the receiver, the applied DGPS method and the distance to the reference station.
There are three available methods for DGPS measurement:
- Real-time positioning: Fixed positioning of the GPS receiver on a defined point and registration of the position. Direct radio contact to the GPS as the two mensurations have to take place simultaneously.
- Use of fixed reference stations (providers are available in many regions and usually with costs)
- Real-time: reception of the correcting information via radio broadcasting and instant, corrected positioning in the field.
- Postprocessing: Measuring with GPS, recording of all points with the exact time of measurement. Subsequent correction of all points based on this information, which is recorded by fixed referenced stations.
The service provides a real-time connection for the most regions, which is broadcasted via the radio broadcasting network for defined frequencies (VHF and long wave). The density of the stations account for up to 60 km. Sapos provides different data sets, which may be used for real-time correction or postprocessing, depending on format and transmission.
- sapos.de - Satellitenpositionierungsdienst der deutschen Landesvermessung (german)