What is DGPS?
Differential GPS (DGPS), or indeed any differential global navigation satellite system GNSS2(DGNSS) in general, greatly increases the potential accuracy of either GPS or GLONASS. By determining the major error sources at a base (or reference) station and applying the corrections thus derived to any other station (which may be mobile), these errors can be largely eliminated. If the correction values are supplied to the mobile station in real time, its position can also be determined in real time to the required accuracy. With code-only observations one metre accuracy can be achieved. With carrier phase observations the accuracy can be increased to better than one centimetre, so this carrier phase differential (CPD) GPS has sometimes been described as precise DGPS (PDGPS).
1 GLONASS: Russian satellite navigation system, similar to the American GPS.
2 GNSS: Global Navigation Satellite System, teh generic term for all satellite navigation systems such as GPS and GLONASS.
Correction data formats
GNRT, in its base station configuration, is designed to generate the necessary correction signals for receiver observations in real time. The most widely used international standards in this area are defined in formats developed by the Radio Technical Commission for Maritime Services (RTCM)3. RCTM-2.0 essentially deals with DGPS (code-only) corrections, while RTCM-2.1 incorporates several enhancements, especially for PDGPS (carrier phase) corrections. With RTCM-2.2, the GLONASS system is also supported. In between RTCM 3.x is in use.
A GNRT base station generates all corrections in a form compatible with the RTCM standard. In addition, for certain supplementary information, GNRT makes use of the free entry field facility in RTCM format, in order decisively to improve the accuracy and availability of the correction signal. The resulting format, known as Geo++Ã‚Â®-RTCM++, enables a simple DGPS to achieve an accuracy of a few decimetres. For PDGPS applications (with centimetre accuracy), the use of RTCM++ with the same information content results in a reduction of data volumes by a factor of four compared with the use of RTCM-2.1/2.2. This makes real-time position fixing possible even with restricted communications bandwidths, for example at 2400 baud (bits per second) in the VHF band (150 MHz).
GNRT works best with the correction signals broadcast (in RTCM-AdV format) from reference stations being set up by the association of German state survey departments (AdV), which similarly allows RTCM++ to be used.
3 RTCM: Radio Technical Coommission for Maritime Services RTCM Spezial Commitee No. 104 has published different versions of RTCM recommended standards for differential NAVSTAR GPS services, indicated here as RTCM 2.x as well as RTCM 3.x.