DGPS vs RTK

Standalone GPS positioning relies only on the receiver range measurements and is limited to an accuracy of a few meters. The error budget of range measurements includes components, which are correlated for all the receivers operating in the same area (e.g. satellite clock and orbit errors and atmospheric delays). These errors can be removed if a static receiver with a known position (base station) transmits information about these errors to a user receiver (rover). The category of positional techniques based on this principle is called “differential positioning”.

In standard DGPS (Differential GPS) technology, only corrections to C/A code pseudoranges are being transmitted, which brings rover positional errors down to values about 1m. DGPS is widely used by navigation users, in particular for urban transportation and in offshore areas. The remaining DGPS error source is multipath, which can be reduced by the use of special multipath mitigation methods, such as Septentrio’s APME technique, used in PolaRx2.

High-precision navigation/surveying applications require RTK (Real-Time Kinematic) technology, which is based on the use of carrier phase. Carrier phase measurements are extremely precise (down to the fractions of millimeter), but they contain an unknown integer initialization constant, the so-called “phase ambiguity”. Therefore RTK positioning has to resolve integer ambiguities to achieve the high level of precision. PolaRx2 implements a state-of-the-art ambiguity resolution algorithm, a modification of the well-known LAMBDA method, developed at Delft University.

The compensation of atmospheric effects in traditional DGPS/RTK is not complete and the induced errors increase with the baseline due to error decorrelation. For baselines of more than 15-20 km, ambiguity fixing is less reliable during periods of high ionospheric activity. There exist a number of so-called Network RTK techniques, where information from a network of base stations is used to better predict the variations of ionosphere delays and orbit errors. In Europe a particular type of Network RTK based on so-called FKP corrections is widely used. FKP corrections contain information, which allows the user to compensate satellite orbit and ionospheric errors as a function of baseline length and effectively fix ambiguities for longer baselines than with a usual RTK.

Internationally accepted data transmission standards for DGPS are defined by RTCM (Radio Technical Commission for Maritime Services), particularly by its Special Committee SC-104. Currently version 2.3 of the DGPS standard is widely used. At the time of this writing (May 2005), the new version of the standard, 3.0, has recently been released. The version of RTCM3.0 for Galileo is now in the making. Except for RTCM, there exist other proprietary DGPS standards, such as Trimble’s CMR.

Due its high sensitivity and low measurement noise, PolaRx2 receivers are widely used as DGPS/RTK rovers/base stations, in particular for precise farming, reference networks and survey applications. The same receiver can be configured either as a base station or as a rover by user commands. Our users will always have access to the latest developments in the DGPS/RTK technology. At the time of this writing, the users of PolaRx could use RTCM 2.3 and 3.0, CMR, FKP. Through its membership in RTCM SC-104 committee, Septentrio is participating in the further development of DGPS/RTK standards.