Correction data formats
GNREF is designed for the computation in real time of the necessary correction signals for observations using a very wide variety of types of receivers (code, carrier-phase, single- or dual-frequency, GPS and/or GLONASS). The most widely used international standards in this area are defined in the RTCM format, developed by the Radio Technical Commission for Maritime Services (RTCM). 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. GNREF can generate all corrections compatible with the RTCM standard.
In addition, for certain supplementary information, GNREF 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 RTCM++, enables a simple DGPS, even without carrier phase measurement, to achieve an accuracy of better than one metre. For PDGPS applications (with centimetre accuracy), the use of RTCM++ 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).
For reference station services such as the SAPOS DGPS service, being set up by the association of German state survey departments (AdV),it is also possible to encode the correction signal in a special format such as RTCM-AdV, so as to exercise access control and hence to introduce a charging procedure. At the mobile station the RTCM correction data are converted back into RTCM-2.1/2.2 without loss of information. (Similar systems are also emerging in other countries and internationally).
Types of receivers
GNREF is designed to be receiver-independent. Thus, in principle, all major receiver types, including Ashtech, Leica, Topcon (Javad) and Trimble among others, are supported by GNREF (see below). This includes both dual and single frequency receivers, and all available information (code or carrier phase) can be processed. As well as GPS, GNREF also supports both GLONASS and hybrid GPS/GLONASS receivers.
GNREF can transmit the computed correction signals through a variety of channels, or even several channels simultaneously. Connection options currently available include by modem, including connections over mobile telephone networks such as those belonging to D-Netz, or by VHF or UHF radio transmission using a radio modem. The latter is especially suitable for local reference stations with only one, or a small number of, roving stations, while the former is in use primarily in connection with permanent reference stations, for example those of national or regional mapping organizations. By means of time-slice technology, multiple transmitters can work with a single frequency in spite of overlapping areas of operation. Further interfaces such as Internet connections using TCP/IP or NETBIOS can be exploited by means of the GNCOM communications module.
GNREF offers the possibility of storing all of the raw data and correction signals generated at a reference station. This may be necessary for post-processing, or desirable in general as proof of observations or as a back-up record. The raw data at the receiver can generally be stored in either receiver-specific or standard RINEX format, or as RTCM++, RTCM-AdV or Geo++®-Minimum RTCM. It can then be prepared for downloading over the Internet (using GNWEB).
GNREF operates in a graphic user interface environment and is presented to the user through various status and control windows.
GNREF is organized so that it can also be operated by data transmission from a remote control station. Access can be effected either from fixed computer networks or using special modem connections. The supplementary GNROM software permits the central configuration, control and monitoring of all of a service supplier’s GNREF stations.
Linking of reference stations
With the addition of GNNET software, several GNREF reference stations can be linked so as to improve accuracy and increase redundancy. The reference stations then communicate with each other in real time in order to derive optimal correction values for a given area of operation and to further reduce the effect of the distance of roving stations from reference stations. Through GNNET the so-called area correction parameter is generated. This is transmitted over the data link to the reference stations and included with the normal RTCM signal to the users. Alternatively RTCM corrections and area correction parameters can be transmitted direct from a central station. It is also possible to generate virtual reference station (VRS) data. All of the data links can be managed with the GNCOM supplementary software, using network protocols such as TCP/IP or NETBIOS.
With the GNCIM and GNRIM supplementary software, extensive functions for integrity monitoring are made available so as to maximize reliability, especially of the real-time transmission. By making comparisons between the stations involved, any problems can be quickly recognized and the station responsible can be identified, so that appropriate measures can be taken automatically.
Prediction of correction data
With the GNPRED supplementary software, RTCM correction data can be predicted by the roving user for anticipated reception epochs. Thus the effect of the latency, or lack of currency, of the correction data in an RTK solution is minimized.
GNREF is independent of receiver type, and supports the major international standards for GPS and DGPS (RINEX, NMEA, RTCM). GNREF makes use of all the potential of a modern PC-based multi-tasking operating system (OS/2 or Windows NT) and its graphic user interface. GNREF is based on modular architecture and thus has the flexibility to accept enhancements by the addition of new components at any time.
Today GNREF is already installed in many reference stations, for example those of the German state survey departments’ satellite positioning service (SAPOS®), and suffices for all the requirements of a reliable and permanent DGPS base station. Moreover, future developments for the further improvement of the DGPS service will be capable of integration with GNREF.
So called area correction parameters (Flächenkorrekturparameter FKP) are generated by GNNET. These FKPs are transmitted to the reference stations and from there transmitted to the rover users, together with the normal RTCM signals. GNREF can alternatively provide correction data for a so called virtual reference station (VRS), computed by GNNET.