Concept and Comparison Of VRS , MAX , iMAX , FKP , NEAREST in GPS Networks
The virtual reference station (VRS), or virtual base station (VBS), idea, introduced by Trimble, is that a base station is artificially created in the vicinity of a rover receiver. All baseline-length-dependent errors, such as abnormal troposphere variation, ionospheric disturbances and orbital errors, are reduced for this VRS. The rover receiving VRS information has a lower level of these errors than a distant base station. The VRS is calculated for a position, supplied by the rover during communication start-up, with networking software. The VRS position can change if the rover is far away from the initial point. The format for sending the rover’s position is standard NMEA format. Most rovers receive VRS data for a calculated base station that is within a couple of metres away. The VRS approach requires bi-directional communication for supplying the rover’s position to the networking software.
For broadcast communication mediums, pre-defined cells, which may be created manually by the network operator, can be used to transmit master-auxiliary corrections, known as MAX, to the rovers. The rover user can connect to the correction service that is most relevant for their geographic location. Depending on the size of the network, multiple cells can be defined to optimise the transmission of data by reducing the number of stations that are contained in the correction messages. In the case of two-way communications, Leica GPS Spider will automatically select the optimum sites for the cell used to generate master-auxiliary corrections for each rover. This correction service is referred to as Auto-MAX. By choosing the most appropriate cell configuration, Auto-MAX corrections minimise the bandwidth required to transmit the corrections. The master station is always chosen as the station nearest to the rover. The auxiliaries are chosen from the surrounding stations to provide the best possible set of corrections for the rover’s position. With Auto-MAX even the largest reference networks can be fully serviced with a single communication channel. The MAX corrections contain the full information from the cell and therefore provide the maximum level of accuracy and reliability for the rover. With MAX, the network operator has to ability to transmit corrections using both two-way and broadcast communication technologies.
The iMAX idea, introduced by Leica Geosystems, is that networking software corrections, based on the rover’s position, are calculated as with VRS. However, instead of calculating the base station observations for the provided position, or another position closer to the base station, original observation information is corrected with the calculated corrections and broadcast. VRS works so that although the rover is unaware of errors the VRS is taking care of, there still might be ionospheric remains in the base station observations. iMAX provides actual base station position information. The rover may assume the base station is at a distance and open its settings for estimation of the remaining ionospheric residuals. The iMAX method may trigger the rover to open its settings further than required since the networking software removes at least part of the ionospheric disturbances. However, compared to VRS above, this approach is safer since it notifies the rover when there might be baseline-length-dependent errors in the observation information. iMAX requires bi-directional communication to the networking software for supplying the base station observation information.
With FKP methodology, a model of the distance dependent errors (“Flächen-Korrektur-Parameter” FKP) is transmitted to the rover. The interpretation of the FKPs and the individualization of the corrections are done at the rover, i.e. the rover can for itself compute the individualized corrections in the same way as VRS/PRS or alternatively use its own algorithms, which might be better adjusted to the RTK algorithms of the rovers. Thus the FKP method provides to the rover more information than VRS method, so that the rover in principal can get more from the received data. FKPs describe the (horizontal) gradients of the corrections. The correction of a (real) reference station is used in combination with the FKPs to compute the individualized corrections for the rover position.
In the simplest case the FKPs describe a linear dependency of the corrections from the position (linear FKP). The FKPs then define an “inclined plane” for the corrections, centered at the (real) reference station. Per satellite and per frequency two parameters (inclination in North-South and in West-East) are required. FKP models of higher order are possible. Note that the validity of linear FKP is limited to about 100km radius, due to the fact that the physical error sources would lead to noticeable non-linear effects over longer distances.
In this case, the system remains the same as a base and rover mode and acts as a single station, and the rover device is connected to the nearest reference station and takes corrections.
It is suggested that you use this method if you are out of the network.