SKPOS®

The concept of the Nearest reference station

From its launch until 2024, the Slovak Real-Time Positioning Service provided services exclusively using the network RTK method in the concept of a virtual reference station (VRS). This concept compensates the lack of the classic RTK method, where the accuracy decreases with increasing distance from the reference station. At the time of launch of the SKPOS operation, the SKPOS infrastructure consisted of only 21 reference stations that received data only from the GPS and GLONASS satellites. Therefore, it was an appropriate decision to provide services only in the VRS concept.
With the increasing number of reference stations and with the launch of Galileo and BeiDou satellite systems, generating a network solution in the VRS concept has become more and more difficult, and it is not possible to generate a network solution from all received satellites. Therefore, in the vicinity of reference stations, it may currently be more appropriate to use data directly from the reference station with the full number of satellites.

The concept of the nearest reference station is based on the automatic selection of the nearest reference station according to the position of the moving rover. The rover sends its approximate position in the form of NMEA GGA messages to the SKPOS control center via the Internet just like the VRS concept. The software in the SKPOS control center automatically identifies the nearest SKPOS reference station based on this position and sends corrections directly from this station back to the rover in the RTCM 3.4 MSM7 standard. The service is available via mountpoint SKPOS_CM_NS_34_MSM7, while it does not use a network solution and does not generate a virtual reference station, but corrections are sent directly from the nearest reference station. Due to the different distribution of masses in the atmosphere and different conditions between the location of the reference station and the moving rover, the method is reliable up to a maximum distance of 20 km from the reference station.

Advantages of the nearest reference station concept:

• data is sent from all satellites that are tracked by the nearest reference station,
• less demanding on the computing power of the control software, as there is no need to generate VRS with complex algorithms,
• the user does not need to select a specific reference station from the list, because the control software automatically identifies the nearest reference station based on the approximate location sent by the user,
• the SKPOS administrator still receive the information about the location and quality of measurements, as it is necessary to send this information in the form of NMEA GGA message, just like with the VRS concept.

Disadvantages of the nearest reference station concept:

• the user have to carefully monitor the distance of the rover from the nearest reference station from which it receives corrections, as the accuracy of its measurement decreases with increasing distance from that station,
• the concept is reliable up to a distance of 20 km from the reference station,
• the user also have to monitor the status of the reference stations, since in the event of a failure of the nearest reference station, he will receive data from a more distant one, which may cause a decrease in accuracy because of the greater distance and different conditions.

If you measure in a distance of maximum 20 km from the reference station and your GNSS equipment is capable to track all GNSS, or if you measure in difficult conditions (e.g. the horizon is significantly obscured by trees, buildings or other obstacles) and you want to use as many satellites as possible.

No. The concept of the nearest reference station is an additional service to the network solution when we need to reach a larger number of satellites.

Due to different conditions at the measurement location and at the reference station, the results may not be sufficiently accurate and reliable. Therefore, we do not recommend measuring at a distance of more than 20 km.

Before measuring, it is necessary to check the distance of the location of interest from the reference station in the map below or determine the distance based on the published coordinates of the reference stations.

In the rover settings, it is necessary to select the mountpoint SKPOS_CM_NS_34_MSM7. Next, for Trimble receivers, select the broadcast format "Multi station (RTCM)" and for Leica receivers, select Receive RTK corrections from the "nearest" network. For other brands of receivers, after choosing the SKPOS_CM_NS_34_MSM7 mount point, the setting must be consulted with the equipment distributor. The remaining settings are the same as for other SKPOS services.

No. The concept of the nearest reference station is available to everyone with a valid SKPOS_cm contract.

Test of the Nearest reference station concept

The aim of the test was to compare the qualitative parameters of the currently used VRS concept with the concept of the nearest reference station. The measurement was carried out at class C points of the National Spatial Network (ŠPS) in various parts of Slovakia. The premise of the nearest reference station concept test was that with increasing distance from the reference station, the accuracy of the measurement will decrease with the concept of the nearest reference station. The test measurements were carried out in six locations at 33 points of the ŠPS and were located mainly in the areas with the greatest distance from the reference stations.

Locations where the nearest reference station concept was tested

At each point of the ŠPS, two measurements were performed in the concept of VRS and two measurements in the concept of the nearest reference station. In both cases, corrections were received in RTCM 3.2 MSM5 format, which included GPS, GLONASS, Galileo and BeiDou satellite systems. The measurement was carried out under ideal, but also under very difficult conditions (obstacles above horizon). A comparison of the results of all performed test measurements obtained using the VRS concept and the nearest reference station concept can be found in the table below.

Testing showed an average increase of 4 satellites, a 1 second reduction in initialization time, and a 0.1 reduction in PDOP. If the measurement is made at distance of less than 20 km from the nearest reference station, the results are practically identical to those obtained using the VRS concept. At a distance greater than 20 km, the results using the nearest reference station concept achieve satisfactory values in general, but number of outliers increased. Therefore, following the performed tests, the concept of the nearest reference station is suitable to be used as a supplement to the used VRS concept for measurements located within 20 km of the reference station. Using the concept of the nearest reference station, we can track more satellites, which can be advantageous for precise measurements in more difficult conditions, with significant obstacles above the horizon.

• RTK measurement test of the nearest reference station concept in SKPOS - Text | Presentation
• Testing of the SKPOS services - student bachelor thesis

Map of locations within 20 km from reference stations