Before delving into the specific advantages of PRS30 or PRS32 compared with VRS30 or VRS32, for example, it is important to understand the difference between these two types of mounting points linked to the format of the corrections specific to NRTK technology:

• PRS (Pseudo Reference Station): PRS uses the FKP approach to individualise ionospheric and geometric errors on a single physical reference station to calculate corrections in real time. The position of the PRS station used to calculate the corrections is offset by several kilometres from the user’s position.

• VRS (Virtual Reference Station): In contrast, the VRS uses the same error compensation technique but the position of the VRS station is located in the user’s working area with an initial vector of 0 km corresponding to the user’s first approximate position. These virtual stations are designed to compensate for the effects of atmospheric disturbance and other sources of error, but the difference in interpretation of the corrections by the mobile receiver plays a major role here.

• Baseline: Distance between the user’s GNSS receiver and the reference station. Accuracy is often correlated with its length. The greater the distance to the station, the worse the accuracy of the receiver. However, this is not always the case and there are thresholds where this distance has no significant impact for the user.

Thanks to the monitoring carried out by our engineering teams, we have observed that during major ionospheric disturbances, PRS correction profiles are better interpreted by GNSS receivers. This significant improvement is mainly due to the method of calculating the algorithms used by our servers, combined with that of the GNSS mobile receivers.

In this way, the length of the baselines is processed in the best possible way by combining the algorithm used on the TERIA side and that of the mobile receiver.

When a user positions himself in a VRS or PRS network, the receiver calculates his position based on a model generated from a multitude of reference stations. Each of these stations acts in a specific way depending on its equipment, its geographical position, the environment of the installation and the atmospheric conditions (meteorological and ionospheric). All these data are combined using several models in real time.

During strong ionospheric disturbances, the fluctuations are greater and the gradient is larger in terms of both temporality and topography. So the models, adjusting in real time, are able to model errors with larger residuals.

In future articles, TERIA will provide information for qualifying TERIA GNSS corrections, enabling users to quantify this information in real time as a function of their position.

The PRS solution is based on VRS technology. Thanks to a technical trick, it enables the mobile receiver to better evaluate model errors and residuals.

In standard RTCM messages, these residuals are transmitted to the mobile receiver via specific messages. In RTCM 3.4, this information also includes the GALILEO and BEIDOU constellations, in addition to GPS and GLONASS, which have been available since previous versions.

As mentioned above, in VRS mode, the distance between the mobile receiver and the VRS virtual station is 0 km. However, by observing this zero distance, the algorithms can unfairly consider that the corrections are perfect with no residuals. So, by virtually creating a factor of several kilometres, the mobile receiver is forced toestimate errors, as if it were operating in conventional RTK mode. This trick hassignificantly improved user performance, both in terms of fixed RTK availability and accuracy.

When ionospheric activity remains low, the difference in performance between VRS and PRS is unquantifiable, so it is especially useful to migrate to a PRS solution when ionospheric activity becomes high.

As we wrote in our article on solar activity, all receivers are affected by the ionosphere, some more than others. It is therefore important for the user to remain vigilant when taking measurements. You can check the ionospheric disturbance index at any time via our Android application iTERIA.

To complete this post, it should be noted that LEICA receivers use IMAX / MAX technology, so it is preferable to use a mounting point associated with this technology: IMAC32 or MAC30.