When using a network approach, expressing reliably GNSS position and velocities in a given reference frame (ITRF2014, IGS14, ETRF2000 or ETRF2014) requires the identification of ‘stable’ and ‘reliable’ reference stations. The choice of these reference stations can have a non-negligible impact on the estimated positions and velocities and of course on the derived geodynamic interpretations.

The EPN network has been set up to offer such reference stations: the reference positions and velocities of the EPN stations are kept up to date and are available through the EPN multi-year position and velocity solution. However, not all EPN stations are by definition suitable of reference stations.

To help the identification of the best EPN reference stations, a new station classification was developed (More about the station classification). In addition of the classification, we also developed a web tool to help the user to select the the most optimal EPN reference stations in a considered area and for a given observation time.

This page explains what is behind the pre-selected EPN stations proposed by the tool.

Figure 1: Algorithm that allows to define if a station is recommended, usable or not recommended as reference station.

The user enters the begin and end date of his densification solution. Based on this time span, the actual observation time of a station in the EPN solution and the class of that station, different rules are applied (Figure 1) in order to define if this EPN station is recommended, usable, not recommended or excluded as reference station.

The ideas behind those rules are:

• The user is applying the same outliers and discontinuities as the EPN multi-year solution.
• For position solutions, stations with noisy time series or exhibiting large seasonal signals are not the best candidates as reference station. That is the reason why C2, C5 and of course C6 stations are not recommended.
• For position and velocity solutions:
  • Stations with less than 3 years in the considered time span are excluded.
  • If the station has the same observations in the EPN and in the densification solution, the chance of having a good agreement between both is maximized. Therefore, only EPN stations with the C6 label are not recommended as reference stations. C5 stations are usable while C0, C1, C2, C3, C4 are recommended as reference stations. Of course, this is only valid if the same rejection and discontinuity policy as in the EPN solution is applied.
  • If the station has not the same observations in the EPN and in the densification solution, then the user must choose the most stable EPN stations.
    The shorter the time series is, the best and the most stable the EPN station should be.
    • If the time series of the station has more than 6 years of data within the considered time span, then C0 and C1 are recommended. C2, C3 and C4 stations are usable and C5 and C6 are not recommended.
    • If the time series of the station has less than 6 years of data within the considered time span, then C0 and C1 are recommended. C2 is usable but C3, C4, C5 and C6 are not recommended.