u-blox ZED-F9P Integration Manual page 111

ZED-F9P - Integration Manual
The picture below shows the network of reference stations used to build up a service covering the
whole of Switzerland.
Figure 81: VRS network in Switzerland
The network of receivers is linked to a computation center, and each station contributes its raw data
to help create network-wide models of the distance-dependent errors. The computation of errors
based on the full network's carrier phase measurements involves, first of all, the resolution of carrier
phase ambiguities and requires knowledge of the reference station positions.
The latter is usually determined as part of the network setup.
This generated VRS data is then sent to the user through a wireless connection, often using the
Networked Transport of RTCM via Internet Protocol (NTRIP). Finally, just as if the VRS data had
come from a physical reference station, the rover receiver uses standard single-baseline algorithms
to determine the coordinates of the user's receiver, in real-time kinematic or post-processed modes.
The main purpose of a VRS station is to reduce the baseline distance between the rover and
the reference station in order to efficiently remove spatially correlated errors using differential
processing, and to incorporate error corrections obtained from the reference stations network.
The Rover receiver must send its own actual position back typically using the NMEA GGA message.
Usually if the VRS system does not receive a GGA message it will not provide the RTCM data to
the Rover. The VRS concept allows a less dense reference network without accuracy degradation
because the multiple reference station network better models the spatially correlated GNSS errors
over longer baselines.
As a result the maximum distance between the rover and the nearest reference station can be
extended beyond the typical 10 ~15 kilometers without accuracy degradation of the single reference
station case. Another benefit of a VRS is that the reference data are free of site-specific errors such
as multipath, because the VRS computation assumes that the virtual station is situated at an ideal
location.
UBX-18010802 - R02 10 Appendix Page 111 of 114
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