Precise Real-Time Positioning Using Network RTK - Global Navigation Satellite Systems: Signal,Theory and Applications - page 170

selected as the nearest continuously operating reference station to the user. The range
difference between the VRS (point i) and master station (point j) with respect to satellite ( s )
reads:

∆R R R

(6)

Where:
R satellite to VRS range
R satellite to the master station range
∆R range difference
The interpolated distance-dependent errors, dispersive δ∆r and non-dispersive δ∆r or

their total, at the VRS are added to the master station's observations to generate the VRS
observations on a satellite-by-satellite and epoch-by-epoch basis for L1 and L2 frequencies,
such that:

I

∆R δ∆r

I δ∆r ∆T /λ

(7)

P P ∆R δ∆r

I δ∆r ∆T

(8)

where ΔT ji represents the difference in the modelled part of the troposphere, which is
usually subtracted before computation of the network errors, and thus need to be
reconsidered. Previous testing of the VRS systems for kinematic ground surveying showed
that system positioning accuracy was typically 1-2 cm in plane coordinates and 3-5 cm in
height (El-Mowafy et al ., 2003). The raw measurements or their corrections are sent to the
rover in RTCM version 2.1 format using message types 18/19 or 20/21, respectively. If the
former types are used, the constructed VRS observations are computed by the network
centre whereas if the latter message types are used, the user has to compute the VRS
observations.

A variation to the VRS concept is the Pseudo Reference Station approach (PRS), where the
virtual reference station is taken at a pre-selected grid point instead of the approximate
position of the user. The virtual observations in this case will also refer to a non-physical
reference station. At start of the survey, the baseline length is typically a few metres for the
VRS approach and could be several kilometres for the PRS method.

The basic advantages of the VRS mode are that it does not need software changes in the user
receiver, and no special formats and conventions are needed. However, a main drawback of
the method is the presence of a restriction on the number of users according to the capacity
of the network processing centre due to the fact that VRS observations are customised for
each user. For kinematic applications, re-determination of VRS may be needed according to
the distance between the user and the VRS. In addition, if RTCM message types 18/19 are
used, the user will have no information about error sizes, which always helps in
interpretation and analysis of positioning results.

5.2 The Area-Parameter Corrections-Flächenkorrekturparameter (FKP) method
The F lächen k orrektur p arameter (FKP) or “Area-Parameter Corrections” method represents
the network information using coefficients of a surface centred at the location of a physical
reference station (Wübbena and Bagge, 2006). Raw reference station observations and