Guidelines for the use of GNSS in surveying and mapping

In addition, phase centre variations (section 5.3.3)
may not be included in the software models. In
real-time surveys, the manufacturers often use a
bespoke data format for the transmission between
base and rover. Thus, it is extremely unlikely that a
rover unit from one manufacturer will work with the
base station from another. Even if such an
arrangement is claimed to work by the
manufacturer, it is not recommended as best
practice.
Apart from the above two reasons it may simply be
found on completion of the fieldwork that data from
one manufacturer’s receiver will not load correctly
into processing software from another
manufacturer. Data may subsequently be available
in RINEX format, but manufacturer-specific
information can be lost in conversion to this format.
Advanced processing techniques may be needed
for RINEX datasets, as specialist knowledge can be
needed to reduce the possibilities for error.
5.3.3 Antenna phase centre variations
Until the late 1990s, antenna phase centre
variations were not modelled as part of the GNSS
processing techniques. Such variations of the
electrical phase centre of a GNSS antenna are a
function of the elevation and azimuth angle
between the receiver/antenna and a satellite. These
variations are particularly evident in the vertical
direction and differ in amplitude and phase for
different types of GNSS antenna. If the same type
of antenna is used on a relatively short baseline,
the antennas’ phase centre variations will be
cancelled out because both antennas have
practically the same elevation angle to a particular
satellite. However, as baseline length increases, the
elevation angles to a satellite from both ends of a
baseline differ and the antenna phase centre
variations become more significant. Furthermore, if
different GNSS receiver/antenna types are being
used, then the different antenna phase centre
variations will certainly not be cancelled.
In studies, Schupler and Clark used laboratory
(chamber) tests to determine the elevation and
azimuth angle dependent antenna phase centre
variations for a Trimble 4000 SST/SSE and a Rogue
Dorne-Margolin antenna. They showed that the
vertical differences between these two antenna
types can reach 35mm at an elevation angle of
approximately 55 degrees. Even more important,
for high accuracy height determination, is the fact
that this effect can be worsened through the use of
tropospheric scale factors, without antenna phase
centre modelling.
Figure 5.1 below shows the phase centre variations
between the Trimble GNSS antenna and the Rogue
GNSS antenna. These biases feed directly into the
end co-ordinates and so are very important to
quantify and correct for in high-accuracy surveying.
The two curves show how the phase centres vary
on both the L1 frequency and dual (L1/L2)
frequencies, plotted against satellite elevation. It
should be noted that this diagram is just one
example of this particular operational difficulty, and
antennas from other manufacturers will exhibit
different behaviour when compared against each
other. In some cases the differences may be
negligible, whilst in others they may be more than
those shown in the diagram.
Figure 5.1: Vertical differences in the phase centres of a Trimble 4000 SST/SSE Geodetic antenna and a
Rogue Dorne-Margolin antenna
40 | GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING