Guidelines for the use of GNSS in surveying and mapping

The phase centre variation is now modelled in one
of two ways; either relative to a well-defined
standard antenna, or absolutely through using
either real or simulated signals. The ability of a
software processing package to be able to include
these antenna models may be of particular benefit
to users. It allows the combination of the user’s
receiver data with data from existing networks of
permanent GNSS receivers, which may be of many
different types. It is recommended that the surveyor
confirms that this facility is available within any
software to be used for advanced processing of
projects which include data from different receiver
types. For RTK use, it is also important to
understand which antenna is being used at the
base – if this is different from the rover, then the
antenna phase centre bias should be taken into
account for the most demanding accuracies.
(Surveyors should make themselves aware of the
current (2009) advances within absolute phase
centre research and its applications. More
information can be found at
www.research.cege.ucl.ac.uk/GNRG/index.html)
5.3.4 Use of GNSS under vegetation cover
GNSS has been proven to still work well under
vegetation cover, such as a tree canopy. However,
there may be a significant loss of signal and a
corresponding increase in the signal/noise ratio
(SNR). This is particularly dependent on the
moisture content of the vegetation, and on the type
of vegetation. The higher the water content, the
greater the signal loss. At the limit this may result in
complete signal loss, which will cause the loss of
satellites from the position solution, particularly in
dynamic surveys when the GNSS equipment is
moving to capture the survey data.
In these cases it is far better to establish tertiary or
secondary control using a medium precision survey
method at the edges of the vegetation where there
is a clear view of the sky. Further control can then
be added under the vegetation canopy using a
traditional traverse and spirit levelling method.
Detail survey observations should also be carried
out under the vegetation canopy using a traditional
method, as RTK methods are not satisfactory in
these marginal conditions.
GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING |41