Geology Reference
In-Depth Information
over intervals ranging from 10 to 45 minutes. This is the highest level of
post-processed GPS accuracy. The base station, which must log dual-
frequency GPS data at a frequency no lower than 0.03 Hz, must be
located at a precisely surveyed point within 10 km if accuracies of a few
centimetres are required.
5.
In some countries there are networks of GPS reference stations (VRS
or
Virtual Reference Stations
) that, if sufficiently dense, can provide the
high-accuracy corrections required for RTK positioning. These are only
available as subscription services.
There are, of course, advantages and disadvantages to each method. Ded-
icated base stations introduce complications into survey operations that field
crews may well feel they can do without, and post-processing is therefore
often favoured. Government systems are free but currently cover only lim-
ited areas. Use of a VRS relies on a cell-phone link to allow corrections to be
continuously downloaded, and this may not be available throughout the sur-
vey area. The charges for network systems such as VRS and for commercial
satellite services are significant, but can be well worth paying if productivity
is improved by eliminating the need to set up base stations. Carrier-phase
data are more difficult to collect in environments where satellite signals are
subject to interruption, and under these conditions the use of code-phase
data alone may be the more practical option. Remember that the theoretical
locational accuracy of a given GPS system is not necessarily achievable at
your site.
15.2.4 Ensuring a GPS is fit for purpose
The locational accuracy required for a geophysical survey obviously de-
pends on its objectives. These should especially be borne in mind when
using instruments with built-in GPS receivers. The fact that a receiver is
integrated into a geophysical instrument should not be taken as evidence
of its adequacy for all surveys using that instrument. Moreover, there is no
point in generating very accurate target co-ordinates using sub-metre RTK
positioning if a hand-held non-DGPS system will subsequently be used to
dig or drill on those targets.
Most GPS receivers provide a quality-control parameter such as '%Pre-
cision' for positions recorded in real time, and can be programmed so that
locations are not recorded if this parameter falls outside a predetermined
range. Planning software provided by most instrument vendors can be used
to predict when a factor called the DOP (dilution of precision), which is
based on the number and positions of satellites in the sky and their rela-
tionships to each other, will fall outside an acceptable level. The lower the
DOP, the better the signal quality. The DOP can be further broken down into
