Geology Reference
In-Depth Information
horizontal HDOP, vertical VDOP and time TDOP. VDOP may not be im-
portant if elevations are not critical, but where an accurate 3D position is
required, as in the case of the void detection survey in Table 15.1, a minimum
of four satellites are needed. Most receivers also record a signal-to-noise ra-
tio (SNR) and the number of satellites actually used to determine position.
To ensure that recorded co-ordinates remain within acceptable tolerances,
a reference point, which could also be a geophysical drift-measurement or
nulling base station, should be revisited twice daily for comparison of the
computed positions.
15.2.5 GPS as a timing device
Every GPS receiver in contact with a satellite is automatically synchronised
to Greenwich Mean or Coordinated Universal Time (GMT or UTC). GPS
receivers can therefore be used for synchronising base station and roving
field magnetometers, or different instruments on multi-instrument platforms.
So precise are the times that they can even be used to synchronise transmitters
and receivers in electromagnetic surveys, removing the need for a cable link
to provide a phase reference.
15.2.6 GPS-plus
No single navigation system can provide accurate positioning all the time and
everywhere. The coming decade is likely to be the decade of GPS-plus rather
than GPS-only, with integrated GPS
Inertial Navigation Systems (INS),
GPS
+
GLONASS, Galileo and COMPASS and GPS
+
WiFi, to name but
a few of the possibilities. GPS
+
INS combines the GPS receiver with a
gyroscope, distance encoder, 3D accelerometers and a 2D digital compass to
infill any gaps in GPS coverage. It can be particularly useful where satellites
are obscured in parts of a survey area by topography or buildings. Multiple
technologies are currently expensive, but during the lifetime of this edition
will almost certainly fall in price and become more readily accessible.
+
15.2.7 GPS-minus
Reliance on new technology can be taken too far. In a micro-gravity survey
on a flat site where elevations of closely-spaced stations are required to
centimetre accuracy, the traditional optical level and graduated staff will
often still provide the required information in the quickest, cheapest and
most accurate fashion.
