Geology Reference
In-Depth Information
is generally to 1 m, or its rough equivalent (0.000 01
◦
) in latitude and
longitude.
Accuracies of the order of 1-2 m can be obtained using differential GPS
(dGPS or DGPS) methods. These apply distance corrections from external
reference systems operating at surveyed locations and can eliminate errors
that are common to both the base station and field receiver. The corrections
canbeappliedby
post-processing
, but can also be carried out in real-time
using corrections transmitted via satellite or from a local base station to a
field receiver.
Even greater accuracies can be obtained using the phase of the carrier wave
on which the PRC is superimposed. The GPS carrier frequency of 1.57 GHz
implies, in principle, an ability to provide accuracies better by several orders
of magnitude than the 1-MHz PRC modulation but, because each cycle of
the carrier wave is almost identical to the previous one,
cycle-skipping
is
possible, introducing errors that are multiples of 20 cm. Real-time kinetic
(RTK) systems eliminate cycle-skipping by using a base station receiver and
transmitter in a known location to re-broadcast the phase of the carrier to the
mobile unit(s). Dual-frequency receivers that track a second carrier-wave
phase signal (L2) as well as the standard code- and carrier-phase (L1) signal
are used if even higher accuracy is required.
A technique called kinematic carrier-phase tracking (KCPT) utilises the
L1 information to provide centimetre-scale locational accuracies but may re-
quire long (20-30-minute) initialisation periods to give an accurate position.
RTK solutions can also be obtained using additional KCPT measurements
on L2, providing the highest real-time accuracies currently available.
Multi-path errors
(i.e. reflections from topography or buildings that pro-
vide alternative paths of different lengths) and variations in the proper-
ties of the atmosphere can significantly reduce accuracy. The main at-
mospheric effects occur in the ionosphere and depend on the magnitude
and variability of the ionisation. They are thus most severe during periods
of high solar activity, and particularly during magnetic 'storms' (see Sec-
tion 3.2.4).
It is now often possible to obtain fixes through the canopy in rainforests,
but buildings or solid rock between receiver and satellite are still insuperable
obstacles. Interestingly, GPS receivers installed on smart phones can outper-
form even the most expensive RTK systems in the urban jungle, due in part
to multi-path errors and weak direct satellite signals in the urban environ-
ment and thanks to technologies such as assisted-GPS (A-GPS), where cell
phone transmitters also broadcast GPS satellite orbit information. Between
2007 and 2010, more GPS receivers were built into cell phones than were
used in all the other applications put together.
