Geoscience Reference
In-Depth Information
The PPS has access to both codes and provides accuracies down to the
meter level.
Differential GPS
Selective availability (SA), the deliberate degradation of the point position-
ing accuracy by “dithering” (i.e., distorting on purpose) the satellite clock
(called
δ
-process) and manipulating the ephemerides (called
ε
-process), has
led to the development of differential GPS (DGPS). Only the basic idea is
explained here.
DGPS is based on the use of two (or more) receivers, where one (station-
ary) reference or base receiver is located at a known point and the position
of the (mostly moving) remote receiver is to be determined. Using code pseu-
doranges, at least four common satellites must be tracked simultaneously at
both sites. The known position of the reference receiver is used to calculate
corrections to the observed pseudoranges. These corrections are then trans-
mitted via telemetry (i.e., controlled radio link) to the roving receiver and
allow the computation of the rover position with far more accuracy than for
the single-point positioning mode.
Using DGPS based on C/A-code pseudoranges, real-time accuracies at
the 1-5 m level can be routinely achieved. Phase-smoothed code ranges yield
the submeter level (Lachapelle et al. 1992). Even higher accuracies can be
reached by the use of carrier phases (precise DGPS). For ranges up to some
20 km, accuracies at the subdecimeter level can be obtained in real time (De-
Loach and Remondi 1991). To achieve this accuracy, the ambiguities must
be resolved “on the fly” and, therefore, (generally) dual-frequency receivers
are required. Furthermore, five satellites per epoch are required.
After the deactivation of SA in May 2000, DGPS must be seen from a
different viewpoint. The increased point positioning accuracy achieved with
a single receiver may suce for some kinds of applications.
Relative positioning
At present, highest accuracies are achieved in the relative-positioning mode
with observed carrier phases. Relative positioning is associated with base-
lines, i.e., the three-dimensional vector between a known reference station
and the location to be determined. Processing a baseline requires that the
phases are simultaneously observed at both baseline endpoints (Fig. 5.1).
Originally, relative positioning was only possible by postprocessing data.
Today, (near) real-time data transfer over short baselines is routinely possi-
ble, which enables real-time computation of baseline vectors and has led to
the real-time kinematic (RTK) technique.
