5.5.4 Modern GPS Surveying: Field Procedures
"STOP & GO" GPS SURVEYING TECHNIQUES
|Centimetre accuracy positioning during very short static observation
periods (<1minute) ... receiver moves carefully from point to point ...|
This is a true kinematic technique because the receiver continues to
track satellites while it is in motion. It is known as the "stop
& go" (or semi-kinematic) technique because the coordinates of
the receiver are only of interest when it is stationary (the "stop"
part), but the receiver continues to function while it is being moved (the
"go" part) from one stationary setup to the next. There are in
fact three stages to the operation:
- The initial ambiguity resolution: This is carried
out (generally in static mode) before the "stop & go" survey
commences. The determination of the ambiguities can be carried out using
any method, but in general it is one of the following:
- A conventional static (or "rapid static") GPS survey determines
the baseline from a fixed receiver to the first of the uncoordinated sites
occupied by the second "roving" receiver. An ambiguity-fixed
solution provides the integer values of the ambiguities.
- Setup both receivers over a known baseline, possibly surveyed previously
- Employ a procedure known as "antenna swap". Two tripods are
setup a few metres apart, each with an antenna on them (the exact baseline
length need not be known). Data is collected by each receiver for a few
minutes (tracking the same satellites). The antennas are then carefully
lifted from the tripods and swapped, that is, the receiver 1 antenna is
placed where the receiver 2 antenna had been, and visa versa (see Figure
below). After a few more minutes the antennas are swapped again (Figure
- The most versatile, and most recent, technique is to resolve the ambiguities
"on-the-fly" (that is, while the receiver is turned on but the
receiver/antenna is moving).
Figure 1. The antenna swap procedure for initialising ambiguities.
- The receiver in motion: Once the ambiguities have
been determined the survey can begin. The roving receiver is moved from
site to site, collecting just a few minutes of phase data. It is very
important that the antenna continues to track the satellites. In this
way the resolved ambiguities are valid for all future phase observations,
in effect converting the ambiguous carrier phase data to unambiguous "carrier-range"
or "phase-range" data (by applying the integer ambiguities as
data corrections). As soon as the signals are disrupted (causing a cycle
slip) then the ambiguities have to be reinitialised (or recomputed). This
can most easily be done by bringing the receiver back to the last surveyed
point, and redetermining the ambiguities by the "known baseline"
- The stationary receiver: The "carrier-range"
data is then processed in the double-differenced mode to determine the
coordinates of the roving receiver relative to the static reference receiver.
The trajectory of the antenna is not of interest, only the stationary
points which are visited by the receiver.
The technique is well suited when many points close together have to
be surveyed, and the terrain poses no significant problems in terms of signal
disruption (usually an audible signal is emitted by the receiver when
it has lost lock on the satellites). The survey is carried out in the manner
illustrated in the Figure 2 below, and the ambiguities reinitialised using
any of the techniques shown in Figure 3.
Figure 2. Field procedure for the "stop & go" surveying technique.
Figure 3. A variety of reinitialisation techniques for "stop &
go" or "kinematic" surveys.
One particular negative characteristics of this technique is the requirement
that phase lock must be maintained by the roving
receiver as it moves from site to site. This requires special hardware mounts
on vehicles if the survey is carried out over a large area.
An additional requirement is that the stationary reference receiver
must continue to track all the satellites being tracked by the roving receiver.
The accuracy attainable is about the same as for the "rapid static"
technique. As with the "reoccupation" technique, the receiver
must have the ability to handle data files from several different sites.
The software then has to sort out the recorded data for the different sites,
and to differentiate the "kinematic" or "go" data (not
of interest) from the "static" or "stop" data (of interest).
It can be implemented in real-time if a communications link is provided
to transmit the "carrier-range" data from the reference receiver
to the roving receiver(s).
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© Chris Rizos, SNAP-UNSW, 1999