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 by GPS.
  • 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 1).
  • 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" method.
  
• 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