Sect_5.5.2

5.5.2 Modern GPS Surveying: Field Procedures

RAPID STATIC GPS SURVEYING

Static positioning with short observation times of 5-20 minutes (vs 1-2 hours) ... giving centimetre accuracies!

Also referred to as fast-static or quick-static. The following characteristics distinguish rapid static techniques from other methods of GPS surveying:

Observation time requirements: These are significantly shorter than for conventional GPS surveying, and are a function of baseline length, number of satellites being tracked and satellite geometry. Typically the receivers need only occupy a baseline for a period of 5-20 minutes (the lower value corresponding to baselines <5km and tracking six or more satellites; the upper value for longer baselines up to 20km, and/or where tracking is to only four satellites).
Hardware requirements: Different GPS "products" have different hardware requirements. In some systems only dual-frequency phase measurement is sufficient, in system configurations dual-frequency pseudo-range measurements are also required. To date there is no "mixing" of receivers and software as in the case of conventional GPS surveying. The exact configuration is of course dependent on the software being used. For example, if the software for rapid static is quite sophisticated then there is less reliance on specialised top-of-the-line hardware. (For example, rapid static results have been obtained with single frequency phase data.) However, for other rapid static "products", full four observable instrumentation (L1 & L2 phase, P1 and P2 pseudo-range; or L1 & L2 phase, C/A pseudo-range and P1-P2 observable) is a prerequisite.
Specialised software: The basis of this technique is the ability of the software to resolve the ambiguities (determine their integer values) using a very short observation period. There is a variety of software, with different characteristics and levels of sophistication, but the fundamental requirement is a fast ambiguity resolution capability.

The field procedures are much like those for conventional static GPS surveying except that the occupation times are shorter, the baselines should be comparatively short, the satellite geometry favourable and signal disturbances such as multipath should be a minimum. Since rapid static surveying is a relatively new technique it is not possible to define exactly how much data needs to be collected in order to produce quality baselines every time, and with high reliability. Instrument manuals typically give guidelines. Some receivers give an audio and/or visual indication when enough data has been collected in the field (but this cannot be confirmed until the data is downloaded and processing is completed). If the real-time mode is employed (the base station transmits tracking data to the mobile unit where it is processed immediately) then the "data quantity gamble" for rapid static GPS surveying can be overcome.

Such a technique is well suited for short range applications such as control densification and engineering surveys, or any job where many points need to be surveyed (see Figure below). Unlike the "kinematic" and "stop & go" techniques there is no need to maintain lock on the satellites when moving from one station setup to another.

Field procedure for the "rapid static" surveying technique.

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