5.2.4 GPS Survey Planning

INSTRUMENTATION & PERSONNEL CONSIDERATIONS

Instrumentation considerations as they relate to the project planning process include:

• Number of available GPS receivers: the larger the number of receivers in a session, the larger the number of directly connected stations, and hence a better network, faster progress and a less expensive survey. However, there may be a restriction on the availability of receivers and field parties, and, in addition, the logistical problems quickly multiply. The optimum number of receivers appears to be of the order of four to six.
  
• Receiver type: all geodetic GPS receivers produce, in principle, similar datasets and hence similar final accuracies. The mixing of different brands of receivers however can cause problems. For example, most data processing software is instrument-specific. This is particularly true for modern GPS surveying techniques (section 5.5.1).
  
• Single or dual-frequency receivers: dual-frequency instruments permit compensation for the ionospheric delays on the GPS signals, hence they are essential for high accuracy applications. They are usually of little benefit for baselines <30km. dual-frequency instrumentation is generally necessary if modern "rapid static" survey techniques are being used. Guidelines may be found in the prescribed GPS survey "standards & specifications" (section 10.2.5). Mixing of single and dual-frequency receivers, as in the situation of mixing receiver types, is not recommended.

Sample field deployment plan.

Session	Time	Field parties

		A	B	C	D
295-1	1000 1100	7352	20882	62755	15007
295-2	1110 1210	7352	20882	20888	70936
295-3	1515 1615	15001	20877	20888	70936
295-4	1800 1900	15001	20877	70418	62500

As noted earlier, many errors in field procedure can be detected if multiple station occupancies are incorporated into the GPS survey plan. Errors may also be minimised through adequate training of field personnel. If the field staff are unfamiliar with the GPS instrumentation to be used for a survey it is even more important to insist on pre-mission testing as part of the survey planning procedure (section 4.4.2). This testing could be:

• over a micro-network of several metres in extent, for example, a square or other regular figure, for which the "ground-truth" can be directly obtained by precise distance and/or angle measurement, or
• over a special permanent test network, perhaps previously surveyed by GPS, or
• a zero-baseline, or
• an effective simulation of the planned survey, in which lines of similar length are surveyed using procedures close to those to be applied during the actual survey. This is the most expensive option.

Such testing will establish:

• the proper functioning of antennas, receivers, data storage, cables, power supplies,
• health and availability of satellites,
• the competence of field parties,
• the proper functioning of field and office software and procedures, and
• the competence of the computing staff.

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© Chris Rizos, SNAP-UNSW, 1999