5.3.3 GPS Survey Reconnaissance

STATION SELECTION, ACCESS & MARKING

This information should be clearly stated in words as well as described in some graphical form. This is critical for minimising down-time due to difficulties in finding stations, or if access involves caretakers (for example, to visit the roofs of buildings), etc.

As with details of the station access and point description, the area around the site should be studied carefully. Depending on the aim of the project, and its accuracy requirements, this task may be very elaborate and include, for example:

• Investigating the provision of on-site power.
• Testing soil stability and defining the appropriate antenna mount (tripod, pillar, etc.).
• Noting the presence of any potential multipath causing structures.
• Noting any UHF, TV, radio, microwave or radar transmitters (they could affect a receiver's operation).
• Establishing permanent monumentation -- using previous marks helps avoid this.
• Establishing nearby azimuth marks.
• Clearing the area of possible obstructions caused by trees or shrubbery.
• Taking photographs of the surrounding area, including any tree cover.

What is a good site? No signal obstructions above 20°. No multipath causing surfaces, such as metallic fences, structures and water surfaces. No nearby electrical installations, such as high tension cables, radio/radar/TV transmitters.

In some countries, account may have to be taken of the season. For example, tree cover may be much thicker during summer than at other times of the year (Figure below). Care should therefore be taken to ensure that site selection (during reconnaissance) is not carried out during the time of year when density of foliage is less than what will be experienced during the survey.

In spring, the branches of trees may still be bare (left), while in summer the trees are covered with thick trees.
(LACHAPELLE & HENRICKSEN, 1995)

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© Chris Rizos, SNAP-UNSW, 1999