To schedule a GPS survey the following factors need to be taken into account:

• Satellites are not normally tracked below an elevation of 15° to 20° due to large atmospheric refraction errors at low elevation angles.
• There is a 24 hour observation window for GPS.
• At least 30 minutes of four (or more) satellite coverage is the accepted norm for standard static GPS surveys. There are periods when many more satellites are visible.
• The satellites' positions in the sky are predictable. They can be computed and output in a convenient graphical form, and taken out into the field during reconnaissance.
• A popular representation of satellite availability is the skyplot, which is a plot of satellite tracks on a zenithal projection centred at the GPS ground station. The satellite azimuth and elevation is shown as a function of time. Figure below is an example of a skyplot with a whole day's satellite groundtracks plotted.

Figure 1. An example of a GPS skyplot.

Skyplots can be used during reconnaissance to ensure that the satellite signals from the selected satellites can be acquired by the receiver. The obstructions at a site can be plotted on a zenith plot independently of a skyplot as in Figure 2 (or a photograph of the zenith could be taken with a fish-eye lens!), and during the survey planning stage a decision can be made on which satellites to be tracked at the sites by comparing Figures 1 and 2 (remembering that only common satellites between sites are useful!). Skyplots annotated with possible obstructions are also useful during actual data tracking because if the data quality is poor (as indicated by the signal-to-noise ratio displayed by the receiver during operation), the possible source of interference can be identified and an appropriate note made to assist subsequent data processing.

Figure 2. Plotting signal obstructions on a skyplot format.

How to best use skyplots during reconnaissance? While GPS manufacturers software can generate the plots, it is not convenient to go into the field with a computer and printer! A compromise is therefore to use a manual method based on a series of skyplots (generated for a representative site such as the centroid of the network), for each hour. At each site the obstructions can then be mapped, and if there are a significant number of potential signal obstructions, they may be visually compared with the various one hour skyplots and the best tracking period selected (that with the minimum obstructions).

How may the obstructions be mapped during reconnaissance? The basic tools are a compass and a clinometer, and every field party should be equipped with them. However, if the site location has not yet been fixed with certainty, as may be the case with sites in urban environments, the reconnaissance assumes greater importance. As there may be many candidate locations for each site, some shortcut techniques should be considered. For example, keep several graphs handy that relate distance to and height of a potential obstruction, to the elevation angle that it subtends (see Figure 3). Efficient reconnaissance techniques can be developed if the heights of buildings and trees are known, and distances are easily measured using, for example, a handheld laser rangefinder.

Figure 3. Graph a) shows height of an obstruction as a function of its distance and
elevation angle, and
Graph b) shows the elevation angle of an obstruction as a function of its distance and height.

The pattern of signal obstructions at a site is very sensitive to the height of the antenna. By increasing the height of the antenna in urban areas using some form of telescopic pole, the extent of signal obstruction may be greatly reduced, as dramatically indicated in Figure 4 below.

Figure 4. A sequence of skyplots showing obstruction masks and groundtracks of visible satellites for antenna heights of 1.5, 5 and 10m respectively. Note the number of visible satellites increases from four (at an antenna height of 1.5m) to nine (at an antenna height of 110m).

When preparing skyplots the following factors should be kept in mind:

• Only approximate coordinates of the intended GPS station are required. Only one such plot need be produced if the GPS network has sides of the order of 100km.
• One skyplot can be used for an entire observing campaign, as the satellite tracks across the sky repeat daily, except that the satellite passes over the same ground point four minutes earlier each day.
• The preparation of skyplots requires approximate satellite ephemerides. Different skyplot generating software may require different satellite data. The most convenient is to use an old Navigation Message (collected during a previous GPS survey or during pre-mission testing). This will contain the necessary GPS Almanac data.
• The Navigation Message or Almanac need not be the most up-to-date. For planning purposes even messages a month old (or longer) may still be used. (Check that satellites have not been moved, or new ones launched in the meantime!)