possible ionospheric and tropospheric delays, which can occur as the signal travels from the satellite to
the receiver antenna.
f. Determine the Number of Required Receivers. The minimum number of receivers required to
perform a differential GPS-S is two. The actual number used on a project depends on the project size
and the number of available instruments and operators. Using more than two receivers will often
increase productivity and field-observation efficiency. Some kinematic applications require two
reference receivers (set at known points) and at least one rover.
g. Determine Personnel Requirements. Personnel requirements are also project-dependent. Most
GPS equipment is compact and lightweight and only requires one person per station setup. However,
when a station is not easily accessible or requires additional power for a data link, two individuals may
be required.
h. Determine Vehicle Requirements. Normally, one vehicle is required for each GPS receiver used.
Vehicles should be equipped to handle the physical conditions that may be encountered while
performing the field observations. In most cases, a two-wheel-drive vehicle should be adequate. If
adverse site conditions exist, a four-wheel-drive vehicle may be required. Adequate and reliable
transportation is important when the observation schedule requires moving from one station to another
between observation sessions.
i. Determine Electrical Requirements. Adequate power should be available for all equipment (such
as, receivers, computers, and lights) that will be used during the observations. PCs, software, and data-
storage devices should be available for on-site field data reduction. Other equipment should include
tripods, tribrachs, tape measures, flags, flashlights, tools, equipment cables, a compass, and an
inclinometer. A data link is also needed if real-time positioning is required.
j. Determine Observation Schedules. Planning a GPS-S requires that the surveyor determine when
satellites will be visible for the given survey area. The first step in determining observation schedules is
to plot the satellite visibility for the project area. Even when the GPS becomes fully operational, a full
two-hour coverage of at least four satellites may not be available in all areas. Station occupation during
each session should be designed to minimize travel time and to maximize the overall efficiency of the
survey. Determination of session times is based mainly on the satellite-visibility plan, with the
following factors taken into consideration:
The time required for safe travel between survey sites.
The time to set up and take down the equipment before and after the survey.
The time of the survey.
EN0593
5-22
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