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Promark2 User’s Guide
signals transmitted by satellites approximately 21,000 km from earth. Being of relatively high
frequency and low power, these signals are not very effective at penetrating through objects
that may obstruct the line-of-sight between the satellites and the GPS receiver. Virtually any
object that lies in the path between the GPS receiver and the satellites will be detrimental to the
operation of the system. Some objects, such as buildings, will completely block out the
satellite signals. Therefore, GPS can not be used indoors. For the same reason, GPS cannot be
used in tunnels or under water. Other objects such as trees will partially obstruct or reflect/
refract the signal; reception of GPS signals is thus very difficult in a heavily forested area. In
some cases, enough signal can be observed to compute a rough position. But in virtually every
case, the signal is not clean enough to produce centimeter-level positions. Therefore, GPS is
not effective in the forest.
This is not to say that your ProMark2 surveying system can only be used in areas with wide-
open view of the sky. GPS can be used effectively and accurately in partially obstructed areas.
The trick is to be able to observe, at any given time, enough satellites to accurately and reliably
compute a position. At any given time and location, 7-10 GPS satellites may be visible and
available for use. The GPS system does not require this many satellites to function. Accurate
and reliable positions can be determined with 5 satellites properly distributed throughout the
sky. Therefore, an obstructed location can be surveyed if at least 5 satellites can be observed.
This makes GPS use possible along a tree line or against the face of a building but only if that
location leaves enough of the sky open to allow the system to observe at least 5 satellites.
For the above reasons, make every effort to locate new points to be established in areas where
obstructions are at a minimum. Unfortunately, the site location is not always flexible. You
may need to determine the position of an existing point where, obviously, the location is not
debatable. In situations were an existing point is in a heavily obstructed area, you may be
forced to establish a new point offset from the existing point, or preferably a pair of intervisible
points, and conventionally traverse to the required point to establish it’s position.
Be aware that obstructions at a GPS data collection site will affect the observation time
required to accurately determine it’s location. Obstructed areas will require longer observation
times. The Observation Timer function of the ProMark2 will automatically extend observation
times at obstruction sites but in some cases, it may not extend the observation period long
enough. You will have to use your own judgement of observation times when surveying
obstructed site. Your judgement will improve through experience.
For large surveys utilizing 3 or more ProMark2 receiver systems, you may want to recon all of
our site locations as part of your survey planning. This will eliminate any delays during the
actual execution of the survey if problems are encountered finding an appropriate site. The
more receiver systems utilized during the survey, the harder the task of coordinating the data
collection becomes. Remember, data must be collected simultaneously between points where
a vector is desired. If one receiver operator is late in starting data collection due to problems
with site location, this could cause problems.
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