Leaders in Pipeline Survey Systems
OPERATOR’S MANUAL
Issue 4.04 - 1207 34
4.9. Notes on GPS
GPS is an abbreviation for Global Positioning System. There are 24 satellites in low-earth orbit.
The GPS receiver in C-Scan receives data from a number of these (maximum 12) depending on
which satellites are ‘in-view’.
When the GPS unit is first turned on, the GPS unit attempts to acquire the satellites ‘in-
view’. Acquisition times vary from 10 seconds to 45 minutes. If the detector has been
moved a large distance or it is the first time the detector has been used it is more likely
that the acquisition time will be nearer to 45 minutes. It is important that the detector
(with GPS ON) be switched on for at least 15 minutes after a position is displayed. This
will ensure a fast turn-on next time.
The minimum number of satellites needed to calculate a position is 3. The more satellites that
are acquired, the more accurate the displayed data will be.
It is very important to check whether the position displayed is the current position. This can be
checked by pressing GPS SAT key in the search screen.
This will show the GPS status and the number of satellites acquired. In the example (above
right) the status is GPS:GOOD SAT:#6.
If GPS: BAD is shown it means that the current position is incorrect, or that the satellites have
not been acquired or have lost acquisition.
If GPS: OLD is shown it means that the data is more than 10 seconds out of date. As the data is
normally updated every 1 second, this could mean that acquisition is about to be lost.
If a point is stored with a BAD or OLD GPS position, the position displayed is likely to be that of
the previous point. This would create a distance of zero. C-Scan picks up on this and will not
allow the operator to use the GPS position; however, it will be recorded. The operator must then
enter the distance manually, or go back to the SEARCH screen and wait for acquisition.
Loss of GPS acquisition is usually caused by the antennae being temporarily obscured; either
by buildings or being inside a vehicle. If the GPS has been on since a good position indication,
the re-acquisition time is a few seconds.
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