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PCCC GIS I Exercise 2 (AV9.1) - Locate your car with GPS, map it with GIS v2, Apr 2007 - page 3 / 31

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You are asked for the closest big city (choose Newark or New York using the Cities button):

  • Click OK. The Plan-Session window will open, and a Status screen will come up.

  • Select Graphs > Number SVs and PDOP. The following screen will appear, showing the

number of satellites (Nsats) visible from Midnight to Midnight on that date:

  • Print this double graph that shows the Nsats (number of satellites) over the next 24 hours.

Step 2 - Collect field data with GPS receivers Procedure: Go outside, and get away from buildings and thick overhanging tree cover.

A

general rule of thumb is that the GPS antenna, in the unit or external, must "see blue sky".

It is

not necessarily leaf cover and moisture that knock out a GPS signal.

When in dense forest it is

critical to get an extension pole and mount an external antenna 10 feet off the ground to get above the first layer of tree branches. Buildings present major problems in urban areas too.

  • Turn the receiver on with the power button

(lower RH corner of receiver).

  • Press SYS until you see a circle with black squares to check your satellite coverage. It might take a few (3-5) minutes for the receiver to reach out and "find" the signals from the satellites, so just be patient. Use your Nsats printout as a guide: As long as you are not obstructed, you should ultimately be able to see the same number of satellites on the GPS unit as that predicted by the Nsats graph.

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