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Cofrin Center for Biodiversity


There are 25-30 functioning satellites in the US Global Positioning System. Each one orbits the earth twice a day following one of six prescribed orbital paths. That means that the number of satellites that are visible to a GPS receiver and the geometry of the satellite constellation are changing constantly. For some kinds of work, this variability needs to be taken in to account and measures for working around it are referred to as "planning".

Why would the GPS gods choose such vague terminology? The fact that the same function was called "mission planning" back when the system was owned by the Defense Department may have something to do with it. Anything more informative and they would have to kill you.

Recreation-grade GPS Work
The geometry of the GPS satellite constellation is not usually a significant factor when preparing for natural resources field work. The worst possible geometry is not catastrophic for the normal performance of the system. However, an awareness of constellation geometry can be very helpful on projects requiring high quality mapping work in tree cover.
Trimble's GNSS Planning Online website offers a fast and easy way to predict when satellite geometry might be a problem. The example shown at right is for the UW-Green Bay campus on Thanksgiving morning, 2013. PDOP (position dilution of precision) is represented by the green line and values over 2.0 are bad news for working in tree cover.

Submeter GPS Work
What is true of planning for recreation grade work is also true for submeter work. The Internet-based planning utility described above can be used, but submeter receivers have a built-in planning function.