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For highest precision, use your GPS during optimum GPS weather
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Where are those GPS Sats?
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The GPS satellite constellation attempts to ensure at least six satellites are visible from most parts of the world at any time. Depending on the local terrain, buildings, urban canyons, etc. the actual visibility of the GPS satellites from your position may be much less than needed for optimum accuracy from your GPS unit.
Fortunately, Trimble has a free software tool available that makes knowing the "GPS Weather" straight forward. It is called "Trimble Planning Software" and is available from...
Since the satellites orbits change a tiny amount over their lifetime, you will need to ensure you have the latest Epheremis File also available from Trimble's web site at...
After downloading and installing the program, the first thing you will want to do is load up the lastest SSF Epheremis file by going to the toolbar, Almanac, Import, SSF.
Then you will want to setup your local position in the File | Station menu. For now just pick one of the default Station Names. I set mine to New York. I set the date to July 7, 2006. I set the start time to 8AM, the duration to 12 hours and the interval to 10 minutes. I also set the Elevation cutoff to 15 degrees. Select OK. Now we are ready to analyze this day's GPS coverage in New York.
Select Graphs | Visible Satellites | GPS. You should see something like this...
This graph represents the number of satellites visible to your receiver over a twelve hour period. Notice the time around 1700 and 1800 hours. Those red areas represent a time when there are only 5 satellites in view above the 15 degree horizon mark. For casual GPS use, this may be just fine. For those times when you are using GPS for precise measurements, you might have some difficulity during the low satellite number times of the day.
Let's have a look at another graph called Position Dilution of Precision or PDOP.
PDOP is the figure of merit based on how spread out in the sky the GPS satellites are. The wider the angle between them, the better the trig accuracy resulting in a lower PDOP. Plus, comparing it to the visibility graph, a high number of viewable satellites does not mean PDOP is going to be good. Look at the 1400 hour time. PDOP soars to over 20 making its position accuracy much worse than usual.
There are many other graphs available to view with this tool. Also, in the Station Editor window, you can define a horizon outline using the "Obstacles" tool to represent the conditions of your GPS location.
As you can see, the "GPS Weather" is important to understand. This software is a must have tool if you are serious about getting the most out of your GPS equipment and need it for precise location information.
Being free software, this program represents a great value.
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