I had thought that GPS satellites were geosynchronous, thus allowing a person to know where he or she is whenever turning on a GPS unit. However, I learned this morning from a surveyor -- as he fretted about how quickly the maximum number of satellites overhead was diminishing -- that this is definitely not the case. True, there are GPS satellites above throughout the day, but the number (and thus the density, and thus the accuracy of readings) of overhead GPS satellites changes changes throughout the day, and certain times of day are, thus, more accurate than others.
Well, when might one know when the greatest accuracy for a particular region might be? Well, there are some websites that show information about where satellites are, but I don't know if any that actually will say, "For coordinates X & Y, the greatest number of GPS satellites overhead will be at time Z." However, the satellite position webpage will tell you where the satellites are right now, and this satellite prediction website will tell you which satellites will be "overhead" at a calculated future time (location is automatically determined by your IP address). Not as user-friendly (and non-geeky) as I basic user like me would like, but it is still very useful, and I will have to make sure that I use it to figure out when future GPS work will be most accurate (given the canopy cover, which will seriously limit the usability of the GPS until the late fall).
2 comments:
Most proper GPS units do have a screen showing you how many satellites they're acquiring, and the related precision of the positioning. Try Garmin's Rino series.
I've always seen those bar graphs on the Garmins that show satellite acquisition. However, I thought that they were just trying to link up with the satellites in geosynch. I should have paid more attention to what it all meant, I suppose...
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