s, again, there's a good sky view on the other side.
A GPS may work okay in a wooden structure on an upper level and near windows, in open forest, a canyon or even in a city between tall buildings, but the accuracy will be compromised.
And the GPS won't work well in thick forest and not at all in metal buildings away from windows, in tunnels, caves, under water, enclosed vehicles or in parking garages, for example.
For the best accuracy, your GPS should receive signals from six satellites but no less than four. Your unit will have a screen that displays the number of satellites it's making a connection with at any time.
How A GPS Works
Here are the basics:
Your GPS unit communicates with satellites, which send out radio signals. Since the speed of those radio signals is a known factor, the receiver is able to use that speed to calculate its distance--and therefore the distance of the one holding it--from a satellite. This calculation puts you and the GPS in an imaginary sphere.
So imagine yourself holding your GPS and standing in a big circle (See: "A" in the image below) on the ground. This reflects the information from a single satellite.
The GPS will do the same thing with the other satellites with which it can communicate via a clear line-of-sight. So for the second satellite, imagine another circle which intersects with the first. (See: "B" below) This second calculation will put your near the intersection of the two circles (See: "A^B").
Then Add a Third satellite
and another intersecting circle.....
This third calculation will place you in the intersection of the three circles. This is considered a "2D" (or two-dimensional) mode, figuring your location horizontally only. In this case, you can input your elevation, if you know it, for more accuracy with the GPS unit. Keep in mind that 2D navigation can be unreliable.
If your GPS can communicate with 4 or more satellites, your location will be more accurate and you'll be in "3D" mode, which will include an elevation calculation. This is what you're aiming for.
Your unit display will indicate whether you are in 2D or 3D mode. If it's displaying 2D, try moving around and getting into a more open area, free of obstacles like tall trees, to hopefully get to 3D mode if at all possible.
**You should always check your "EPE" or "Estimated Position Error," which will display on your GPS, usually right on the same screen that indicates how many satellites it's communicating with--the satellite page. This will also show you how accurate, or perhaps inaccurate, the position fix is. An EPE of 20 feet or less is good. The lower the EPE the better.
A Word On Satellite Configuration
And why it matters
Where in the sky the satellites your GPS is communicating with are located relative to where it -- and you -- are located is important when it comes to the accuracy of the information the unit will provide. And your handheld unit will display that configuration on the satellite page. Again, this is page where you can determine how many satellites the GPS is connecting or communicating with and the Estimated Position Error (EPE). So this is yet another piece of information right on one screen.
The satellite screen is the first screen you should use when you fire up your GPS.
The satellite configuration is usually depicted within two concentric circles on the screen, with the satellites represented by dots or numbers. The inner circle on the screen represents the sky more above you than to the sides. The outer circle represents the horizon.
If your GPS is connecting with four or more satellites and is in 3D mode, and your EPE is low, that means your position fix will be good. And a good satellite configuration is even better.
What you want to see, ideally, is that one or two satellites are overhead -- within that inner circle -- with the rest spread out around the sky, somewhere in that outer circle. If all of those satellite "dots" or numbers are in the outer circle, with none over head, try moving around and see if you can improve that configuration.
Buying The Right Handheld GPS
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