How does a GPS receiver device determine your location? The basic process is simple in principle. The receiver gets signals from 4 or more satellites within line of sight that contain enough information to determine the distance between you and each satellite. That information combined with knowledge of each satellite’s location is sufficient to triangulate your location. The distance calculation is done by calculating the time the signal took to reach the receiver. The main reason for needing 4 or more satellites rather than just 3 is because the GPS receiver’s time device is not accurate enough per se, so it needs extra information to correct this. In good open sky conditions you can expect a modern GPS device to be accurate to within about 10 metres on average.
So how does this affect you at the receiving end. Firstly you need to have 4 or more satellites in view. Out on the tops this is pretty much always but as you descend into valleys reception can vary. GPS signals are designed to penetrate cloud and rain and modern receivers (including phones) work fairly well in dry bush and less so in wet bush. Even then you can expect small random errors in position and elevation which will often lead to a track log that appears to zig- zag slightly more than your actual travel.
So what happens when reception is marginal. Well the best case is when the device simply signals no reception – then you know where you stand. Very occasionally you can get into a situation where you are in a gorge or valley where there are 2 or 3 satellites in view and one just below your horizon opposite a steep valley wall beside you. Sometimes that hidden satellite is in just the right position to bounce its signal off the opposite wall to your receiver. A receiver can reject anomalous bounce signals there are enough direct signals but otherwise it will perform the calculation as if it had line of sight. In other words it could silently reposition you at the perceived position where you would be if the bounced signal was line of sight – perhaps somewhere up on the valley wall. Usually this is pretty obvious when it happens but I have read of the occasional party ignoring common sense and blindly following their GPS.
I’ve been using GPS exclusively for navigation in the hills for nearly 20 years now and it has helped me out numerous times but you should always treat it as just another tool to go alongside the one that lives between your ears.