Thanks for the intersting read!!
nice to know
The Wikipedia and Trimble pages give more insight.
One of the keys to GPS operation is that the GPS box uses correlation techniques -- it knows exactly what bit pattern it should be "seeing" for each bird. So it can take a bit pattern and slide it around (in time) looking for the highest correlation. These correlation techniques allow GPS receivers to work with very, very low signal levels.
Early GPS receivers (such as my GPS 45) use serial, one channel at a time receivers (correlators). Start-up times for these units can be half an hour or more. Better code, faster processors, better hardware resulted in parallel receivers, with up to 12 active channels at a time. Modern parallel receivers can cold start in a couple of minutes or less.
But you only need 4 birds to get a GPS fix -- why would you want to listen to 12?
Using more than 4 birds lets you overdetermine the solution, refining your location. Older receivers with a lower number of channels (8 say) use different models to select the subset of currently visible birds to give the best fix.
...And now that you've got your overdetermined solution, and know where you are, and what time it is, you can dig out your spherical trig book to find distances between two points on a sphere. Oh, wait -- not quite spherical; we'll have to use the WGS-84 geoid.
...And then there's all the maps! What fun!
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