h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

As a navigational aid GPS is great. There are a few pitfalls though because it's a radio based system where the signals are being beamed at VHF frequencies (somewhere round 1300 or 1500MHz I think). If you're on a flat plain or the top of a mountain or tall building and so can see a large area before the horizon you'll get a good fix from several satelites quite quickly. If you travel from that point to somewhere built up (a city street, canyon or forest) then due to the way radio works your reciever won't be able to see all the satelites. It's just like you can't see all the sky when walking along a built up street or mountain gully. The reciever will also be picking up reflections from the sides of the area you're in. It can compensate for this because of the way the system works but it just takes longer to fix on the satelites it can see.

Just a couple of points to think about when you're climbing up that gully in the mountains or walking along the skyscraper lined street that your GPS reciever might not be the best navigational aid to tell you where you are.

I find that an inexpensive system called Verbal Orientation (VO) often provides a 'fix'. When navigating through a built-up area where satellite radio signals are unreliable simply use the VO unit to 'ask' a 'person' if they know where THEY are. If the result is positive then you can assume that your location is effectively the same to within 1-2 metres, depending on your relative position to the 'person.'

You still need a nav ref for the Person though. In some cases they can provide this for you. In others they can't. Still it gives you some kind of relative position though when you can't get a true fix to the baseline.

Unless you stick a flag on his head.

In my researh for an artical I'm writing (A2638488) I've found there is a way around the problem of getting GPS signal in a built up area.

Basically by looking for signals for fixed devices such as wireless access points and comparing them to a location database you get a rough location for yourself to compliment your GPS device

this is a url to it http://www.placelab.org/