Posted on 01/16/2008 1:04:09 PM PST by Squidpup
I just got a gps last spring and had fun geochaching with my family this last summer. Picked up a traveling bug I need to plant somewhere else, pronto.
Just on the off chance that we might need it, I carried a simple GPS on a car trip up the Oregon coast just after that very bad storm in early December. We also had a cell phone, a two-meter ham handheld, and a fifty-watt capable two-meter ham mobile transceiver. We might have gotten stranded but I was pretty confident that I could summon help and tell them where we were.
OK, I will have a stab at it.
Cell-phone based geolocation uses triangulation based on the cell phone being able to be seen by more than one cell tower. They are accurate in places like a city, to within 50 or 100 feet.
In such a remote area, there would not be the overlap of cell towers needed to triangulate the position. GPS is a different beast, uses satellites in orbit, and is accurate down to about 3 feet or less.
A cellphone doesn’t have a GPS receiver (for picking up location signals from GPS satellites) unless it is specifically built into the telephone. Adding GPS usually adds significantly to the cellphone’s cost. Further, a GPS receiver doesn't emit a signal (to triangulate on), it just receives ones that are already in the air and computes its location using timing differences from the multiple satellites it “sees” overhead. GPS is superior to the cellphone triangulation method because the likelihood of getting three or more GPS satellite signals is very high due to the extensive coverage in geosynchronous orbits around the Earth and because the calculation algorithm used in determining location is fairly precise.
Pitt said their cellphones didn’t have the GPS capability. Since he was one of the two lost hikers, maybe you should believe him.
Cool. Geekiness saves lives.
GPS satellites are not in geosynchronous orbits, they are in low Earth polar orbits, and their accuracy is not as good as triangulating from cell towers can potentially be. A few years ago a special geosynchronous WAAS satellite was added that sends out a correction signal. This information is computed by land stations that know their exact location and compare it to their GPS calculated location, which is usually in error. The gravity of the Earth is not uniform and the GPS satellite positions vary constantly from their predicted positions.
Rather than just timing signals a system could be implemented to measure the phase of a radio wave when it hits. This would improve the accuracy down to a tiny fraction of an inch. Some very precise surveying equipment uses this technique.
Thanks for correcting me on that. I was thinking of television and radio communications satellites which are in geosychronous orbits.
The last message I ever got from a bottle said "you're the disco king, ask her to dance..."
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