Imagine you're a (sort of) GPS hand set, receiving signals from ground-based transmitters at certain Geocache sites, rather than from satellites.
The transmitters send out a radio signal every second, on the second, at precisely the same time as each other, thanks to their highly accurate atomic clocks.
The speed of radio waves in air is about 299,700,000 metres a second (Info from Wikipedia). In other words, they take 3.33667 nanoseconds to travel 1 metre. (There are 1,000,000,000 nanoseconds in a second.)
For this reason the signals get to the receiver at different times depending on the distance they have travelled.
At GZ, you would get the following time differences for the signals, in nanoseconds relative to the first to arrive, from the following sites:
| |
GC code |
Cache name |
Delay,
(nanosec) |
|
Update, 7/Dec/2014 |
| |
GC2FT7W |
Stirling DECCA |
0 |
|
Page archived but still available here |
| |
GCVH2Z |
Salty Lump |
14,217 |
|
|
| |
GC3FV2B |
Valley View |
22,640 |
|
|
| |
GCR1YB |
Buchlyvie Muir View |
30,700 |
|
|
| |
GCR6X2 |
The Mast |
60,286 |
|
Cache moved 3 mtrs 24/Aug/2014; delay updated |
| |
GC1N77Q |
South Black Hill |
117,410 |
|
|
| __ |
__________ |
____________________ |
_ |
|
|
Using these differences and the locations of the transmitters, you can calculate a location which would give the differences your receiver detects.
That position is GZ.
You don't have to know, in advance, the exact, absolute time the signals left the transmitters or arrived from them. (You would need your own atomic clock for that.) You just have to measure the time DIFFERENCES to a reasonable degree of accuracy.
Neat, eh ?
Go for it.
Notes
A real GPS works not in two, but in three dimensions; it has to work with the positions of moving satellites; and it has to allow for the changing speed of radio waves as they come from the vacuum of space into the atmosphere. What a little marvel a real GPS receiver is!
To solve this puzzle you could get away with using four transmitters but six are given in case one or two disappear.
When placed, the cache contained a keyring, miniature compass/whistle/light, bouncy ball, 4 pesetas.
The cache is NOT at the posted coordinates. (Surprise !)
Bring gloves in case of nettles.
The cache is near a shepherd's track with easy walking.
Don't let your dog worry any sheep.
The area is popular with walkers so be discreet.
Enjoy the view.