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A cache by zeusfaber Send Message to Owner Message this owner
Hidden : 06/05/2013
4 out of 5
2.5 out of 5

Size: Size:   small (small)

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Geocache Description:

A nod to a family of pre-GPS electronic navigation systems.

Key information

The cache is not particularly close to the location listed nor to the other two waypoints.  Apart from the church locations, the only information needed to find it is that it is:

1596 m closer to St Martin’s than it is to St Cyr’s
2233 m closer to St James’ than it is to St Cyr’s.

You’re looking for a one litre cliplock box.

Original cache contents included the usual swaps and a £1000 note for FTF.
No clues in the following – just here for interest

Why on Earth would anyone use differences in distance to describe a position?

Almost all electronic navigators use the fact that the further away you are from a radio beacon, the longer it takes for a signal to get from it to you (another 3 nanoseconds or so for every extra metre).  If you know what time the signal was sent and you’ve brought an atomic clock with you, you can use the time you receive the signal to calculate your distance from the beacon.  Do the same with two other beacons, and you have a position fix.

That atomic clock is a major practical problem so, until GPS came up with a neat trick using one or two extra beacons and some hefty computing power to keep a cheap crystal clock adjusted to the necessary accuracy, engineers needed to find a way round not really knowing what time it was.

It’s always much easier to measure small time differences than it is to know the absolute time (counting out five seconds without using a watch is easier than guessing the right time to the nearest five minutes).  Back in the 1930s, somebody had the bright idea of making two beacons broadcast pulses simultaneously and getting the receiver to measure the difference in arrival time of the two pulses – something which translates into how much further you are from one beacon from the other (this should be starting to look familiar now).  Add another beacon and you’ve got enough information for a fix.

Several systems used this approach, starting with the RAF’s GEE in 1941 and including Omega and Decca (one of these was my first professional run-in with troublesome Nav Systems and the other was stunningly accurate when it worked – which wasn’t often).  GPS  made almost all of these Hyperbolic Navigation systems obsolete, and pretty much the only one left now is LORAN (for LOng RAnge Navigation) which has been through a few iterations since its introduction in 1942 and is still maintained in the UK as a fallback in case the GPS constellation dies.

Additional Hints (Decrypt)

Arkg gb n sbex. Jurer jbhyq lbh unir chg vg?

Decryption Key


(letter above equals below, and vice versa)

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