1 Terry: {pausing in her trek across Nepal to check her GPS unit} Well. Here's the GPS coordinates Jane gave me.
2 Terry: {walking on} Then she said to walk another kilometre north. Why didn't she just give me the coordinates for that point?
3 Terry: {looking at the GPS unit as she walks} Huh. That's weird. It's giving odd readings, like I'm on the other side of the mountain range.
4 Terry: I wonder what could be causing that? {A yeti watches Terry from behind a rock.}

GPS, or Global Positioning System is a system allowing accurate navigation on Earth with the use of artificial satellites. Basically, each satellite emits a signal indicating its position and time. From anywhere on Earth you can receive the signals from three or more of these satellites. Since the speed of light is known, you can calculate your distance from each satellite, which pinpoints your latitude, longitude, and also altitude to within a few metres. It's all pretty nifty, really.

Except when local weirdness interferes with the signals, such as what the Bermuda Triangle is reputed to do.

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2016-11-12 Rerun commentary: I suppose it was a bit optimistic to say that you can receive GPS signals "anywhere on Earth". This is true for most places on the surface of the Earth, but not really true in caves, or indoors, or in parts of the concrete canyons of modern cities. You pretty much need a line of sight to the satellites, as the radio signals don't penetrate solid objects very far.

I gave Terry a hiking stick as she walks through the Himalayas. I've noticed in Europe, particularly in Germany, a lot of people walking in the countryside with such sticks. These aren't really a thing where I live in Australia. And apparently they're called trekking poles, not hiking sticks. Shows you how much I know about them.