GPS? Global positioning system.

What does it do? Tells you where you are - within 100 yards.

Do I care? Not if you're walking down the high street. But you might, if you were lost in the mountains or sinking in the Channel.

What does it give you? A grid reference, usually, or latitude and longitude. Also distance travelled, speed and time to destination.

Not a substitute for map-reading, then? No. It helps you plot your own way out of trouble or give a precise location to the rescue services.

How does it work? By picking up radio signals from US military satellites.

A bit like those nifty Cruise missiles? Sort of. They've degraded the signal for you and me, though. The military version is really pinpoint.

What's the physics? They locate microwaves from four different satellites and plot them against each other.

Sounds expensive. You can get a good one for £150. Some are less than £100. The price has dropped tenfold in 10 years.

Who makes them? Garmin and Magellan are the market leaders.

Where do I buy them? Electronics and specialist outdoor shops: or see "dealer locator" on www.garmin.com or www.magellangps.com

Points to watch: Avoid models with only one or two receivers: multi-channel systems are more reliable and quicker.

Do they work everywhere? Efficiency is reduced among high buildings and in deep canyons, rain forests and heavy snowfalls.

What's the alternative? Carry a sextant; get lost.