GPS (Global Positioning System) devices enable you to locate exactly where you are in the world. The system sounds magical, but is simply based on the mathematical principle of triangulation. Using signals from three or more of the 24 satellites which orbit Earth, GPS's can pinpoint their own position. With four signals, altitude can be calculated as well.
Although such a system is very precise, the satellites are owned by the US Department of Defence, and they don't like the idea of civilians - or their enemies - being able to use them. So, they introduce some error into the equation - selective availability - which lessens the precision and means GPS units that you can buy will only be accurate to about 25 metres.
Garmin is a well established manufacturer of GPSs and just a year ago most units were quite bulky and cost several hundred pounds. About the same size and weight of a mobile phone, the new eTrex is ultra-portable, and at 150g you'll hardly notice it in your backpack. Price has come down a lot too, with this unit costing £130.
What you get is a rugged, waterproof and sporty-looking piece of kit which has excellent build quality - essential when you're throwing it around out on the trail. What it will do for you in addition to determining your location is create, name and save locations (waypoints) in memory, allowing you to navigate back to a point any time you like.
Once you start moving, it can provide other data like speed, direction, time and distance to destination, and more. The eTrex is not intended to replace basic navigation, just support it - so you should make sure you are capable of navigating without it. This is important for several reasons: signals from satellites can be affected or blocked by cloud or bad weather, and the unit itself could fail or batteries could wear out. On two alkaline AAs, the eTrex operates for about 22 hours in battery save mode.
The eTrex is designed with ease of use in mind, and there are just five rubberised buttons on the sides to control all the functions. It can store up to 500 waypoints, and one route with up to 50 waypoints. The eTrex gives you even more peace of mind with its automatic track log which saves up to 10 tracks, enabling you to retrace your path.