I won't delve deeply into the details of how GPS works. That would take ages and you might already know. But in case you don't - suffice to say that there's a bunch of satellites orbiting the earth. Each one sends out a time signal along with it's unique identifier. GPS devices know about each of the satellites and where they are in relation to each other so, by working out the time difference between each of the signals sent by each satellite, the GPS receiver can work out where on earth it is (literally!).

Garmin are well known for their GPS offerings. In recent years they've turned it into a massive business. The eTrex range is one of the longest-standing in Garmins line-up and it is aimed squarely at people who enjoy the outdoors. The old yellow eTrex was, and indeed still is, at the bottom of the range. This is not to say it's a bad product, but after a few years of use, I started to realise it's limitations.

The improvement over the eTrex comes in the form of the eTrex Summit. Only one step up in the range, it's much the same in terms of appearance. A sleek, waterproof grey/silver case about the size of an older mobile phone, the Summit boasts the same capacity for 500 waypoints, but this one can store up to 20 routes (again of 50 waypoints each). It also seems to have a larger track log memory, meaning that you can record your trail for longer before it starts to get over-written. In practice, there's about enough track log memory for about 6 hours of walking.

While the interface remains the same, the hardware (and software) has been upgraded from the base model. Whereas the base eTrex would only give you a heading when you were on the move, the Summit comes with an electronic compass for accurate bearings even when stationary. This does take it's toll on the battery life but can come in handy, especially when you have lost the GPS signal. In addition to the compass, the Summit also improves on the GPS-based altimeter function with the addition of a full barometric altimeter. This allows for much more accurate estimation of elevation and gives an excellent calculation of ascent/descent rates. Both of these new functions come with their own GUI modes, the compass being the most familiar and the altimeter being the least useful. Other display modes will show you how far/long to your next waypoint, how fast you're traveling, your average speed, maximum speed, elapsed time and distance walked - all of which can be both inspiring and demoralizing, depending on your frame of mind!

It's probably worth mentioning that there are a reasonable number of accessories for the eTrex range. I bought a handlebar mount for mine so that I could plot mountain bike routes as well as hill-walking. Unfortunately it's more than a little unstable and the batteries tend to jolt free meaning everything turns off - most unsatisfactory. Other options include mobile-phone alike carry cases, dashboard mounts and cigar-lighter power adapters.

It is the lack of base-map information that keeps the cost of the unit so low while still providing all the navigation features of the more expensive models. Where the Summit really does come into it's own is when it is used in conjunction with some mapping software on a PC (such as the excellent Memory-Map navigator). Get yourself a USB/serial cable and hook it up to your PC. Create your maps on reliable OS-based digital maps and export them to the unit, waypoints and all, for future use. It's then a simple task of selecting the appropriate route when you reach the start point and the GPS will guide you all the way round (if needed).

3 years ago I paid around £235 for mine. IN a recent check on prices I've seen it available for as low as £140 which is probably about as good as you will get. So if you're looking for a rugged, handheld GPS that's ideally suited to hill-walking in the UK, you could do a lot worse than the eTrex Summit.

It just remains for me to remind you that a GPS should not be relied upon 100% for navigation. The signals are only accurate to within around 15m, but this isn't always good enough and there is always the chance that the batteries could run out etc. Spare batteries are essential, especially in the winter when the cold saps them of their power at an alarming rate. But regardless of how many spare batteries you carry, you should ALWAYS venture out in the hills properly equipped with a compass and the relevant OS map.