Choosing a GPS unit

DIVE takes a look at waterproof handheld Global Positioning System (GPS) units. These useful devices are great navigation aids and have a multitude of features, but there are caveats attached to them. They should always be used in conjunction with paper charts and should not be solely relied on to navigate a passage. Indeed, when you start up a unit, you will notice that the manufacturer usually displays a similar message requiring you to confirm that you understand this point. The reasons for this are twofold. First, they are electronic pieces of equipment and can fail and, perhapsmore importantly, most leisure GPS devices navigate in direct lines – unless you specifically enter waypoints to, say, plota course around a headland. Also, they won’t automatically route you around other hazards such as rocks, shallow water and shipping channels.

What is GPS?

The Global Positioning System is a constellation of between 24 and 32 orbiting satellites, owned and maintained by the US Department of Defense, which beam microwaves to GPS devices – also commonly known as ‘satnav’ (from Global Navigation Satellite System,or GNSS) – here on Earth.

How does it work?

In layman’s terms, all the satellites ‘know’ exactly where they are in relation to a place on Earth. The GPS device simultaneously gathers this information from several satellites, does some number crunching and then can deduce where it is located. The accuracy can be astonishing – to within 5m or so is not uncommon – however, several things can effect this.

First, in times of conflict, the US Department of Defense can simply detune the network. A more common problem is atmospheric conditions, or ifa direct line of sight to the satellites is obscured by cliffs or parts of a vessel, for instance. A good navigator should always carry charts and keep a keen watch, using the GPS as confirmation rather than trusting it implicitly.

Choosing a GPS unit

Most modern hardboats and RIBs will have fixed GPS units installed, many combined with a chartplotter and sounder, but it is always advisable to havea backup. Moreover, if you regularly charter different vessels, you may want tohave all your own waypoints portable so you can always have them to hand, in which case a mobile device is much more practical.

When venturing out to sea, it is highly advisable to keep salt water firmly away from any electronic equipment, soit is essential to buy a waterproof unit. Splashproof devices are fine for the odd shower on land, but salt water will soon corrode them. The specification to look for is IPX7, which means it can withstand immersion in one metre of water for 30 minutes.

The next thing to check is whether you can operate all the buttons when wearing gloves – some even have touchscreens that can beused while wearing the thickest of neoprene gloves. Also, look at the display, not only in the shop but outside in bright sunlight – is the screen still clear?

Finally, the majority of GPS units on the market don’t come with marine software cartography as standard, so you will need to purchase this separately. It usually comes on a tiny memory card, and you simply need to make sure the chart software is compatible with the model you wish to buy. Expect to pay around £150 for it.

>>> Page 2: Top tips for using a GPS unit