Without a signal from sufficient satellites your device won’t be able to locate your position. GPS receivers generally have an obvious icon that indicates whether they’ve got a lock or not, while most also have a separate screen showing satellite positions, the strength of the signal being received and the accuracy of the position.

With some older GPS receivers satellite reception can be adversely affected by a combination of climatic conditions and physical environment. In particular, reception in locations adjacent to high objects can drop, or be lost completely. Trees, canyons and cliffs, all common features in the outdoors, could cause one of these receivers to lose signal reception. In these cases a slight change in position may be all that is required to relocate the satellites.

Most of the current generation of GPS units are much more tolerant of obstructions, with tree cover in particular far less of a problem. The antenna is sometimes a limiting factor – compact GPS units like Garmin’s eTrex or bike-specific Edge series use a flat “patch” antenna, which is inherently less sensitive than the splendidly-named quadrifilar helix antenna that you’ll find in, say, a Garmin GPSmap unit. The more sophisticated helix antenna will get a signal under trickier circumstances than the patch, but there are clear packaging advantages to the simpler option – a bar-mounted GPS receiver with a thumb-sized aerial sticking out of it vertically probably wouldn’t be popular.

Finding a Signal

Tips on getting the best possible signal:

• It will take longer to get a 3D fix the first time you use your GPS or if you have moved more than about 600 miles from your last GPS location
• Look around you before you switch on your GPS and choose the most open location
• If you’ve got a compact GPS with no visible antenna, it’ll probably work best held flat (such as – happily – on a bar mount). If it’s got a sticky-out aerial, point it upwards
• Stay as far away as you can from trees, buildings, hills and high-sided vehicles
• Give your GPS time to log on – turn it on while you’re getting everything else ready, and if you’ve driven to the trail pop it on the car roof while it locates itself. Don’t leave it there, though…
• Do not start moving until you have 3D signal. Look at the status bar or at the the satellite page to see this

Tips on getting the best possible position fix

If you want to be as certain as possible of your position, perhaps to mark it so you can return later, you need to follow these simple rules:

• Make sure you have a 3D position fix
• Give your GPS time to settle in a stationary position and you will find that the accuracy improves
• Use the ‘averaging’ function, if your GPS has one, to give the best possible position fix

False Signals

Occasionally through no fault of your own a GPS may receive a false signal and therefore give an error in position and/or speed. There are all sorts of ways that this can happen – a satellite may be in slightly the wrong place, its clock might be a tiny bit out, the signal may get bounced off things on its way to the receiver rather than arriving in a straight line, atmospheric conditions can affect it and so on. If a GPS starts doing strange things when receiving a poor signal always question your position, check your map and, if necessary, move to a location where you can get a clear view of the sky and re-establish a good signal before proceeding.