So you've just finished a run, you've checked your route on your watch, and... wait, why does it look like you spent several miles running in a river? And why did you have to spend 20 minutes waiting around by your front door before it even established a GPS lock?
There are several reasons why your watch might have trouble pinpointing your location, adn the device itself might be working perfectly well. As Garmin explains, even the best GPS watches aren't 100% accurate, and are susceptible to various technological and environmental factors.
Obstructions
Trees, buildings, tunnels, and cliffs can all throw off your GPS watch's accuracy. Your watch may be unable to establish a clear line of sight with a satellite, or the signal might be reflected off a nearby surface, meaning it appears to be originating from a different location. This can cause 'GPS drift', where your watch follows your general route, but with less precision.
Space weather
Space weather is a result of activity on the sun's surface, including solar wind and flares, and it might be the reason your GPS watch is misbehaving. As the Space Weather Prediction Center explains, GPS radio signals travel from your device to a satellite in orbit and back again, passing through the Earth's ionosphere in the process. When space weather is 'quiet', GPS positioning can be accuarate to within a meter or so, but when solar winds and flares occur, the ionosphere is disturbed, which can result in readings that are tens of meters off your true location.
Electrical fields
If you're near the Earth's magnetic equator, you might find that your GPS watch sometimes struggles to find a lock. This is due to current systems and electric fields that create 'bubbles' in the ionosphere, which cause GPS signals to 'ripple', or scintillate. This effect is most noticeable just after sunset.
Accidental radio interference
There might be a man-made reason why your GPS watch is struggling to keep tabs on your location. GPS devices use specific ultra-high frequency radio bands. Your country's communications bureau (such as the FCC in the US, or Ofcom in the UK) will set power limits for transmitters in the bands near GPS to help prevent accidental jamming, but interference can still happen – particularly if someone nearby is using something like imported amateur radio equipment that doesn't comply with local regulations.
Deliberate GPS jamming
GPS jammers can cause havoc with everything from air traffic control to emergency pagers used by doctors, and are therefore illegal to use in many countries (including the UK, and the US where the FCC is clamping down on them). However, they do exist, and can be as small as a cigarette lighter. As Military-Aerospace Electronics explains, GPS signals are quite weak, so even a simple jammer can cause disruption over a very large area.
It's more likely that your watch's GPS is being interrupted by accidental interference or space weather, but there's also a chance you might be caught in the range of a jammer.
Bad design
If your GPS watch consistently fails to report your location accurately, sadly the problem might be with the device itself. For example, if the aerials within the watch case are too close together (something that could easily happen with something as small as a watch), they can cause interference with one another. This could be a problem if your watch was imported via a route that meant it wasn't certified by your country's communications bureau.
Bad maps
Your watch is only as good as its software, and if it's using inaccurate maps, there's nothing you can do about it other than ensuring you keep the software up to date so you receive any new and improved maps as soon as they're available.
- Best Garmin watches: for any budget, and any sport
