The science of GPS
GPS satellites are always on the move – they are not geostationary. If your GPS receiver knows where to look for them, it can speed reception. The satellites transmit several signals, one of which is almanac data, which provides the orbital information for the satellites in the system. Most modern GPS receivers store several days worth of almanac data, which can help speed acquisition, by predicting where to search for the satellites.
So if you haven’t used your GPS for several days, it may be slow to lock onto satellites. And since the GPS constellation is always shifting, there are good reception days and bad reception days, which is why I say it MAY be slow to lock.
This can be exacerbated by other things as well, such as:
- Being in an environment where signals are blocked (urban canyons are notorious for this)
- In my own experience, GPS receivers are slower to lock when moving, so it can help to wait for a good signal before putting your car in drive
- Moving your GPS a great distance is also problematic (see “The airport problem” below)
What to do if your receiver won’t lock? The answer is really quite simple. Power your GPS on, and let it sit under open sky, stationary, for half an hour. If you don’t have a good spot outdoors, put it on your dashboard, giving it as much of a view of the sky as possible. DO NOT turn it off as soon as it locks onto satellites; it needs time to download the almanac.
Special case: The airport problem
Flying cross-country or around the globe? Sorry to anthropomorphize, but your GPS ends up confused about where it is — it’s looking for satellites in relation to where you last turned it on. So here’s my suggestion. Use your GPS the morning of or the day before your trip; let it guide you to the airport. As soon as possible, after your flight and you are outside of the airport, turn it on. I often do this while riding a shuttle bus to pick up a rental car. Another option is to set it in a hotel window, but remember, don’t turn it off as soon as it locks. Give it a full half hour to download the satellite almanac data.
Cell phone pros and cons
Mobile phones use assisted GPS (A-GPS), meaning they are able to get GPS satellite almanac data from cell towers, which happens much faster than downloading it from satellites (which transmit data at a paltry 50 bits per second). Unfortunately many cell phones struggle with GPS reception in the absence of a cellular signal, so I wouldn’t exactly call them superior in this regard.