In order for GPS receivers to function, they need to know three critical pieces of information. These are the current date and time, the receiver's location on the earth and the location of the satellites in orbit.
When a GPS receiver has these three pieces of information, it performs a normal startup procedure, called a "warm start," and will typically begin displaying a position in less than one minute. When a GPS receiver does not have this information, however, it must perform a much slower "cold start" routine that can take up to 20 minutes.
The wide variance between the warm and cold starts highlights the significant challenge that a GPS receiver faces when it must search the entire available sky for a GPS signal that is not much stronger than the overall background noise. Here are some tips that you can use when initializing your GPS receiver.
Clear View of the Sky
If a GPS receiver can locate one satellite, it greatly aids the initialization process because the GPS receiver gets precise time and starts receiving the location of other satellites, from that initial one.
The GPS receiver has a better chance of locating that first satellite if it has the opportunity to see all of the available satellites in the sky. When tall vertical objects like buildings or trees block satellite signals, the GPS receiver must still search the entire sky, but it now has fewer opportunities of finding that first satellite, because there are fewer available signals.
Initialize Before Hitting the Backcountry
Since there will likely be plenty of obstructions in the backcountry, initialize your GPS receiver at home and let it track satellites for at least 15 minutes. This will give your GPS receiver the most up-to-date information on each satellite's position in the sky, which will greatly aid its ability to acquire GPS signals in challenging environments.
This information, called an almanac, is good for about four hours before it starts becoming degraded and less effective. For best results, initialize your GPS receiver within four hours of your trip and let it track satellites for 15 minutes to ensure that it has a complete almanac.
Force a Cold Start
GPS receivers use two different search methodologies, depending on whether they are performing a warm or cold start. Warm starts are much quicker than cold starts because the GPS receiver knows exactly where to look in the sky to acquire a satellite signal.
Sometimes, however, GPS receivers get confused and think they have enough information about the current date and time, their location on the earth, and the location of the satellites in orbit to perform a warm start. When some of the data is wrong, the GPS receiver may never acquire any satellites because it is using a narrow search routine in the wrong area of the sky.
If your GPS receiver does not acquire any satellites within 5 to 10 minutes, force it into its cold start routine, which will cause it to search the entire sky. Check the documentation specific to your GPS model to see how to reset it, or clear its memory.
Article Written By Roy Scribner
Roy Scribner is based in Silicon Valley, where he writes about outdoor recreation topics for various online media outlets, while moonlighting as the business development manager for a defense aerospace firm. Roy and family are avid RV'rs and campers, averaging 5,000 miles every year in the Western United States.