Despite the fact that the technology relies on a clear view of the sky to function, there are many times when it appears that your GPS device continues to function while in a tunnel. This isn't because the satellite signal is penetrating the tunnel, or because the tunnel is equipped with a signal boosting tool. This is because of forward-thinking on the part of GPS manufacturers, who in anticipating the problem of tunnels have equipped GPS devices with a unique solution -- the ability to predict your course and continue to display directions.
When you enter a tunnel, your GPS no longer tracks your position in real-time. Instead, your GPS extrapolates your course, route and current speed and presents you with an estimated position. Simply put, your GPS device gives you an educated guess based on the last available data before you entered the tunnel.
The data used to extrapolate your current position while underground is generated using positional and speed data and calculated using a mathematical algorithm. Each manufacturer utilizes a different algorithm, meaning that the calculations for determining relative position varies. This may result in differing underground performance from one manufacturer to the next, but generally shouldn't negatively affect performance too greatly.
Line of Sight
The reason your GPS system requires these measures in the first place lies with the GPS system's largest weakness -- a necessity for line of sight communication. Orbiting satellites can only "look" straight down at the Earth, meaning that without a clear, unobstructed view of the sky your device can't be "seen" by the satellite for signal acquisition. In addition to tunnels, large buildings and structures, mountains and trees can potentially obstruct this view.
While your GPS can generally handle generating a course while you're out of reach, it isn't infallible. If you know your route will require extensive underground travel, you should take the time to carefully memorize the underground portions of your trip. This way, should your GPS fail to provide accurate directions, you can still stay on course until you exit the tunnel.
- Photo Credit Jupiterimages/Creatas/Getty Images