How Does GPS Work in a Tunnel?by Richard Asmus
Global Positioning System receivers calculate their locations by analyzing signals that they receive from satellites. These signals don't pass through solid objects. A GPS in a vehicle may have an external antenna, or it may pick up enough of bounced signal out of the air to operate. If signals in a tunnel are too weak, the GPS may still function, depending on its quality and features.
The U.S. Department of Defense built a system of satellites that orbit the earth primarily for military purposes. But today the DOD allows anyone on earth to use the signals they transmit to locate themselves using GPS receivers free of charge. Twenty-four satellites that orbit the earth every 12 hours constantly send signals. Up to three spare satellites may also be in orbit, ready to take over if any fail. The orbit pattern of the satellites allows all points on earth to receive signals from at least four satellites at all times.
A GPS receiver needs signals from at least four satellites to determine longitude, latitude and altitude information for any point on earth. The GPS signals can't pass through solid objects, but receivers can pick up and use bounced signals to operate. In a tunnel, it's highly unlikely that four signals will be present and strong enough to operate even the most basic GPS receiver. If you turn on a GPS receiver while in a tunnel, it probably won't work and will give you an indication that it's not receiving signals.
A basic GPS receiver only gives basic location information for its present location. But more elaborate receivers accept programs that can superimpose your location onto a graphic map or satellite photographs of the earth. Many programs track your location and report it at all times while moving. If the signal is temporarily lost, the program gives a warning. Depending on the program, the map information remains intact with your last location shown on the presentation. However, some programs may continue to track.
A GPS program with a dead-reckoning feature will continue to estimate your motion using the last location, direction and velocity information. When you exit the tunnel, the program picks up the location information anew and continues to report your progress. It may appear that the GPS continued to receive a signal, but it did not. You may not even notice the signal-loss warning that came on. Some units with audio programming may tell you your signal is temporarily lost.