The GPS satellite network is composed of approximately 24 satellites that orbit the earth at about 8,700 miles per hour. Despite the impressive nature of it all, the GPS network has at least one foe that gets in the way of its radio signals to the Earth -- solid matter. The reason for this is based on a number of factors, and technology developers are working on solutions.
The 12,000-mile distance between GPS satellites and the earth means that satellite signals must travel a long way, and that's part of the problem. Perhaps you've experienced the drama of losing a cell phone signal in a parking garage or basement -- and that signal comes from a cell phone tower that's perhaps only a few miles away. Satellite signals don't travel well through solid materials, which is why you can't get reception inside buildings, similar to why you would get poor reception under a dense tree canopy.
The distance problem is compounded when considering that to get an accurate GPS reading, a GPS receiver needs a signal from at least four satellites. These satellites help determine the receiver's latitude, longitude and altitude. Once the position is known, a receiver can calculate speed and distance to destination. Getting a signal from one satellite in a building may be difficult enough, but having to get signals from three more adds to the difficulty of getting reception inside a building.
Line of Sight
GPS signals travel by line of sight, which means that while they can pass through transparent materials like glass and clouds, they can't go around solid structures, like buildings and tunnels. It's also a reason GPS units have trouble in canyons, where maybe signal from some satellites can reach the receiver but not signal from all four.
Despite the challenges a GPS signal faces, developers have ways to circumvent them. For example, in March 2013 Apple bought the company WiFiSlam, which has developed a sort of indoor GPS system that could be used in shopping malls, for example. Instead of signal from satellites, this technology uses wireless signals to triangulate the position of a receiver, which could be a smartphone. Beyond that, GPS units in automobiles do their own guesstimate calculations in moments when they lose GPS signal, such as when driving through a tunnel.
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