How GPS Was Designed to Work
GPS was designed to work outdoors. The signal from the GPS satellites is extremely weak, and the satellites are far away. The total radiated power from each satellite is just 27W, about the same power from a dim lightbulb. The satellites are more than 20,000-km high (about 55 times higher than the space shuttle flies). When the signal reaches the GPS receiver on the Earth, the received signal power is about 100 attowatts; “atto” means 10–18, and it is not a commonly used prefix. We typically express such low powers in terms of decibels. But it is useful to mention the attowatt just this once, to get a feel for how weak the received GPS signal really is. The received signal power is 100 attowatts when the receiver is outdoors; when the receiver moves indoors, the signals rapidly get weaker, by 10–100 times in a house, and by 100–1,000 times or more in a large building. However, it is not just indoors where electronic tracking devices has signal problems; the weak signal is a...