Sending and Receiving Signals

Radio signals from the satellites travel at the speed of light. It takes less than one-

tenth of a second (only 65 to 85 milli­seconds) for a radio signal to travel from a GPS satellite to a GPS receiver on Earth. A GPS receiver picks up the signals and measures the time that they took to travel from the satellites. It then multiplies these times by the speed of light to calculate the distance to each satellite. Knowing how far it is from the satellites enables the receiver to pinpoint its own location.

The satellites transmit on two differ­ent radio frequencies, L1 and L2. The signals may slow down a little as they travel through the atmosphere, and this can cause an error in calculating a posi­tion. Because of this, the simplest GPS receivers are accurate to within about 30 to 60 feet (about 9 to 18 meters). More advanced receivers can correct errors caused by the atmosphere, and so they are more accurate. These receivers can calculate their position to within about 15 to 30 feet (about 5 to 10 meters).