GPS for Dummies.pdf - Engineering Surveyor

Chapter 3: GPS Fundamentals51 Satellites Ground stations ReceiversFigure 3-1:GPS iscomposedof satellites,groundstations, andreceivers.Eyeing satellitesIn GPS jargon, a satellite is the space segment. A constellation of 24 GPS satellites(21 operational and 3 spares) orbits about 12,000 miles above the Earth(as shown in Figure 3-2). The satellites zoom through the heavens at around7,000 miles per hour. It takes about 12 hours for a satellite to completely orbitthe Earth, passing over the exact same spot approximately every 24 hours.The satellites are positioned where a GPS receiver can receive signals from atleast six of the satellites at any time, at any location on the Earth (if nothingobstructs the signals).A satellite has three key pieces of hardware: Computer: This onboard computer controls its flight and otherfunctions. Atomic clock: This keeps accurate time within three nanoseconds(around three-billionths of a second). Radio transmitter: This sends signals to Earth.