History 1957 – American Physicists monitoring Sputnik – noticed doppler effect. Explain doppler TRANSIT, OMEGA, Timation, etc 1973 – Defense Navigation Satellite System (DNSS) 1983 – President Reagan made GPS freely available to the public after  Korean Air Lines Flight 007, a Boeing 747 carrying 269 people, was shot down after straying into the USSR’s prohibited airspace. Selective availability – May 1 2000, Clinton Now – cars, planes, phones, etc Tech Wiki: The GPS concept is based on time and the known position of specialized satellites. The satellites carry very stable atomic clocks that are synchronized with one another and to ground clocks. Any drift from true time maintained on the ground is corrected daily. Likewise, the satellite locations are known with great precision. GPS receivers have clocks as well; however, they are usually not synchronized with true time, and are less stable. GPS satellites continuously transmit their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time). Time and known position. atomic clocks, relativity, corrected daily Transmits pseudorandom code known to rx, TOT. Receiver time aligns to get TOA 4 sats – 3 for position, 1 for time. How does the receiver know the location of the satellites? Uses beyond location Trimble Guide – Here