1. Determining the location of a survey ship or aircraft, usually with respect to geodetic coordinates but sometimes with respect to reference beacons whose geodetic locations may not be known. Positioning is sometimes divided into (a) celestial navigation, locating oneself by observing celestial bodies, which sometimes includes satellite navigation such as the global positioning system (GPS); (b) piloting, determining position with respect to geographical points, including many radio-navigation methods; and (c) dead reckoning, positioning by the extrapolation of track and direction from a previously known point of departure, including inertial positioning, Doppler-sonar, and Doppler-radar methods. Some modern positioning systems measure traveltime or differences in traveltime from reference stations, some measure the phase in standing-wave patterns set up by pairs of transmitter stations, some measure Doppler frequency shifts, some measure the direction of strongest signal. See Figure L-6. Features of some systems are listed in Figure P-10. 2. Locating reflecting points; see migration.