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North:
Compass North—The uncorrected direction indicated by the north-seeking end of a compass
needle.
Grid North (GN)—An arbitrarily selected direction of a rectangular grid. In grid navigation, the
direction to the 180° geographical meridian from the pole is almost universally used as standard grid
north.
Magnetic North (MN)—The direction indicated by the north-seeking pole of a freely suspended
magnetic needle, influenced only by the earth's magnetic field.
True North (TN)—The direction from an observer's position to the geographic North Pole. The
north direction of any geographic meridian.
Parallax error—See Celestial Observation Errors.
Personal error—See Celestial Observation Errors.
Pitch—The rotation of an aircraft about its lateral axis.
Pitot-Static Tube—A device that consists of a pitot tube and a static port which measures pressures in
such a way that the relative airspeed of an aircraft may be determined.
Polar Distance—Angular distance from a celestial pole or the arc of an hour circle between the celestial
pole or the arc of an hour circle between the celestial pole and a point on the celestial sphere. It is
measured along an hour circle and may vary from 0° to 180° since either pole may be used as the origin
of measurement.
Precession:
Apparent—The apparent deflection of the gyro axis, relative to the earth, due to the rotating effect
of the earth and not due to any applied force.
Induced (Real)—A precession resulting from a torque, deliberately applied to a gyro. The gyro
precesses 90° from the point of applied pressure in the direction of rotation.
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Of the Equinox—The average yearly apparent movement of the first point of Aries to the west.
Precomputed Curve—A graphical representation of the azimuth or altitude of a celestial body plotted
against time for a given assumed position (or positions) and computed for subsequent use for celestial
observations. Used in celestial navigation to determine position or to check a sextant.
Pressure Altitude Variation—The pressure difference, in feet or meters, between mean sea level and
the standard datum plane.
Pressure Line Of Position (PLOP)—A line of position determined by pressure pattern formulas,
plotted parallel to the effective heading of the aircraft, and indicating the net crosswind displacement.
Procedure Turn—An aircraft maneuver in which a turn is made away from a designated track followed
by a turn in the opposite direction, both turns being executed at a constant rate so as to permit the
aircraft to intercept and proceed along the reciprocal of the designated track.
Projection (Chart, MAP)—Any systematic arrangement of meridians and parallels portraying the
curved surface of the earth upon a plane.
Pulse Duration—In radar, measurement of pulse transmission time in microseconds; that is, the time
the radars transmitter is energized during each cycle. Also called pulse length and pulse width.
Pulse-Length Error—A range distortion of a radar return caused by the duration of the pulse.
Pulse Repetition Frequency—In radar, the number of pulses that occur each second. Not to be
confused with transmission frequency which is determined by the rate at which cycles are repeated
within the transmitted pulse.
Pulse Recurrence Time (PRT) Or Rate (PRR)—The interval of time, in microseconds, between the
transmission of two successive radar or radio pulses.
Radar Beacon (RACON)—A receiver-transmitter combination which sends out a coded signal when
triggered by the proper type of pulse, enabling determination of range and bearing information by the
interrogating station or aircraft.
Radar Beam—A directional concentration of radio energy.
Radar Nautical Mile—The time required for a radar pulse to travel out 1 nautical mile and the echo
pulse to return (12.4 microseconds).
Radio Compass (ADF)—A radio receiver antenna which is used to measure the bearing to a radio
transmitter.
Radio Frequency (RF)—Any frequency of electrical energy above the audio range which is capable of
being radiated into space.
Radio Navigation—Radio location intended for the determination of position or direction or for
obstruction warning in navigation.
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Radome—A bubble-type cover for a radar antenna.
Range Control—The operation of an aircraft to obtain the optimum flying time.
Range Definition—The accuracy with which a radar set can measure range, usually a function of pulse
shape.
Range, Maximum—The maximum distance a given aircraft can cover under given conditions by flying
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