Aviation Electronics Technician 1 - Historic Naval Ships Association

DOPPLERIn the following text we will discuss theAN/APN-153(V) navigation set. For more informationon the basics of the Doppler theory, refer toAviation Electronics Technician 2 (Organizational),NAVEDTRA 12330.The AN/APN-153(V) navigation set is aself-contained, airborne, pulsed Doppler radarsystem. It is designed to provide navigation data tothe navigation computers onboard various aircraft.This system automatically and continuously providesground speed and drift angle information to thecomputer. This information is then used for the deadreckoning of the aircraft without the aid of windestimates or true airspeed.The system operates without the use of groundinstallations over an unlimited geographical range.Weather does not affect the system performance. Thenavigation set is accurate from 40 feet to 50,000 feetover land and water that has a sea state of 1 or greater.Ground speed is accurate from 80 to 800 knots, withthe drift being accurate to 40 degree right or left. Thissystem is difficult to jam because of the directivityand narrow width of the microwave beamtransmission and the variation of PRF.Theory of OperationThe Doppler system determines the ground speedand drift angle components by measuring thefrequency shift in received echoes. In other words,the system beams signals to the ground, receives thereturn echoes, and then measures the frequency shiftproduced by the relative motion between the aircraftand the earth. Since the aircraft moves both along itslength and across its length, more than one beam isrequired. The AN/APN-153(V) uses four. These fourbeams strike the ground at the corners of a rectangle.The system is pulsed so that only two beams(diagonally opposite) are transmitted or received at atime. Of these, aircraft motion shifts the forwardbeam up in frequency and the rearward down infrequency. These two shifts are compared, and adifference signal for the pair is formed. Then theother two beams are used. The two difference signalsare then compared, and an azimuth motor rotates theantenna to make and keep them equal. This keeps theantenna aligned with the ground track. Since the driftangle is derived directly from antenna position, driftangle accuracy is not affected by signal quality,terrain, or sea state, as long as any echo at all isreceived. Once the antenna is aligned with the groundtrack the measured frequency shift is used to deriveground speed. The drift angle and the ground speedinformation is then sent to the navigation computer asinputs for dead reckoning navigation.Major ComponentsThere are three major components in theAN/APN-153(V) system. They are the Receiver-Transmitter RT-680A/APN-153(V), AntennaAS-1350/APN-153(V), and the Control IndicatorC-4418A/APN-153(V).RECEIVER-TRANSMITTER.— The RT-680A/APN-153(V) contains the transmitter, receiver, andthe frequency tracker circuits (fig. 2-16, view A). It isessentially a conventional radar system that uses amagnetron power oscillator whose PRF is varied by asawtooth voltage. The receiver is a superheterodynereceiver, where the two signals are amplified, mixed,and detected. The resultant detector signal is a singleaudio signal. This audio signal is then filtered,amplified, and sent to the frequency tracking circuits.Here, the signal is mixed with the output frequencygenerator in the main tracking loop. Any differencebetween the two is amplified and phase detected. Theresulting voltage is fed to the frequency generator,which makes its frequency equal to the receivedaudio. The received audio is a function of theDoppler shift, and, therefore, is the ground speed.ANTENNA.— The AS-1350/APN-153(V)contains the microwave plumbing, pitch and rollrotary couplers, antenna arrays, and the pitch and rollservo networks that maintain the arrays in levelposition during aircraft motion (fig. 2-16, view B).The antenna pitch and roll data used for leveling areobtained from the course attitude data transmittergroup of the aircraft.The antenna takes the RF pulses from the RT andradiates them in two patterns emitted alternately athalf-second intervals. It then receives the returnechoes and feeds them to the RT. The antenna alsotakes the signal from the detector in the main trackingloop to position the antenna arrays parallel to theaircraft’s ground track. The array position, whichnow represents aircraft drift angle, is fed to a servofollow-up in the control panel.CONTROL INDICATOR.— The C-4418A/APN-153(V) contains the controls and indicatorsrequired for system operation (fig. 2-16, view C).The control indicator applies system power, selects2-18