DOPPLER In the following text we will discuss the AN/APN-153(V) navigation set. For more informa- tion on the basics of the Doppler theory, refer to Aviation Electronics Technician 2 (Organizational), NAVEDTRA 12330. The AN/APN-153(V) navigation set is a self-contained, airborne, pulsed Doppler radar system. It is designed to provide navigation data to the navigation computers onboard various aircraft. This system automatically and continuously provides ground speed and drift angle information to the computer. This information is then used for the dead reckoning of the aircraft without the aid of wind estimates or true airspeed. The system operates without the use of ground installations over an unlimited geographical range. Weather does not affect the system performance. The navigation set is accurate from 40 feet to 50,000 feet over land and water that has a sea state of 1 or greater. Ground speed is accurate from 80 to 800 knots, with the drift being accurate to 40 degree right or left. This system is difficult to jam because of the directivity and narrow width of the microwave beam transmission and the variation of PRF. Theory of Operation The Doppler system determines the ground speed and drift angle components by measuring the frequency shift in received echoes. In other words, the system beams signals to the ground, receives the return echoes, and then measures the frequency shift produced by the relative motion between the aircraft and the earth. Since the aircraft moves both along its length and across its length, more than one beam is required. The AN/APN-153(V) uses four. These four beams 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 a time. Of these, aircraft motion shifts the forward beam up in frequency and the rearward down in frequency. These two shifts are compared, and a difference signal for the pair is formed. Then the other two beams are used. The two difference signals are then compared, and an azimuth motor rotates the antenna to make and keep them equal. This keeps the antenna aligned with the ground track. Since the drift angle is derived directly from antenna position, drift angle accuracy is not affected by signal quality, terrain, or sea state, as long as any echo at all is received. Once the antenna is aligned with the ground track the measured frequency shift is used to derive ground speed. The drift angle and the ground speed information is then sent to the navigation computer as inputs for dead reckoning navigation. Major Components There are three major components in the AN/APN-153(V) system. They are the Receiver- Transmitter RT-680A/APN-153(V), Antenna AS-1350/APN-153(V), and the Control Indicator C-4418A/APN-153(V). RECEIVER-TRANSMITTER.— The RT-680A/ APN-153(V) contains the transmitter, receiver, and the frequency tracker circuits (fig. 2-16, view A). It is essentially a conventional radar system that uses a magnetron power oscillator whose PRF is varied by a sawtooth voltage. The receiver is a superheterodyne receiver, where the two signals are amplified, mixed, and detected. The resultant detector signal is a single audio signal. This audio signal is then filtered, amplified, and sent to the frequency tracking circuits. Here, the signal is mixed with the output frequency generator in the main tracking loop. Any difference between the two is amplified and phase detected. The resulting voltage is fed to the frequency generator, which makes its frequency equal to the received audio. The received audio is a function of the Doppler shift, and, therefore, is the ground speed. ANTENNA.— The AS-1350/APN-153(V) contains the microwave plumbing, pitch and roll rotary couplers, antenna arrays, and the pitch and roll servo networks that maintain the arrays in level position during aircraft motion (fig. 2-16, view B). The antenna pitch and roll data used for leveling are obtained from the course attitude data transmitter group of the aircraft. The antenna takes the RF pulses from the RT and radiates them in two patterns emitted alternately at half-second intervals. It then receives the return echoes and feeds them to the RT. The antenna also takes the signal from the detector in the main tracking loop to position the antenna arrays parallel to the aircraft’s ground track. The array position, which now represents aircraft drift angle, is fed to a servo follow-up in the control panel. CONTROL INDICATOR.— The C-4418A/ APN-153(V) contains the controls and indicators required for system operation (fig. 2-16, view C). The control indicator applies system power, selects 2-18