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),
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.
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
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
aircrafts 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