nal, a basic pulse radar can detect a weak signal from
a moving target in the presence of strong signals from
large targets, such as landmasses and heavy seas.
Pulse-Doppler radars use the Doppler shift signal.
These radars can detect an aircraft flying close to a
hill or mountain where the strong landmass echo
would block detection with a basic pulse radar. Cir-
cuitry in the pulse-Doppler radar normally would
reject the stationary target, allowing easy detection of
the weak signal from the moving target.
The ranging system of a pulse-Doppler radar is
more complex than that of a pulse or a frequency-
modulated, continuous-wave (FM-CW) radar. A
pulse-Doppler radar senses both range and velocity by
time-sharing its waveform between these functions.
To detect a Doppler frequency from the target echo,
most pulse- Doppler radars use a much higher pulse-
repetition frequency (PRF) than basic pulse radars.
Higher PRF decreases the pulse-repetition time
(PRT) between pulses, resulting in the possibility of
a target echo returning at the time of the next trans-
mission. This, in turn, results in a blind spot in the
range. If the echo from the first pulse returns after the
second pulse is transmitted, then a range ambiguity
occurs.
The range blind spot and ambiguities can be com-
pensated for by changing the PRF over a wide range.
For example, the fire-control computer could adjust
the radar PRF based on the expected range of a
designated target. If the designation were for a target
at 50 nmi (100 kyd), the PRF could be changed so that
the second pulse would not occur until enough time
had elapsed for the target echo to return for that range,
plus an additional range interval for the acquisition
and tracking gates. Varying the PRF over a wide
range by computer control can resolve range ambi-
guities and blind ranges.
DOPPLER SHIFT THEORY.— A Doppler shift
allows distinguishing between the target and the trans-
mitter leakage. The amount of Doppler shift is deter-
mined by the radial velocity of the target since the
radial velocity is the apparent speed that the target is
closing on or going away from the radar.
A target can move in any direction and in a wide
range of speed; therefore, the radial velocity can
change considerably. If the target is moving at a 900
angle to the radar, then no Doppler shift is produced.
However, if the target moves straight at or away from
the radar, radial velocity will equal the actual target
speed.
The amount of Doppler shift is also dependent on
the wavelength resulting from the transmitter fre-
quency. A target radial velocity that produces a spe-
cific Doppler shift at 5,000 MHz would produce twice
as much at 10,000 MHz.
DOPPLER SHIFT DETECTION.— Pulse-Dop-
pier radars can detect moving targets by the Doppler
shift. Moving-target indication (MTI) is used pri-
marily to detect moving targets with pulse-Doppler
search radars.
Since stationary targets produce no Doppler shifts,
the return signal echo has the same frequency and
phase as the transmitted pulse. However, moving tar-
gets do produce Doppler shifts; therefore, the return
signal echo has a different phase from that of the
transmitted pulse.
To use this principle, pulse-Doppler radars must
be able to compare the echo signal with a reference
signal that is in phase (coherent) with the transmitted
signal. A means of storing or controlling the trans-
mitted phase is required to provide coherent detection.
One method uses a magnetron for the transmitter.
This requires that the local oscillator be stable for a
small fraction of a cycle during one pulse period. A
sample of the transmitted and stable local oscillator
(STALO) signals are fed to the coherent oscillator
(COHO). This locks in the phase of the COHO until
the next transmitter pulse. Figure 2-2 is a diagram of
coherent MTI with a phase-locked COHO oscillator.
2-3
