compared. Phasing scanning causes an actual move-
ment of the radar beam with respect to the antenna
axis. In addition, electronic scanning includes conical-
scan-on-receive-only (COSRO) scanning techniques.
Monopulse Scanning
With monopulse (simultaneous lobing) scanning,
range, bearing, and elevation angle information of a
target is obtained from, as the name implies, a single
pulse. This type of tracking radar normally produces
a narrow circular beam of pulsed RF energy at a high
pulse-repetition rate (PRR). Each pulse is divided into
four signals that are equal in amplitude and phase.
The four signals are radiated at the same time from
each of four feed horns that are grouped in a cluster.
The radiated energy is focused into a beam by a
microwave lens. In turn, energy reflected from the
target is refocused by the lens into the feed horns. The
amount of the total energy received by each horn
varies, depending on the position of the target relative
to the beam axis. The four targets are at different posi-
tions with respect to the beam axis. A phase inversion
takes place at the microwave lens similar to the image
inversion in an optical system.
The amplitude of returned signals received by
each horn is continuously compared with those re-
ceived in the other horns, and error signals are gen-
erated that indicate the relative position of the target
with respect to the axis of the beam. Angle servo cir-
cuits receive these error signals and correct the posi-
tion of the radar antenna and the director to keep the
beam axis on target.
An important advantage of a monopulse-tracking
radar over a radar using conical scan is that the instan-
taneous angular measurements are not subject to er-
rors caused by target scintillation. Scintillation is the
rapid fluctuation of the echo signal amplitude as the
target maneuvers or moves, resulting in radar beams
bouncing off different areas of the target and causing
random reflectivity, which may lead to tracking er-
rors. A monopulse-tracking radar is not subject to this
error because each pulse provides an angular meas-
urement without regard to the rest of the pulse train;
therefore, scintillation does not affect the measure-
ment.
An additional advantage of monopulse tracking is
that no mechanical action is required, such as a
scanner. Figure 2-6 (on page 2-8) shows monopulse
variations of received energy with target positions.
Phasing Scanning
Phased array antennas use the phasing scanning
method. This method controls the phase of the RF
signals fed to multiple feed horns, dipoles, or radia-
tors. The angular position of the beam is determined
by the relative phase of the signals at each element.
When the phase of the signals applied is changed, the
beam can be steered without moving the antenna.
Another method of phasing uses changes in the
transmitter frequency.
Changing the frequency
changes the wavelength; thus, with a fixed length of
waveguide between the elements, the phase relation-
ship changes as the frequency changes.
Phased arrays can be used for tracking by a
monopulse-receiving technique. The array is divided
into quadrants, with each quadrant equivalent to one
of the four horns. The sum of all four quadrants is
compared to the sums and differences of different
quadrants, just as in monopulse scanning. This tech-
nique is also adaptable to receive-only antenna sys-
tems.
COSRO Scanning
Scanning the received signal by electronically
switching between the antenna elements (feed horns)
to produce a conical antenna receive pattern is called
conical-scan-on-receive-only (COSRO). COSRO
scanning is used with monopulse transmissions and
with single-beam transmissions.
Angle errors are produced in much the same way
as mechanical conical scanning. However, COSRO
scanning is less effective than monopulse scanning,
2-7
