Factors Affecting Radar Operation and Radar Losses 145

5.24 RADAR SYSTEM LOSSES

There are several types of losses in the radar system. The individual loss is insignificant.
Dtthe combined losses are significant in the estimation of performance of any ratio.
The types of losses are:
Plumbing losses Transmission line losses
" Duplexer losses " Polarization losses
" Antenna losses
" Scanning losses
Beam shape losses
Radome losses
" Medium losses Phased array losses
" Digital phase shifter losses Losses due to unmatched filter
" Integrator losses Constant false-alarm rate re
ceiver losses
" threshold level losses
" Straddling losses
Limiting losses
Filter straddling losses
" Sampling losses Coilapsing losses
"Operator losses Equipment losses
" Propagation losses Pulse width losses
" Squint losses
5.24.1 Plumbing Losses
These are the losses that exist in microwave
circulators, directional compo.1ents of radar like duplexers,
couplers, waveguides, bends and joints.
5.24.2 Aransmission Line Losses
Ihe transmission lines and waveguides are used to
connect the
line losses exist due to tw0-waytransmitter the radar
antenna. In radars, transmission to
transmission line losses in X-band waveguide is about 0.5 propagation. Typical
dB/meter.
5.24.3 Duplexer Losses
Ihe losses exhibited by the duplexer are
nese different during transmission and reception.
about 2losses
dB. are specified by the manufacturers. Typical losses due to duplexer is
5.24.4 olarization Losses
The
misdesign
Of
antenna polarization always exhibit some losses due to
the undesired directions.
radiation in
5.24.5 Antenna Losses
The
adiatiantenna
rHere, losses are mostly indicated by lossy resistance. This is different from
onR, isresistance. The real part of the antenna
resistance consists of R, and R,
antenna gain. radiation resistance and R, is lossy resistance. This loss is reflected in
 Fundamentals of Navigational Aids
Radar Engineering &
146
Beam Shape Losses
modulated in amplitude by the antenna
In radar operation, a number of pulses areequation is hence not constant. This leads to
taken in the radar
beam shape. The gain
losses in radar range.
The beam shape loss isgiven by
N
Beam shape loss = (N-1)/2
1+2 exp-5.55
n=l (N, -1'
Here, N= Number of pulses integrated.
N= Number of pulses received within one way half-power beam width.
A typical beam shape loss is about 2 dB when N= 11.
$.24.7 Scanning Losses
The antenna gain is not the same while transmitting and receiving in the direction of
the target. During scanning, it creates an additional loss, called the scanning loss.
5.24.8 Radome Losses
Radome, made with materials of good electromagnetic properties have low
However., they result in reduction in bandwidth and gain, high VSWR andlosses.
antenna noise temperature. These losses are called radome high
radome has a two-way typicaltransmission loss of 1.2 dB losses. Metal space frame
frequencies. between L-band and X-band

5.24.9 Medium Losses

Radar operation depends on the free space or
As the strength of an echo
also medium between the radar and the target.
depend on the
the atmospheric changes
medium losses. result in losses in the type of atmosphere above the
received echo. These losses areground;
calleu
5.24.10 Phased Array
Phased array radars have
Losses
distribution. These additional
in the form ofnetworks result in networks
to provide the required phase
appear
reduction in gain. additional transmission losses. These losses
5.24.11 Constant
The small
number of
False-Alarm Rate Receiver Loss (CFAR)
losses in reference calls
signal-to-noise ratio. called
The typical such loss delay-line taps in CFAR increases the
level is about 2.0 dB.
 Factors. Affecting Radar Operation and Radar Losses 147

5.24.12 Integrator Losses

Some integrators result in losses. For example, the binary window detector exhibits a
theoretical loss of about 1.5 dB to 2.0 dB. The loss in signal-to-noise ratio due to
hinaryinformation isabout l dB to 2.5 dB.
K4.13 Threshold Level Losses
This loss depends on the preset threshold level in the receiver. It is about a fraction of
a dB.

's.24.14 Limiting Losses

The limiter circuits used in the radar receiver result in losses called limiting losses.
For example, pulse compression circuits to eliminate amplitude fluctuations in the
signal.
The typical loss in S/N due to hard limiter ranges between 1.1 dB to 6.4 dB
depending on code length and number of pulses integrated. Digital soft limiter has
0.4 dB to 0.5 dB loss whereas digital hard limiter has losses ranging between 2.0 dB
and 8.6 dB.

5.24.15 Straddling Losses

Straddling losses are also called the range straddling losses. These losses exist when
the range gates are not centered on the pulse. Practically, they also exist when the
pulses are wider than optimum.
5.24.16 Filter Straddling Losses
These losses exist in Doppler filter band when the signal spectral lines are not centered
on the filter. These losses exist in both analog and digital processing.

15.24.17 Sampling Losses

These losses represent the difference between the sampled value and the maximum
pulse amplitude. It is about 2 dB when the sampling rate is one per pulse. If the
Sampling rate is two per pulse,then the loss is about 0.5 dB. When the sampling rate
Sper pulse, then the loss is about 0.2 dB.

8.24.18 Collapsing Losses

Tne collapsing loss is defined as the ratio of the integration loss for M+ Npulses to
the integration loss for N pulses. That is,

L, (M +N)
Le (M, )=
L, (N)
Here, Lc(M, N)=The collapsing loss.
L,(M +N = The integration loss for M+Npulses.
L,(N) =The integration loss for Npulses.
148 Radar Engineering & Fundamentals of Navigational Aids

The collapsing loss for alinear detector is greater than that for asquare law
detection when the number of pulses integrated is small and the collapsing ratio is
large.
K24.19 Operator Losses
The early radar outputs arc integrated by the operators and there occurs operator loss
due to manual data interpretation. In the modern radars, decision making is done
automatically by a computer and hence operator losses are included.
5.24.20 Equipment Losses
The cquipment behaves differently in factory and field conditions due to equipment
degradation and interference surroundings. It is difficult to make preciseestimation of
such losses.

5.24.21 Propagation Losses

The losses are accumulated as the transmitted signals propagate in free space. The
different phenomena encountered are reflection from earth, refraction from atmosphere.
diffraction and other atmospheric effects. These effects make the transmitting and
receiving echo signals attenuated.
5.24.22 Pulse Width Losses
The radar antennas are often associated with long transmission line structures. Hence,
it takes considerable time to commence radiation. This
happens for a significant
fraction of the pulse. This time is called fill time. During this time, antenna
radiate efficiently and hence losses exists. This loss is called pulse width does not
loss.
5.24.23 Squint Losses
When the beam has squint from the axis, the antenna gain in
islower than the peak gain. The squint the direction of the target
losses due to this is about 3 dB to 9 dB.

5.25 RADAR
CROSS-SECTION (RCS) OF TARGETS, o
The radar cross
the magnitude ofsection
is the target's
relative size. It represents
the echo signal returned
It is defined as the
reflecting/scattering
to the radar by the
target.
ratio
angle to the incident power of power reflected
towards the radar receiver per unitsolid
density per 4T.
Power reflected towards the radar
receiver/Unit
Incident power density/4n
solid any