Graphics Programs Reference
In-Depth Information
Radars in the L-band are primarily ground based and ship based systems that
are used in long range military and air traffic control search operations. Most
ground and ship based medium range radars operate in the S-band. Most
weather detection radar systems are C-band radars. Medium range search and
fire control military radars and metric instrumentation radars are also C-band.
The X-band is used for radar systems where the size of the antenna consti-
tutes a physical limitation; this includes most military multimode airborne
radars. Radar systems that require fine target detection capabilities and yet can-
not tolerate the atmospheric attenuation of higher frequency bands may also be
X-band. The higher frequency bands (Ku, K, and Ka) suffer severe weather
and atmospheric attenuation. Therefore, radars utilizing these frequency bands
are limited to short range applications, such as police traffic radar, short range
terrain avoidance, and terrain following radar. Milli-Meter Wave (MMW)
radars are mainly limited to very short range Radio Frequency (RF) seekers
and experimental radar systems.
1.2. Range
Figure 1.1 shows a simplified pulsed radar block diagram. The time control
box generates the synchronization timing signals required throughout the sys-
tem. A modulated signal is generated and sent to the antenna by the modulator/
transmitter block. Switching the antenna between the transmitting and receiv-
ing modes is controlled by the duplexer. The duplexer allows one antenna to be
used to both transmit and receive. During transmission it directs the radar elec-
tromagnetic energy towards the antenna. Alternatively, on reception, it directs
the received radar echoes to the receiver. The receiver amplifies the radar
returns and prepares them for signal processing. Extraction of target informa-
tion is performed by the signal processor block. The targetÓs range, , is com-
puted by measuring the time delay, , it takes a pulse to travel the two-way
path between the radar and the target. Since electromagnetic waves travel at
the speed of light,
R
t
3 0 8
, then
c
=
×
m
sec
c t
2
R
=
--------
(1.1)
1
---
where
is in meters and
is in seconds. The factor of
is needed to
R
t
account for the two-way time delay.
In general, a pulsed radar transmits and receives a train of pulses, as illus-
trated by Fig. 1.2. The Inter Pulse Period (IPP) is , and the pulsewidth is .
The IPP is often referred to as the Pulse Repetition Interval (PRI). The inverse
of the PRI is the PRF, which is denoted by
T
τ
,
f r
1
PRI
1
---
f r
=
----------
=
(1.2)
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