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CR5CBCC User Manual
Budapest
10 June 2021
Page 3 of 17
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CR5CBCC_User_manual.docx
2
Explanation of CW Radar function
Continuous-wave radar is a type of radar system where a known stable frequency continuous-
wave radio energy is transmitted and then received from any reflecting objects. Continuous-
wave (CW) radar uses Doppler, which renders the radar immune to interference from large
stationary objects and slow moving clutter.
Maximum distance in a continuous-wave radar is determined by the overall bandwidth and
transmitter power.
This bandwidth is determined by two factors.
•
Transmit energy density (watts per Hertz)
•
Receiver filter size (bandwidth divided by the total number of filters)
Frequency-modulated continuous-wave radar (FM-CW)
–
also called continuous-wave
frequency-modulated (CWFM) radar
–
is a short-range measuring radar set capable of
determining distance. This increases reliability by providing distance measurement along with
speed measurement, which is essential when there is more than one source of reflection
arriving at the radar antenna. This kind of radar is often used as "radar altimeter" to measure
the exact height during the landing procedure of aircraft. It is also used as early-warning radar,
wave radar, and proximity sensors. Doppler shift is not always required for detection when FM
is used.
In this system the transmitted signal of a known stable frequency continuous wave varies up
and down in frequency over a fixed period of time by a modulating signal. Frequency difference
between the receive signal and the transmit signal increases with delay, and hence with
distance. This smears out, or blurs, the Doppler signal. Echoes from a target are then mixed
with the transmitted signal to produce a beat signal which will give the distance of the target
after demodulation.
A variety of modulations is possible, the transmitter frequency can slew up and down as follows:
•
Sine wave, like air raid siren
•
Sawtooth wave, like the chirp from a bird
•
Triangle wave, like police siren in the United States
•
Square wave, like police siren in the United Kingdom
Range demodulation is limited to 1/4 wavelength of the transmit modulation. Instrumented
range for 100 Hz FM would be 500 km. That limit depends upon the type of modulation and
demodulation.
Sawtooth modulation is the most used in FM-CW radars where range is desired for objects that
lack rotating parts. Range information is mixed with the Doppler velocity using this technique.
Modulation can be turned off on alternate scans to identify velocity using unmodulated carrier
Pressure compensation element