SeaKing Sidecan Sonars
0374-SOM-00013, Issue: 01
73
© Tritech International Ltd.
Appendix B. CHIRP Signal Processing
There are several advantages of
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Digital Sonar Technology
(DST) which allows the use of CHIRP signal processing technology in order to
improve the images generated by the sonar.
In monotonic (single frequency burst) sonar, the range resolution is determined by
the length of the transmitted pulse. The smaller the pulse is, the greater the resolution
achievable and vice-versa. The smallest pulse length is typically 50 micro seconds
and velocity of sound in water is approximately 1500 metres/second which gives a
range resolution of 37.5mm. This result effectively determines the ability to resolve
separate targets.
Target
Seperation
T1
T2
Time
Transmitted Pulse
Target 1 echo
Target 2 echo
Sonar range
resolution
Target
seperation
Combined echo
(seen by the receiver)
Using the example above, if two targets are less than 37.5mm apart then they cannot
be distinguished from each other. The net effect is that the system will display a single
large target, rather than multiple smaller targets.
CHIRP signal processing overcomes these limitations by sweeping the frequency
within the burst over a broad range of frequencies throughout the duration of
transmission pulse. This creates a signature acoustic pulse - the sonar knows what
was transmitted and when. Using pattern-matching technology, it can now look for
its own unique signature being echoed back from targets.
Transmitted signal
Transmitter
circuit
Receiver
circuit
Received decoded signal
pulse duration
In a CHIRP system, the critical factor determining range resolution is now the
bandwidth of the CHIRP pulse which means the range resolution is given by:
Range resolution =
velocity of sound
2
x
bandwidth
The bandwidth of a typical
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CHIRP system is 50kHz
