Page 418 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

The ability of the acoustic system to resolve targets is determined by the pulse length; this, how- ever, has its drawbacks. To get enough acoustic energy into the water for good target identification and over a wide variety of ranges, the transmission pulse length has to be relatively long. The equa- tion for determining the range resolution of a conventional monotonic acoustic system is given by:
Range resolution 5 velocity of sound=ðbandwidth 3 2Þ
In a conventional monotonic system at moderate range, a typical pulse length is 50 μs and velocity of sound (VOS) in water 1500 m/s (typical). Therefore, the range resolution 5 37.5 mm. This result effectively determines the range resolution (or ability to resolve separate targets) of the monotonic acoustic imaging system (Figure 15.23).
10 5 On5 00 5
Monotonic
10
5 10
Pulse length
Pulse length
CHIRP
15.2 Sonar types and interpretation 411
Off
On
Off
10
12345 12345
Time in s Time in s
FIGURE 15.22
Comparison of monotonic (left) versus CHIRP (right) sonar techniques.
Tx pulse
Net effect!
Rx pulses
FIGURE 15.23
Example of inability of monotonic sonar to distinguish between close proximity targets.
Intensity
Intensity