Page 420 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

In summary, CHIRP techniques provide the following advantages:
• Greatly improved range resolution compared to fixed-frequency sonars
• Larger transmission pulse lengths for increased operating ranges
• Improved discrimination between closely spaced targets
• Improved noise rejection and SNRs
• Reduced power consumption, from high-speed digital circuitry
15.2.5.2 Acoustic lens sonar
For an explanation of acoustic lens sonars, the concept will first be introduced followed by an example of a commercially available product using this technology.
From Loeser (1992) comes a general explanation of acoustic lens technology:
Acoustic lenses simplify the beam-forming process. The liquid lens is a spherical shell, filled with a refractive medium, that focuses sound energy in the same manner an optical lens focuses light energy. Sound waves incident on the lens are refracted to form a high-intensity focal region. The refraction is caused by a difference in acoustic wave speed in the lens media and surrounding water. The focusing ability is set by its diameter as measured in wavelengths for the frequency of interest.
A single hydrophone located in the focal region of the lens forms a highly directive beam pat- tern without the necessity of auxiliary beam-forming electronics.
The Sound Metrics name for their acoustic lens products are MIRIS and DIDSON (the latest iteration of this product series is the ARIS), referenced here as an example of the acoustic lens technology (Figure 15.25).
FIGURE 15.25
The DIDSON acoustic lens sonar.
15.2 Sonar types and interpretation 413