An autonomous underwater vehicle (AUV) uses active sonar pulses to map fine detail on the ocean floor. The AUV operates in seawater where the speed of sound is v=1.52×103ms−1. Engineers want the system to resolve objects as small as 10cm on the bottom, which lies 120m below the vehicle.
(a) Compute the minimum sonar frequency that will produce a wavelength no greater than the 10cm resolution requirement.
(b) For a normal mapping pass, the AUV emits a short pulse and records the returning echo. Calculate the minimum time interval that must elapse between the emission of two successive pulses so that echoes from the seafloor do not overlap.
(c) The AUV later moves into colder water where the speed of sound decreases while the sonar continues operating at your frequency from part (a). Discuss qualitatively how the change in sound speed affects (i) the system’s spatial resolution and (ii) the echo-return time.
(d) Give one practical reason, unrelated to the calculations above, why sonar systems designed for very deep-ocean exploration often operate at lower frequencies than the value you obtained in part (a).