SONAR systems are generally ineffective for locating objects in air because
(a) Because the speed of sound is lower in air than in water, making distance measurements unreliable.
(b) Because the law of reflection of sound does not hold accurately for gases.
(c) Because ultrasonic waves are strongly absorbed in air, so very little energy returns to the detector.
(d) Because air completely blocks all sounds having frequency above 20kHz.