RajivConfident, ultrasound is very difficult to focus into a narrow beam and the speed it travels is subject to air pressure and humidity. It's range is generally very short, a few hundred metres at best.
Airports use highly focussed rotating dishes that send pulsed microwave signals and according to the direction and time for an echo to return can calculate the distance and direction of an object such as an aircraft. The principle is called RADAR (RAdio Direction And Ranging) and is accurate, relatively immune to atmospheric conditions and works over many Km distance.
I have this vision in my mind of Pokhara airport having a huge trumpet spinning on the control tower roof, swatting the planes out of the air. :shock:
Brian.