A coastal measuring station employs ultrasonic echo-ranging to determine distances under water.
(i) Explain the basic physical principle that allows the station to measure distance by timing the returning echo of the pulse.
(ii) Justify mathematically why the measured time of the round trip of the pulse is divided by 2 when calculating the one-way distance.
(iii) State any two environmental factors that can change the numerical value of the speed of sound in sea water and briefly describe how each one affects a distance measurement.