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Reflection of electromagnetic waves is transmitted and  reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction. The reflected signal is also called scattering. Backscatter is the term given to reflections in the opposite direction to the incident rays. Electromagnetic energy travels through air at a constant speed, at approximately the speed of light (300,000 km/sec). This constant speed allows the determination of the distance between the reflecting objects (airplanes, ships or cars) and the radar site by measuring the running time of the transmitted pulses.This energy normally travels through space in a straight line and will vary only slightly because of atmospheric and weather conditions. By using  radar antennas this energy can be focused into desired direction. Thus the direction of the  objects can be found out.