Radar waves cannot penetrate solid objects – they are reflected by solids – and the surface of the earth is no exception. Because the earth is curved, a ground-based radar can only detect an aircraft once it has cleared the horizon and is no longer hidden behind the earth. Hence a high-flying aircraft will be detected by a ground radar before a low-flying aircraft. To avoid detection, many military aircraft and missiles use very low flight profiles, a technique called terrain masking. Many modern weapons like cruise missiles use this tactic, making their long-range detection by ground-based radar difficult. Helicopters and strike aircraft flying very low to achieve the same effect are said to have a nap of the earth flight profile.
To detect low-flying objects, a radar must be located above them, with a line of sight to the intruder. This can be achieved to a limited degree by locating the radar system on top of a hill, or on top of tall masts (as with ships). However, the only long-range solution is to mount a radar on an aircraft flying above the intruders.
The technical challenge encountered is caused by the mass of radar returns that result from pointing a radar at the ground. The solid ground and all the objects on it reflect the radar waves, and are picked up by the radar creating confusing clutter on the radar screen. It is difficult or impossible to separate the radar image of the low-flying intruding aircraft from the general ground clutter.
Look-down/shoot-down radars have been enhanced with electronic programmes that process the radar image and search for moving objects, which are detected by looking for Doppler shifts in the radar return. The radar removes all static and slow moving objects (e.g. the ground, buildings, automobiles) from the display, and shows only moving objects. Since the radar is linked to the aircraft's fire control system, it can provide targeting information to weapons once it has detected a moving object.
Look-down/shoot-down radars provide a combat aircraft with the ability to engage targets flying below them. This is highly desirable, as it allows an aircraft to detect and attack targets whilst maintaining the tactically advantageous position conferred by superior altitude.
Most modern fighter aircraft possess look-down/shoot-down capable radars.
However, this using of the Doppler Effect to separate moving fighters gives the target a way of eluding the searching radar. If the target aircraft flies perpendicular to the radar beam, its speed relative to the attacker become the same as the ground, and there is potential for the system to lose it. This is known as "beaming" because it is maneuvering so that the radar is placed in the aircraft's beam (which is the position perpendicular to the aircraft's flight direction).
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