Look Down, Shoot Down
for the capability of an aerial radar
to find and engage targets flying a nap of the earth
Radar is a line of sight system. It emits a radio wave in a known direction and then detects a target based on the echo of that wave. Distance is determined by the amount of time delay caused by the returning wave. Velocity is determined by frequency shifts in the wave, which confrom to the doppler effect.
We all know the earth' surface is curved. Radar waves cannot penetrate through solid objects. Such objects, including the earth's surface will reflect the radar wave. So a land-based can detect an airborne target only when it rises above the horizon. For this reason the detection range of a radar is partly a function of altitude. The higher the approaching aircraft the greater the distance by which it can be detected.
Aircraft and weapon designers know this, as do soldiers. One way of evading radar detection is to fly very low, using the Earth's own terrain features to hide the approaching aircraft or missile. This is called terrain masking. The US cruise missiles, the European Panavia Tornado and the French exocet missile all employ this technique. Fixed wing aircraft and helicopters flown this way are said to be flying a nap of the earth profile. Helicopters excel at this, but the speed of a jet flying at less than 100 meters above the ground makes it a very formidable attacker.
One solution is to raise the radar. Naval radars are usually mounted as high as possible, and hills readily sought for land based radars. But this only minimizes, not eliminates, the blockage caused by the earth's curvature. The only real solution is to move the radar up in the sky, so it can see far over the horizon. Thus airborne radars were born.
But the earth reflects radar energy, as does everything on it. This creates what is known as clutter. Trees, people and moving cars can all be detected. Radars can be tuned to produce a picture of the ground, which is very useful for bomber aircraft and for map making. But a low-flying aircraft can easily be lost in the clutter.
Digital signal processing made possible by the microcomputer offers one solution. Radars can be programmed to ignore everything that is not in motion, and instead concentrate on Doppler signals. Essentially, the radar is told not to display anything that isn't moving. Militarily this is very useful, as it can be used to identify moving objects of all types, including trucks, trains, tanks and of course airplanes and missiles.
Look down, shoot down radars, such as those aboard the US F-15 Eagle, use doppler screening to detect, track and attack targets. Essentially the radar is tied in with the fighter's fire control system and permits the attack of low-flying aircraft from above. This is a good way to attack, as the attacking fighter is in a much superior position, particularly in regard to energy maneuverability.
Today, all current and future generation fighter aircraft are equipped with and employ 'Look Down, Shoot Down' radars. Adding the capability to a modern digital radar really only requires some additional programming, and perhaps some enhanced computer capability. For this reason, 'Look Down, Shoot Down' systems should become near ubiquitous.