Warmatrix

Radar is a detection system that uses radio waves. It sends out radio pulses and captures any reflections, measuring the distance through the time that has passed between the two.
As early as 1886 CE the German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. Several engineers took up the challenge and produced prototype radar systems. It took until 1934 CE before the technology was advanced enough to produce working detection systems. Several countries developed them independently of each other. The USA was the one to coin the acronym "r.a.d.a.r.", which stands for RAdio Detection And Ranging. This soon became a word in itself.
During World War II radar systems were rapidly improved. They were used on land to monitor battlefields even in fog, rain and snow; at sea by ships to locate other ships; on and under land to detect mines. Their most important role was and still is to act as early warning systems for approaching aircraft and missiles. Radar played an important part in the Battle of Britain, giving early warning of German bomber approaches. Early radar systems were large. They progressively got smaller, until aircraft themselves could be outfitted with them too. The advantage of airborne radar is that, operating from high altitude, its horizon is further away.
Radar uses a wide range of frequencies. Low-frequency waves have little trouble penetrating fog, rain or other atmospheric distortions, but are not very accurate, while high-frequency waves are the opposite, precise but prone to electromagnetic noise. A common defense against radar is 'chaff', aluminum strips cut to a quarter of the radar wave length. This was first used in 1943 CE. After World War II engineers started to use 'jamming', by sending counter-radar pulses to a tracking radar to confuse its input signal. During the Vietnam War the USA used the first anti-radar missiles, which use radar against itself, by homing in on the radar waves and trying to destroy the source. Radars fight back by varying their frequencies, among others.
After World War II the combination of radar systems and surface-to-air missiles became so effective that no military aircraft was safe in a well defended enemy zone. Aircraft either had to fly extremely high, out of range of missiles, or extremely low, ducking under the radar waves. In the 1980's CE the advantage shifted partially back to the aircraft with the introduction of stealth airplanes. But detection technology is catching up in an ongoing weapons development race. Bistatic radars disconnect sender and receiver, potentially increasing accuracy; low-frequency radars are making a comeback because they are less sensitive to stealth measures; computer algorithms are used for advanced pattern recognition.