The concept of using radio waves to detect objects goes back as far as 1902, but the practical system people know as radar began in the late 1930s. British inventors, aided by research from other countries, developed a rudimentary warning system that could detect objects moving towards the coastline of England. The system used high-frequency radio waves to detect German planes and calculate their distance. This purpose lead to the term, which is actually an acronym for RAdio Detection And Ranging.
The principle behind this technology may sound confusing at first, but a simple experiment can demonstrate the basics. A person with a very accurate stopwatch and super hearing is facing the side of a mountain somewhere in the distance. She holds the stopwatch in one hand and starts timing as she screams as loudly as she can towards the mountain. When the first echo of her voice can be heard, she stops timing. She has now become a basic radar unit — since she knows how fast sound travels, she can calculate the distance between herself and the mountain by using the elapsed time on the stopwatch.
Radar works on many of the same principles demonstrated in this experiment. Instead of one person screaming, a powerful radio beam is sent out at a specific frequency. When this burst of radio energy strikes a solid object, at least part of that energy will be reflected back to the transmitter. This signal may not be very strong, but a sensitive electronic receiver can amplify it. The transmitter and receiver are usually mounted close together, much like a person's mouth and ears.
By calculating the speed of the radio waves and the time it takes for the signal to bounce off the object and hit the receiver, a radar operator can gauge the distance between himself and the object. Moving the transmitter to different points allows the operator to receive multiple returns. All of these individual reflections are combined to estimate the size of the object or objects being struck.
The technology has improved considerably since the days of World War II, but the underlying principles are still the same. Calculations about an object's speed and direction are made from the results of transmitter and receiver data. When a radar antenna is seen spinning in place, it is sending out thousands of signals and receiving them just as quickly.
The radio frequencies on modern systems are now largely in the microwave range, unlike the shortwave radio frequencies used by the British inventors. Radar jammers use matching frequencies to confuse the receivers looking for authentic ones, but microwave frequencies are much more difficult to jam.