The word radar started out as an acronym for RAdio Detection And Ranging, but the word has become so commonplace these days (and the applications of radar have broadened so much) that it is generally taken as its own word.
The main use for a radar (as its name suggests) is to use electromagnetic (radio) waves to detect targets from a remote location, and determine characteristics of the targets, such as how far away they are and how fast they are moving. Many other target characteristics are able to be measured as well. These are as incredible and varied as:
-- the size of the target
-- the direction of the target (ranges are actually the radial distance from the radar system; interferometry permits determining the angle from which a target's signal arrived so that the target may actually be located in space)
--the temperature and composition of atmospheric particles, even as far away as the region of space where high Earth-orbiting satellites are found.
The basic idea behind how a radar works is this: a radio pulse is transmitted into the air. It travels out at the speed of light (it's an electromagnetic wave), bounces off a target, and then returns back to the radar to be received. The transmitted signal is said to illuminate the target. When we receive a reflected pulse, we can measure how long the pulse took to travel out to the target and then back to us. Since we know the speed of light, we can then calculate how far away the target is. This is determining the target's range. The other measurable target characteristics mentioned above (and more) are determined in a similar fashion: by applying what we know about physics and how radio waves behave, we can take careful measurements of the reflected signals we receive to find out all sorts of things about the target that reflected the wave.
Typical quantities which characterize radars are their:
-- carrier frequency: Just like the frequency of a radio station you tune in on your stereo, this is the frequency of the overall radar wave. It determines what type of targets will be able to be seen by the radar.
-- operating frequency: how often pulses are transmitted. This determines how quickly the "picture of a target can be taken". In the jargon, we say the target's characteristics are sampled. The operating frequency also determines how far away the radar can see things and still determine their range.
-- operating power: how "loud" a signal is transmitted into the air. The more powerful the radar, the easier it is to see objects.
Typical characteristics of radars are their:
-- range resolution: what's the smallest feature they can see?
-- sensitivity: how "quiet" a signal can they pick up?
One concept that is very important for radars is that of Doppler shift. The main idea here is that, depending on how fast a target is moving, it will actually change the frequency of the radar's transmitted wave when it reflects it. The usual example for illustrating Doppler shift is to imagine a police car with its siren on, which is traveling toward you very fast. The siren will seem to decrease in pitch as the car passes you; this is because as the car was moving toward you, it was compressing the sound wave ahead of it so that the wave peaks became closer together, thus increasing the wave's frequency and the pitch that you heard. Similarly, when a radio wave with a particular frequency travels out and bounces off a moving target, its frequency will become shifted in a predictable manner according to the speed of the target and the direction it's traveling.
So the two most predominant things measured by radars are target range and velocity. It would then seem natural to display the radar data in a range vs. Doppler (velocity) plot. For each range visible to the radar, then, the power of the received signal is displayed as a function of velocity. One can then distinguish individual targets as bright spots at a particular range and at a particular velocity in the plot.
For an example of some real radar data, check our Real-Time Data.