BBC - h2g2 - Radar Technology - Signal Processing

The main task of a radar's signal processor is making decisions. After a signal has been transmitted, the receiver starts collecting returns, with those originating from near objects arriving first because time of arrival translates into target range. The signal processor places a raster of range bins over the whole period of time, and now it has to make a decision for each of the range bins as to whether it contains an object or not.

This decision-making is severely hampered by noise. Atmospheric noise enters into the system through the antenna, and all the electronics in the radar's signal path produces noise too. The common metaphor is to speak of a 'noise carpet', 'noise floor' or 'grass' and it is easy to understand the metaphor by taking a look at a radar scope.

Even if atmospheric attenuation can be neglected, the return from a distant object is incredibly weak. Target returns are often no stronger than twice the average noise level, sometimes even buried under it. It is quite a problem to define a threshold for the decision whether a given peak is noise or a real target. If the threshold is too high then existing targets are suppressed - that is, the probability of detection P_D will drop. If the threshold is too low then noise peaks will be reported as targets and the probability of false alarms P_FA will rise. A common compromise is to have some 90% probability of detection and a false alarm rate of 10^-6. This means that a real target will be detected in 9 out of 10 attempts, and that the radar is allowed to produce a single false alarm over a period of 24 hours.

A clever means to maintain a given P_FA is known as CFAR, for Constant False Alarm Rate. Rather than keeping the threshold at a fixed point, CFAR circuitry inspects one range bin after the other and compares the signal level found there with the signal levels found in its neighbouring bins. If the noise level is rather high in all of them (because of precipitation) then the CFAR circuit will raise the threshold accordingly.

Further tasks of the signal processor are:

Combining information. Secondary Surveillance Radars like those located on airports can ask an aircraft's transponder for information like height, flight number or fuel state. Pilots may also issue a distress signal via the transponder. The ground radar's signal processor combines this data with its own measurements of range and angular direction and plots them all together on the appropriate spot on the scope.
Forming tracks. By correlating the data sets which were obtained in successive scan cycles, the radar can calculate a flight vector which indicates an aircraft's speed and expected position for the next scan period. Airport radars are capable of tracking hundreds of targets simultaneously, and flight safety depends heavily on their reliability. Military tracking radars use this information for gun-laying or guiding missiles into a calculated collision point.
Resolving ambiguities in range or Doppler measurements. Depending on the radar's pulse repetition frequency (PRF), the readings for range, Doppler, or even both, are ambiguous. The signal processor is aware of this and selects a different PRF when the object in question is measured again. With a suitable set of PRFs, ambiguities can be eliminated and the true target position can be determined.
Ground Clutter Mapping. Clutter is the collective term for all unwanted blips on a radar screen. Ground clutter originates from buildings, cars, mountains and so on, and a clutter map serves to rise the decision threshold in areas where known clutter sources are located.
Time and power management. Within a window of some 60�x40�, phased array radars can instantly switch their beam position to any position in azimuth and elevation. When the radar is tasked with surveying its sector and tracking dozens of targets at the same time, there is the danger of either neglecting part of the search sector or losing a target if the corresponding track record isn't updated in time. Time management serves to maintain a priority queue of all the tasks and to produce a schedule for the beam-steering device.
Power management is necessary if the transmitter circuitry runs the danger of overheating. If there's no backup hardware then the only way of continuing regular operation is to use less power when less power is required, say, for track confirmation.
Countering interference. Interference can be a) natural, or b) man-made. Natural interference can be heavy rain or hailstorms, but also weird propagation conditions like ducting. Ducting is the same to a radar as is a mirage to some lost soul in a desert: the radar beam bounces off the boundary between two layers of air at different temperatures.
Man-made interference, if created on purpose, is also called jamming and is one of the means of Electronic Countermeasures. Unintentional man-made interference is a problem when radars of the same type are deployed without proper planning and their signals get into each other's ways. In particular, Automotive Radars have to deal with the latter type of interference.

In summary, the signal processor is the brains of a radar. Usually, it is represented by a very capable computer with Megabytes of software and as the above list shows, a lot of things keep it busy.