Radar is a detection system. It uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects.
REAL APERTURE RADAR:
Real aperture radar (RAR) is a form of radar that transmits a narrow angle beam of pulse radio wave in the range direction at right angles (90 degrees) to the flight direction and receives the backscattering from the targets which will be transformed to a radar image from the received signals.
RAR transmits a narrow angle beam of pulse radio wave in the range direction at right angles to the flight direction (called the azimuth direction) and receives the backscattering from the targets which will be transformed to a radar image from the received signals. Usually the reflected pulse will be arranged in the order of return time from the targets, which corresponds to the range direction scanning.
The resolution in the range direction depends on the pulse width. However if the pulse width is made small, in order to increase the resolution, the S/N ratio of the return pulse will decrease because the transmitted power also becomes low. Therefore, the transmitted pulse is modulated to chirp with a high power but wide band, which is received through a matched filter, with reverse function of transmission, to make the pulse width very narrow and high power .This is called pulse compression or de-chirping. By making the pulse compression, with an increase of frequency f in transmission, the amplitude becomes times bigger, and the pulse width becomes 1/TDf narrower. This method is sometime called range compression.
There are some limitations in the real aperture radar. One limitation is the fact that the equipment required to transmit a very short, high-energy pulse is difficult to build. For this reason, most long range radar systems use the “chirp” approach which is an alternative method of pulse
compression by frequency modulation. In the case of the chirp technique, instead of a short pulse with a constant frequency, a long pulse is emitted with a modulated frequency. The frequency modulation must be processed after reception to focus the pulse to a much shorter value. For the user, the result is the same as if a very short pulse had been used throughout the system.