Detection of the purposes - one of tasks of the radar-tracking system because it is necessary for safety of flight of the aircraft. For all time spent in air the sounding of space by radar signals for the purpose of detection of meteoformations, moving targets, features of a land relief and also buildings and constructions which can be on a flight trajectory is carried out.
The correct detection of the purpose is influenced by several factors: the size and the effective area of dispersion (EAD) of the purpose, its situation concerning the antenna, the level of noise, type of the directional pattern of the antenna and also characteristic of the processing device of the accepted signal - the coordinated filter or the correlator. Also that matters, the target is moving or motionless (it is more difficult to find a moving target).
First of all in the direction of the purpose it is necessary to radiate the probing signal. This task is carried out by the irradiating antenna which will transform an electric signal from the transmitter to e/m the field. The choice of an emitted signal is made according to prior information on the purpose. In other words, the type of a signal depends on a type of the purpose which needs to be found. Radar targets can be various: from droplets of water (hydrometeors) of which clouds, to enemy planes consist.
The signal from the irradiating antenna extends in the direction of the purpose, is reflected from it and the second stage - reception of the reflected signal comes to the reception antenna, therefore. During a way to the purpose and back the signal changes, more precisely, its parameters change: amplitude and a phase, and in case of the movement of the radar-tracking system concerning the purpose - also frequency. Time of arrival of the reflected signal and its phase allow to judge the range at which the purpose concerning the radar is located, and the difference of frequencies between the radiated and accepted signal (so-called Doppler shift) comprises information on speed of the found object concerning the aircraft. The reception antenna carries out transformation of the field to an electric signal which comes to the processing device. In the processing device (the coordinated filter or the correlator) there is a transformation of the accepted signal directed to increase in the relation signal/noise. It is necessary to reduce the probability of false alarm and to increase the probability of the correct detection of the purpose. At the output of the processing device by convolution of a signal with its pulse characteristic the correlation function of the accepted signal having usually pronounced maximum answering the purpose turns out. If this maximum exceeds the set threshold, the purpose is found.