28 luglio 2016

How a radar works

A radar equipment allows to detect the position of an object by comparing a reference signal of a transmitter and the return signal (eco radar), transmitted again by the object and analyzed by a receiver. A weather radar allows to detect, inside a volume of atmosphere, the presence of hydrometeors, i.e. raindrops, that influence the propagation of a radar beam through electromagnetic waves. A weather radar is thus constituted by a transmitter of high power electromagnetic waves.

The radiation sent out is concentrated in a beam, generally of width between 1 and 2 degrees, through an antenna that works as a receiver of the energy sent out by hydrometeors. The pointing direction of the antenna may vary both in azimut and elevation, so to carry out volumetric scans.

The radiation sent out by a weather radar is made by a train of impulses, generated by a modulator of impulses, with a particul repetition frequency called Prf, Pulse repetition frequency. A radar, usually, transmits a millionth second impulses, separated by intervals of a millisecond, during which the signal radiated again can go back to the receiver, before being the next impulse gets sent out. Based on the intensity of the eco-radar, the elaboration system assigns an intensity to precipitations in the atmosphere that allow the user to identify the importance and localization of the phenomena.

Precipitation may be detected both horizontally and vertically to allow to get information on the extension on the soil and the vertical development of clouds. An immediate visualization of images produced is fundamental for an early and approximated movement speed of precipitations. The choice of operating with a given frequency is the result of a compromise between conditions of the propagation of the electromagnetic wave in the atmosphere and techical building requirements that characterize the tool. Infact the wave lenghth, and thus the frequency, affects  the reduction of the signal both during propagation and the sizing phase of the antenna.

C band radar is the most used at our latitude, in order to guarantee good observation performances and allows to obtain a beam with an opening span of 4 metres. C band weather radars can provide qualitative information up to about 200 Km. Precipitation estimates are though possible for distances up to 100 Km.

The Civil Protection Department uses also X band radars, with smaller dimensions, thus more versatile and transportable, that guarantee the same performances , but the quantiative applicability doesn't reach 80 km.

With a 1 Km resolution, a weather radar produces information equal to the one produced by 10,000 pluviometers or rain gauges. Real-time measurement of precipitations is carried out with an automatic pluviometers network, that provide just a point information, not enough representative for the whole hydrographic basin. A complete radar coverage allows to calculate precipation quantities more accurately over the whole surface, helping the improvement of water management.

Besides operational aspects, radar information are fundamental to study precipation regime, especially the intense ones, for general climate research of a given area, with purposes of environmental safeguard, physical cloud research, as well as nowcasting.