5N32 Duga [Arc] - Background
The ionosphere has a strong influence on the personality of radiowaves which propagate beneath, within, or through it; and this influence derives from the spatial and temporal non-uniformity in the refractivity of the magneto-ionic medium. Over-the-Horizon (OTH) radars employ either a ionospheric forward scatter (receiver in front of the transmitter) or backscatter (part of the target reflections bounce back to the combined receiver/transmitter site) mode. Operating in the HF band these systems are vulnerable to ionospheric variations and these variations seriously degrade their usefulness.
Distances well beyond the horizon can be reached by exploiting the effect of ionosphere reflection, since in the HF band the electro-magnetic waves are gradually bent through the ionosphere. The ionosphere extends from about 90 to 2000 km altitude and modifies the propagation path of radar signals in a frequency dependent manner. The ionosphere acts like an electromagnetic mirror at these frequencies, and the radar signal becomes trapped in the Earth-ionosphere waveguide. The signal is reflected back by the Earth to the radar system that can perform the detection operations. This type of radar achieves the remarkable advantage of a very wide and time continuous coverage that ranges from 600 km up to 3000 km. The cost-effectiveness of such a system is outstanding.
Such a HF radar is a very complex system because it is characterized by a set of features which are very unusual if compared to ordinary microwave radars. Transmission frequency must be selected upon the ionosphere propagation behavior in the wide band [3-30 MHz]. Ionosphere channel behaviour depends on date, sun activity and spatial coordinate. Therefore, ionosphere propagation is very changeable from night and day.
Because the returning target signal is submerged in the scatter return of the surrounding target area, high transmitted power, pulse coding, and advanced data correlation are required. The key to successful employment of this system concept is real-time knowledge of ionospheric characteristics over the search path, which can involve any number of great-circle paths thousands of km long, some of which traverse the polar regions and the auroral zones.
In the HF band, radar performances are heavily affected by background noise, which is mainly due to external noise. More precisely, the external noise is composed by atmospheric noise, cosmic noise and man-made noise. Internal noise caused by thermal effect is almost neglectable. Designers must deal with heavy propagation losses due to the very long travelling distances as well as strong absorption losses mainly due to the D layer of the ionosphere. The whole loss contribution can be up to 100-150 dB.
The apparently simple propagation mechanism hides the complexity of the ionosphere structure. This implies a challenging target localization that could be achieved by a smart system calibration combined with a three dimensional reconstruction of the signal path through the ionosphere. The HF band is by far the most sensitive to ionospheric variation' due to solar disturbances since it requires all layers of the ionosphere for propagation. Primary reflection occurs in the E and F regions with the D region acting as a variable attenuator.
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