Study On Some Problems In HF Radar Signal Processing

Various radars working at different frequencies have been widely used in many domains of human activities, among which high-frequency (HF) radars have received particular attention in recent years for their unique abilities of over-the-horizon (OTH) detection. By sky or ground wave propagation, the detection range of the HF radars breaks through the limit of the curvature of the earth, which makes it possible for long-range air early-warning and target monitoring. At the same time the unique abilities in detecting low-detectable targets of the HF radars makes them powerful equipments for detection of hermit aircrafts, small airplanes in heavy land and sea clutters, and even intercontinental missiles. The costs of HF OTH radars are much lower than those of planet-borne systems. Just for these advantages, HF OTH radars now have become important military equipments of many powerful countries. Besides military objective, HF radars also play important roles in the national economic construction. HF radars can be used for measurements of ocean state parameters such as ocean surface currents, waves and wind. And comparing with other traditional measuring equipments such as buoys and satellites, they have remarkable advantages in their wide detection ranges and real-time and all-weather operation capabilities. HF radars have also been successfully used in floating iceberg detection. HF radars have great significance for ocean environment forecasting, fishing, seeking and rescuing, traffic management, and bridge construction. Moreover, the real-time monitoring provided by HF radars has particular significance for a near-sea country to maintain its rights and fulfill its obligation in the 200-miles (370-kilometers) exclusive economic zone (EEZ).Our country has so long a coastline that the HF radars are much applicable in the ocean environment and target monitoring. Strengthening of researches and development of HF OTH radars will help provide a powerful safeguard for the national securities and economic construction.There are basically two technology trends for HF OTH radars, one is employingconventional large-scale phased antenna array and forming narrow beams for high azimuth resolution, and the other is employing small-scale array and broad beam while using modern spatial spectral estimation algorithms to achieve the same azimuth resolution. The signal processing of narrow-beam radars are in the given direction to which the beam is pointing, so the received interferences and clutters only come from that direction and thus the signal-to-noise ratio (SNR) is relatively high, and target detection is only need to be performed on the range-Doppler spectra, which has a relatively low demand. However, the broad-beam radars will receive all the interferences and clutters in all directions, so the SNR is relatively low, and besides target detection on the range-Doppler spectra, target snapshots need to be formed for their spatial spectral estimation, respectively. In comparison, the narrow-beam radars ask lowly for the signal processing but the system costs are high and it is difficult for the localization and maintenance, while on the contrary the broad-beam radars cost lowly and are easy to maintain and transport, but the signal processing complexities are greatly increased. Generally speaking, target detection with a broad-beam radar is much more difficult than with a narrow-beam radar. Take detection in Doppler domain for example, spatial domain pre-filtering in narrow-beam radars can greatly lower the noise and interference in a given direction, while in broad-beam radars even the target signals in different directions may interfere with each other. It is still an important task to find out ways for better detection performance with broad-beam radars. Because of the complexity of the operation environment, there is large amount of interference, so effective interference suppression will be the key to improvement of the radar detection performance, and first of all full acknowledgement about the interference forms in tem...