Study On Adaptive Interference Mitigation Technique In High Frequency Surface Wave Radar

High frequency surface wave Radar (HFSWR), utilizing vertical polarized electromagnetic wave which follows the curvature of the earth along the air-water interface with low propagation loss on highly conductive ocean surface, can detect and track targets beyond the horizon such as vessels on the sea or aircrafts at low altitude. It provides a new way to manage and explore oceanic resources in civil projects. Therefore, many countries devoted much energy to developing HFSWR in recent years. Ionosphere has proved to be the biggest obstacle to achieve consistently good performance in long-range detection and surface vessels tracking for HFSWR. On one hand, the ionosphere reflects the radar's own signal, which returns to the receiver through complex modulation and path and forms ionospheric clutter; on the other hand, the ionosphere propagates and reflects the long-range radio signal to radar receiver, and forms skywave radio interference. Both ionospheric clutter and sky radio interference raise the floor of power spectrum and destroy the detection background, which cause huge difficulty for target detection and badly affect the detection range. Therefore, it is very crucial in HFSWR to effectively mitigate the ionospheric clutter and skywave radio interference, increase the signal to interference and noise radio (SINR) of target echo, improve the performance of detection, and increase the detection range. The current research was carried out under the circumstance mentioned above and the major contributions include:1. Characteristics of ionospheric clutter in equator were analyzed based on long term observation and abundant experimental data for the first time.The inducement, path and propagation characteristics of ionospheric clutter in HFSWR were analyzed. The clutter was classified into near vertical incident (NVI) clutter and sea propagated clutter based on propagation path. The Characteristics of ionospheric clutter were investigated in time, range and velocity domain, especially, spatial properties were studied with emphasis. The similarities and differences were compared (especially vertical clutter and sea propagated clutter). These studies provided the idea and possible approach for ionospheric clutter mitigation. 2. HFSWR antenna array amplitude and phase errors were real-time blind calibrated for the first time. And the compensation of receiver mismatch was proposed.HFSWR works in a short wave band. The antenna array amplitude and phase errors were very big which affect the sidelobe and null depth of radar beam. So it is imperative to calibrate the array antenna under this situation. Isolated targets with big SNR were selected as the calibration source to measure and calibrate array antenna error, and a suitable target blind search algorithm was studied. The antenna amplitude error was reduced from±2dB to±0.5dB and phase error was reduced from±20o to±3o. Narrow pulse sequence compensation method was proposed to compensate the mismatch of receiver amplitude and phase response. After the compensation, good match filter result was achieved and the receiver amplitude mismatch was reduced from 0.776dB to 0.005dB, and the phase mismatch was reduced from 42o to 0.02o. The calibration of receive system errors decreases the energy of ionospheric clutter from sidelobe, and guarantees the effective cancellation of clutter and interference.3. Based on the method of cable phase delay,new receive antenna was built to suppress the NVI ionospheric clutter at near zenith dirction. Sidelobe cancellation methods based on horizontal polar antenna and vertical beamformer were proposed to mitigate NVI ionsopheric clutter.New receive antenna was built to improve the mitigation ability of near zenith direction. The energy of NVI ionospheric clutter was reduced without increasing the complexity of signal processing. Based on horizontal polar antenna and vertical beamformer, NVI ionospheric cluter suppressed by the sidelobe cancellation method was discussed. Results show satisfactory suppression.4. A mitigation technology of sea propagated ionospheric clutter was studied and achieved. Detection before cancellation (DBC) method based on the exclusion of target and sea clutter was proposed for the case of weak clutter and small antenna errors. And recursive variational diagonal loading detection before cancellation based on multiple detections was proposed for the case of strong clutter or big antenna errors for the first time. Satisfactory results were achieved in real system. Detection before cancellation (DBC) method based on the exclusion of target and sea clutter was proposed to resolve target loss and floor hoist after traditional single notch filter auxiliary ionospheric clutter sidelobe cancellation for the case of weak clutter and small antenna errors. The sample data was trained to get the pure ionospheric clutter data by excluding the target and sea clutter information; for the case of strong clutter or big antenna errors, recursive variational diagonal loading detection before cancellation (RVDL-DBC) based on multiple detections was proposed for the first time. The small target was detected step by step through the multiple detections and cancellations, and effective cancellation results were achieved. The method keeps the target property after clutter cancellation. Finally, the whole clutter mitigation method was summarized. Experimental results showed that the method could effectively suppress ionospheric clutter without losing any target information, increase the SNR of target, improve radar detection background and meet the requirement of ionospheric clutter mitigation.5. Robust least squares radio interference mitigation method was proposed, and super resolution methods were discussed for radio interference mitigation.Based on the idea of convex optimization, the sample of horizontal auxiliary channel was modified, and the best performance of interference rejection in the worst case error circumstance under the limit of maximum error bound was obtained. A robust least squares method for skywave radio interference mitigation was proposed to settle the optimal weight coefficient effectively.In summary, the crucial techniques in ionospheric clutter and skywave radio interference mitigation of HFSWR were achieved. Radar detection background was improved; all-weather ability of radar was ensured; and the overall performance was improved. The current study is extremely useful for the development of whole radar system and has great potential to be employed in applications.