Research On Multidomain Collaborative Ionosphere Clutter Mitigation Methods For High Frequency Surface Wave Radar

High Frequency Surface Wave Radar(HFSWR), which works in the short wave band, suffers from many radio frequency interferences(RFI) such as co-channel interference, ionosphere clutter, etc. Among all types of RFI, the ionosphere clutter is a main factor that seriously degrades the detection capability of HFSWR. Traditional ionosphere clutter suppression methods use part of the signal’s information in time domain, frequency domain, spatial domain or polarization domain. However, the effect of clutter cancellation and application scope is limited due to the insufficient utilization of all the information obtained.To solve this problem, an effective measure is to take advantage of spatial and polarization signal processing together. In this work, the spatial and polarization domain suppression methods are first researched with oblique projection as math tools. In this way, the potential of both filtering technology can be developed. Based on that, multi-domain collaborative ionosphere clutter mitigation scheme is further researched, with the purpose to combine time-frequency domain, spatial domain and polarization domain signal processing together. The main content is summarized as follows:1. In chapter 2, based on abundant experimental data of long term observation, the characteristic of ionosphere clutter is analyzed in aspects of features in time domain, Doppler domain, polarization domain and spatial domain and the non-stationary characteristic as well. Then after the principle of oblique projection is given, the basic framework of multi-domain collaborative suppression is briefly introduced, which lays the foundation of the following research work.2. In chapter 3, the interference cancellation method in spatial domain is researched. Currently, spatial domain interference mitigation is the main measure to suppress RFI in HFSWR. However, there are several problems unsolved. First, how to design the optimal filter that could suppress the RFI while keeping the coherent characteristic of the target unchanged requires consideration. Second, how to suppress RFI when there are multiple interferences is need to be addressed. Moreover, limited by the array aperture of HFSWR, the mainlobe or null in space can not be narrow enough. When the ionosphere clutter is close to the target, the signal to interference ratio(SIR) can not be effectively improved.To solve the problems, according to single interference, oblique projection spatial filter(OPSF) and improved oblique projection spatial filter(IOPSF) based on optimal signal to interference and noise ratio(SINR) constraint are first proposed. OPSF could keep the signal unchanged and form space null to cancel interference. In this way, the target coherence can be retained while interference can be suppressed to the maximum extent. Particularly, such suppression ability is effective with interference both outside and inside of the mainlobe, which makes it able to suppress mainlobe interference. Moreover, since the noise gain is large when the principle angle of OPSF is small, IOPSF based on optimal SINR is proposed, which can be used to improve the SINR performance of OPSF under low signal to noise ratio(SNR).According to more ionosphere clutters, multi-interference cancellation method based on OPSF is proposed. When there are many ionosphere clutters from different directions, multi-interference cancellation method is designed in the range-Doppler domain. By constructing multiple parallel OPSF, spatial notches are formed to suppress all the ionosphere clutters respectively. Then logic product is performed to mitigate all the clutters simultaneously.Furthermore, mainlobe-interference suppression OPSF(MS-OPSF) in range-Doppler domain is proposed based on multi-interference cancellation method, when it is unable to accurately estimate the angle of arrival(AOA) of the ionosphere clutters. It is achieved by both increasing the number of spatial notches to cover more unwanted signals from other directions and utilizing the property of single OPSF to suppress interference inside the mainlobe. By setting part of the notches in the mainlobe, MS-OPSF is able to suppress interferences in the mainlobe and good sidelobe suppression ratio can be obtained.3. In chapter 4, the interference cancellation method in polarization domain is researched. The oblique projection polarization filter(OPPF) has demonstrated potential in interference suppression due to the relaxation of orthogonality of subspaces. Currently, OPPF is mainly used in time and frequency domain, however in this work, OPPF is further applied in range-Doppler domain and range-time domain. Range-Doppler domain polarization suppression(RDDPS) is first presented. It is designed for weak ionosphere clutter and implemented in the range-Doppler domain with a floating sample window to perform polarization estimation and ionosphere clutter suppression. Then range-time domain suppression(RTDPS) is proposed. It is designed to suppress the powerful and highly non-stationary ionosphere with a multi-segment polarization estimation and suppression scheme. At last, base on RDDPS and RTDPS, a comprehensive scheme of ionosphere clutter suppression is proposed. This scheme is proved to be effective in mitigating non-stationary ionosphere clutters in polarization domain.4. In chapter 5, the multi-domain collaborative interference mitigation method is researched. The oblique projection spatial-polarization collaborative mitigation(OPSPCM) method is proposed and applied to suppress ionosphere clutter in HFSWR.Although the OPSF and OPPF have advantages in spatial or polarization domain, the performance of such single domain filter degrades a lot when the target and interference gets close in corresponding domain. In order to solve the problem, by using polarization sensitive array(PSA), both spatial and polarization information of the signals can be obtained. And the framework of multi-domain collaborative is therefore established on such array. In this paper, the advantage of oblique multi-domain collaborative filter(MDCF) which is based on PSA is first analyzed. Then the relationship between PSA and the HFSWR array with horizontal polarized channel is analyzed and the equivalent parameter estimation method is researched according to actual array mode. Finally, the OPSPCM is proposed by combining the ideas of RDDPS and MS-OPSF. In this way, the collaborative suppression of ionosphere clutter in both spatial and polarization domain is achieved. Moreover, multi-beam OPSPCM(MB-OPSPCM) method is further proposed. In this way, while the ionosphere clutter mitigation performance is still kept, the observation scope can be enlarged to have the similar terget detection range with conventional HFSWR.Theoretical analysis and results of experimental data demonstrate that, by using interference cancellation scheme proposed in spatial domain and polarization domain, the ionosphere clutter can be suppressed with acceptable performance. Furthermore, the multi-domain collaborative processing is proved to be better in mitigating ionosphere clutters, which demonstrates the feasibility and advantage of multi-domain collaborative processing. It can be concluded that, this work presents an available solution for the next generation anti-interference system for HFSWR and offers some ideas and inspirations for the research in interference suppression based on PSA.