Oblique projection polarization filter (OPPF) can be applied as an effective approach for interference cancellation in high-frequency surface wave radar (HFSWR) and other systems. In order to suppress the nonstationary ionosphere clutter further, a novel OPPF based clutter suppressing scheme is proposed in this paper. The polarization and nonstationary characteristic of the clutter are taken into account in the algorithms referred to as range-Doppler domain polarization suppression (RDDPS) and the range-time domain polarization suppression (RTDPS) method, respectively. The RDDPS is designed for weak ionosphere clutter and implemented in the range-Doppler domain directly, whereas the RTDPS algorithm is designed to suppress the powerful ionosphere clutter with a multisegment estimation and suppression scheme. About 15–23 dB signal to interference ratio (SIR) improvement can be excepted when using the proposed method, whereas the targets can be more easily detected in the range-Doppler map. Experimental results demonstrate that the scheme proposed is effective for nonstationary ionosphere clutter and is proven to be a practical interference cancellation technique for HFSWR.

By exploiting the long-range propagation of vertically polarized electromagnetic wave in the band of 2–15 MHz, high-frequency surface wave radar (HFSWR) is able to receive vessel and low-flying aircraft echoes over the horizon. However, the signal environment of high-frequency (HF) band is far from satisfactory. The powerful shortwave radio interference, ionosphere clutter, and industrial interference that dominate the pure receiver noise in the HF band cause a significant limit of detection capability. Ionosphere clutter, which is often observed to mask multiple successive range and Doppler cells, is one of the main interference sources. In some cases, the power of the clutter is so high that even the target echoes are overwhelmed, resulting in poor detection and tracking performance [

Adaptive beamforming schemes have been developed by using the space information of ionosphere clutter [

However, the ionosphere clutter cancellation method, which meets the requirement of practical HFSWR system, is still an open issue, as the ionosphere clutter is usually nonstationary. In this paper, two types of clutter cancellation approaches are given to suppress the nonstationary ionosphere clutter. The range-Doppler domain polarization suppression (RDDPS) is designed for weak ionosphere clutter and performed in the range-Doppler domain, while the range-time domain polarization suppression (RTDPS) is designed for strong ionosphere clutter. The procedure of clutter polarization estimation and clutter suppression in segments for the RTDPS is emphasized in our research. And the segmentation parameter optimization is also discussed in detail to obtain a balance between the clutter cancellation and the target restoration. The specific utilization of RDDPS or RTDPS depends on the type of the clutter at the corresponding range cell.

The remainder of this paper is organized as follows. In Section

Suppose the received wavefront is a completely polarized wave and its polarization angle and polarization angle difference are

Suppose the polarization steering vectors of the target and interference are

The fundamental property of this polarization oblique projection operator can be given as

The scalar form of (

Usually, the received signal is a linear combination of the target

When scalar form is required, the filtering process can be rewritten as

According to (

Equations (

If the estimation accuracy is uncertain, in order to analyze the performance loss of OPPF that was caused by the estimation error, the mean square error (MSE) of the target can be defined as

Supposing the target signal, interference, and noise are uncorrelated, (

Supposing the polarization steering vectors of the target and the interference are both accurately estimated, according to (

In this case, we have

Ignoring the noise term, the MSE is mainly determined by

According to [

Therefore, (

Equation (

Supposing the target signal is accurately estimated while the interference is not, we have

Substituting (

The MSE can be simplified as

Equation (

Moreover, fixing the SIR and SNR and the power of the target, according to (

To apply an OPPF in ionosphere clutter cancellation, it is necessary to accurately estimate the polarization state of the target and the ionosphere clutter, so that the clutter can be suppressed to the maximum whereas the target can be effectively restored. With regard to the nonstationary characteristic of the ionosphere clutter, the polarization estimation methods and ionosphere clutter cancellation scheme should be properly designed.

Ionosphere clutter, which is a kind of self-generated interference in HFSWR, is the radar signals reflecting back from the ionosphere. The polarization plane of the electromagnetic wave is usually rotated by the ionosphere along the radio propagation path, which makes the high power vertically polarized radar signals change into elliptically polarized ones. However, the echoes of targets arriving along the surface of ocean remain vertically polarized. Evident difference between the elliptical polarized ionosphere clutters and the vertically polarized target signals can be found in the dual-polarized HFSWR. That is, the power of the horizontally polarized component of ionosphere clutter is close to that of the vertically polarized part or even stronger. However, the power of the horizontally polarized component of the target echoes propagating along the ocean surface is quite weak compared with that of the vertically polarized component [

Affected by the irregular activities of the free electrons and charged ions, the electron concentration of the ionosphere is unstable, which leads to the nonstationary feature of the ionosphere clutter [

The ionosphere clutter is not stable in a CIT, and its duration time varies from several pulse periods to the whole CIT.

In each single pulse, some clutters occupy less range cells whereas some occupy more.

The nonstationary feature also leads to the different frequency spread in Doppler spectrum; some clutters locate in multiple successive Doppler cells while some exist in fewer Doppler cells.

The ionosphere clutter is usually elliptically polarized but its polarization state is unstable in the whole CIT [

According to the characteristics of ionosphere clutter discussed above, a novel OPPF based polarization cancellation scheme for nonstationary ionosphere clutter is proposed. Figure

Because of the weak power of radar echoes, range processing and Doppler processing need to be carried out first. Range-Doppler maps are generated from the raw data of each horizontal and vertical channel, respectively.

The state of all the samples in the range-Doppler map is then estimated, including the power value and polarization parameters. By using the basic characteristic of the signals in the range-Doppler map, the target and other clutters, such as ground clutter, ocean clutter, and ionosphere clutter, can be preliminarily separated and classified.

Based on the power estimation, the type of ionosphere clutter is identified cell by cell in range domain. The specific utilization of RDDPS or RTDPS at each range cell is determined by the clutter type at each range cell.

RDDPS and RTDPS algorithms, designed for different types of ionosphere clutter, will be used to suppress the clutter. The former one is designed for the ionosphere clutter with low power and the filtering process is carried out in range-Doppler domain directly. The latter one is designed for powerful clutter, and the filtering process is mainly carried out in the range-time domain. In a practical HFSWR system, the two types of ionosphere clutter may exist simultaneously and locate at different range cells. Therefore, the selection of RDDPS or RTDPS is decided by the clutter type at the corresponding range cell. Because of the adaptive scheme in this part, an optimal filtering effect can be obtained.

After all the range cells are treated with the procedure in (4), the final range-Doppler map is composed for RDDPS, whereas a range-Doppler processing is needed for RTDPS to obtain the final range-Doppler map.

Structure of ionosphere clutter cancellation scheme.

As drawn in Figure

After the parameters have been estimated, all the signals will be classified. Firstly, a power value gate is set by

The ionosphere clutter is checked cell by cell. For the range cells that ionosphere clutter exists in, polarization cancellation scheme will be applied. In order to decide which polarization filtering algorithm will be taken, define function

The criterion of algorithm selection is based on the relationship of the power between the target and the ionosphere clutter. At the corresponding range cell, if the maximum value of the ionosphere clutter power is smaller than the average power of the target, the clutter is considered to be weak and

However, in certain range cells, the target may be nonexistent; the value of

To illustrate the potential advantages of range-Doppler domain suppression, three main aspects are given as follows.

Due to the time-frequency invariance of the signal’s polarization characteristic [

In range-Doppler domain the power of each signal can be concentrated on limited range and Doppler cells, respectively, which significantly increases the SNR and interference to noise ratio (INR). In this way, the polarization estimation accuracy of both the target and the clutter will be improved effectively. As mentioned in Section

In range-Doppler domain, the multiple clutters and targets can be isolated in different cells, providing a possibility of individual polarization estimation and suppression for each clutter, and the polarization degree of the signals is improved.

Based on the analysis above, polarization filtering in range-Doppler domain is considered to be more suitable to process the ionosphere clutter with weak power. The improved SIR and INR will significantly enhance the estimation accuracy and the performance of the filter.

The illustration of RDDPS is drawn in Figure

Diagrammatic illustration of RDDPS.

In the range-Doppler window, the polarization estimation is performed cell by cell in range for

For simplicity, suppose

Equation (

When the actual number of the clutters is less than

The scheme of RDDPS takes advantage of the characteristic of the ionosphere clutter in range-Doppler domain and hence is suitable for ionosphere clutter cancellation of low power in HFSWR. However, the nonstationary characteristic of ionosphere clutter will degrade the performance of RDDPS, as a result of the decline of clutter’s polarization degree. To solve this problem, another scheme which is referred to as range-time domain polarization suppression (RTDPS) is proposed.

Rather than estimating the polarization state in a CIT, the nonstationary characteristic of ionosphere clutter can be relaxed when the clutter is observed under a smaller time scale. By dividing the time domain data into many segments, the polarization degree in each segment can be improved. Thus, for a RTDPS, data segmentation is performed prior to the estimation and filtering process. The range-Doppler map is used to find the position (range and Doppler frequency) of the clutter, and the polarization state of the clutter is estimated in the corresponding range-Doppler cell of each segment, whereas the filtering process is performed in the range-time domain. To guarantee the estimation accuracy, a higher INR is required to compensate the decreased coherent integration time in each segment. That is why the RTDPS is more suitable for suppressing the strong ionosphere clutters. For the case of single clutter in the corresponding range cell, RTDPS is applied in the mode of single-notch mode, whereas a multinotch filtering mode of RTDPS will be taken when more clutters exist. It is necessary to emphasize that the notch is in the Doppler domain rather than in the polarization domain, which is completely different from the definition in [

As shown in Figure

Principle of single-notch RTDPS.

Based on the results of the pretreatment step, a Doppler notch is set first to cover the region of several successive Doppler cells where the ionosphere clutter locates. In this way, the polarization steering vector of the ionosphere clutter can be estimated from the samples in the Doppler notch. After being processed by RTDPS, the ionosphere clutter in this area will be suppressed heavily, forming a “notch” in the Doppler spectrum.

After the Doppler notch has been set, the time domain data is divided into

Suppose the sum of time domain samples at the current range cell

In each segment, it is convenient to decompose

Meanwhile, the highly polarized target

Then the OPPF operators of each segment can be obtained as

After the Doppler processing, the final output of single-notch RDTPS can be expressed as

The second component in (

By dividing the range-time domain data into segments, the estimation and suppression scheme is performed in separate segments and the polarization degree of the ionosphere clutter in each segment can be increased. As a result, the estimation accuracy of the clutter’s polarization will be improved and the clutter can be suppressed more completely as the completely unpolarized part is unable to be suppressed.

Multinotch RTDPS is implemented for more clutters at the current range cell, as drawn in Figure

Principle of multinotch RTDPS.

Supposing the sum of ionosphere clutters is

Supposing the width of the Doppler notch is

For each segment, the polarization steering vector of the ionosphere clutter within the Doppler notch can be estimated by

The estimated values are determined by the sample that has the maximum power within the Doppler notch.

Derived from the experimental HFSWR data, Figure

Selection of polarization parameters in Doppler notch.

In Figure

To improve the polarization stationarity or the polarization degree of the clutter in each segment, more segments number should be used. As given in Figure

Polarization stationarity under different segment numbers.

Therefore, a satisfied performance of clutter cancellation can be only obtained by properly selecting the number of time domain segment. There is a trade-off between the improvement of polarization degree and reduction of estimation error, which should be balanced in practical application.

To evaluate the performance of the proposed method, experimental data of a dual-polarized HFSWR in the east coastal region of China [

Set a group of successive notches that cover the ionosphere clutters region locating in Doppler cells 132–195 at the range cell 157. Figure

Clutter cancellation performance by different segment numbers.

In each segment, the target signal with an ideal polarization will be recovered without any distortion. However, the polarization of some targets may be different with an ideal one and a distortion of amplitude and phase will be introduced in these segments. As a result, the targets (or Braggs) are also suppressed when the segment number increases (see Doppler cell 200 in Figure

Therefore, while the segmentation of RTDPS improves the performance of clutter cancellation, the coherent integration of the targets with polarization mismatching will be influenced. Then the segment number should be balanced between the clutter suppressing and target restoration. In our simulation, the segmentation number is set as

Figure

Results of RDDPS and RTDPS at range cell 69.

In Figure

Figure

Results of RTDPS at range cell 158.

Figure

The original and the processed range-Doppler map.

In this paper, a novel ionosphere clutter cancellation scheme is proposed. Two types of ionosphere clutter cancellation methods, RDDPS and RTDPS, are designed based on the theory of OPPF. RTDPS is suitable in dealing with powerful ionosphere clutter, whereas the RDDPS performs a stable filtering effect to suppress the clutter of low power. The selection of RDDPS or RTDPS algorithm is determined by the clutter type at the corresponding range cell. Combining the RDDPS and RTDPS together, the ionosphere clutters can be suppressed deeply. Theoretical analysis and experimental results demonstrate that the proposed scheme is valid and the SIR can be increased more than 15 dB. It is indicated that the proposed method may serve as a suitable method for ionosphere clutter cancellation in a practical HFSWR system.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This project is sponsored by the National Natural Science Foundation of China (no. 61171180).