Giuseppe Fabrizio

Robust adaptive beamforming for HF surface wave over-the-horizon radar

Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia.

Spatial adaptive subspace detection in OTH radar

The work presented here addresses the problem of target detection against spatially structured interference composed of jamming plus noise, where for practical reasons, the received target wavefront may also deviate from the traditional plane wave model. This detection problem arises in over-the-horizon (OTH) radar systems where spatially distributed targets often compete for detection against directional interference that is spread over the entire range-Doppler search space. Conventional detection processing schemes are compared with a recently proposed adaptive subspace detector (ASD) that takes both the spatial structure of the interference and the possibility of target wavefront distortions into account. Experimental array data recorded by the Jindalee sky-wave and Iluka surface-wave OTH radar systems, located in central and northern Australia respectively, is used to evaluate detection performance.

Stap for Clutter and Interference Cancellation in a Hf Radar System

An alternative skywave line-of-sight (SkyLOS) high frequency radar architecture has been proposed for early detection and tracking of ballistic missiles. It consists of a skywave over-the-horizon (OTH) radar augmented by one or more ground-based systems for line-of-sight reception in the illuminated region. The line-of-sight systems provide additional Doppler-time profiles of the target with different observation geometry to improve flight trajectory estimation. Targets compete for detection against powerful clutter and interference from manmade and natural sources. We introduce a practical STAP technique to deal with operational signal environments and demonstrate its performance on live SkyLOS data

Non-stationary interference cancellation in HF surface wave radar

High frequency (HF) interference in surface wave over-the-horizon (OTH) radars typically exhibits a time-varying or non-stationary spatial structure. Adaptive beamformers that update the spatial filtering weight vector within the coherent processing interval (CPI) are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal de-correlation of the clutter which severely degrades sub-clutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses non-stationary interference without degrading sub-clutter visibility is proposed. The new algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HF surface wave (HFSW) OTH radar, located near Darwin in far north Australia.

A multipath-driven approach to HF geolocation

Abstract A method is proposed for HF emitter geolocation and mode group-range estimation when the signal of interest is received via skywave propagation by a uniform linear array (ULA) of antennas at a single site. Ionospheric multipath and the geometrical relationships that constrain such propagation are exploited to derive a new class of estimator for a problem that inherently produces an ambiguous solution when the modes are considered individually. The introduced multipath-driven approach resolves the coning ambiguity and opens up the possibility to perform meaningful HF single site location (SSL) with a ULA. The method is also applicable to reduce estimation errors in traditional HF-SSL systems based on two-dimensional apertures. Field-data gathered in a controlled experiment with known truth is used to validate the method.

Passive radar in the high frequency band

Passive radar systems using emitters of opportunity for target detection and tracking have received significant interest recently, especially those which exploit frequency modulated (FM) radio stations and TV transmitters as signal sources. This paper is concerned with passive radar systems that utilize signal sources in the high frequency (HF) band (3-30 MHz), where due to long-distance ionospheric propagation, the transmitter may be located beyond the line-of-sight (i.e. over-the-horizon). To investigate this application, a two-dimensional (L-shaped) antenna array with a digital receiver per element was tuned to simultaneously record cooperative and non-cooperative HF signal sources within the receiver bandwidth. This data was processed by conventional and adaptive methods to determine the detection performance of the system for a dedicated aircraft target. GPS data from the cooperative aircraft provided accurate ground truth of the target flight path and velocity, enabling its bi-static range/Doppler and direction of arrival (azimuth/elevation) versus time profiles to be calculated and compared with radar measurements. The experimental system and data processed in the paper is exclusively from the DSTO HF radar program.

Challenging Issues in Multichannel Radar Array Processing

This paper is focused on multichannel radar array processing and addresses some challenging issues concerning: the adaptation in nonstationary and nonhomogeneous environments, the value of Space-Time Adaptive Processing (STAP) in an operational High Frequency (HF) radar system, the benefits resulting from the joint use of multiple (not colocated) transmitters and receivers, and the parallel implementation of the multichannel processor. The work follows on where the importance of a multichannel processing is demonstrated with reference to airborne, ground-based, and Over The Horizon (OTH) radar systems.

adaptive mitigation of spread clutter in high frequency surface-wave Radar

High frequency surface-wave (HFSW) radars are cost-effective systems for maritime surveillance and sea-state mapping of ocean areas in the Exclusive Economic Zone (EEZ). The effective mitigation of disturbances, either from interference sources or unwanted radar echoes scattered from the ionosphere, is fundamental to ensure the successful operation of such systems. This paper addresses the mitigation of ldquoionospheric clutterrdquo spread in delay (i.e. range) and Doppler frequency by adaptive spatial filtering for the problem of target detection in surveillance applications. An alternative adaptive processing method that jointly exploits the spatial and temporal information of range-gated signals, over receivers and pulses, is proposed and applied to experimental data from an Australian HFSW radar. Target detection performance is compared against conventional and standard adaptive beamforming techniques. The author thanks the High Frequency Radar Branch of the Defence Science and Technology Organization (DSTO), Australia, for supporting this work and making the experimental data available.

A New Data Extrapolation Approach Based on Spectral Partitioning

In this letter, we propose a new linear predictive (LP) data extrapolation approach. It involves partitioning the spectrum into multiple spectral subbands and using a different autoregressive (AR) process to model each subband. The new extrapolation approach is then combined with the classical discrete Fourier transform (DFT) to produce a new hybrid LP-DFT spectral estimator to address the detection and estimation problem of multiple sinusoids in a discrete data sequence. Simulation results demonstrate the superiority of the proposed hybrid technique over an existing popular hybrid LP-DFT technique, where a single AR process is used to model the entire spectrum of the data sequence.

Geolocation of HF skywave radar signals using multipath in an unknown ionosphere

A method is proposed to jointly estimate the ground position of a point source, which transmits or scatters a known HF radar signal, and the virtual heights of reflection in an unknown ionosphere using the group-range and cone angle-of-arrival measured for each propagation mode by a multi-channel receive system connected to a uniform linear array (ULA) of antennas. Specifically, the method exploits multipath propagation of HF signals in the ionosphere and the geometrical relationships that constrain such propagation to derive a new estimator for a coordinate registration (CR) problem that inherently produces an ambiguous solution if the modes are considered individually. Field data gathered by the Australian Jindalee OTH radar in a controlled experiment with known truth information is used to validate the method.