Benjamin T. Root

HF‐over‐the‐horizon radar ship detection with short dwells using clutter cancelation

Coherent sea-clutter cancelation is shown to be an effective method for the exposure of ships with HF OTH (high-frequency over-the-horizon) sky wave radar. A sinusoidal model has been successfully used to approximate and subtract the first-order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s). (Short dwells are required for aircraft surveillance over the vast HF radar coverage area.) The new clutter canceling algorithm has been successfully applied to both synthetic and real OTH data from the U.S. Navys Relocatable Over-the-Horizon radar (ROTHR) (AN/TPS-71), and results are presented.

HF radar ship detection through clutter cancellation

High-frequency over-the-horizon (HF-OTH) radar uses ionospheric refraction to detect targets at thousands of kilometers. A timely surveillance of large areas requires relatively short dwells (coherent integration times) of less than 10 seconds. The resulting resolution in the Doppler Fourier transform is sufficient for detecting fast targets such as aircraft, but slow targets such as ships require integration times 12-30 seconds long to distinguish the ship peaks from the much more powerful ocean clutter. By modeling the first-order clutter as sinusoidal and subtracting it, we are able to expose ships in dwells as short as 3 seconds that would otherwise be masked by the mainlobe spread of the clutter. This technique is applied to data from the US Navys relocatable OTHR (ROTHR). This approach is an alternative to superresolution techniques and may sometimes be more robust.

Maneuvering target detection in over-the-horizon radar by using adaptive chirplet transform and subspace clutter rejection

In over-the-horizon radar (OTHR) target detection, the signal-to-clutter ratio (SCR) is very low, typically from -50 dB to -60 dB. Furthermore, for maneuvering targets, such as aircraft and missiles, Doppler frequencies of their radar return signals may be time-varying. In this case, the Fourier transform based techniques and super resolution spectrum estimation techniques may not work well since they use sinusoidal signal models. We propose a signal subspace clutter rejection algorithm combined with an adaptive chirplet transform technique for maneuvering target detection with OTHR. Simulation results of adding simulated maneuvering targets into raw OTHR clutter data are presented to illustrate the effectiveness of the proposed algorithm. The simulation results show that moving targets with -53.5 dB SCR can be detected.

High-frequency over-the-horizon radar ship detection through clutter cancellation: an alternative to high-resolution spectral estimation

High-resolution spectral estimation is the resolution of spectral components that cannot be distinguished in the Fourier transform. This paper presents what is, in effect, an unusual techniques to perform high-resolution spectral estimation. By canceling powerful and obscuring ocean clutter, weak ship targets are exposed in the short-time Fourier transform of a particular kind of radar data. This achieves the same result as high-resolution spectral estimation, but it also preserves the desirable properties of the Fourier transform. A specific clutter-canceling algorithm has been developed and successfully tested on data from the US Navys Relocatable Over-the-Horizon Radar. A side-by-side comparison of this algorithm with Tufts-Kumerisan forward-backward linear prediction, a popular high-resolution spectral estimator, will be presented. Although both techniques performed well at detecting the ship and estimating its Doppler frequency, the clutter cancellation techniques seems to provide a better estimate of the ship power relative to the peak of the clutter. This property would be useful in target identification.

Joint time-frequency analysis of over-the-horizon radar data

This paper examines the application of joint time-frequency analysis (JTFA) to the detection of weak targets that are close to the powerful ocean clutter in signals received by over-the-horizon radar (OTHR). Issues of interest are the detection of targets near time-varying clutter and the use of JTFA to model the clutter for the purpose of excision. Comparisons are made between the spectrogram and the particular JTFA algorithm applied here, namely, the Smoothed Pseudo Wigner-Ville Distribution (SPWVD). Results are shown for real and synthetic signals. The SPWVD of the real signal has been successfully modeled by a synthetic signal, which increases our understanding of the behavior of this transform when applied to OTHR clutter data.

Ionospheric distortion mitigation techniques for over-the-horizon radar

High-Frequency radar detects targets at thousands of kilometers over the horizon by refracting its beam from the ionosphere. A disturbed ionosphere may distort the signal severely, especially in auroral and equatorial regions. The powerful ground clutter spreads in doppler and masks targets. This distortion is sometimes assumed to take the form of a random complex time-varying distortion function multiplying the time-domain signal. Simple and effective techniques have been developed to mitigate this distortion provided that either the amplitude or the phase of the distortion predominates. The general case of severe amplitude and phase distortion is much more difficult. The techniques are highly model- dependent but are sometimes reasonable for HF radar signals. An emphasis is placed on making the algorithms efficient, so that they can run in real time and keep up with the flood of radar data. The distortion model is first analyzed by phase-screen concepts that model the physics of the electromagnetic propagation through the turbulent ionosphere. To date the techniques have been tested on simulations, since the I/Q data collected thus far do not exhibit the kind of distortions for which these techniques are applicable.

Coherence in over-the-horizon radar

The coherence of successive dwells from an Over-the-Horizon (OTH) High-Frequency (HF) radar (3-30 MHz) is investigated using a technique based on the spectrogram. Although the data are coherent within a dwell, it is not known if coherence is preserved from one dwell to the next, due to possible limitations of the signal processor. (Incoherence imposed by the propagating medium is not considered here.) Land clutter consisting of a sufficiently clean complex sinusoid should reveal the coherence of data across dwells. Unfortunately, the ionosphere imposes multipath and distortion such that it is difficult to obtain sufficiently clean dwells, but a few cases indicate a linear spectral phase offset on the second dwell consistent with a virtual shift of the time origin that can, in principle, be easily compensated for.

Autocalibrating the Wink 2D random array

This paper describes an initial attempt to calibrate a large, random, sparse, high-frequency, 2-dimensional array using the transmissions from radio stations. A semi-quantitative discussion is presented of various intuitive ideas for calibration, along with samples of typical results from numerical testing using synthetic and real data. First, a theoretical discussion of the effect of calibration errors is provided, in which a distinction is made between mild and severe calibration errors. Then a variety of techniques are suggested for both cases. For mild calibration errors, in which true peaks are still apparent in the spectrum, a simple approach is presented where the information in the true peaks is used to approximate the field at the array, from which the correct calibration can be deduced. This technique will converge to the correct solution with sufficient independent data sets. For severe calibration errors, in which the spectrum contains only speckle, several techniques are proposed to obtain a crude calibration of the array. One technique fits a plane wave to the uncalibrated receiver voltages. Another technique forces or assumes a plane wave at the array and then deduces the error by comparing different data sets. The third technique uses a Monte Carlo approach to generate the calibration weights, and a discussion of the correct interpretation of the results is provided. If this crude initial calibration can reduce the calibration errors to the mild case, then the calibration can continue in a two-step procedure using the techniques for the mild case.

RADARC model comparisons with Amchitka radar data

Data collected with the AN/TPS-71 relocatable over-the-horizon radar (ROTHR) has been used to validate the RADARC Model. For the first 10 days of January, April, and July, 1200 and 0000 UTC, the vertical ionograms, oblique backscatter soundings, radar ground backscatter amplitudes, and noise levels have been compared with model predictions. The differences between the model and median h′ƒ soundings are small enough to have negligble effect on predictions. The coverage predicted for oblique backscatter soundings agrees well with observed median data except for sporadic E effects which were underestimated. Medians of observed ground backscatter levels agree fairly well with predictions. The actual noise that controlled radar performance was the most significant deviation from the model; all nighttime and January day noise was higher than that predicted, and this was due to spread-in-Doppler clutter appearing as noise.