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.
KEYWORDS: Time-frequency analysis, Radar, Target detection, Doppler effect, Chemical species, Fourier transforms, Frequency modulation, Data modeling, Signal detection, Astatine
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.
In over-the-horizon radar (OTHR) moving target detection, the signal to clutter ratio (SCR) is low. One method to detect a moving target is to first reject the clutter and improve the SCR before the detection, such as the adaptive Fourier transform developed by Root when a target moves uniformly. When a target does not move uniformly, the Fourier based techniques for the target detection including super resolution techniques may not work well. In this paper, we replace the Fourier transform by the adaptive chirplet transform in the Doppler processing in OTHR when a target moves non-uniformly.
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.
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.
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 Navy's 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.
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