The over-the-horizon radar is able to detect ships over very large oceanic areas. Detection performance is sometimes reduced because of smearing of the sea echo due to variations of the Doppler shift imposed by the ionosphere. In this paper we propose a alecontamination procedure, based upon maximum entropy spectral analysis of sea scatter Bragg lines, to correct the phase of the signal before performing classical fast Fourier transform processing. The improvement in quality of the signal after phase correction results in a sharpening of spectral peaks. In the example presented the signal was chosen to contain spectral lines of a transponder installed on board a ship, to be used as reference lines. The decrease in the spectral width is significant for the sea scatter Bragg lines and for the transponder lines. As the Bragg lines sharpen, the ship echo is more easily detected in the spectrum. The proposed decontamination procedure involves more computation load than the simpler frequency shift method used for sea state monitoring, but, on the other hand, it can handle Doppler shift variations during coherent integration time.
The effects of geomagnetic pulsations on the Doppler shifts of HF backscatter radar echoes are studied. Three mechanisms are considered: (1) variation of the phase refractive index under the influence of the varying magnetic component, (2) modulation of the plasma density, and (3) electrodynamic drift of the ionosphere. It is shown that the electrodynamic drift is probably the most efficient mechanism to account for the measured Doppler shifts. In this model the Doppler shift on F propagation modes is sensitive to the eastward electric field of the wave, while E propagation modes are sensitive to the northward and eastward components with weights depending on the reflection height. Pi 2 and Pc 3 effects on the Doppler shift of backscattered echoes are presented in the experimental part. In one example of continuous pulsations it is found that the coherence of ground magnetic variations and HF echo Doppler shifts is high near specific frequencies in the spectra. The coherence of radar measurements in resolution cells about 100 km apart is nearly unity and the cross-phase spectra show very small phase differences. An example of the effects of Pi 2 pulsations on F and E s propagation modes is presented. and Garriott [ 1964-[, Duffus and Boyd [ 1968], Klostermeyer and Rb'tt•jer [1976], Menk et al. [1983] and Sutcliffe and Poole [1984]. Rishbeth and Garriott [1964] have proposed two possible interpretations of the Doppler effect associated with the pulsations. The first alternative is a dynamo effect in the E region with a polarization electric field transmitted upward in the F region and resulting in a E x B drift. The second alternative is the motion of the F layer under the direct influence of the magnetohydrodynamic (MHD) wave. Rishbeth and Garriot [1964] have calculated the corresponding Doppler shifts, but they were found to be usually smaller than the measured ones. These ideas were further considered by Duffus and Boyd [1968] and K!osterrneyer and Rb'tt•ler [1976], who have found that the hydromagnetic wave interpretation explained their observations well. Sutcliffe and Poole [1984] have considered two models to interpret their measurements at vertical incidence. In the first one the phase refractive index/• varies with the magnetic component of the MHD wave. In the second one the ionosphere undergoes a cross-field drift under the influence of the varying electric field of the wave, which is similar to the second alternative of Rishbeth and 183 184 BOURDILLON ET AL.: EFFECTS OF GEOMAGNETIC PULSATIONS ET AL.: EFFECTS OF GEOMAGNETIC PULSATIONS 187
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