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Measured ionospheric Doppler spreading of HF ground backscatter
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1983
[PDF-249.66 MB]
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Description:Doppler spectra from ionospherically propagated ground backscatter are presented. These spectra show how the ionosphere distorts HF backscatter radio waves by frequency spreading. For example, such frequency spreading degrades ocean waveheight measurements by mixing first- and second-order parts of the sea-echo spectrum. The ground backscatter spectra presented here show that ionospheric Doppler spreading is so ubiquitous that its effect should never be disregarded, even when a narrow azimuth beam radar such as ours is used. Nevertheless, Doppler spreading varies so quickly that it usually pays to wait for it to diminish. For example, had we been measuring ocean waveheight under the ionospheric conditions present during our 25 minutes of ground backscatter measurements, the waveheight errors would have varied from about 20% to about 200% if the significant waveheight had been about 0.5 m. In fact, the waveheight error would have decreased from 200% to 20% in 5 minutes. The ionospheric Doppler spreading varies considerably as the reflection region in the ionosphere varies also. Phase spectra are also shown, and in contrast with surface-wave sea-echo spectra, average skywave backscatter phase spectra have significant trends. Although multipath combined with differential ionospheric motions probably cause the observed Doppler spreading, the multipath structure is not apparent in the observed spectra, which appear more continuous than discrete.
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Rights Information:CC0 Public Domain
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