On the variability of Doppler spectra in HF groundwave radar remote sensing over the ocean surface: An investigation based on pulsed and frequency modulated sources

An examination of the fluctuations of the Bragg peaks in high frequency (HF) radar received backscatter spectra from the ocean surface is considered in the light of recent cross section models. It is well known that there are likely several factors giving rise to this phenomenon. In the first part of the research work, the effect that is purely caused by the use of a pulsed Doppler radar waveform is examined. The radar received signal is assumed to be scattered from a time-varying random rough surface represented as a zero-mean, Gaussian random process. Numerical examinations of the existence and distribution of Bragg fluctuations are carried out first. Various radar operating frequencies and pulse widths are selected to simulate the time series for different conditions. Doppler spectra (power spectral density, PSD) are estimated as periodograms. The centroids of the Bragg peaks are obtained and compared with theoretical values and the Bragg fluctuations from time to time are observed. The statistical properties of the centroid positions are indicated and compared with resolutions of the fast Fourier transform (FFT) algorithm to reveal their significance. Then the physical processes which underlie the observed variability of the HF sea echo over short time periods are examined. It will be seen that, by implementing both numerical and analytical techniques, interrogation of such a surface via a pulsed signal inherently leads to temporal variation of echo power at every frequency position of the Doppler spectrum.;The second part of the research work is an investigation of the Bragg fluctuations when HF radar uses the frequency modulated waveforms, which include frequency modulated continuous wave (FMCW) and frequency modulated interrupted continuous wave (FMICW) sources. Such waveforms are often employed in practical radar systems. The mathematical expressions of the cross sections for the FMCW and FMICW waveforms are worked out and their properties are addressed. Then, similar to the previous analysis for the pulsed waveform, time series of the radar received electric field signals are simulated and the PSDs are calculated. Centroid positions of the Bragg peak regions are located and compared to the theoretical values. Statistical properties of the Bragg fluctuations are investigated and comparison with respect to that for the pulsed waveform is carried out. Field data from the Wellen Radar (WERA) are used to verify the simulation results and the fluctuations of the Bragg peaks for the FMCW waveform.