Bragg Resonance in Measured L-band Radar Clutter Doppler Spectra from Inland Water

A highly developed Bragg resonance phenomenon was observed in measurements of vertically polarized L-band radar backscatter recorded from the surface of a small inland fresh-water reservoir in central Massachusetts. These measurements were obtained with Lincoln Laboratory's LCE (L-band Clutter Experiment) radar sited on Wachusett Mt. from which the reservoir surface was visible at 4-km range and at 4° depression angle, on a day of light winds with the radar looking obliquely downwind. The highly developed Bragg resonance in the backscatter from the reservoir surface caused strong Bragg spikes to exist in the clutter Doppler spectrum from the reservoir at low (viz., +3.4 and -3.8 Hz) but nonzero-Doppler frequencies. The sharp narrow first-order Bragg spikes rose 20 to 25 dB above adjacent spectral levels in the clutter spectra from the reservoir surface. The maximum amplitude of the dominant Bragg spike at -3.8 Hz was 40 to 45 dB stronger than tree clutter at -3.8 Hz in nearby spatial cells and only 10 dB weaker than the peak zero-Doppler tree clutter in the same nearby cells. This strong Bragg resonance at vertical polarization was persistent in time (15 min) and space (all seven range gates for which the reservoir surface was visible to the radar) throughout the course of measurements on the light-winds measurement day. Spectral results are presented for both cross-polarized and both copolarized combinations of linear polarization. Bragg spikes were observed to exist across this polarization matrix of measurements, although at horizontal polarization the overall backscatter from the reservoir surface, including the Bragg resonant component, was much weaker than at vertical polarization. On a second measurement day of stronger winds and changed wind direction (such that the radar was looking upwind), significant Bragg resonance still existed in the backscatter from the reservoir surface but was weaker than on the light-winds day. Strong Bragg spikes at low but nonzero-Doppler frequencies from small inland bodies of water might potentially cause false alarms for moving target indicator or other Doppler signal processing techniques designed for target detection in ground clutter. Accesion For NTIS CRA&i DTIC TAB Unannounced Justification -I