Abstract: A frequency surveillance method for monitoring background atmospheric noise evels in unoccupied channels in an operating frequency band of a high frequency communication system. In the method, the unoccupied channels are scanned a number of times to obtain analogue signals. The obtained signals are transformed to a digital form and averaged. The averaged digital signals are filtered to remove substantially all radio frequency interference. Absolute signals are then produced by scaling or comparing the filtered signals with a known noise level. The background noise levels are obtained by further averaging the absolute signals. A visual display unit is used to display the background noise levels as a function of frequencies. A frequency surveillance apparatus is also disclosed.

Abstract: A method of obtaining oceanographic and meteorological data from high frequency radar spectral information including the steps of transmitting a plurality of signals to a remote geographic location, recording signals backscattered from the said location, generating a family of Doppler clutter spectra from the recorded backscattered signal and analyzing the Doppler clutter spectra in terms of a model or models to generate oceanographic and meteorological data estimates. The Doppler spectra are categorized according to Suitability for Deatiled Analysis (SDA). The SDA takes account of the clutter-to-noise ratio in the spectrum, the amplitudes of the Bragg-lines, multimode propagation, spectral broadening and other spectral parameters. Spectra with a low SDA are inappropriate for extracting directional wave spectrum estimates of reasonable accuracy but are useful for obtaining wind direction estimates.