High frequency radar and its application to fresh water

a r t i c l e i n f o Keywords: Remote sensing High frequency radar Coastal processes Instrumentation High frequency (HF) radar has become an important tool for remotely mapping the spatial distribution and temporal evolution of waves and currents of the nearshore coastal ocean. Its acceptance along ocean coasts has resulted in the development of several commercially available systems and a planned nationwide coastal network to routinely measure coastal currents. Because HF radiation is known to propagate less efficiently over fresh water than seawater, it has been largely overlooked as a viable tool for freshwater application. However, its potential utility in freshwater was clearly demonstrated by a deployment along Lake Michigan as part of the 1999–2001 Episodic Events Great Lakes Experiment. As part of this experiment, the University of Michigan Multi-frequency Coastal Radar consistently produced reliable near surface current measurements to a range of approximately 25 km offshore showing strong correlation with both in-situ measurements and numerical hind-casts. This paper provides background on HF radar technology, a summary of the current state of the art with respect to freshwater and describes the results of a recent experiment to measure the propagation of HF radar signal over freshwater using CODAR Ocean Sensors SeaSondes, operating at 5 and 42 MHz with 21 W and 90 W average radiated powers, respectively. The effective offshore range for these radars was found to be 18 km at 5 MHz and 4–5 km at 42 MHz. These findings are consistent with currently available models for the prediction of propagation loss, verifying that they can reliably be used to estimate ranges in freshwater settings. Introduction The coherent backscatter of high frequency (HF) radar waves (10 to 100 m wavelengths) at grazing incidence from the ocean surface was first observed by Crombie (1955). He found that the peak in the backscattered Doppler spectrum was located approximately at the frequency of ocean surface gravity waves with wavelengths on the order of one half the radar wavelength. These findings indicated that radio waves reflected from the sea obey Bragg scattering in which the ocean surface acts as a diffraction grating. To first order, the strong HF echo arises from a Bragg scattering interaction with ocean waves traveling radially with respect to the radar and having a wavelength of one half the radar wavelength. Observed Doppler spectra, as shown in Fig. 1, reveal the dominant, first order Bragg …