Coastal Ocean Dynamics Applications Radar - How Does IT Work? (Theory Behind Its Functioning)

How Does It Work? (Theory Behind Its Functioning)

CODAR operates using sky transmission of waves in the HF band (3–30 MHz), as electromagnetic waves in this band have wavelengths that are commensurate with wind-driven gravity waves on the ocean surface. According as the customer necessities, it can be used in single or multi-frequency mode. As the ocean has a rough surface, when a high frequency signal reaches the ocean surface, a portion of the incident energy is scattered back towards the source and the receiver measures the reflected signal. This backscattering (or reflection) produces an energy spectrum at the receiver, even if the energy source is single-frequency, because of the shape and motion of the sea surface. Interpreting the spectral returns for various transmit frequencies is the key to extracting information about the ocean and, specifically, to measure surface currents.

As a consequence of the Bragg’s scattering law, the strongest received return comes from ocean waves that are traveling directly toward or away from the radar source and whose wavelength is exactly one-half as long as the transmitted radar wave. The return signal is then processed and its spectral analysis provides the sea-echo Doppler spectrum, where two dominant peaks at different frequencies can be recognized).

Displacement of these peaks away from their known frequencies is called “echo Doppler shift” and permits to obtain the radial velocity of a surface current, that is the scatter velocity along the line between the hit surface and the radar. In fact, the magnitude of this component of the velocity is proportional to the amount of the shift. Therefore, CODAR measures this Doppler frequency shift (along with the distance from the radar to the sector and directional angle) to estimate the radial velocity over a sector of the sea surface.