Unique Nature of HF Radar
High-frequency (HF) radio
formally spans the band 3-30 MHz (with wavelengths between 10 meters at the upper
end and 100 meters at the lower end). For our radars, we extend the upper limit
to 50 MHz. A vertically polarized HF signal is propagated at the electrically
conductive ocean water surface, and can travel well beyond the line-of-sight,
beyond which point more common microwave radars become blind. Rain or fog does
not affect HF signals.
The ocean is a rough surface, with water waves of many different periods.
When the radar signal hits ocean waves that are 3-50 meters long,
that signal scatters in many directions. In this way, the surface
can act like a large diffraction grating.
But, the radar signal will return directly to its source only when
the radar signal scatters off a wave that is exactly half the transmitted
signal wavelength, AND that wave is traveling in a radial path either
directly away from or towards the radar. The scattered radar electromagnetic
waves add coherently resulting in a strong return of energy at a
very precise wavelength. This is known as the Bragg principle, and
phenomenon 'Bragg scattering'. At the SeaSonde's HF/VHF frequencies
(4-50 MHz), the periods of these Bragg scattering short ocean waves
are between 1.5 and 5 seconds.
What makes HF RADAR particularly useful for current mapping is that
the ocean waves associated with HF wavelengths are always present.
The following chart shows three typical HF operating frequencies and
the corresponding ocean wavelengths that produce Bragg scattering.
25 MHz transmission -> 12m EM wave -> 6m ocean
12 MHz transmission -> 25m EM wave -> 12.5m ocean wave
transmission -> 60m EM wave -> 30m ocean wave
So far three facts
about the Bragg wave are known: its wavelegnth, period, and travel direction.
Because we know the wavelength of the wave, we also know it's speed very precisely
from the deep water dispersion relation.
The returning signal exhibits a Doppler-frequency shift. In the absence
of ocean currents, the Doppler frequency shift would always arrive
at a known position in the frequency spectrum.
But the observed Doppler-frequency shift does not match up exactly
with the theoretical wave speed. The Doppler-frequency shift includes
the theoretical speed of the speed of the wave PLUS the influence
of the underlying ocean current on the wave velocity in a radial path
(away from or towards the radar).
The effective depth of the ocean current influence on these waves
depends upon the waves period or length. The current influencing the
Bragg waves falls within the upper meter of the water column (or upper
2.5 meters when transmitting between 4-6 MHz). So, once the known,
theoretical wave speed is subtracted from the Doppler information,
a radial velocity component of surface current is determined.
By looking at the same patch of water using radars located at two
or more different viewing angles, the surface current radial velocity
components can be summed to determine the total surface current velocity
Is it that simple?
The basic physics relating the HF radar signal to
the nature of the ocean waves and currents is beautifully simplistic, but the
task of mapping surface currents with a modern radar sensor is more complex.
Continue reading for more details.