**The
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 it's 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 the 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 wave

12 MHz transmission -> 25m EM wave -> 12.5m ocean
wave

5 MHz transmission -> 60m EM wave -> 30m ocean wave

So far three facts about the Bragg wave are known: its wavelength,
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. 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 vector.Basic HF Radar for
Current Mapping At a SeaSonde HF radar station there is
one transmitting antenna and one receiving antenna unit.
The antennas are connected to the radar transmit chassis
and receive chassis, which are controlled by a small desktop
computer.

The transmitting antenna is omni-directional, meaning that
it radiates a signal in all directions. The receive antenna
unit consists of three co-located antennas, oriented with
respect to each other on the x, y, and z-axes (like the
sensors on a pitch and roll buoy). It is able to receive
and separate returning signals in all 360 degrees.

For mapping currents, the radar needs to determine three
pieces of information:

1. Bearing of the scattering source (which we'll refer to
as 'Target'),

2. Range of the Target, and

3. Speed of the Target

To determine bearing, range and speed of the Target, a time
series of the received sea echo is processed.

**The first determination is Range to target.**

The distance to the patch of scatterers in any radar depends
on the time delay of the scattered signal after transmission.
The SeaSonde employs a unique, patented method of determining
the range from this time delay. By modulating the transmitted
signal with a swept-frequency signal and demodulating it
properly in the receiver, the time delay is converted to
a large-scale frequency shift in the echo signal. Therefore,
the first digital spectral analysis of the signal extracts
the range or distance to the sea-surface scatterers, and
sorts it into range 'bins' (typically set to 32 bins, but
capable up to 64 bins). In HF versions of the SeaSonde,
these bins are typically set between 1 and 12 km in width.
In the VHF version of the SeaSonde, these bins are typically
set between 300 m and 1.5 km.

**The second determination is Speed from Doppler of
the target.**

Information about the velocity of the scattering ocean waves
(which includes speed contributions due to both current
and wave motions) is obtained by a second spectral processing
of the signals from each range bin, giving the Doppler-frequency
shifts due to these motions. The length of the time series
used for this spectral processing dictates the velocity
resolution; at 12 MHz for a 256-second time-series sample,
this corresponds to a velocity resolution ~4cm / s. (The
velocity accuracy is a separate quantity; it can be better
or worse than this depending on environmental factors.)
The SeaSonde or any radar can measure only the velocity
component from Doppler 'radial' to the radar from the target
on the ocean, meaning that component pointing toward or
away from the radar.

**The third determination is the Bearing of the target.**

After the range to scatterers and their radial speeds have
been determined by the two spectral processing steps outlined
above, the final step involves extraction of the bearing
angle to the patch of scatterers. This is done for the echo
at each spectral point (range and speed) by using simultaneous
data collected from the three colocated directional receive
antennas. The complex voltages from these three antennas
are put through a 'direction-finding' (DF)algorithm to get
the bearing. The particular, patented algorithm adapted
and perfected for the SeaSonde is referred to as MUSIC.
At the end of these three signal-processing algorithms,
surface-current radial speed maps are available in polar
coordinates. That is, the radial speeds on the ocean are
specified vs range and bearing about the origin, which is
the radar site.

Radial data is produced at interfals varying between 18
minutes for the low-frequency systems to 4 minutes at the
upper frequencies. These data are then averaged over a user-selected
time period (typically an hour), to create a radial vector
map at the radar station. A computer called the central
data combining station, located at the users office, connects
to the radar station computer at user-selectable time intervals,
and retrieves the radial vector map data files.

**From radial speed map to total surface current velocity
vector map**

Radial speed maps from each radar site alone are not a complete
depiction of the surface current flow, which is two-dimensional.
This is why at least two radars are normally used to construct
a total vector from each site's radial components. At the
central data combining station, the radial vector maps from
multiple radar stations are merged to create a total velocity
vector current map.