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
colocated 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
1. Bearing of the scattering source (which we'll refer to as 'Target'),
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
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
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
The central data combining station does more than combine radial vector
maps to create total vector maps. It archives current, wave, and diagnostics
data on disk, both internal and external. In some cases, it creates
JPEG or GIF files of outputs for real-time internet transfer to the
users website. It can display wave data if that is being collected.
Finally, it can be used as a base to connect to the remote radar sites
and direct their operations as well as install or retrieve special
files, as though the operator were standing in front of the remote
station monitor itself.