NEW Advanced Feature!
Enhancing
SeaSonde Networks With Bistatic / Multi-Static Functionality
New Multi-Static Software for SeaSondes
Multi-static function is a straightforward augmentation that will allow
expanded and improved current-mapping coverage inside a SeaSonde network.
This patented technique has been under development at CODAR for over ten
years and is now available for commercial release. The subsequent sections
describe the technology and how it can be used to enhance both new and
existing SeaSonde networks.
Defining Monostatic & Bistatic
Every SeaSonde, and all other commercial ocean observing HF radars, are
backscatter -- or monostatic -- radars. This means transmitter and receiver
are co-located together. When the transmitter is positioned away from the
receiver by tens of kilometers, this unconventional variation is called "bistatic".
The Bistatic Geometry Made Simple
A backscatter radar measures observables like currents in a polar coordinate
system. Contours of constant time delay are range circles about the radar.
Doppler shift from the transmitted frequency gives a velocity component
along bearing spokes from the radar, and these are called "radials",
i.e., perpendicular to a radial circle. A bistatic radar measures observables
in an elliptical coordinate system, where constant time delay contours
of the echo are ellipses. This family of ellipses has the transmitter and
receiver locations as the ellipse focal points. Doppler shift gives a component
of velocity that falls on hyperbolas passing perpendicular through the
ellipses. These velocities are referred to as "ellipticals".
The measurements below illustrate radials and ellipticals measured off
the coast of New Jersey with a bistatic-enhanced SeaSonde.
An example of "radials" and "ellipticals" for
a New Jersey-sited 25-MHz SeaSonde with a buoy-mounted bistatic
transmitter is shown in the above figure. These are measured
and processed simultaneously.
|
Why Bistatic Radars Are Uncommon in HF Ocean Observing
In any radar, the transmit signal must be coherent with the receiver signal
generator. That's easy when they are together, because the same signal
source can be used for both. When they are separated, accurate frequency
and time synchronization is the drawback. CODAR invented and patented a
methodology based on GPS timing along with CODAR’s unique, patented
FMCW (frequency-modulated continuous wave) gated signals to control the
exact sweep time of multiple transmitters down to nanoseconds so all transmitters
can occupy the exact same frequency channel. This enables a single receive
antenna to process unambiguously scattered signals from multiple transmitters.
The signals from various transmitters are identified and separated in the
demodulation phase.
Defining Multi-Static: Simultaneous Monostatic & Bistatic
Operation
Bistatic wouldn't have significant value if a network still ended up with
the same quantity of data, the only noticeable difference being that transmitters
and receivers are separated from each other (switching from monostatic
to bistatic). However, this is not the end of the process; With the CODAR-patented
methods, one receiver can see its own backscatter echoes and produce radial
maps, and can also see those signals from several other appropriately placed
transmitters and process each of those transmitter signals to produce ellipticals,
all at exactly the same time (not sequentially by on/off switching). This
is called" multi-static". It can both extend coverage and increase
data density inside monitoring area.
 |
 |
| Coverage and quality of four backscatter
Long-Range SeaSondes off the coast of New Jersey. Dark
red indicates best quality of total vectors; yellow going
to white indicates poor or no cove |
Enhancement obtained by adding a
transmitter on a buoy 150 km offshore, operating multi-statically
with the four SeaSondes on the left. Higher quality (darker
color) and greater coverage area. |
|
The Bottom Line: Benefits of Multistatic Function
One can expect an extension in coverage area. This can vary between 30%
(e.g., where only the existing coastal SeaSonde radar transmitters are
used) up to 100% if stand-alone bistatic transmitters are judiciously placed
(along coast, on buoys, islands or offshore structures). CODAR staff have
tools to help predict and optimize this coverage based on your existing
or proposed network. One can expect more robust, accurate current vectors,
and fewer gaps within the existing coverage area. More measurements from
different angles of the current field at a point will lead to a more accurate
total vector. The figures above show an example where a buoy transmitter
is added to augment an existing four-SeaSonde coastal network off New Jersey.
Here the coverage area has more than doubled, and the darker shading denotes
more accurate total vector mapping.
Transmit Sources For Bistatic / Multi-static Networks
Scenario #1: Utilizing Transmit signals from other SeaSonde Remote Units
A transmitter from one conventional, backscatter SeaSonde remote unit can
be the source for bistatic echoes for any of the other nearby SeaSonde
receivers (inside a different SeaSonde remote unit). If a network of overlapping
backscatter SeaSondes is already in place, producing total vector maps
among them -- and they have our GPS-assisted timing package called "SHARE" --
this can be the foundation of a multi-static network. In this case, without
adding any additional hardware the SeaSonde network can be converted into
a mutli-static network by installing CODAR’s new bistatic/multi-static
signal processing software package at some or all of the receiver stations
inside network. For example, four adjacent backscatter SeaSondes can be
converted into as many as ten multi-static echo sources for the same patches
of sea.
Scenario #2: Adding Stand-alone Bistatic Transmitters to
a SeaSonde Network
 |
Shown here is SeaSonde bistatic
transmitter mounted onto buoy off New Jersey coast.
|
|
As orientation between transmitters and receivers affects the bistatic
coverage area, there can be benefit to placing additional transmitter(s)
at strategic locations. The stand-alone bistatic transmitters offered by
CODAR consist of a transmitter, transmit antenna, and an Iridium communication
link for simple remote control of transmitter. These units cost less than
a complete SeaSonde Remote Unit and require less space and infrastructure,
allowing operation inside an even wider variety of environments. The fact
that there is no receive antenna reduces siting constraints. Absence of
a computer and receive system lowers the overall power requirement significantly
(the bistatic transmitter and Iridium link require approximately 100 watts
power total). Refer to SeaSonde Bistatic Transmitter Product Information
Sheet for further details.
What Is Needed?
If this is an existing SeaSonde backscatter network outfitted with SHARE
technology, it needs only software to convert into a Multi-static network:
one software package for for each receiver (SeaSonde remote unit) that
is to receive bistatic echoes from any number of transmitters. This software
package will be configured and keyed to that single receiver. For example,
suppose one SeaSonde remote unit is to receive echoes from three other
coastal SeaSonde remote unit transmitters, you need only one software package
for that receiving SeaSonde remote unit and it can produce one set of radials
and two or three sets of ellipticals. Additional software packages are
required for each additional receiver that will be processing data bistatically
or multi-statically indie the network. CODAR staff can help select which
SeaSonde remote unit transmitters will work best with others in the vicinity,
with a special visualization code. This shows the expansion in coverage
area, as well as the increase in robustness within the existing backscatter
map region. One may consider stand-alone transmitters along with the existing
or proposed coastal network. These bistatic transmitters could be on buoys,
on an offshore platform or island, or at a coastal point. Again, CODAR
staff can help decide if and where such an option might be desirable, using
the visualization code mentioned above. In this case, it will be necessary
to purchase the additional bistatic transmitter hardware, while the need
for receiver Bistatic/Multi-static data processing software packages remains
the same. Contact CODAR for further details.
|
