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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.
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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.
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| 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. |
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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
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Shown here is SeaSonde bistatic transmitter
mounted onto buoy off New Jersey coast.
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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.
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