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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.

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.


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