Satellite Global Positioning Systems (GPS) are best known for their ability to precisely determine location in space. But equally valuable, and even more precise, are GPS time signals, which when processed through special commercial receivers, provide time or frequency precision of one part in 10^-12.

CODAR engineers have created a method that exploits the GPS clock function to allow series of transmitters to all operate within the same allotted channel bandwidth simultaneously. It does so by very precisely controlling the start time of each SeaSonde transmitter's frequency sweep, and staggering the sweep starts so each signal can be identified and separated during demodulation in the receiver. One frequency channel can serve all of the radars without mutual interference or performance degradation.

This will minimize the impact on radio spectral utilization, making it easier to obtain government radio broadcast authorizations, and also to find channels free of external interference. More radars can be added to a network with no demands for additional bandwidth.

This GPS Timing Synchronization feature is being offered as an Optional Add-On product for the Standard and Hi-Res SeaSondes (12-50 MHz). Because of the limited availability of radio spectral space, and the distances traveled by radio broadcasts at lower frequencies, this GPS will be included in all Long Range SeaSondes (4-6 MHz).

Below are two other ways which GPS Synchronization can be used to expand SeaSonde utility. We intend to add these system features to our commercial product list in late 2002, and are presently refining and field-testing various prototypes.

GPS Synchronization for Future Bi-Static Operation:
A SeaSonde receiver and transmitter need not be located at the same place. Instead, transmitters can be placed away from the receive antenna (referred to as a Bi-Static Configuration). With bi-static operation, the transmitter unit does not need a computer controlling it, and is not sensitive to temperature or its environment, which means it can be placed at many locations where a regular backscatter radar cannot not work. For example, a Bi-Static Transmitter has been placed on a buoy powered with solar-powered batteries off the New Jersey Coast. It was installed December 2001 and is still in operation.

GPS Synchronization in Future Multi-Static Operations:
Controlling the exact sweep time of multiple transmitters will allow for a single receive antenna to process unambiguously scattered signals from multiple transmitters. The various transmitters will be identified and separated in the demodulation phase. Each transmitter's signal will produce a set of scalar current vectors on an elliptical coordinate grid, confocal to that TX and its RX. So, if several transmitters are positioned correctly, then a single receiver can produce total vector current maps.

For more information on the GPS Timing Synchronization Feature, please contact our office

Long Range SeaSondes Operating on 3 U.S. Coastlines -
October 2000

Continuous, Real-Time mapping of ocean surface currents has been raised to the next level in the United States with the installation of Long Range SeaSondes along the Pacific and Atlantic coastlines, as well as modified Long Range units in the Gulf of Mexico. The Long Range SeaSondes are able to map currents beyond the continental shelf, coverage from shore averaging 190 km in the day, less during nighttime when environmental noise levels increase.
 
The Long Range design is the latest addition to the SeaSonde family of coastal HF radars. providing the largest coverage area (approx. 190 km) with the coarsest resolution (3 - 6 km).
 
Below are sample radial maps produced from Long Range SeaSondes.



^
Radial map above produced by Long Range SeaSonde unit at 6pm, 26July2000 GMT. The range shown here is about 190 km, the average range of the unit during daytime. At night, the environmental noise level increases causing a reduction in range.

Upper plot below shows radial surface velocity contours as observed by Moss Landing, CA Long-Range SeaSonde at beginning of storm, with wind vector measured at M3 mooring shown at white arrow. Lower plot shows radial currents eighteen hours later after wind rotated so it is coming from the West. Coverage in lower plot extends slightly beyond 200 km.