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Pushing the Range
of Your SeaSonde
Hardware configuration options to expand the observable
range of your SeaSonde,
potentially beyond 300 km
Operating Frequency
One easy way to achieve greater range is to
operate at a lower frequency band. This has
been known for decades, and is the reason
CODAR engineers designed the SeaSonde
for operation across a wide band, ranging from 4.4
MHz up to 50
Click image to enlarge
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MHz. The lowest of the SeaSonde
transmit frequency bands (within 4.4 - 6 MHz)
allows for greater ranges than the higher bands
without any increase in radiated power. In general,
the average daytime observable range achieved by a
SeaSonde operating near 4-6 MHz (Long-Range
mode) is typically 160-220 km. Actual coverage
varies on many factors, such as exact antenna
placement, sea conditions, external noise as well as
some user selectable software settings.
Twin Transmit Antennas
Another method for increasing observable range is
adding a second transmit (TX) antenna. A single
monopole antenna exhibits a uniform,
omnidirectional transmit signal pattern, meaning
that the radiated power is equally distributed across 360
degrees-- including sections over land. The simple addition of a second
transmit antenna allows for (beam control) and
through proper orientation & phasing the antennas can direct more power
transmitter signal power out towards sea. It is
most common for this configuration to be used on the lower SeaSonde bands
(5-14 MHz), but is possible at any frequency.
This hardware configuration option is available when ordering a new SeaSonde
Remote Unit or as a retrofit. The offcial
name is Twin Transmit Antenna Configuration.
The directionality of the beam fan produced by the TX antenna pair can be
controlled by adjusting the spacing between the
two elements. Spacing the two elements closer together will cause the signal
strength to "fan out", so that the power is
spread across a wider angle. As the two antennas are pulled apart, the resulting
beam pattern becomes narrower, sending a
stronger signal out towards a more specific angular sector. This produces
even greater range, but inside a narrower angle
sector. The antenna spacing (and hence the TX beam fan spread) is customized
at the radar site and optimized for that
SeaSonde user's goals.
SeaSonde Coverage to 334 km Shown
Click image to enlarge
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Dual Transmitter - Twin Transmit
Antenna Configuration for
Maximum Range
A third way to extend range is to increase the
TX radiated power. However, when it comes
to increasing power, one quickly reaches the
point of diminishing returns on investment.
The SeaSonde transmitter output power is 40
watts average, which is low and also highly
practical from a design and results perspective.
Increasing the output power of a single
transmit power supply creates more internal
heat, placing more wear on itself and other
nearby parts inside the transmitter chassis.
Hence these other parts, including heat sinks,
must be upgraded to perform under this
greater heat stress. This drives the price of a
radar up to an uncomfortable cost level.
Instead of increasing power from a single transmitter,
CODAR offers a more practical solution –– that is
addition of a second transmitter. This SeaSonde Remote
Unit configuration is referred to as SeaSonde Dual
Transmitter –– Twin Transmit Antenna
Configuration. In this setup, two transmitters are
connected to a set of twin TX antennas. This doubling
of the transmit power and the focusing its beam out
towards sea can make a dramatic difference in daytime
observable range, extending it by as much as 90 km. To
put this into perspective, the range increase is the same
as would result from a single transmitter with a single
antenna that had its power increased from 40 to nearly
200 watts.
A deployment of the special configuration units several
years ago in the Gulf of Mexico had radars positioned at
bottom of the Southwest Pass in Louisiana and on an oil
platform to the west. Daytime average observable
ranges for these units consistently stayed above 240 km
and at times the Southwest Pass radar unit had ranges up
to 340 km were reached.
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Click images to enlarge |
| A demonstration of range improvement was conducted
in September 2008. Data from a Long-Range SeaSonde unit operating
in Bodega, California is shown here. Normal range of this unit
is 170-190 km (shown at right). When the system hardware is modified
to Long-Range SeaSonde with the dual transmitter - twin TX antenna
configuration the current maps extend out past 300 km in certain
sectors (shown at far right). |
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Data Produced by
Long-Range SeaSonde |
Data Produced by
Dual TX- Twin TX Antenna
Long-Range SeaSonde |
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The odd-shaped Vermilion 31A oil
platform,
nearly 1000 meters long, had plenty of space
for the
Long- Range twin TX antennas
and receive antenna.
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SeaSonde antennas at Southwest Pass
were installed in marshland only accessible by boat. |
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Total vector current map produced by
combining the Vermilion & Southwest Pass radials,
extending to 334 km offshore.
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