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| HF
Radar |
| What
is an HF Radar?
High
Frequency (HF) Radar systems are a type of oceanographic tools
that provide detailed data about the ocean surface. These
systems monitor a very large area of the ocean, on the order
of hundreds of square miles, as opposed to traditional oceanographic
sensors, such as buoys, which only record data from a single
point location. HF radars are land-based systems which transmit
electromagnetic waves over the ocean. When these electromagnetic
(EM) waves hit the ocean surface, they are reflected in many
different directions by surface waves. However, electromagnetic
waves that hit surface waves of exactly half their wavelength
are reflected back to the radar system. These backscattered
waves provide measurements of important phenomena such as
surface currents and waves. |
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| Principles
of Operation:
HF radars
operate based on the Doppler effect
principle. When an electromagnetic wave hits a surface
wave, the EM wave's wavelength is altered via interference
with the surface wave's wavelength. Depending on whether the
surface wave is traveling towards or away from the shore,
the EM wave's wavelength will either decrease or increase.
Based on this shift in wavelength, know as a Doppler shift,
the wavelength, and hence velocity, of the surface current
can be determined.
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Figure
1.
HF radar systems emit electromagnetic waves of uniform
wavelength. When these EM waves hit surface waves of
half their wavelength, they are reflected back to the
radar station, as shown here. |
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Figure
2. The surface wave will interfere with the
electromagnetic wave and alter its wavelength. If the
surface wave is traveling towards shore, the EM wave's
wavelength will decrease. If the surface wave is traveling
away from shore, the EM wave's wavelength will increase.
This phenomenon is known as Doppler
shift. |
| Each
individual HF radar station can only collect data along
radial directions. For instance, a single station can
only determine the speed of waves approaching or receding
along radial directions. In order to determine the true
direction and velocity of surface waves, at least two
HF radar systems must share overlapping coverage. When
two radar systems are monitoring the same region, one
records the "x" component of velocity, and the
second records the "y" component, enabling true
velocity to be graphed in the x-y plane. Thus, in order
to have a strong network of HF radar coverage, many systems
must be stationed incrementally along the coast, providing
continuous overlapped coverage. |
Figure
3. This
map depicts the radial coverage from the Pritchards
Island HF radar site. By only using data from one HF
radar system, we only get information on the speed of
the currents approaching and receding along radial directions. |
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| Figure
4. This figure shows two current HF radar systems,
brand name WERA, in the Southeast area. The first is stationed
at Pritchards Island, SC; this coverage zone is shown in blue.
The second system is stationed at St. Catherine's Island,
GA; this coverage zone is shown in purple. The overlapping
region is where true two-dimensional surface current data
is recorded.
Read
more about the SC / GA HF Radar project here.
Furthermore,
the range of an HF radar system depends on the wavelength
of the electromagnetic waves emitted. As wavelength increases,
coverage area increases. So, long-range HF radar systems use
a low frequency (long wavelength), and can achieve coverage
that extends several hundred miles off-shore. However, these
systems do not have good coverage near-shore or in small areas
such as ports. Therefore, high-frequency (short wavelength)
systems are used to monitor smaller coverage areas near shore.
In general, there are three classifications of HF radar systems:
long-range, standard-range, and short-range.
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| Figure
5. This figure depicts existing HF radar coverage along
the eastern coast of the United States. The mid-Atlantic region
has extensive coverage, comprised of nested long, standard,
and short range systems. This network provides detailed data
regarding both near-shore and off-shore conditions. In contrast,
the Southeastern region has very sparse coverage. |
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HF
Radar Capabilities
HF Radars
can measure the following information:
- Surface
Current Magnitude & Direction
- Wave
Height & Direction
Furthermore,
data is reported in near real-time, which allows access to
current ocean conditions.
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Figure
6. This map shows wave height and direction, as
recorded by an HF radar system located near Key Largo, FL.
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HF
Radar Applications
HF Radar
systems can provide detailed information about surface current
trends over large areas of ocean. This detailed, real-time
data is essential in many situations, such as the following:
- Minimizing
environmental destruction from hazardous spills by accurately
understanding surface flow patterns
- Improving
extreme weather tracking and prediction by monitoring current
ocean conditions
- Expediting
maritime search and rescue by providing rescuers with accurate,
details surface current and wave information
- Improving
Homeland Security by monitoring ship traffic
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Author: Megan Schuler |
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