How Does the COOLroom Work?
- CODAR Lets Us Check
Out the Ocean Without Ever Leaving the Shore
If you have ever dropped something in the ocean, you have probably
seen how the ocean tends to carry it away from you and you've
watched it bob up and down in waves. The movement you've seen
is caused by the influence of the ocean's surface currents and
waves on the dropped object.
Wind and surface currents affect all objects on the surface of
the ocean. The
direction of surface current movement is the result of the interaction
forces, including salinity and density currents, land and sea
gravity and global rotation. For that reason, locating an object
floating in the
ocean can become a complex problem. It requires gathering and
several pieces of data.
In the COOLroom, oceanographers determine
surface currents and wave heights and frequency using information
gathered by a radar system called Coastal Ocean Dynamics Application
Radar, sometimes called Coastal RADAR, or CODAR for short. The
computers in the COOLroom interpret the data and then re-present
them as real-time maps of the ocean using arrows to indicate currents.
At certain beaches off New Jersey, they are also able to provide
real-time charts displaying wave height and speed.
How does CODAR work?
The Rutgers CODAR network is comprised of two systems:
- A long-range system using low frequency, that measures the surface
currents over an extended area larger than the size of New Jersey.
- A standard system that measures the sea surface current between
the towns of Brant Beach and Brigantine, New Jersey. This system
can only take measurements of a small area of the ocean but has
the advantage of producing high-resolution data and the system
can be easily moved to study other coastline currents in the future.
The flow of a particular parcel of water in the ocean is called
a current, and in order to map them, we measure the ocean for
its speed and direction or velocity. The CODAR systems take these
measurements by bouncing radio waves off the surface of the ocean.
Each CODAR site has two antennas: the first transmits a radio
signal out across the ocean surface and the second listens for
the reflected radio signal after it has bounced off the ocean's
waves. By measuring the change in frequency of the radio signal
that returns, the CODAR system determines how fast the water is
moving toward or away from the antenna. The standard system can
also determine the height and frequency of the waves near the
However, each antenna site can only determine how fast the water
is moving toward or away from that antenna. But the water might
actually be moving away and to the right or left at the same time
or, in other words, at an angle. Therefore, in order to determine
the ocean's actual direction, the COOLroom computers process measurements
taken of the same spot of the ocean at the same time from two
different antenna sites. Then the computers combine the readings
to come up with the net direction of the ocean.
Once the speed and direction of an object are
determined, it’s possible to predict where the object might
be headed in the near future. By using vectors, each distance
and time may be estimated or calculated with significant precision.
With the use of long-range CODAR, scientists
now can take measurements of surface water as far as 100 miles
offshore. In the future, buoys will be added even further offshore,
increasing the range significantly.
To learn how to read a CODAR
Surface Velocity Map go to the Control
Room and click on the CODAR
Check out the COOLroom to find out: today's
ocean surface currents and the
surface conditions off Tuckerton.
The COOLroom also archives
CODAR data if you want to research
a past date.
So, who uses this stuff?
People other than oceanographers find these surface velocity maps useful. The Coast Guard's Sea and Rescue uses them to track disabled and lost ships for survivors. The food that fish feed on tends to get trapped where two areas of water meet in a convergence zone and, therefore, fishermen find those areas on the velocity map where currents are pushing the water together. It is easier to steer a boat with the current rather than against it, so boaters and sailors use these images to track surface currents.
And, of course, surfers use them to figure out which beach has the biggest waves.
Doppler Shift Theory
Doppler Shift explains the change
in frequency of a signal when it is bounced off a moving object.
For example, a train whistle sounds differently depending upon
whether it is moving toward you or away from you and how fast
it is going.
One way to explain this is by imagining yourself throwing tennis
balls at a stationary object. The distance and time it takes for
each ball to leave your hand, strike the object, and then return
would be the same for each tennis ball. Thus, if you throw a ball
every two seconds, the balls should return to you in two second
Now imagine that the object is moving toward you. The first ball
leaves your hand, strikes the object, and returns. By the time
the second ball makes contact with the object, the object has
moved closer. Thus the return trip for the second ball is shorter
than the trip for the first ball. As a result, if the starting
intervals are two seconds between tennis balls, the return interval
will be less than two seconds.
This shift in time interval is called Doppler Shift. If the return
is less than the release interval, the target object must be moving
toward you. Conversely, if the return interval is greater than
the release interval, the object must be moving away.
CODAR works the same way. If two different
CODAR stations record how long it takes for their signal to bounce
off a target and return to the station, scientists can tell whether
the target is moving toward, or away from the station. All the
stations then combine their data (the component data), and determine
the actual resulting motion of the object.
to Josh Kohut describe how CODAR
uses radio waves to "listen" for which way the ocean