HF-Radar Technology

High Frequency (HF) radar is used by ocean researchers to measure surface current velocity fields near the coast. A HF radar system can measure surface currents with a top resolution of 1 km with a maximum range of 200 km, the two quantities being inversely proportional. The temporal resolution is usually on the order of an hour. The resulting surface plots provide a much higher resolution in space than previous techniques like current meter arrays. These vector plots allow mesoscale features, like coastal eddies, to be resolved with much more accuracy than an array of current meters. All the HF radars used in the Mid Atlantic are the SeaSonde manufactured by Codar Ocean Sensors.

5 mHz 13 mHz 13 mHz old 25 mHz

So how does HF-Radar work?

The basic mechanics of a HF radar system lies in the analysis of a backscattered radio wave. A HF radar system works very much like a radio station in that it emits a radio signal. While a radio station does not monitor the signal that is scattered back to the station, a CODAR site uses this back-scattered radio wave to calculate surface currents.
If the ocean were completely flat, no signals would be back-scattered. Since the ocean is not flat, it scatters the radio signal in many different directions. It is the scattered signal that is directed back to the receive antenna that is used to measure the surface currents.
The data acquisition of the system can be broken down in to three main components in order of accuracy:

I. Current Velocity of Target

The most accurate measurement of a HF radar is the radial velocity of the surface currents.  The radar can only measure the component of the surface current that is along a radial line from the receiver.  It performs this measurement by calculating the Doppler shift of the received signal from the transmitted signal.  The radar measures the velocity of the ocean wave as well as the underlying current.  The theoretical speed of the ocean wave is calculated using linear wave theory.  Thence the underlying current speed is calculated by removing  the velocity of the ocean wave from the velocity measured by the radar.

II. Range to Target

Most conventional radar systems measure the distance to a target by measuring the time delay of the return signal. If the speed of the signal and the time is known, then the total distance traveled can be calculated. The range to the target would then be half the total distance. The problem with this method is that the SeaSonde system needs to be resolved to very fine grid points (about 1 km). Since it does not take very long for a signal traveling at the speed of light to move 1 km, a very sensitive watch is needed. CODAR overcomes this problem by sending out a frequency modulated (fm) signal.

III. Angular Direction of Target

The direction of the target is determined using the signal received by three different antennas. The three antennas include two loop antennas and a monopole. Each antenna has a different beam pattern. The monopole receives the same signal independent of the incoming direction, omnidirectional. Signal information received by the monopole can therefore be used to normalize the information collected by the two loop antennas. The signal received by the two loop antennas is dependent upon the incoming direction. They are oriented ninety degrees to each other so that they can be used in combination to determine the incoming direction of the signal. When information from the two loop antennas are normalized with the monopole signal, the MUSIC algorithm is used to determine the direction of the signal. This process is referred to as Direction Finding and allows a CODAR system to have a directional resolution of one degree.


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CODAR Sites Location

name longitude latitude frequency photos

MAB Regional - 5MHz

NAUS -69.9472 41.8443 5Mhz photo
NANT -69.9719 41.2498 5Mhz photophotophotophoto
MVCO -70.5268 41.3498 5Mhz photophotophoto
BLCK -71.5509 41.1527 5Mhz photophotophotophoto
MRCH -72.7455 40.7887 5Mhz photophotophotophoto
HOOK -73.9838 40.4332 5Mhz photophotophotophoto
LOVE -74.1171 39.7362 5Mhz photophotophotophotophotophoto
BRIG -74.3621 39.4074 5Mhz photophotophotophoto
WILD -74.8532 38.9537 5Mhz photo photo photo photo
ASSA -75.1529 38.2050 5Mhz photophoto
CEDR -75.5923 37.6729 5Mhz photo
LISL -75.9226 36.6917 5Mhz photophotophoto
DUCK -75.7501 36.1803 5Mhz photo
HATY -75.5199 35.2572 5Mhz photophoto

NJ Coastal - 13 Mhz

SEAB -73.9727 40.3617 13Mhz photophotophotophoto
BELM -74.0052 40.1961 13Mhz  
SPRK -74.0720 39.9352 13Mhz  
BRNT -74.1983 39.6156 13Mhz  
BRMR -74.3612 39.4082 13Mhz  
RATH -74.6664 39.1926 13Mhz  
WOOD -74.7931 38.9877 13Mhz  

Eastern Long Island Sound - 25Mhz

BISL -71.5518 41.1526 25Mhz photo
MISQ -71.8042 41.3229 25Mhz photo
MNTK -71.8567 41.0710 25Mhz photophoto

Western Long Island Sound - 25Mhz

GCAP -73.6238 40.9826 25Mhz photo
STLI -73.5873 40.9087 25Mhz photo

NY Harbor - 25Mhz

BRZY -73.8826 40.5617 25Mhz photophotophotophoto
SILD -74.1245 40.5436 25Mhz photophotophotophoto
HOSR -73.9836 40.4322 25Mhz photophotophotophoto

Delaware Bay - 25Mhz

CMPT -74.9608 38.9314 25Mhz photophoto
HLPN -75.0889 38.7936 25Mhz photophoto

Chesapeake Bay - 25Mhz

SUNS -75.9722 37.1372 25Mhz photo
CBBT -76.0627 37.0462 25Mhz photo
VIEW -76.2432 36.9499 25Mhz photophoto
CPHN -76.0168 36.9308 25Mhz photo

HF-Radar Data

1. University's HF Radar sites

2. MARACOOS links

3. History of MARACOOS HF Radar Quality Control/Quality Assurance