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.
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.
MARACOOS CODAR Sites
CODAR Sites Location
1. University's HF Radar sites
- University of North Carolina - HFRadars, Sea Surface Currents
- Old Dominion - Surface Current Mapping in the Lower Chesapeake Bay
- Rutgers - CODAR Surface Current Maps
- Stevens - NY Bight Apex Present Surface Velocity
- University of Connecticut - Block Island Sound Codar Viewer, Western Long Island Sound Codar Viewer
- University of Rhode Island - Ocean Surface Currents (CODAR)
- University of Massachusetts Dartmouth/SMAST - Nauset Station Radials
2. MARACOOS links
- Rutgers - New Optimal Interpolation Product
- MARACOOS HF Radar Site Owners - Excel File
- MARACOOS HF Radar Blog - password protected.
3. History of MARACOOS HF Radar Quality Control/Quality Assurance
- Websites: - MARACOOS Radial Plots
- MARACOOS WebServers: NAUS, BLCK, SILD, HOSR, HOOK, SEAB, BRIG, WILD, VIEW, ASSA, LISL, CBBT
- Documents: HFradarConsortiumPilot_Mar2006.doc
- Figures: Radial Site Analysis - Missing Data for Oct 2009, Nov 2009, Dec 2009, Jan 2010, Feb 2010, Mar 2010
- Presentation: Results from Mid Atlantic High Frequency Radar Network