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High Frequency Radar: Sea Surface Current and Wave Sensors
 
Radar antenna and processing electronics
Instrument Type: CODAR Ocean Sensors SeaSonde HF Radar System
Description: High-frequency (HF) radar uses radio-wave backscatter to map surface currents over wide swaths of the coastal ocean. These are commonly referred to as HF Radar Instruments and take advantage of the propagation of radio waves over large distances out and back across the surface of the conducting sea water, and they have resonant interactions with ocean surface waves (i.e., the ones that can make people seasick). Back at the shoreline, a measurement of the frequency offset (i.e., Doppler shift) of the reflected radio waves provides a remotely sensed measurement of the speed of the ocean waves and surface currents. Sea surface current data is measured with SeaSondes, compact HF radars designed by CODAR Ocean Sensors, Ltd. They use patented FMCW (frequency modulated continuous wave) signal processing and crossloop direction finding to measure ocean surface currents and wave parameters of the coastal ocean. Each station consists of a transmitter, receiver, 2 antennas, and data acquisition and processing computers. A transmitter broadcasts a frequency modulated radio frequency pulse at 2Hz (two pulses per second). The Doppler shifted return signal (sea echo) is detected with a compound cross loop/monopole receive antenna and the signal is processed into estimates of surface current speed and direction and wave heights, period and direction. BML's three 12 MHz stations have a spatial resolution of 2 km and create hourly maps of surface currents out to a distance of 30-50 km (20-30 miles) along a 65 km (40 mile) length of shore. Onshore wave directional spectral data are also produced. Total coverage area for the 3- radar array is approximately 2800 km2. BML's two 5 MHz stations have a spatial resolution of 5 km and create hourly maps of surface currents out to a distance of 90-200 km (55-125 miles). Onshore wave directional spectral data are also produced.
Locations:
5 MHz - Bodega Marine Laboratory: 38° 19' 10.1" N 123° 4' 25.0" W - Installed: 2 February 2007
5 MHz - Point Arena: 38° 55' 42.4" N 123° 43' 40.0" W - Installed: 16 June 2007
12 MHz - Bodega Marine Laboratory: 38° 19' 2.3" N 123° 4' 20.9" W - Installed: May 2001
12 MHz - Point Reyes: 38° 2' 49.7" N 122° 59' 20.9" W - Installed: June 2001
12 MHz - Salt Point Point State Park: 38° 34' 0.5" N 123° 19' 53.7" W - Installed: 2002
12 MHz Specifications - Surface currents:
Map spatial resolution: 2 x 2 km
Map temporal interval: hourly; currents averaged over one hour
Map area coverage: 65 km along shore x 30 km offshore, variable
Map vector accuracies: speed < 7 cm/s, direction < 10°
Specifications - Waves:
Target range: 3 km from coast around each radar
Significant wave height accuracy: 7 to 15%
Dominant on-shore direction accuracy: 5° to 12°
Dominant wave period accuracy: 0.6 s
Minimum detectable significant wave height: 1 m
Wave temporal interval: hourly; waves averaged over one hour
Specifications - Radiated signal:
Output radiated power: 80 watts peak, 40 watts average
Operating frequency range: 12-14 MHz
Sea Surface Current Data
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