16
mode. To address this shortcoming, specialized single depth transducers emit three
or four acoustic beams horizontally from the acoustic head providing data on current velocity at the depth of the sensor thus filling the bin blanked in the profiler
mode (Fig. 2.3).
Remote High Frequency Radar Surface Current Sensing The report by Barrick
and collaborators in 1977 entitled “Ocean Surface Currents Mapped by Radar,”
based on a body of observations dating back to 1955, constituted a breakthrough in
the field of ocean current measurement allowing the creation of synoptic maps of
nearshore surface currents from ground-based stations. These new radars operating
in the high frequency (HF) and very high frequency (VHF) radio bands (well below
conventional microwave radar bands) are known as high frequency radar (HFR).
Commercial HFR in the USA builds upon the direction-finding design of Barrick
et al. (1977).
The method relies on the phenomenon of Bragg scattering whereby radiation
backscattered off recurrent features is amplified through positive interference when
the radiation wavelength is exactly twice that of the recurrent target pattern and the
backscatter angle is exactly 180°. Bragg scattering is well known for its application
in X-ray crystallography where distances to be measured are on the order of 10
−10
m.
Ocean surface waves exhibit wavelengths on the order of tens of meters, for which
distances HF/VHF radio at frequencies from 3 to 300 MHz span the environmentally relevant range. Return echoes, when plotted as absolute power returned (in dB)
against displacement from the transmission (Tx) frequency (in MHz), are visualized
as Bragg peaks, displaced by a few kHz to either side of the central transmit Tx
frequency denoting Doppler displacement by approaching or receding ocean surface waves. Underlying currents however cause further displacement since the
waves are traveling along a moving medium. Since the speed of deep water ocean
waves is precisely known, the residual Doppler displacement can be attributed to
the underlying current. In practice, a Tx/Rx pair is emplaced as close as possible to
the seashore avoiding interfering structures or vegetation (Fig. 2.4).
Fig. 2.3 Acoustic Doppler current profiler. One of three profiling transducers is visible in the head
well. Two transducers of the three in the single depth array are visible mounted flush to the housing
cylinder
2 Electronic Sensors and Instruments for Coastal Ocean Observing
mode. To address this shortcoming, specialized single depth transducers emit three
or four acoustic beams horizontally from the acoustic head providing data on current velocity at the depth of the sensor thus filling the bin blanked in the profiler
mode (Fig. 2.3).
Remote High Frequency Radar Surface Current Sensing The report by Barrick
and collaborators in 1977 entitled “Ocean Surface Currents Mapped by Radar,”
based on a body of observations dating back to 1955, constituted a breakthrough in
the field of ocean current measurement allowing the creation of synoptic maps of
nearshore surface currents from ground-based stations. These new radars operating
in the high frequency (HF) and very high frequency (VHF) radio bands (well below
conventional microwave radar bands) are known as high frequency radar (HFR).
Commercial HFR in the USA builds upon the direction-finding design of Barrick
et al. (1977).
The method relies on the phenomenon of Bragg scattering whereby radiation
backscattered off recurrent features is amplified through positive interference when
the radiation wavelength is exactly twice that of the recurrent target pattern and the
backscatter angle is exactly 180°. Bragg scattering is well known for its application
in X-ray crystallography where distances to be measured are on the order of 10
−10
m.
Ocean surface waves exhibit wavelengths on the order of tens of meters, for which
distances HF/VHF radio at frequencies from 3 to 300 MHz span the environmentally relevant range. Return echoes, when plotted as absolute power returned (in dB)
against displacement from the transmission (Tx) frequency (in MHz), are visualized
as Bragg peaks, displaced by a few kHz to either side of the central transmit Tx
frequency denoting Doppler displacement by approaching or receding ocean surface waves. Underlying currents however cause further displacement since the
waves are traveling along a moving medium. Since the speed of deep water ocean
waves is precisely known, the residual Doppler displacement can be attributed to
the underlying current. In practice, a Tx/Rx pair is emplaced as close as possible to
the seashore avoiding interfering structures or vegetation (Fig. 2.4).
Fig. 2.3 Acoustic Doppler current profiler. One of three profiling transducers is visible in the head
well. Two transducers of the three in the single depth array are visible mounted flush to the housing
cylinder
2 Electronic Sensors and Instruments for Coastal Ocean Observing