18
0.5 km for high frequency/short range systems to up to 6 km for mid- and long range
systems. Units operating in the mid-frequency at 13 MHz provide detailed surface
short range current maps at 2 km resolution and longer range views at 6 km resolution (Fig. 2.7).
Since seawater is electrically conductive, HF radiation propagates as a ground
wave that can be viewed as a half-wave propagating along the surface and extending
at half-wave intervals above the water surface. This property allows transmitted
radiation to project beyond the horizon along the geoid and backscattered Bragg
signals to be detected at the source. Transmit and receive antennae may be fitted
with horizontal ground plane antennae orthogonal to the vertical omnidirectional
antenna to enhance signal transition from the ocean surface to the Rx antenna.
High frequency radar efficacy is highly dependent on salinity since ground wave
propagation depends on the electrical conductivity of the aqueous ionic medium.
Experimental range determination operating a 42 MHz system in San Francisco
Bay was performed by plotting the hourly rate of effective radial returns against
salinity. The resulting response is of a rectangular hyperbolic form ranging from a
null intercept at practical salinity 3 to a maximum of about 1000 possible returns per
hour at salinity above 25. Experiments with a 4.5 MHz long-range unit over Lake
Michigan, USA, yielded a maximum effective range out to about 20 km, while the
range of such a unit operating over ocean water is greater than 200 km. See: https://
www.ioos.noaa.gov/wp-content/uploads/2015/12/use_hfr_freshwater_paper2012.
pdf (Accessed Sept 9 2017).
An alternate means of achieving directionality using HF is that of the beam forming phased-array system. This system offers superior performance to that of the
direction-finding design in nearshore applications (Gurgel et al. 1999) but requires
Fig. 2.5 Directional HFR
induction coils in the Rx
antenna platform
discriminate orientation of
the returning signals
2 Electronic Sensors and Instruments for Coastal Ocean Observing
0.5 km for high frequency/short range systems to up to 6 km for mid- and long range
systems. Units operating in the mid-frequency at 13 MHz provide detailed surface
short range current maps at 2 km resolution and longer range views at 6 km resolution (Fig. 2.7).
Since seawater is electrically conductive, HF radiation propagates as a ground
wave that can be viewed as a half-wave propagating along the surface and extending
at half-wave intervals above the water surface. This property allows transmitted
radiation to project beyond the horizon along the geoid and backscattered Bragg
signals to be detected at the source. Transmit and receive antennae may be fitted
with horizontal ground plane antennae orthogonal to the vertical omnidirectional
antenna to enhance signal transition from the ocean surface to the Rx antenna.
High frequency radar efficacy is highly dependent on salinity since ground wave
propagation depends on the electrical conductivity of the aqueous ionic medium.
Experimental range determination operating a 42 MHz system in San Francisco
Bay was performed by plotting the hourly rate of effective radial returns against
salinity. The resulting response is of a rectangular hyperbolic form ranging from a
null intercept at practical salinity 3 to a maximum of about 1000 possible returns per
hour at salinity above 25. Experiments with a 4.5 MHz long-range unit over Lake
Michigan, USA, yielded a maximum effective range out to about 20 km, while the
range of such a unit operating over ocean water is greater than 200 km. See: https://
www.ioos.noaa.gov/wp-content/uploads/2015/12/use_hfr_freshwater_paper2012.
pdf (Accessed Sept 9 2017).
An alternate means of achieving directionality using HF is that of the beam forming phased-array system. This system offers superior performance to that of the
direction-finding design in nearshore applications (Gurgel et al. 1999) but requires
Fig. 2.5 Directional HFR
induction coils in the Rx
antenna platform
discriminate orientation of
the returning signals
2 Electronic Sensors and Instruments for Coastal Ocean Observing