17
In newer mid- and short-range systems, a single Tx/Rx antenna is now used but
paired Tx/Rx antennae are still required for long-range systems. Long-wave, low
frequency HFR operating at 5 MHz is naturally tuned to 30 m wave length ocean
waves whereas an intermediate HF range of 12 MHz reflects off 12.5 m waves.
Nevertheless, as for many systems here discussed, there is a tradeoff between range
and resolution with shorter wavelengths providing greater resolution at the cost of
shorter range and vice versa. A horizontal range of up to 200 km from the coast at
resolution of 6 km is achieved with the low frequency systems, while high frequency systems can provide resolution down to 0.5 km at ranges limited to
15–30 km. Directional discrimination of return signals received at a single Rx
antenna is achieved by means of orthogonal induction coils in addition to the omnidirectional vertical whip antenna (Fig. 2.5).
Only wave fronts directly approaching toward, or receding from, the antenna will
produce significant Bragg scatter since those traveling at an angle to or parallel to
the antennae emplacement will scatter Bragg waves away from the Rx antenna or
not at all. Consequently, the current vector resulting from backscatter of radiation
emitted by a collocated Tx antenna at any given point within the field of the Rx
antenna will be radial to this location (Fig. 2.6).
A Tx/Rx emplacement at a judicious distance (tens to hundreds of km) from the
first allows the construction of a second radial plot. Resultant surface current vectors can be computed from the superposition of radial vector fields from the two
sites. Distance from the target to the antennae emplacement is estimated by the time
delay of the return signal. Resulting data is averaged within bins that can vary from
Fig. 2.4 Installing a high frequency surface current radar antenna emplacement on the west coast
of Puerto Rico. Left: The receiving antenna mounted at the end of a dock. The housing at the top
of the support mast holds orthogonal induction coils (see Fig. 2.5). Right: Transmit antenna
mounted on the beach. Workers in the background are burying the Rx antenna cable run.
Omnidirectional Tx and Rx antenna are mounted vertically and ground planes extend laterally
2.2 Electronic Sensors and Instruments for Ocean Observing
In newer mid- and short-range systems, a single Tx/Rx antenna is now used but
paired Tx/Rx antennae are still required for long-range systems. Long-wave, low
frequency HFR operating at 5 MHz is naturally tuned to 30 m wave length ocean
waves whereas an intermediate HF range of 12 MHz reflects off 12.5 m waves.
Nevertheless, as for many systems here discussed, there is a tradeoff between range
and resolution with shorter wavelengths providing greater resolution at the cost of
shorter range and vice versa. A horizontal range of up to 200 km from the coast at
resolution of 6 km is achieved with the low frequency systems, while high frequency systems can provide resolution down to 0.5 km at ranges limited to
15–30 km. Directional discrimination of return signals received at a single Rx
antenna is achieved by means of orthogonal induction coils in addition to the omnidirectional vertical whip antenna (Fig. 2.5).
Only wave fronts directly approaching toward, or receding from, the antenna will
produce significant Bragg scatter since those traveling at an angle to or parallel to
the antennae emplacement will scatter Bragg waves away from the Rx antenna or
not at all. Consequently, the current vector resulting from backscatter of radiation
emitted by a collocated Tx antenna at any given point within the field of the Rx
antenna will be radial to this location (Fig. 2.6).
A Tx/Rx emplacement at a judicious distance (tens to hundreds of km) from the
first allows the construction of a second radial plot. Resultant surface current vectors can be computed from the superposition of radial vector fields from the two
sites. Distance from the target to the antennae emplacement is estimated by the time
delay of the return signal. Resulting data is averaged within bins that can vary from
Fig. 2.4 Installing a high frequency surface current radar antenna emplacement on the west coast
of Puerto Rico. Left: The receiving antenna mounted at the end of a dock. The housing at the top
of the support mast holds orthogonal induction coils (see Fig. 2.5). Right: Transmit antenna
mounted on the beach. Workers in the background are burying the Rx antenna cable run.
Omnidirectional Tx and Rx antenna are mounted vertically and ground planes extend laterally
2.2 Electronic Sensors and Instruments for Ocean Observing