27
Acoustic Anemometers
Acoustic anemometers are similar to acoustic current meters in that they rely on
orthogonal paired transducer arrays but dissimilar in that they measure time-offlight rather than echo return. Taking advantage of their two-way functionality,
members of a pair of acoustic transducers alternate their Tx and Rx functions. The
time-of-flight difference reveals the net wind component along the array. Readings
from the orthogonal pair allow computation of the resulting wind vector. These
instruments operate with very low energy draw with a maximum range of wind
velocity of 270 km.h
−1
(Fig. 2.11).
Satellite-Borne Scatterometers
Satellite-borne scatterometers provide an effective means of estimating wind fields
over large swaths of the ocean by measuring the return radiation scattered off ocean
waves when irradiated obliquely. A patch of ocean is repeatedly irradiated with
microwave radiation (C and Ku radar bands) at different azimuth angles, and the
radar cross section (σ 0 ), essentially the intensity of the return radiation relative to
the irradiation power, is recorded. Absolute intensity provides a measure of the
average wave height. The directional component is derived from the differential
azimuth readings since, assuming a steady wind field over the course of the readings, for any given wave height, σ 0 , will be greatest when the slanted beam points
toward incoming waves and minimal when the wave field is orthogonal to the
antenna orientation. In practice, a rotating antenna is used to achieve quasi-synoptic
differential azimuth readings required for calculation of the vector component.
Scatterometer satellites travel in near-earth polar orbits covering swaths of width up
to 1800 km with instrument footprints on the order of 25–50 km.
Fig. 2.11 Acoustic
anemometer. Four
ultrasonic transducers are
visible
2.2 Electronic Sensors and Instruments for Ocean Observing
Acoustic Anemometers
Acoustic anemometers are similar to acoustic current meters in that they rely on
orthogonal paired transducer arrays but dissimilar in that they measure time-offlight rather than echo return. Taking advantage of their two-way functionality,
members of a pair of acoustic transducers alternate their Tx and Rx functions. The
time-of-flight difference reveals the net wind component along the array. Readings
from the orthogonal pair allow computation of the resulting wind vector. These
instruments operate with very low energy draw with a maximum range of wind
velocity of 270 km.h
−1
(Fig. 2.11).
Satellite-Borne Scatterometers
Satellite-borne scatterometers provide an effective means of estimating wind fields
over large swaths of the ocean by measuring the return radiation scattered off ocean
waves when irradiated obliquely. A patch of ocean is repeatedly irradiated with
microwave radiation (C and Ku radar bands) at different azimuth angles, and the
radar cross section (σ 0 ), essentially the intensity of the return radiation relative to
the irradiation power, is recorded. Absolute intensity provides a measure of the
average wave height. The directional component is derived from the differential
azimuth readings since, assuming a steady wind field over the course of the readings, for any given wave height, σ 0 , will be greatest when the slanted beam points
toward incoming waves and minimal when the wave field is orthogonal to the
antenna orientation. In practice, a rotating antenna is used to achieve quasi-synoptic
differential azimuth readings required for calculation of the vector component.
Scatterometer satellites travel in near-earth polar orbits covering swaths of width up
to 1800 km with instrument footprints on the order of 25–50 km.
Fig. 2.11 Acoustic
anemometer. Four
ultrasonic transducers are
visible
2.2 Electronic Sensors and Instruments for Ocean Observing