IEEE Geoscience and Remote Sensing Magazine - December 2016 - 39

sea/oceanic features, including swell waves and internal waves. The Bragg model applies only to slightly
rough surfaces under low wind conditions (it is often
used to describe the scattering from ripples in the twoscale model). The Bragg model has been used to interpret the ocean currents by high-frequency Doppler
radar measurements at large incidence angles [15].
To adequately explain the microwave scattering signature of water surfaces and apply its features to remote
sensing, a set of experiments (i.e., experimental verification of the combined frequency, azimuth, and incidence angles) and wind speed variations of the NRCS
are required [6]. For that study, a scatterometer, a radar
designed for measuring the surface scatter characteristics,
is used. The scatterometer sends microwave signals to the
surface and measures the power backscattered from it.
The NRCS can be calculated with the parameter of the
radar range equation using the following equation:
vc =

(4r)3 R 4 Pr
,
Pt G 2 m 2 A

(1)

where vc is the NRCS, R is the range to the target, P r
is the power received, P t is the power transmitted, G is
the antenna gain, m is the radar wavelength, and A is the
illuminated area. The NRCS refers to how much radar
power is reflected back to the scatterometer. Observing
the water surface at various incidences and wind-relative
azimuth angles and at different wind speeds enables the
scatterometers to measure the water-surface backscattering signature as a function of such parameters.
The NRCS can be converted into wind vectors using a
so-called geophysical model function, an empirical relation
between NRCS and the wind vector

©istockphoto.com/olyniteowl

vc = f (U, i, a, p, m),

DECEmBER 2016

ieee Geoscience and remote sensinG maGazine

(2)

where U is the wind speed, i is the incidence angle, a
is the azimuth illumination angle relative to upwind
direction, and p is the polarization of the radiation.
Fundamental scatterometer wind retrieval is based on
a geophysical model function generated from backscatter measurements that are collected from airborne or
spaceborne platforms over areas of the sea/ocean where
wind fields are available simultaneously from in situ
buoys or other independent sources.
The history of water-surface scatterometry began
during World War II, when marine radar operators observed noise on their radar screens that obscured small
boats and low-flying aircraft [16]. They called this noise
sea clutter. In fact, it was the backscatter of the radar
39



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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - December 2016

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