IEEE Geoscience and Remote Sensing Magazine - December 2016 - 48

polarization [28]-[30]. Therefore, only this case has been
MUlTIPOlARIzATIOn And
considered in the article in
MUlTIFREqUEnCY
detail. Multipolarization and
IMPlEMEnTATIOn TO ThE
multifrequency implementaAIRbORnE WEAThER RAdAR,
tion to the AWR, while posWhIlE POSSIblY OFFERInG
sibly offering superior performance, will require more
SUPERIOR PERFORMAnCE,
significant technical modiWIll REqUIRE MORE
fication of the radar. NeverSIGnIFICAnT TEChnICAl
theless, such AWR functionMOdIFICATIOn OF
ality enhancement will allow
ThE RAdAR.
its wider use in studying
sea clutter as well as in improving current geophysical
model functions and developing new geophysical model
functions for other undocumented incidence angles and
for standalone and joint wind measurements over water.
COnClUSIOn
This study has shown that an AWR employed in the
ground-mapping mode as a scatterometer can be used for
remotely measuring the water-surface backscattering signature and recovering the near-surface wind vector over
water from the azimuth NRCS curves obtained, in addition to its typical meteorological and navigation applications. The azimuth NRCS curve can be obtained with
AWR during a circular track flight when the azimuth direction of the beam relative to the aircraft current course
is fixed. The fixed beam should be pointed to the outer
side of an aircraft turn to observe a greater area of the water surface and to obtain a greater number of independent
NRCS samples. The azimuth beam direction should be
perpendicular to the aircraft current course or should at
least tend to perpendicular position when the scanning
sector is narrower than ±90°. The NRCS curve and wind
measurement is started when a stable horizontal circular
flight at a given altitude, speed of flight, roll, and pitch has
been established. The measurement is finished when the
azimuth of the measurement starting point is reached. To
obtain a greater number of NRCS samples for each sector
observed, several consecutive full-circle 360° turns should
be completed. Since the scan plane is horizontal because
the antenna is stabilized, the aircraft roll should not exceed the maximum value allowed for ensuring antenna
stabilization and, consequently, the constancy of the incidence angle.
The azimuth NRCS curve can also be obtained when
an aircraft equipped with AWR makes a two-stage horizontal rectilinear flight, at first in one given direction and
then in the opposite direction. A two-stage measurement
is performed to obtain two 180° azimuth NRCS data
sets that form the entire 360° azimuth NRCS data set.
The first stage is started when a stable horizontal rectilinear flight at a given altitude and speed of flight has
been established. It is finished when a required number
48

of NRCS samples for each azimuth sector observed is obtained. After the aircraft turns for 180°, the second stage
is started when a stable horizontal rectilinear flight at
given altitude and speed of flight has been established.
The stage is finished when a required number of NRCS
samples for each observed azimuth sector is reached. To
obtain a greater number of NRCS samples for each sector,
several consecutive two-stage measurements could also
be performed.
Both of the considered methods are applicable for a research measurement. A classical, circular track flight for the
measurement of the entire 360° azimuth NRCS curve takes
a relatively long time, as its duration cannot be shorter than
the aircraft turn for 360°. Also, it is not so convenient for a
pilot, as opposed to a two-stage rectilinear track flight, especially under amphibious aircraft or seaplane safe landing
on a water surface.
AWR in the mode of the wind vector measurement
should use the horizontal transmit and receive polarization,
as the difference in the upwind and downwind NRCS values
at this polarization is greater than at the vertical transmit
and receive polarization [2], [6], [24]. It should also provide
for the incidence angle of selected cells i " 45c, which is
explained by better usage of the anisotropic properties of
water-surface scattering at medium incidence angles [24] as
well as by power reasons. For the water surface, the NRCS
falls radically as the incidence angle increases and assumes
different values for different conditions of sea state or water roughness; while, for most other types of terrain, the
NRCS decreases slowly with the increase of the beam incidence angle [28]. Otherwise, the incidence angle of selected
cells should be in the range of validity of the NRCS model
function (12) and should be out of the shadow region of
water backscatter.
The principle considered and the algorithms proposed
in this article can be used for the enhancement of the
AWR, the development of an airborne radar system for
operational measurement of sea roughness characteristics, and the estimation of wind speed and direction over
water. They may also be used for ensuring the safe landing of amphibious aircraft or seaplanes on a water surface,
e.g., under search-and-rescue missions or fire fighting in
the coastal areas and fire-risk regions complementing a
terrain-avoidance system [28], [52].
ACKnOWlEdGMEnTS
We would like to acknowledge the financial support of this
work by the Russian Science Foundation (project no. 16-1900172). We would also like to express our sincere thanks to
the University of Pavia, Italy, and the Technical University
of Košice, Slovakia, for the research opportunities provided
and to the reviewers for their useful suggestions. Alexey
Nekrasov would like to thank the Cariplo Foundation, the
Landau Network-Centro Volta, and the National Scholarship Program of the Slovak Republic for the support of his
exchange visits.
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