312 research outputs found
Theory of synthetic aperture radar ocean imaging: A MARSEN view
This paper reviews basic synthetic aperture radar (SAR) theory of ocean wave imaging mechanisms, using both known work and recent experimental and theoretical results from the Marine Remote Sensing (MARSEN) Experiment. Several viewpoints that have contributed to the field are drawn together in a general analysis of the backscatter statistics of a moving sea surface. A common focus for different scattering models is provided by the mean image impulse response function, which is shown to be identical to the (spatially varying) frequency variance spectrum of the local complex reflectivity coefficient. From the analysis has emerged a more complete view of the SAR imaging phenomenon than has been previously available. A new, generalized imaging model is proposed
Specular point scattering contribution to the mean Synthetic Aperture Radar image of the ocean surface
n general, the return signal scattered from the ocean surface used to form synthetic aperture radar (SAR) images contains contributions from at least two scattering mechanisms. In addition to resonant Braggâtype scattering, specular point scattering becomes important as the angle of incidence becomes small ( âČ 20°). In this paper we include the specular point rough surface scattering mechanism in a model for the mean SAR image of the ocean surface and examine the effects of this scattering mechanism theoretically. We find that the complete mean SAR intensity image consists of a sum of images due to specular point scattering and Braggâtype resonant scattering. Because surface specular points have a short coherence time and move with considerable velocities, the contribution to the mean image due to these scatterers is of low azimuthal resolution and is displaced from the actual sea surface, typically by several SAR resolution cells. The bandwidth of this image can easily exceed the bandwidth of a typical SAR processor, leading to a loss of mean image intensity. The local backscatter crossâsection modulation is strong and nonlinear in the slope of the longwave field in the SAR range direction. At small incidence angles, this causes the specular point return from wave slopes tipped toward the SAR to become much brighter than the Braggâscattering return. Taken together, these effects are capable of producing azimuthally oriented streaks in SAR images, such as have been observed by Seasat. We present numerical estimates of coherence time, azimuthal displacement, crossâsection modulation, etc., computed using the parameters of the Seasat and shuttle imaging radarâB SARs as well as typical parameters for an airborne X band SA
Microwave backscattering theory and active remote sensing of the ocean surface
The status is reviewed of electromagnetic scattering theory relative to the interpretation of microwave remote sensing data acquired from spaceborne platforms over the ocean surface. Particular emphasis is given to the assumptions which are either implicit or explicit in the theory. The multiple scale scattering theory developed during this investigation is extended to non-Gaussian surface statistics. It is shown that the important statistic for the case is the probability density function of the small scale heights conditioned on the large scale slopes; this dependence may explain the anisotropic scattering measurements recently obtained with the AAFE Radscat. It is noted that present surface measurements are inadequate to verify or reject the existing scattering theories. Surface measurements are recommended for qualifying sensor data from radar altimeters and scatterometers. Additional scattering investigations are suggested for imaging type radars employing synthetically generated apertures
Technical background, chapter 3, part B
A description is given of the physics of electromagnetic scattering from the sea and a guideline is presented to relate an observable (such as the radar cross section) to the hydrodynamics or physical properties of the sea. As specific examples of the interdisciplinary science of electromagnetics and geophysical oceanography, the physics is discussed in connection with data provided by three instruments: namely, the scatterometer, the altimeter, and the imaging radar. The data provided by each instrument are discussed in context with specular point and Bragg scattering theories. Finally, the degrading effect of extraneous sources of noise is discussed as a limiting mechanism of the accuracy of the ocean surface measurement
Aircraft and satellite measurement of ocean wave directional spectra using scanning-beam microwave radars
A microwave radar technique for remotely measuring the vector wave number spectrum of the ocean surface is described. The technique, which employs short-pulse, noncoherent radars in a conical scan mode near vertical incidence, is shown to be suitable for both aircraft and satellite application, the technique was validated at 10 km aircraft altitude, where we have found excellent agreement between buoy and radar-inferred absolute wave height spectra
Theoretical modeling of dual-frequency scatterometer response: improving ocean wind and rainfall effects
Ocean surface wind is a key parameter of the Earthâs climate system. Occurring at the interface between the ocean and the atmosphere, ocean winds modulate fluxes of heat, moisture and gas exchanges. They reflect the lower branch of the atmospheric circulation and represent a major driver of the ocean circulation. Studying the long-term trends and variability of the ocean surface winds is of key importance in our effort to understand the Earthâs climate system and the causes of its changes. More than three decades of surface wind data are available from spaceborne scatterometer/radiometer missions and there is an ongoing effort to inter-calibrate all these measurements with the aim of building a complete and continuous picture of the ocean wind variability.
Currently, spaceborne scatterometer wind retrievals are obtained by inversion algorithms of empirical Geophysical Model Functions (GMFs), which represent the relationship between ocean surface backscattering coefficient and the wind parameters. However, by being measurement-dependent, the GMFs are sensor-specific and, in addition, they may be not properly defined in all weather conditions. This may reduce the accuracy of the wind retrievals in presence of rain and it may also lead to inconsistencies amongst winds retrieved by different sensors.
Theoretical models of ocean backscatter have the big potential of providing a more general and understandable relation between the measured microwave backscatter and the surface wind field than empirical models. Therefore, the goal of our research is to understand and address the limitations of the theoretical modeling, in order to propose a new strategy towards the definition of a unified theoretical model able to account for the effects of both wind and rain. In this work, it is described our approach to improve the theoretical modeling of the ocean response, starting from the Ku-band (13.4 GHz) frequency and then broadening the analysis at C-band (5.3 GHz) frequency. This research has revealed the need for new understanding of the frequency-dependent modeling of the surface backscatter in response to the wind-forced surface wave spectrum. Moreover, our ocean wave spectrum modification introduced to include the influences of the surface rain, allows the interpretation/investigation of the scatterometer observations in terms not only of the surface winds but also of the surface rain, defining an additional step needed to improve the wind retrievals algorithms as well as the possibility to jointly estimate wind and rain from scatterometer observations
Recommended from our members
Microwave scattering from surf zone waves
Wave breaking in the surf zone is an important forcing
mechanism on the generation of nearshore currents and in the driving
of sediment transport. At the same time, wave breaking can have
significant spatial and temporal variability that needs to be
accounted for in the description of nearshore processes. Remote
sensors are best suited to collect wave breaking measurements due to
their large footprint and synoptic capabilities, but in order to
extract quantitative wave parameters a proper understanding of the
imaging mechanisms is essential. Microwave sensors have been shown
to be able to measure wave parameters in deep water, but in the surf
zone many of the assumptions the algorithms are based upon do not
hold. Additionally, the dynamics of breaking waves are different and
may affect in a yet determined way the signal.
This dissertation first intends to address an observational gap
regarding surf zone microwave measurements. A novel combination of
synchronous, large coverage marine radar, calibrated pulsed Doppler
radar and video observations from a field site enable the analysis
of the evolution and characteristics of the wave signature. The
combined data sets yield superior discrimination rates between
breaking and non-breaking waves. Discrimination also allows the
study of the microwave scattering by source, where active breaking
is separated from remnant foam and steepening waves. Results show
that the backscattered power from breaking waves, specifically from
the wave roller, is a several dB larger than that of foam and
steepening waves and independent of the environmental conditions and
polarization state. While similar results have been obtained for
deep water waves and variety of scattering models have been
proposed, it is found that none of the models can describe all the
data. Additionally, most of the models neglect the roller
morphology. Therefore, in the last section a scattering model is
introduced, in which the roller is treated as a volume where a
collection of water droplets embedded in air can scatter
incoherently. Multiple interactions of the scattered fields between
particles and the boundaries are also accounted for. Though the
model formulation is complex, it depends on a few physical
parameters (diameter, volume fraction, medium permittivity) and no
calibration constants. Comparison against data shows that the model
does a reasonable job in predicting the observed scattering levels,
polarization response and grazing angle dependencies, although is
not capable to reproduce the maximum scattered levels observed and
predicts polarization ratios always less than unity
Detection and characterization of deep water wave breaking using moderate incidence angle microwave backscatter from the sea surface
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1990The importance of wave breaking in both microwave remote sensing and
air-sea interaction has led to this investigation of the utility of a Ku-Band CW
Doppler scatterometer to detect and characterize wave breaking in the open ocean.
Field and laboratory measurements by previous authors of microwave backscatter
from sharp-crested and breaking waves have shown that these events can exhibit
characteristic signatures in moderate incidence angle measurements of the radar
cross-section (RCS) and Doppler spectrum. Specifically, breaking events have been
associated with polarization independent sea spikes in the RCS accompanied by
increased mean frequency and bandwidth of the Doppler spectrum.
Simultaneous microwave, video, and environmental measurements were made
during the SAXON experiment off Chesapeake Bay in the fall of 1988. The
scatterometer was pointed upwind with an incidence angle of 45 degrees and an
illumination area small compared to the wavelength of the dominant surface
waves. An autocovariance estimation technique was used to produced time series
of the RCS, mean Doppler frequency, and Doppler spectral bandwidth in real-time.
The joint statistics of the microwave quantities indicative of breaking are
used to investigate detection schemes for breaking events identified from the video
recordings. The most successful scheme is based on thresholds in both the RCS
and the Doppler bandwidth determined from joint distributions for breaking and
non-breaking waves. Microwave events consisting of a sea spike in the RCS
accompanied by a large bandwidth are associated with the steep forward face of
waves in the early stages of breaking. The location of the illumination area with
respect to the phase of the breaking wave, the stage of breaking development, and
the orientation of an individual crest with respect to the antenna look-direction
all influence the detect ability of a breaking event occurring in the vicinity of the
radar spot. Since sea spikes tend to occur on the forward face of waves in the
process of breaking, the whitecap associated with a given sea spike may occur
after the crest of the wave responsible for the sea spike has passed the center of
the illumination area. Approximately 70% of the waves which produce whitecaps
within a distance of 5m of the bore sight location are successfully identified by a
threshold-based detection scheme utilizing both RCS and bandwidth information.
The sea spike statistics are investigated as functions of wave field
parameters and friction velocity u*. For VV and HH polarization, the frequency
of sea spike occurrence and the sea spike contribution to the mean RCS show an
approximately cubic dependence on u*, which is consistent with theoretical
modelling and various measures of whitecap coverage. The data also suggest that
the average RCS of an individual sea spike is not dependent on u*. At high
friction velocities (u*â40-50cms-l), the contribution of sea spikes to the mean RCS
is in the range of 5-10% for VV and 10-20% for HH. The wind speed dependence
of the percentage of crests producing sea spikes is comparable to that of the
fraction of breaking crests reported by previous authors. The percentage of wave
crests producing sea spikes is found to vary approximately as (Re*)1.5, where Re*
is a Reynolds number based on u* and the dominant surface wavelength. This
result agrees with measurements of the degree of wave breaking by. previous
authors and is shown to be consistent with a cubic dependence on u *. Models
for the probability of wave breaking as a function of moments of the wave height
spectrum are compared to our results. The Doppler frequency and bandwidth
measurements are also used to inquire into the kinematics of the breaking process.This work was funded by grants from the MIT Sloan Basic Research Fund, the
National Science Foundation (Physical Oceanography), and the Office of Naval Research
(Physical Oceanography). Additional funding was provided by the National Aeronautics and
Space Administration through the Graduate Student Researchers' Fellowship Program
- âŠ