Simultaneous observations of ocean surface winds and waves by Geosat radar altimeter and airborne synthetic aperture radar during the 1988 Norwegian Continental Shelf Experiment (original) (raw)
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Atmosphere-Ocean, 1994
Measurements of ocean directional wave spectra, significant wave height, and wind speed over the Grand Banks of Newfoundland were made using the combined capabilities of the radar ocean wave spectrometer (ROWS) and scanning radar altimeter (SRA). The instruments wereflown aboard the NASA P-3A aircraft in support of the Grand Banks ERS-) Synthetic Aperture Radar (SAR) Wave Experiment. The NASA sensors use proven techniques, which differ greatlyfrom SAR, for estimating the directional Iong-wave spectrum; thus they provide a unique set of measurements for use in evaluating SAR performance. ROWS and SRA data are combined with spectra from the SAR aboard the Canadian Centre for Remote Sensing (CCRS) CV-S80 aircraft, the first-generation Canadian Spectral Ocean Wave Model (CSOWM) hindcast, and other available in situ measurements to assess the ERS-) SAR's ability to correctly resolve wave field components along a 200-to 300-km flight une f or four separate satellite passes. Given the complex seas present on the Grand Banks, the complementary nature of viewing the sea spectrumfrom the perspectives of multiple sensors and a wave prediction model is apparent. The data intercomparisons show the ERS-! SAR to be meeting the expected goals for measuring swell, but the data also show evidence of this remote sensor 's inabiliiy to detect the shorter waves travelling in the azimut/i or along-track direction. Example SAR spectra simulations are made using a non-linearforward transform with ROWS measurements as input. Additionally, surface wind and wave height estimates made using the ROWS altimeter channel are presented. These data demonstrate the utility of operating the system in its new combined altimeter and spectrometer configuration.
Journal of Geophysical Research, 1991
C band radar images of ocean gravity waves off the Norwegian coast were processed into one-dimensional azimuth spectra. These spectra were used to measure the azimuth spectral (width) cutoff on the basis of a least squares fit to a Gaussian spectral shape. The widths were calculated for a range of wave heights (2-5 m) and wind speeds (2-18 m/s) during 3 days in March, 1988. Velocity smearing (rr v) estimates were extracted, independent of R/V and incidence angle, based on an imaging model and the measured azimuth cutoffs with cry values varying from 0.4 to 0.7 m/s. Quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery. We propose a first-order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m. This model is offered as a first estimate of when ocean wave swell will be detected by the C band SAR on board the ERS 1 spacecraft. The model predicts that this swell will be imaged under light winds of the order of 2-4 m/s. Higher wind speeds cause larger smearing, which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track.
Journal of Geophysical Research, 1982
Over the period July 4 to October 10, 1978, the SEASAT synthetic aperture radar (SAR) gathered 23 cm wavelength radar images of some 108 km 2 of the earth' s surface, mainly of ocean areas, at 25-40 m resolution. Our assessment is in terms of oceanographic and ocean monitoring objectives and is directed toward discovering the proper role of SAR imagery in these areas of interest. In general, SAR appears to have two major and somewhat overlapping roles: first, quantitative measurement of ocean phenomena, like long gravity waves and wind fields, as well as measurement of ships' second, exploratory observations of large-scale ocean phenomena, such as the Gulf Stream and its eddies, internal waves, and ocean fronts. These roles are greatly enhanced by the ability of 23 cm SAR to operate day or night and through clouds. To begin we review some basics of synthetic aperture radar and its implementation on the SEASAT spacecraft. SEASAT SAR imagery of the ocean is fundamentally a map of the radar scattering characteristics of-30.cm wavelength ocean waves, distorted in some cases by ocean surface motion. We discuss how wind stress, surface currents, long gravity waves, and surface films modulate the scattering properties of these resonant waves with particular emphasis on the mechanisms that could produce images of long gravity waves. Doppler effects by ocean motion are also briefly described. Measurements of long (wavelength >• 100 m) gravity waves, using SEASAT SAR imagery, are compared with surface measurements during several experiments. Combining these results we find that dominant wavelength and direction are measured by SEASAT SAR within +-_ 12% and +-_ 15 ø, respectively. However, we note that ocean waves are not always visible in SAR images and discuss detection criteria in terms of wave height, length, and direction. SAR estimates of omnidirectional wave height spectra made by assuming that SAR image intensity is proportional to surface height fluctuations are more similar to corresponding surface measurements of wave height spectra than to wave slope spectra. Because SEASAT SAR images show the radar cross section cr ø of-•30 cm waves (neglecting doppler effects), and because these waves are raised by wind stress on the ocean surface, wind measurements are possible. Comparison between wind speeds estimated from SEASAT SAR imagery and from the SEASAT satellite scatterometer (SASS) agreed to within +0.7 m s-• over a 350-km comparison track and for wind speeds from 2 to 15 m s-•. The great potential of SAR wind measurements lies in studying the spatial structure of the wind field over a range of spatial scales of from •<1 km to >•100 km. At present, the spatial and temporal structure of ocean wind fields is largely unknown. Because SAR responds to short waves whose energy density is a function of wind stress at the surface rather than wind speed at some distance above the surface, variations in image intensity may also reflect changes in air-sea temperature difference (thus complicating wind measurements by SAR). Because SAR images show the effects of surface current shear, air-sea temperature difference, and surface films through their modulation of the-•30cm waves, SEASAT images can be used to locate and study the Gulf Stream and related warm water rings, tidal flows at inlets, internal waves, and slicks resulting from surface films. In many of these applications, SAR provides a remote sensing capability that is complementary to infrared imagery because the two techniques sense largely different properties, namely, surface roughness and temperature. Both stationary ships and moving ships with their attendant wakes are often seen in SAR images. Ship images can be used to estimate ship size, heading, and speed. However, ships known to be in areas imaged by SAR are not always detectable. Clearly, a variety of factors, such as image resolution, ship size, sea state, and winds could affect ship detection. Overall, the role of SAR imagery in oceanography is definitely evolving at this time, but its ultimate role is unclear. We have assessed the ability of SEASAT SAR to measure a variety of ocean phenomena and have commented briefly on applications. In the end, oceanographers and others will have to judge from these capabilities the proper place for SAR in oceanography and remote sensing of the ocean. Imaged swath 100 km wide, nearside offset 240 km from subsatellite track Angle of incidence 19 ø to 25 ø Typical resolution 25 to 40 m in both range and azimuth directions depending on processing techniques Radar wavelength 23.5 Contrast ratio 9 dB Transmitter power Peak 1 kW Average 55 W Chirp pulse length 33.4 t•s RF bandwith 19 MHz Pulse repetition rate 1463-1640 Hz Radar antenna Dimensions 10.7 x 2.16 m Gain 35 dB Beam Width 1.73 ø x 6.2 ø Polarizaiton 99% linear horizontal stations (i.e., when the subsatellite track was within about 2000 km of the ground station). Hence, global coverage was not possible. Nevertheless, some 108 km 2 of imagery was collected over both land and sea, as is shown in Figure 3. This imagery is available to interested users through the National Oceanic and Atmospheric Administration's Environmental Data Information Service, National Climate Center, Room 100, World Weather Building, Washington, DC, 20233 (telephone, 301-763-8111). The SAR signal data, recorded on high density digital tape at the various ground stations, was converted into images in two ways: optical imaging and digital imaging. The optical method (discussed in detail by Goodman [
Validation of RADARSAT-2 Polarimetric SAR measurements of ocean waves
2009 IEEE International Geoscience and Remote Sensing Symposium, 2009
Four C-band fully polarimetric synthetic aperture radar (POLSAR) images of ocean waves from the RADARSAT-2 SAR are used to measure ocean slopes and wave spectra. A new technique has been developed to measure wave slopes in the SAR azimuth and range directions. The POLSAR ocean wave parameter measurements were validated with in situ observations form an NOAA National Data buoy Center (NDBC) buoy. The results show that wave parameters measured using the new method are in good agreement with in situ NDBC measurement products.
Synergistic measurements of ocean winds and waves from SAR
Journal of Geophysical Research: Oceans
In this study we present a synergistic method to retrieve both ocean surface wave and wind fields from spaceborne quad-polarization (QP) synthetic aperture radar (SAR) imaging mode data. This algorithm integrates QP-SAR wind vector retrieval model and the wave retrieval model, with consideration to the nonlinear mapping relationship between ocean wave spectra and SAR image spectra, in order to synergistically retrieve wind fields and wave directional spectra. The method does not require a priori information on the sea state. It combines the observed VV-polarized SAR image spectra with the retrieved wind vectors from the VH-polarized SAR image, to estimate the wind-generated wave directional spectra. The differences between the observed SAR spectra and optimal SAR image spectra associated with the wind waves are interpreted as the contributions from the swell waves. The retrieved ocean wave spectra are used to estimate the integrated spectral wave parameters such as significant wave heights, wavelengths, wave directions and wave periods. The wind and wave parameters retrieved by QP-SAR are validated against those measured by the National Data Buoy Center (NDBC) directional wave buoys under different sea states. The validation results show that the QP-SAR SAR has potential to simultaneously measure the ocean surface waves and wind fields from space.
Synthetic aperture radar imaging of ocean waves during the marineland experiment
IEEE Journal of Oceanic Engineering, 1983
X-and L-band simultaneously obtained synthetic aperture radar (SAR) data of ocean gravity waves collected during the Marineland Experiment were analyzed using wave contrast measurements. The Marineland data collected in 1975 represents a unique historical data set for testing still-evolving theoretical models of the SAR ocean wave imaging process. The wave contrast measurements referred to are direct measurements of the backscatter variation betaeen wave crests and troughs. These modulation depth measurements, which are indicators of wave detectability, were made as a function of: a) the settings used in processing the S A R signal histories to partially account for wave motion; b) wave propagation direction with respect to radar look direction for both Xand L-band SAR data; c) SAR resolution; and d) number of coherent looks. The contrast measurements indicated that ocean waves imaged by a SAR are most discernible when X-band frequency is used (as compared to L-band), and when the ocean waves are traveling in the range direction. Ocean waves can be detected by both Xand L-band SAR, provided that the radar surface resolution is small compared to the ocean wavelength (at least I/4 of the ocean wavelength is indicated by this work).
Wave and wind retrieval from SAR images of the ocean
annals of telecommunications - annales des télécommunications
Over the past few years, recognition of the importance of the coastal zone has led to the establishment of international programmes for monitoring the coastal zone environment and its change. The European programme MARSAIS is part of this effort. One important component of such actions aims to better predict the sea surface wind and wave dynamics in these vulnerable regions where most economic marine activity is taking place.
Validation of RADARSAT-2 fully polarimetric SAR measurements of ocean surface waves
Journal of Geophysical Research, 2010
1] C band RADARSAT-2 fully polarimetric (fine quad-polarization mode, HH+VV+HV +VH) synthetic aperture radar (SAR) images are used to validate ocean surface waves measurements using the polarimetric SAR wave retrieval algorithm, without estimating the complex hydrodynamic modulation transfer function, even under large radar incidence angles. The linearly polarized radar backscatter cross sections (RBCS) are first calculated with the copolarization (HH, VV) and cross-polarization (HV, VH) RBCS and the polarization orientation angle. Subsequently, in the azimuth direction, the vertically and linearly polarized RBCS are used to measure the wave slopes. In the range direction, we combine horizontally and vertically polarized RBCS to estimate wave slopes. Taken together, wave slope spectra can be derived using estimated wave slopes in azimuth and range directions. Wave parameters extracted from the resultant wave slope spectra are validated with colocated National Data Buoy Center (NDBC) buoy measurements (wave periods, wavelengths, wave directions, and significant wave heights) and are shown to be in good agreement.