Measurements of Ocean Surface Backscattering Using an Airborne 94-GHz Cloud Radar—Implication for Calibration of Airborne and Spaceborne W-Band Radars

2005 ◽  
Vol 22 (7) ◽  
pp. 1033-1045 ◽  
Author(s):  
Lihua Li ◽  
Gerald M. Heymsfield ◽  
Lin Tian ◽  
Paul E. Racette
Keyword(s):  
Millimeter Wave ◽  
Cross Section ◽  
Incidence Angle ◽  
Surface Wind ◽  
Ocean Surface ◽  
Regional Study ◽  
Specular Scattering ◽  
Cloud Radar ◽  
W Band ◽  
Low Incidence

Abstract Backscattering properties of the ocean surface have been widely used as a calibration reference for airborne and spaceborne microwave sensors. However, at millimeter-wave frequencies, the ocean surface backscattering mechanism is still not well understood, in part, due to the lack of experimental measurements. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE), measurements of ocean surface backscattering were made using a 94-GHz (W band) cloud radar on board a NASA ER-2 high-altitude aircraft. This unprecedented dataset enhances our knowledge about the ocean surface scattering mechanism at 94 GHz. The measurement set includes the normalized ocean surface cross section over a range of the incidence angles under a variety of wind conditions. It was confirmed that even at 94 GHz, the normalized ocean surface radar cross section, σo, is insensitive to surface wind conditions near a 10° incidence angle, a finding similar to what has been found in the literature for lower frequencies. Analysis of the radar measurements also shows good agreement with a quasi-specular scattering model at low incidence angles. The results of this work support the proposition of using the ocean surface as a calibration reference for airborne millimeter-wave cloud radars and for the ongoing NASA CloudSat mission, which will use a 94-GHz spaceborne cloud radar for global cloud measurements.

scholarly journals Understanding Ku-Band Ocean Radar Backscatter at Low Incidence Angles under Weak to Severe Wind Conditions by Comparison of Measurements and Models

2020 ◽  
Vol 12 (20) ◽  
pp. 3445
Author(s):  
Qiushuang Yan ◽  
Chenqing Fan ◽  
Jie Zhang ◽  
Junmin Meng
Keyword(s):  
Wind Speed ◽  
Wave Breaking ◽  
Scattering Theory ◽  
Specular Reflection ◽  
Incidence Angle ◽  
Surface Wind ◽  
Specular Scattering ◽  
Low Incidence ◽  
The Impact ◽  
Ku Band

The rain-free normalized radar cross-section (NRCS) measurements from the Ku-band precipitation radars (PRs) aboard the tropical rainfall measuring mission (TRMM) and the global precipitation measurement (GPM) mission, along with simultaneous sea surface wind truth from buoy observations, stepped-frequency microwave radiometer (SFMR) measurements, and H*Wind analyses, are used to investigate the abilities of the quasi-specular scattering models, i.e., the physical optics model (PO) and the classical and improved geometrical optics models (GO and GO4), to reproduce the Ku-band NRCS at low incidence angles of 0–18° over the wind speed range of 0–45 m/s. On this basis, the limitations of the quasi-specular scattering theory and the effects of wave breaking are discussed. The results show that the return caused by quasi-specular reflection is affected significantly by the presence of background swell waves at low winds. At moderate wind speeds of 5–15 m/s, the NRCS is still dominated by the quasi-specular reflection, and the wave breaking starts to work but its contribution is very small, thus, the models are found in excellent agreement with the measurements. With wind speed increasing, the impact of wave breaking increases, whereas the role of standard quasi-specular reflection decreases. The wave breaking impact on NRCS is first visible at incidence angles near 18° as wind speed exceeds about 20 m/s, then it becomes dominant when wind speed exceeds about 37 m/s where the NRCS is insensitive to wind speed and depends linearly on incidence angle, which cannot be explained by the standard quasi-specular scattering theory.

Estimation of wave-induced contribution to Sentinel-1 Doppler shift observations

2020 ◽  
Author(s):  
Artem Moiseev ◽  
Harald Johnsen ◽  
Johnny Johannessen
Keyword(s):  
Doppler Shift ◽  
Incidence Angle ◽  
Wind Waves ◽  
Surface Wind ◽  
Surface Current ◽  
Ocean Surface ◽  
Reliable Information ◽  
Near Surface ◽  
Ocean Surface Current ◽  
Wave Induced

<p>The Doppler Centroid Anomaly (DCA) registered by microwave Synthetic Aperture Radar (SAR) contains information about ocean surface motion in the radar line-of-sight direction. The recorded signal is associated with the motion induced by the total wavefield (i.e., both wind waves and swell) and underlying ocean surface currents. Hence, accurate estimates of the wave-induced contribution to the observed DCA is required in order to obtain reliable information about underlying ocean surface current. In this study, we develop an empirical geophysical model function for the estimation of the wave-induced DCA. The study is based on two months of Sentinel-1 SAR Wave mode (WV) DCA observations collocated with wind field at 10m height from the ECMWF model and sea state information from the WAVEWATCH III model.</p><p>Analysis of two months of observations acquired over land showed that thanks to the novel Sentinel-1 DCA calibration, the uncertainty in the data does not exceed 3Hz (corresponding to a radial velocity of 0.21/014 m/s in the near/far range. The relationship between the DCA and the near-surface wind is in agreement with previously reported findings under the assumption of fully developed seas; the DCA is about 24% of the range wind speed at 23° incidence angle and decreasing (up to 50%) with increasing incidence angle from 23° to 36°. However, the difference between upwind (i.e., the wind blows towards antenna) and downwind (i.e., wind blows away from the antenna) configurations is inconsistent from study to study. Reliable information about the wave field indeed helps to describe the spread in the DCA, especially at low and moderate wind speeds, and when the ocean surface is dominated by the remotely generated swell.</p><p>The CDOP model is used as a baseline for estimating the wind-wave-induced Doppler shift. Retraining of the CDOP model for the Sentinel-1 SAR observations (CDOP-S) yielded a significantly better fit. Then, we extended the GMF with parameters of the wavefield (significant wave height, mean wave period and direction) in the moment of SAR acquisition. Combining information about near-surface wind and ocean surface wave fields also considerably improves the accuracy of the wave-induced Doppler shift estimates. In turn,  the accuracy of the ocean surface current retrievals are improved as demonstrated by the promising agreement with the near-surface ocean surface current climatology based on multiyear drifter observations.</p>

scholarly journals Analysis of Ku- and Ka-Band Sea Surface Backscattering Characteristics at Low-Incidence Angles Based on the GPM Dual-Frequency Precipitation Radar Measurements

2019 ◽  
Vol 11 (7) ◽  
pp. 754 ◽  
Author(s):  
Qiushuang Yan ◽  
Jie Zhang ◽  
Chenqing Fan ◽  
Junmin Meng
Keyword(s):  
Wind Direction ◽  
Incidence Angle ◽  
Surface Wind ◽  
Sea Surface ◽  
Dual Frequency ◽  
Precipitation Radar ◽  
Ka Band ◽  
Surface Conditions ◽  
Wave Parameters ◽  
Low Incidence

The co-located normalized radar backscatter cross section measurements from the Global Precipitation Measurement (GPM) Ku/Ka-band dual-frequency precipitation radar (DPR) and sea surface wind; wave and temperature observations from the National Data Buoy Center (NDBC) moored buoys are used to analyze the dependence and sensitivity of Ku- and Ka-band backscatter on surface conditions at low-incidence angles. Then the potential for inverting wind and wave parameters directly from low-incidence σ0 measurements is discussed. The results show that the KaPR σ0 is more sensitive to surface conditions than the KuPR σ0 overall. Nevertheless; both the KuPR σ0 and KaPR σ0 are strongly correlated with wind speed (U10) and average wave steepness (δa) with the exception of specific transitional incidence angles. Moreover, U10 and δa could be retrieved from pointwise σ0 near nadir and near 18°. Near 18°; wind direction information is needed as the effect of wind direction on σ0 becomes increasingly significant with incidence angle. To improve the performance of U10 retrieval; especially for low U10; auxiliary δa information would be most helpful; and sea surface temperature is better taken into account. Other wave parameters; such as significant wave height; wave period and wave age; are partly correlated with σ0. It is generally more difficult to retrieve those parameters directly from pointwise σ0. For the retrieval of those wave parameters; various auxiliary information is needed. Wind direction and wave direction cannot be retrieved from pointwise σ0.

scholarly journals Analysis of Radar Cross Section of the Tank and Its Application at Millimeter Wave W-Band

2017 ◽  
Vol 28 (9) ◽  
pp. 756-759
Author(s):  
Hokeun Shin ◽  
Sung Chan Song ◽  
Jihyung Kim ◽  
Yong Bae Park
Keyword(s):  
Millimeter Wave ◽  
Cross Section ◽  
Radar Cross Section ◽  
W Band

Ocean surface wind speed retrieval from C-band quad-polarized SAR measurements at optimal spatial resolution

2020 ◽  
Vol 12 (2) ◽  
pp. 155-164
Author(s):  
He Fang ◽  
William Perrie ◽  
Gaofeng Fan ◽  
Tao Xie ◽  
Jingsong Yang
Keyword(s):  
Wind Speed ◽  
Spatial Resolution ◽  
Surface Wind ◽  
Ocean Surface ◽  
Surface Wind Speed ◽  
Ocean Surface Wind

Near-field cross section imaging based on 2-D NUFFT for millimeter wave

2016 ◽  
Author(s):  
Yingzhi Kan ◽  
Yongfeng Zhu ◽  
Liang Tang ◽  
Hongzhong Zhao
Keyword(s):  
Millimeter Wave ◽  
Cross Section ◽  
Near Field

Fully polarimetric passive W-band millimeter wave imager for wide area search

2013 ◽  
Author(s):  
Jonathan Tedeschi ◽  
Bruce Bernacki ◽  
Dave Sheen ◽  
Jim Kelly ◽  
Doug McMakin
Keyword(s):  
Millimeter Wave ◽  
Wide Area ◽  
W Band
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