C-band Scatterometer Measurements Research Articles

Assessment of C-Band Polarimetric Radar for the Detection of Diesel Fuel in Newly Formed Sea Ice

There is a heightened risk of an oil spill occurring in the Arctic, as climate change driven sea ice loss permits an increase in Arctic marine transportation. The ability to detect an oil spill and monitor its progression is key to enacting an effective response. Microwave scatterometer systems may be used detect changes in sea ice thermodynamic and physical properties, so we examined the potential of C-band polarimetric radar for detecting diesel fuel beneath a thin sea ice layer. Sea ice physical properties, including thickness, temperature, and salinity, were measured before and after diesel addition beneath the ice. Time-series polarimetric C-band scatterometer measurements monitored the sea ice evolution and diesel migration to the sea ice surface. We characterized the temporal evolution of the diesel-contaminated seawater and sea ice by monitoring the normalized radar cross section (NRCS) and polarimetric parameters (conformity coefficient (μ), copolarization correlation coefficient (ρco)) at 20° and 25° incidence angles. We delineated three stages, with distinct NRCS and polarimetric results, which could be connected to the thermophysical state and the presence of diesel on the surface. Stage 1 described the initial formation of sea ice, while in Stage 2, we injected 20L of diesel beneath the sea ice. No immediate response was noted in the radar measurements. With the emergence of diesel on the sea ice surface, denoted by Stage 3, the NRCS dropped substantially. The largest response was for VV and HH polarizations at 20° incidence angle. Physical sampling indicated that diesel emerged to the surface of the sea ice and trended towards the tub edge and the polarimetric scatterometer was sensitive to these physical changes. This study contributes to a greater understanding of how C-band frequencies can be used to monitor oil products in the Arctic and act as a baseline for the interpretation of satellite data. Additionally, these findings will assist in the development of standards for oil and diesel fuel detection in the Canadian Arctic in association with the Canadian Standards Association Group.

High-Resolution Soil Moisture Retrieval With ASCAT

Satelliteborne C-band scatterometer measurements of the radar backscatter coefficient $(\sigma^{0})$ of the Earth can be used to estimate soil moisture levels over land. Such estimates are currently produced at 25- and 50-km resolution using the Advanced Scatterometer (ASCAT) sensor and a change detection algorithm originally developed at the Vienna University of Technology (TU-Wien). Using the ASCAT spatial response function (SRF), high-resolution (approximately 15–20 km per pixel) images of $\sigma^{0}$ can be produced, enabling the creation of a high-resolution soil moisture product using a modified version of the TU-Wien algorithm. The high-resolution soil moisture images are compared to images produced with the Water Retrieval Package 5.5 algorithm, which is also based on the TU-Wien algorithm, and to in situ measurements from the National Oceanic and Atmospheric Administration U.S. Climate Reference Network (NOAA CRN). The WARP 5.5 and high-resolution image products generally show good agreement with each other; the high-resolution estimates appear to resolve soil moisture features at a finer scale and demonstrate a tendency toward greater moisture values in some areas. When compared to volumetric soil moisture measurements from NOAA CRN stations for 2010 and 2011, the WARP 5.5 and high-resolution soil moisture estimates perform similarly, with both having a root-mean-square difference from the in situ data of approximately 0.06 m3/m3 in one study area and 0.09 m3/m3 in another.

A Dense Medium Microwave Backscattering Model for the Remote Sensing of Oil Palm

Very little work has been carried out to examine the potential of synthetic aperture radar (SAR) imaging data for growth monitoring, disease monitoring, and yield prediction of oil palm plantations. In this paper, we present a scattering model for oil palm based on the radiative transfer equations, solved iteratively up to the second order, to better understand the backscattering behavior of oil palm canopies for SAR image interpretation. The oil palm canopy is modeled as a multilayer background host medium embedded with discrete scatterers of different shapes, with elliptic disks representing the leaves and cylinders representing the fronds and trunks. We consider the effects of coherent scattering through the dense medium phase and amplitude correction theory, as well as the near-field interactions between closely spaced scatterers through the Fresnel phase and amplitude corrections. Ground-truth data are used as input parameters for the model. The model is used to calculate the backscattering coefficients of oil palm canopies at different stages of growth and for different polarizations, frequencies, and incident angles. Comparisons between model calculations and C-band scatterometer measurements of multipolarization backscattering coefficients of 4-year-old oil palm canopies in Bangi, Malaysia show good agreement. Our model also predicts that SAR images at L-band are more sensitive to changes in the structure of the canopies, particularly that of the fronds, so will be the best frequency for oil palm growth monitoring and disease detection.

A Monte Carlo Method for Simulating Scattering From Sea Ice Using FVTD

A scattering model based on a Monte Carlo method and the finite-volume time-domain (FVTD) method has been created for sea ice scattering simulations. Statistical methods were used to generate a Gaussian-distributed randomly rough surface. The Polder-Van Santen-de Loor (PVD) model was used to estimate the sea ice dielectric values with inputs based upon actual measured physical variables obtained during field-based experiments and well-known parameterizations. Scattering simulations were performed through an application of the scattered-field (SF) formulation invoked in an FVTD computational engine. Simulated SFs were compared with C-band scatterometer measurements and showed good agreement for copolarized signals in a series of case studies. The developed simulation method has the potential to be used for a variety of sea ice types under different physical conditions.

C-Band Scatterometer Measurements of Multiyear Sea Ice Before Fall Freeze-Up in the Canadian Arctic

Backscatter signatures of multiyear sea ice (MYI) during the late summer and early fall season before the fall freeze-up in the Canadian Arctic archipelago (CAA) have been obtained through the use of a ship-based polarimetric scatterometer. The device operates in C-band, and measurements were conducted in swaths from incidence angles of 20 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> -60 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> . Three characteristic sites on MYI floes were investigated in the high Arctic and the central Arctic regions. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">situ</i> snow and sea-ice thermophysical data were collected at each site in conjunction with local scatterometer measurements. The thermophysical data were subsequently analyzed using dielectric modeling techniques and coupled with the backscattering measurements (sigma <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> ). Observed backscatter values and ratios were found to be in agreement with literature data, with volumetric scattering as the dominant scattering mechanism.

A C-Band Wind/Rain Backscatter Model

With the confirmed evidence of rain surface perturbation in recent studies, the rain effects on C-band scatterometer measurements are reevaluated. By using colocated Tropical Rainfall Measuring Mission Precipitation Radar, ESCAT on European Remote Sensing Satellites, and European Centre for Medium-Range Weather Forecasts data, we evaluate the sensitivity of C-band sigmadeg to rain. We develop a low-order wind/rain backscatter model with inputs of surface rain rate, incidence angle, wind speed, wind direction, and azimuth angle. We demonstrate that the wind/rain backscatter model is accurate enough for describing the total backscatter in raining areas with relatively low variance. We also show that the rain surface perturbation is a dominating factor of the rain-induced backscatter. Using three distinct regimes, we show under what conditions the wind, rain, and both wind and rain can be retrieved from the measurements. We find that the effect of rain has a more significant impact on the measurements at high incidence angles than at low incidence angles

Laboratory measurements of sea ice: connections to microwave remote sensing

The connections between laboratory measurements and remote-sensing observations of sea ice are explored. The focus of this paper is on thin ice, which is more easily simulated in a laboratory environment. The authors summarize results of C-band scatterometer measurements and discuss how they may help in the interpretation of remote-sensing data. They compare the measurements with observations of thin ice from ERS and airborne radar data sets. They suggest that laboratory backscatter signatures should serve as bounds on the interpretation of remote-sensing data. They examine these bounds from the perspective of thin ice signatures, the effect of temperature, and surface processes, such as frost flowers and slush on these signatures. Controlled experiments also suggest new directions in remote-sensing measurements. The potential of polarimetric radar measurements in the retrieval of thickness of thin ice is discussed. In addition to the radar results, the authors discuss the importance of low-frequency passive measurements with respect to the thickness of thin ice.

Microwave backscattering and emission model for grass canopies

Microwave radar and radiometer measurements of grasslands indicate a substantial reduction in sensor sensitivity to soil moisture in the presence of a thatch layer. When this layer is wet it masks changes in the underlying soil, making the canopy appear warm in the case of passive sensors (radiometer) and decreasing backscatter in the active case (scatterometer). A model for a grass canopy with thatch is presented in order to explain this behavior and for comparison with observations. The canopy model consists of three layers: grass, thatch, and the underlying soil. The grass blades are modeled by elongated elliptical discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneous and flat. The distorted Born approximation is used to compute the radar cross section of this three layer canopy and the emissivity is computed from the radar cross section using the Peake formulation for the passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moisture, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-band radiometer measurements at these grasslands.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.

Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data

The angular dependence and polarization behaviour of back-scattering and emission of polar firn at 5.2 GHz and 10.3 GHz were measured during an oversnow traverse in Dronning Maud Land, Antarctica. The signatures emphasize the importance of snow stratification in the interpretation of microwave remote sensing measurements. Highest backscattering coefficients and little angular variations were observed for refrozen firn near the coast. In permanently dry snow, areas with high accumulation rates and homogeneous snow morphology showed low backscattering coefficients and high emissivities. Pronounced layering and related density variations in low accumulation zones resulted in increased polarization differences of brightness temperatures and increased like-polarized backscattering coefficients. This behaviour is confirmed by the analysis of C-band scatterometer measurements of the Active Microwave Instrument aboard the European Space Agency's ERS-1.

On the wind power law and atmospheric stability effects in ocean scatterometry

In this study, C-band scatterometer measurements reported by Keller et al. (1989), are reanalyzed in order to find the exponent of the power law dependence of scattering cross-section on ocean surface wind stress for a 45° incidence angle. The exponent is found by two methods: first, by observing the change in backscatter with change in wind stress over a range of windspeeds from 6 to 13 ms –1 using neutrally stratified conditions, and second, by considering nearly constant windspeed and observing the change of wind stress induced by dramatic variations in the air-sea temperature difference. We employ Monin-Obukhov theory for the second method. Our results indicate that both methods produce a power law exponent with a value near 1.2. Given that both methods produce nearly the same value for the exponent, these results suggest that variations of radar cross-section induced by atmospheric stability, when the model function is defined in terms of windspeed, can be accounted for entirely by Monin-Obukhov theory and variation of the wind stress drag coefficient with stability. It also suggests that if the model function uses friction velocity as the wind parameter, then neither the mean or variance of radar cross-section observations should exhibit a dependence on atmospheric stability, and atmospheric stability becomes an irrelevant parameter to radar scatterometer model functions when friction velocity is used.

C-band scatterometer measurements of a tallgrass prairie

C-band (4.75 GHz) scatterometer measurements were made of a tallgrass prairie in an attempt to determine the relationship between 1) the backscattering coefficient and different expressions of soil moisture and 2) the backscattering coefficient and various canopy parameters. The findings of this study support those made in previous studies in terms of the optimum polarization and view angle selection for soil moisture work. In contrast to previous studies, it was found that view angles of 30° and 45° also produced strong correlations with soil moisture. In addition, there were two new findings regarding the effect of prairie vegetation on the C-band backscattering coefficient. The first was a moderately strong correlation and partial correlation between σ 0 and leaf water potential, which indicates some capability of C-band measurements in detecting extremes in the water status of prairie vegetation under shallow soil conditions. The second was the finding that site differences (primarily differences in vegetation) due to burn treatments appeared to be sufficient to cause significant differences in the sensitivity of σ 0 to soil moisture. The site differences could not be removed by any known expression of soil moisture. These two findings were surprising since previous radar studies had reported minimal vegetation effects when using a frequency and view angles such as those in this study.