Polarimetric Mode Research Articles

VALIDATION OF SPACEBORNE RADAR SURFACE WATER MAPPING WITH OPTICAL sUAS IMAGES

Abstract. The Canada Centre for Remote Sensing (CCRS) has over 40 years of experience with airborne and spaceborne sensors and is now starting to use small Unmanned Aerial Systems (sUAS) to validate products from large coverage area sensors and create new methodologies for very high resolution products. Wetlands have several functions including water storage and retention which can reduce flooding and provide continuous flow for hydroelectric generation and irrigation for agriculture. Synthetic Aperture Radar is well suited as a tool for monitoring surface water by supplying acquisitions irrespective of cloud cover or time of day. Wetlands can be subdivided into three classes: open water, flooded vegetation and upland which can vary seasonally with time and water level changes. RADARSAT‐2 data from the Wide-Ultra Fine, Spotlight and Fine Quad-Pol modes has been used to map the open water in the Peace‐Athabasca Delta, Alberta using intensity thresholding. We also use spotlight modes for higher resolution and the fully polarimetric mode (FQ) for polarimetric decomposition. Validation of these products will be done using a low altitude flying sUAS to generate optical georeferenced images. This project provides methodologies which could be used for flood mapping as well as ecological monitoring.

Spaceborne MIMO Synthetic Aperture Radar for Multimodal Operation

In this paper, we introduce a novel multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) concept for multimodal operation. The proposed system employs waveforms based on the orthogonal frequency division multiplexing (OFDM) technique and digital beamforming (DBF) on receive. Thereby, it becomes feasible to maximize spatial degrees of freedom, which are necessary for the multimodal operation. The proposed MIMO SAR system produces multiple high-resolution wide-swath SAR imageries that are used for coherent postprocessing. Through this paper, we aim to open up a new perspective of using MIMO concept for a wide-swath SAR imaging with high resolution in interferometric and polarimetric modes, based on OFDM and DBF techniques. Therefore, this paper encompasses broad theoretical backgrounds and general system design issues as well as specific signal processing techniques and aspects.

Dissecting the AGB star L2Puppis: a torus in the making

The circumstellar environment of L2 Pup, an oxygen-rich semiregular variable, was observed to understand the evolution of mass loss and the shaping of ejecta in the late stages of stellar evolution. High-angular resolution observations from a single 8 m telescope were obtained using aperture masking in the near-infrared (1.64, 2.30 and 3.74 $\rm\mu m$) on the NACO/VLT, both in imaging and polarimetric modes. The aperture-masking images of L2 Pup at 2.30 $\rm\mu m$ show a resolved structure that resembles a toroidal structure with a major axis of ~140 milliarcseconds (mas) and an east-west orientation. Two clumps can be seen on either side of the star, ~65 mas from the star, beyond the edge of the circumstellar envelope (estimated diameter is ~27 mas), while a faint, hook-like structure appear toward the northeast. The patterns are visible both in the imaging and polarimetric mode, although the latter was only used to measure the total intensity (Stokes I). The overall shape of the structure is similar at the 3.74 $\rm\mu m$ pseudo-continuum (dust emission), where the clumps appear to be embedded within a dark, dusty lane. The faint, hook-like patterns are also seen at this wavelength, extending northeast and southwest with the central, dark lane being an apparent axis of symmetry. We interpret the structure as a circumstellar torus with inner radius of 4.2 au. With a rotation velocity of 10 km s$^{-1}$ as suggested by the SiO maser profile, we estimate a stellar mass of 0.7 M$_\odot$.}

On the Alternate Transmission Mode for Polarimetric Phased Array Weather Radar

AbstractPulse-to-pulse switching of polarizations (alternate transmission mode) is considered for polarimetric phased array radar (PAR). It is argued that the performance of the radar in terms of data quality should match or exceed the achieved standards of the Weather Surveillance Radar-1988 Doppler (WSR-88D). It turns out that the most stringent demand on the radar concerns the surveillance scan at the lowest elevations wherein the polarimetric variables are free of overlaid echoes, while ground clutter is significantly reduced. The scan uses a long pulse repetition time that has repercussion on the standard errors of the polarimetric variables and hence the choice of polarimetric mode. Herein the dwell time of this scan serves as a benchmark for comparisons of the accuracy of estimates. Because weather PAR should provide useful information at low signal-to-noise ratios (SNR) as low as those measured by the WSR-88D, the statistics of polarimetric variables, known at high SNR, is extended to low SNRs. It follows that the alternate mode would not match the performance of the simultaneous mode in the surveillance scans on the WSR-88D. Quasi-simultaneous transmission and reception of horizontally polarized and vertically polarized waves is discussed as a cost-effective alternative.

Preliminary design of the full-Stokes UV and visible spectropolarimeter for UVMag/Arago

AbstractThe UVMag consortium proposed the space mission project Arago to ESA at its M4 call. Arago is dedicated to the study of the dynamic 3D environment of stars and planets. This space mission will be equipped with a high-resolution spectropolarimeter working from 119 to 888 nm. A preliminary optical design of the whole instrument has been prepared and is presented here. The design consists of the telescope, the instrument itself, and the focusing optics. Considering not only the scientific requirements, but also the cost and size constraints to fit an M-size mission, the telescope has a 1.3 m diameter primary mirror and is a classical Cassegrain-type telescope that allows a polarization-free focus. The polarimeter is placed at this Cassegrain focus. This is the key element of the mission and the most challenging one to be designed. The main challenge lies in the huge spectral range offered by the instrument; the polarimeter has to deliver the full Stokes vector with a high precision from the FUV (119 nm) to the NIR (888 nm). The polarimeter module is then followed by a high-resolution echelle-spectrometer achieving a resolution of 35000 in the visible range and 25000 in the UV. The two channels are separated after the echelle grating, allowing specific cross-dispersion and focusing optics for the UV and the visible ranges. Considering the large field of view and the high numerical aperture, the focusing optics for both the UV and the visible channels is a Three-Mirror-Anastigmatic (TMA) telescope, needed to focus the various wavelengths and many orders onto the detectors.

Role of Polarimetric SAR data for discrimination/biophysical parameters of crops based on canopy architecture

Abstract. Synthetic Aperture Radar (SAR) sensors have great potential for a wide range of agricultural applications, owing to their capability of all-weather observation. It is particularly useful in tropical regions in Asia where most of the crops are grown in rainy season. The use of SAR images for the assessment of rice-planted area is in operational stage in Asian countries owing to its characteristic temporal signature however, applications of SAR images for the estimation of biophysical plant variables are challenging, especially for crop scattering and discrimination in case of other tropical crops. Canopy geometry and architecture mainly govern the interaction of microwave signal with the vegetation. In this study evaluation of C-band SAR data at different polarization combinations in linear as well as circular polarimetric imaging modes for rabi crops and other associated landuse has been attempted. Also understanding the scattering response of various crops based on canopy architecture was attempted. The scattering parameters were found to vary for planofiles and erectophiles, partitioning of scattering and absorption were determined.

HST observations of the limb polarization of Titan

Titan is an excellent test case for detailed studies of the scattering polarization from thick hazy atmospheres. We present the first limb polarization measurements of Titan, which are compared as a test to our limb polarization models. Previously unpublished imaging polarimetry from the HST archive is presented which resolves the disk of Titan. We determine flux-weighted averages of the limb polarization and radial limb polarization profiles, and investigate the degradation and cancelation effects in the polarization signal due to the limited spatial resolution of our observations. Taking this into account we derive corrected values for the limb polarization in Titan. The results are compared with limb polarization models, using atmosphere and haze scattering parameters from the literature. In the wavelength bands between 250 nm and 2000 nm a strong limb polarization of about 2-7 % is detected with a position angle perpendicular to the limb. The fractional polarization is highest around 1 micron. As a first approximation, the polarization seems to be equally strong along the entire limb. The detected polarization is compatible with expectations from previous polarimetric observations taken with Voyager 2, Pioneer 11, and the Huygens probe. Our results indicate that ground-based monitoring measurements of the limb-polarization of Titan could be useful for investigating local haze properties and the impact of short-term and seasonal variations of the hazy atmosphere of Titan. Planets with hazy atmospheres similar to Titan are particularly good candidates for detection with the polarimetric mode of the upcoming planet finder instrument at the VLT. Therefore, a good knowledge of the polarization properties of Titan is also important for the search and investigation of extra-solar planets.

Multifeature fusion for polarimetric synthetic aperture radar image classification of sea ice

Sea ice conditions are so heterogeneous, and the differences between the different ice types are less varied than that of land targets, so only using polarimetric or textural features would lead to misclassification of polarimetric synthetic aperture radar (PolSAR) data of sea ice. To support the identification of different ice types, the fusion of textural and polarimetric features would be a good solution. Simple discrimination analysis is used to rationalize a preferred features subset. Some features are analyzed, which include entropy H /alpha α /anisotropy A and three kinds of texture statistics (entropy, contrast, and correlation), in the C- and L-band polarimetric mode. After that, a multiobjective fuzzy decision model is proposed for supervised PolSAR data classification of sea ice, and the targets are categorized according to the principle of maximum membership grade. In consideration of the interference of the correlation among features, the model is based on Mahalanobis distance in which the covariances between the selected heterogeneous features could restrain the interference among redundant features. In the end, the effectiveness of the algorithm for PolSAR image classification of sea ice is demonstrated through the analysis of some experimental results.

3-Pol Polarimetric Weather Measurements With Agile-Beam Phased-Array Radars

Most actual polarimetric weather radars consider either alternate transmission of horizontal and vertical polarizations or simultaneous transmission of both polarizations. Simultaneous reception of vertical and horizontal components is always performed. The way these operating modes have to be modified, to cope with the loss of polarization purity and the increase of polarization channels coupling at some pointing directions when planar phased-array radars (PARs) are used, has already been discussed. Complete correction of these effects showed to be difficult, requiring precise control of the relative amplitudes and phases of the dipoles feeding signals on transmission in a pulse-to-pulse basis. If simultaneous transmission of vertical and horizontal polarizations is considered, complete correction has not been achieved. In this paper, the implementation of a third polarimetric measurement mode based on alternate transmission of three different polarizations with PARs is discussed. It is found that implementation of this method, which provides minimum-variance unbiased linear polarimetric covariance matrix estimates, with phased-array systems only requires simple adjustments at reception.

Equivalent Stacking of Polarimetric Synthetic Aperture Radar Interferograms Based on Analysis of Persistent and Distributed Scatterers

We propose a method of incorporating the copolarized observations (HH and VV observations) of multipolarimetric SAR acquisitions into persistent scatterer interferometry to estimate the surface displacement and the scatterer height. Interferograms obtained from HH and VV observations were equally weighted on the basis that the magnitude of the backscattered signals from persistent scatterers represented by odd-bounce scattering and even-bounce scattering are the same in the copolarized observations. Moreover, because of the coarser slant-range resolution of multipolarimetric SAR data, distributed scatterers (DSs) should be taken into account to obtain a spatial density of pixels that is sufficient to estimate the parameters of interest. To this aim, we applied a small baseline criterion to create the analyzed data sets, and we used space adaptive filtering to improve the signal-to-noise ratio of the DSs. We applied this method to estimate the ground subsidence rate in an area of groundwater extraction in Japan using the SAR images that are acquired by Advanced Land Observing Satellite/Phased Array Type L-band SAR (PALSAR) in the full polarimetric observation mode.

Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

To understand the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment, within the MiMeS project, we searched for magnetic objects among a large sample of massive stars, and build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e.g., abundance peculiarities, Halpha emission), which are also of interest for future studies. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the stars were detected in early-type Bp stars, while the last star was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Halpha emission with respect to the theory of centrifugally-supported magnetospheres. (Abriged)

Optical and infrared polarimetry of the transient LMXB Centaurus X-4 in quiescence

We present the first optical and infrared polarimetric study of the low mass transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed to search for an intrinsic linear polarisation component in the system emitted radiation that might be due, e.g., to synchrotron emission from a compact jet, or to Thomson scattering with free electrons in an accretion disc. Multiband (BVRI) optical polarimetric observations were obtained during two nights in 2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These observations cover about the 30% of the 15.1 hours orbital period. J-band observations were obtained in 2007 with the NICS (TNG) instrument at La Palma, for a totality of 1 hour observation. We obtained 3-sigma upper limits to the polarisation degree in all the optical bands, with the most constraining one being in the I-band (P<0.5%). No phase-correlated variability has been noticed in all the filters. The J-band observations provided a 6% upper limit on the polarisation level. The constraining upper limits to the polarisation in the optical allowed us to evaluate the contribution of the possible emission of a relativistic particles jet to the total system radiation to be less then the 10%. This is in agreement with the observation of a spectral energy distribution typical of a single black body of a K-spectral type main sequence star irradiated from the compact object. Due to the low S/N ratio it was not possible to investigate the possible dependency of the polarisation degree from the wavelength, that could be suggestive of polarisation induced by Thomson scattering of radiation with free electrons in the outer part of the accretion disc. Observations with higher S/N ratio are required to examine in depth this hypothesis, searching for significant phase-correlated variability.

Polarimetric Response of Rice Fields at C-Band: Analysis and Phenology Retrieval

A set of ten RADARSAT-2 images acquired in fully polarimetric mode over a test site with rice fields in Seville, Spain, has been analyzed to extract the main features of the C-band radar backscatter as a function of rice phenology. After observing the evolutions versus phenology of different polarimetric observables and explaining their behavior in terms of scattering mechanisms present in the scene, a simple retrieval approach has been proposed. This algorithm is based on three polarimetric observables and provides estimates from a set of four relevant intervals of phenological stages. The validation against ground data, carried out at parcel level for a set of six stands and up to nine dates per stand, provides a 96% rate of coincidence. Moreover, an equivalent compact-pol retrieval algorithm has been also proposed and validated, providing the same performance at parcel level. In all cases, the inversion is carried out by exploiting a single satellite acquisition, without any other auxiliary information.

Investigation of the Capability of $H\hbox{-}\alpha$ Decomposition of Compact Polarimetric SAR

Recently, there has been an increasing interest in compact polarimetry (CP), which helps to reduce the complexity, cost, mass, and data rate of synthetic aperture radar (SAR) systems while attempting to maintain many capabilities of a fully polarimetric system. In this letter, we provide a comparison of the capabilities of three conventional CP modes to distinguish different physical scattering mechanisms (PSMs) through H-α decomposition. A compact H-α space for dual circular polarimetric mode is proposed on the basis of the distribution centers and densities of different PSMs. Both airborne and spaceborne SAR data are used to validate our method. The experimental results show that the decomposition accuracy of each PSM is improved compared with the results of previous publication, the kappa coefficient increases by nearly 0.1, and the overall accuracy is above 81%.

RADARSAT-2 Beam Mode Selection for Surface Water and Flooded Vegetation Mapping

Synthetic Aperture Radar (SAR) is well known for its ability to map surface water. There are a number of SAR satellites providing data for this application including the Canadian RADARSAT-2 system. RADARSAT-2 has a wide range of beam modes and some users may be intimidated by the variety of choices and have a difficult time deciding on the most appropriate beam mode. This technical note addresses some issues behind beam mode and polarization selection for surface water mapping with RADARSAT-2 and the upcoming RADARSAT Constellation Mission (RCM). This includes the impacts of resolution, wind effects, and the best mode for flooded vegetation detection. The results show that high resolution modes like the single polarized Spotlight are best for accurately delineating the surface water edge and small patches of flooded terrain. The addition of the cross-polarization available in other beam modes can provide useful surface water information in windy or rough surface conditions because there is little effect on the RADAR backscatter compared to the HH single polarization. For accurately delineating flooded vegetation, a polarimetric or compact polarimetric mode is best because the phase is maintained, which allows the user to apply polarimetric decompositions models to help separate the RADAR backscatter.

Categorization of the Glaciated Terrain of Indian Himalaya Using CP and FP Mode SAR

This paper presents the comparison between the potential of synthetic aperture radar (SAR) data in compact polarimetry (CP) versus the fully polarimetric POLSAR mode (FP) for glacier areas. The eigenvalue-based constructed parameters, decompositions and supervised classification schemes were used for this inter-comparison. The comparison is focused on compact polarimetric techniques versus full polarimetric techniques based on both of qualitative and quantitative analysis of the glaciated parameters extraction. Overall performance of CP mode is lower than FP mode for glaciated terrain parameter extraction. Furthermore, it was found over the part of rugged Gangotri glaciated terrain, Indian Himalaya that the reconstruction pseudo polarimetry data is erroneous and highly biased because of data sets do not satisfy the pseudo polarimetric reconstruction conditions in the glaciated terrain.

Polarimetric mode of full polarization interferometric radiometer

全极化综合孔径辐射计（Full Polarization Interferometric Radiometer，FPIR）是一套高分辨率、轻量化、低功耗的X波段一维综合孔径微波辐射计.分析了FPIR的极化理论，推导出FPIR的极化可视度函数.为了获取全极化信息，FPIR系统采用两种不同的天线阵列设计方案.对于每种稀疏天线阵列，设计了最优极化切换序列.设计的最优极化序列不仅使灵敏度的损失达到最小，还使得水平极化、垂直极化和交叉极化亮温的灵敏度达到相同水平.

Polarimetric SAR Response of Snow-Covered Area Observed by Multi-Temporal ALOS PALSAR Fully Polarimetric Mode

This study discusses the capability assessment of fully polarimetric L-band spaceborne synthetic aperture radar (SAR) for detection of seasonal snow covered areas. In this paper, ALOS PALSAR time-series data sets obtained in quad-pol modes have been investigated to evaluate the polarimetric signal scattered from a snow-covered mountainous ecosystem in Niigata, Japan. Results show that changes in the scattering mechanism across the various snow states can be identified from polarimetric parameters. In particular, different polarimetric parameters offer complementary information on the snow properties. Based on the characteristic seasonal changes of polarimetric parameters, a new method to map snow-covered areas is proposed in this study using an information fusion approach. Snow extent can be identified successfully by combining polarimetric indices with an overall accuracy of 74.4% as compared with in situ measurements and 77.0% as compared with optical images.

Monitoring Volumetric Surface Soil Moisture Content at the La Grande Basin Boreal Wetland by Radar Multi Polarization Data

Understanding the hydrological dynamics of boreal wetland ecosystems (peatlands) is essential in order to better manage hydropower inter-annual productivity at the La Grande basin (Northern Quebec, QC, Canada). Given the remoteness and the huge dimension of the La Grande basin, it is imperative to develop remote sensing monitoring techniques to retrieve hydrological parameters. The main objective of this study is to find out if multi-date and multi-polarization Radar Satellite 2 (RADARSAT-2) (C-band) image analysis could detect seasonal variations of surface soil moisture conditions of the acrotelm. A change detection approach through the use of multi temporal indexes was chosen based on the assumption that the temporal variability of surface roughness and natural vegetation biomass is generally at a much longer time scale than that of surface soil moisture (Δ-Index is based on a reference image that represents dry soil, in order to maximize the sensitivity of σ° to changes in soil moisture with respect to the same location when soil is wet). The Δ-Index approach was tested with each polarization: σ° for fully polarimetric mode (HH, HV, VV) and the cross-polarization coefficient (HV/HH). Results show that the best regression adjustment with regard to surface soil moisture content in boreal wetlands was obtained with the cross-polarization coefficient. The cross-polarization multi-temporal index enables precise volumetric surface soil moisture estimation and monitoring on boreal wetlands, regardless of the influence of vegetation cover and surface roughness conditions (bias was under 1%, standard deviation and RMSE were under 10% for almost all estimation errors). Surface soil moisture estimation was more precise over permanently flooded areas than seasonally flooded ones (standard deviation is systematically greater for the seasonally flooded areas, at all analyzed scales), although the overall quality of the estimation is still precise. Cross-polarization ratio image analysis appears to be a useful mean to exploit radar data spatially, as we were able to relate changes in wetland eco-hydrological dynamics to variations in the intensity of the ratio.

The Dominion Astrophysical Observatory Magnetic Field Survey (DMFS)

AbstractIn this paper we present a few results from the first three years of an ongoing survey of globally-ordered magnetic fields in relatively faint (down to V ≈ 9) upper main sequence peculiar stars that we are conducting on the Dominion Astrophysical Observatory (DAO) Plaskett telescope. The DMFS uses the inexpensive DAO polarimeter module, dimaPol, mounted at the Cassegrain focus of the 1.8 m telescope to detect new magnetic stars and determine rotation periods and longitudinal magnetic field curves using medium-resolution (R ≈ 10,000) circular spectropolarimetry of both the Hβ line and metal lines in an approximately 280 Å wide wavelength region centered on Hβ. By concentrating on the mid-B to A-type peculiar stars, the DMFS provides an extension to the ‘Magnetism in Massive Stars’ (MIMES) Large Program which concentrated on similar field detections in more massive stars.