Study of Fully Polarimetric Radiometer

An imaging polarimetric slope sensing instrument for measuring water waves has been developed. From themeasurement of the intensities of three different linear polarization states, it is possible to determine the first three components of the polarization Stokes vector. The slope of the water surface is computed from the measurement of the polarization of reflected light. Unlike in common polarimeters, custom optics are not required in this simple setup consisting of three cameras aligned in parallel and each equipped with a standard polarization filter. The trade-off for the simple setup is the need for more extensive system calibration and image post-processing. The camera setup was fully calibrated (extrinsic, intrinsic, and distortion parameters) with a specialized calibration procedure using a custom built target. The analyzer matrix, which transforms the intensities of the measured polarization states into the Stokes vector components was determined and verified experimentally. A data set collected during an experiment on board the research vessel Meteor is analyzed. In an experiment at the Hamburgische Schiffsbau-Versuchsanstalt, the polarimeter was successfully operated under laboratory conditions. It is shown to be capable of measuring the slope distribution of mechanically generated waves. Elevation power spectra, determined by integration of the slope measurements, show good agreement with reference measurements with a wave wire. Deviations at low wave frequencies due the small size of the polarimeter footprint can be compensated with a transfer function that is derived from the measurements.

The development of the NOAA Polarimetric Scanning Radiometer (PSR) system commenced in the fall of 1995, with the first-generation system operated on the NASA P-3B aircraft to study passive microwave ocean surface wind signatures in March, 1997. The PSR system consist of sets of polarimetric radiometers housed within standardized gimbal-mounted scanhead drums. Each scanhead is rotatable by a gimbaled positioner so that the radiometers can view any angle within 70/spl deg/ elevation of nadir and at any azimuthal angle (1.32/spl pi/ sr solid angle), as well as external hot and ambient calibration targets. The configuration supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. Scanheads are designed for in-flight operation without the need for a radome (i.e. in contact with the aircraft slipstream), thus allowing precise calibration and imaging without superimposed radome signatures. The conical scan mode allows the full Stokes' vector to be imaged without polarization mixing. The PSR has been used in several successful airborne missions, demonstrating the first 2-dimensional ocean surface wind vector mapping, high-resolution hurricane rainband imaging and satellite rainfall rate validation, C-band soil moisture imaging, high-resolution sea-ice mapping, and ocean internal wave imaging. Since its inaugural mission there have been several new hardware developments that have extended the capability of the PSR system in terms of the observable spectrum, polarizations, and compatibility with various aircraft. Currently in progress are developments which will provide the capability to perform wideband airborne hydrological studies with a single suite of synchronized, compatible sensor heads. This suite will include the PSR/CX, PSR/S, and PSR/L scanheads, which collectively extend the capabilities of the original PSR/A scanhead. Four positioners are anticipated to be available for operation in 2003. Each assembly (scanhead and positioner) was designed for integration into several aircraft, including the NASA DC-8, Orion P-3B, and WB-57F, Scaled Composites' Proteus, Airplatforms, Inc. Canberra B-6, U.S. Navy P-3A, and NASA ER-2. Upon completion, the PSR system will provide passive polarimetric microwave imagery at most of the channels in the range of 1.4 to /spl sim/800 GHz that are useful for spaceborne or airborne hydrological remote sensing. Studies planned using the system include vapor-to-runoff phase monitoring of precipitation, estuarian runoff and mixing, targeted forecasting, and cirrus generation by convection.

Realizing the need for lower noise figure and smaller physical size in todays higly sensitive radiometers, this paper presents a new compact analog front-end (AFE) for use with the existing L-band (1400–1427 MHz) radiometer designed and operated by the Technical University of Denmark. Using sub-harmonic sampling to sample directly at the RF-frequency, this radiometer obtains a fully polarimetric response and enables detection and removal of radio frequency interference (RFI). A more compact AFE will enable various desired features, as for example the ability to use the front-end with antenna arrays needing one receiver per antenna (Synthetic Aperture Radiometer, SARad), reduced weight for airborne missions and an easy temperature stabilization, i.e. improved instrument stability. The new front-end possesses an improved system noise temperature of only 76 K (roughly 40 K improvement) measured at an AFE physical temperature of 44°C, thus making the radiometer even more sensitive.

On May 19, 2015, the German research icebreaker Polarstern began a six-week expedition to the Arctic Ocean initiated by the “ART” team, which stands for “Arctic in Rapid Transition”. The expedition PS92 (ARK XXIX/1) “TRANSSIZ” (Transitions in the Arctic Seasonal Sea Ice Zone, Fig. 1.1) conducted ecological and biogeochemical early spring process studies from the shelf to the basins of the European Arctic margin and on the Yermak Plateau, in order to link past and present sea-ice transitions in the Arctic Ocean. The cruise involved scientists from eleven countries in collaboration with research groups from divisions of the Alfred Wegener InstituteHelmholtz Centre for Polar and Marine Research together with scientists from the German BMBF-project ‘Transdrift’, as well as from the French-Canadian projects ‘GreenEdge’. Overall the vessel travelled 3,665 sm and carried out 68 stations with a total of 242 casts. Some groups also used the transit to the research area to study meridional variability of trace gases, algae species and nutrients from temperate regions of the North Atlantic and into the ice-covered Arctic Ocean by using the surface online water system. Once reaching the investigation area, the science parties conducted process studies for rate measurements of productivity, ecosystem interactions and carbon- and nitrogen cycling. By comparing data from the shelf, data from across the shelf-break into the deep basin it was possible to compare carbon export from plankton and sea ice communities as well as identifying the potential characteristics in carbon production, the fate and export, and to identify similarities and differences in ecosystem functioning along topography-, sea ice- and water mass-related gradients. The ice stations (Fig. 1.2) involved coring of a standard set of sea-ice cores for biological, physical and chemical variables as well as for trace gases and geological proxy validation. It further involved the study of sea ice properties and under-ice water and covered the study of trace and greenhouse gases, biodiversity, primary and bacterial production as well as a detailed study of the nitrogen cycle. Short-term moorings were deployed under the ice to determine the vertical carbon flux. A small Remotely Operated Vehicle (ROV) was operated under the ice to focus on spectral radiation measurements, but also to record environmental parameters (e.g. ice thickness, salinity, temperature) and video imaging of the under-ice environment. Light transmission measured with the ROV showed an increase of light penetration during the course of our expedition. The under-ice fauna and other environmental parameters were investigated by using the towed “Surface Under Ice Trawl” (SUIT). Helicopter flights were used to determine the large-scale distribution of sea-ice thickness with an EM-bird along the cruise track, which overall revealed that the average sea-ice thickness was 1.4 m, that is, comparatively thin and similar to summer vales previously observed in Fram Strait. During the sea-ice stations, parallel sampling of pelagic and benthic ecosystems and geological cores were conducted. Light spectra of hyper-spectral radiometers were used to establish the penetration depth of ultraviolet radiation into the different types of oceanic waters from the zodiac and under the ice. Water samples were taken from the water bottles of the CTD rosette to study the chemistry, biology and various geological proxies. A UVP (Underwater Video Profiler System) was deployed to provide detailed vertical profiles of particle distribution, size composition and the zooplankton community. Quantitative sampling of the mesozooplankton, and also foraminifera, which are used as paleo-proxies, were carried out by using multi-nets. For macrozooplankton and nekton, a Multiple-closing Rectangular Midwater Trawl (MRMT) was used. The distribution of macrozooplankton and pelagic fish was monitored continuously on selected transects with Polarstern’s EK60 echosounder. Benthic communities were collected by box corers and the material was further used for experimental and biogeochemical analyses of the benthic surface sediment layers, including sea ice- and paleo proxies. Benthic communities show a clear shelf to basin decrease in the oxygen demand. Overall, the communities also show a clear North South gradient in our study area. TV-Multi-corers for geological measurements were used to get undisturbed core tops of near-surface sediments (Fig. 1.3). For the geological coring, detailed bathymetric mapping and sub-bottom profiling systems (Hydrosweep and Parasound) were used to find suitable coring positions for Kastenlot and Gravity cores. Special emphasis on this cruise was taken to quantify the environmental preconditions for productivity (e.g. nutrients, stratification) which will allow us to be able to improve predictions of the potential annual primary production in a future ice-free Arctic Ocean, as well as improving reconstructions of productivity, sea ice and ocean circulation across the last 1-2 last glacial cycles. Results from the cruise will further improve the understanding of ecosystem functioning and biogeochemical cycles during the transition from spring to summer. The expedition ended on the morning of the 28th June 2015 in Longyearbyen.

Compared with the traditional microwave radiometer, one dimensional synthetic aperture microwave radiometer can effectively improve the spatial resolution of sea surface temperature retrieval. However, the spaceborne one dimensional synthetic aperture microwave radiometer is multiple incidence angle observation for the sea target and the incidence angle changes from 0° to 55°. In order to develop sea surface temperature inversion algorithms suited to one dimensional synthetic aperture microwave radiometer, it is necessary to evaluate the sensitivities of brightness temperature to oceanic and atmospheric environmental elements. Using the sea surface emissivity model and the atmospheric radiative transfer model, we construct a oceanic and atmospheric radiative transfer mode suited to the one dimensional synthetic aperture microwave radiometer. In this paper, the sensitivity changes of C-band vertical and horizontal polarization brightness temperature to oceanic and atmospheric environmental elements at different incidence angles are studied, and the corresponding sensitivity coefficients are calculated. The results indicate that the sensitivities of vertical and horizontal polarization brightness temperature to oceanic and atmospheric environmental elements show different characteristics. With the increase of the incidence angle, the sensitivity of the vertical polarization brightness temperature to the sea surface temperature is enhanced, and the sensitivity to the sea surface wind field is relatively weakened; but the horizontal polarization brightness temperature is opposite. The vertical and horizontal polarization brightness temperature errors caused by the errors of atmospheric water vapor content and cloud liquid water content increase with the increasing of the incidence angle, however, even at a large incidence angle of 55°, the errors of vertical and horizontal polarization brightness temperature are still less than 0.12 K. If the accuracy of sea surface temperature inversion is higher than 1 K, the calibration precision of one dimensional synthetic aperture microwave radiometer should be better than 0.6 K. Overall, the results of this paper are of great significance to the research of sea surface temperature retrieval with multiple incidence angle for one dimensional synthetic aperture microwave radiometer.

WindSat is a satellite-based multi-frequency polarimetric microwave radiometer designed to measure the fully polarimetric radiometric brightness temperature (TB) at 10.7, 18.7, and 37.0 GHz, and linearly polarized TB at 6.8 and 23.8 GHz. The primary goal of WindSat is to demonstrate the capability of polarimetric microwave radiometry in remote sensing of the ocean surface wind vector. Sea surface temperature, water vapor, and cloud liquid water, are among some of the other geophysical parameters that can also be measured. Solution of an inverse method to retrieve these environmental parameters from the polarimetric radiometer measurements requires a forward model that characterizes the measured brightness temperature due to emission from the surface and the intervening atmosphere, and its transmission through the atmosphere. This paper concentrates predominantly on the atmospheric component of the Windsat forward model. This includes two separate but related algorithms. We have developed a complete radiative transfer model that calculates the upward and downward atmospheric radiation, including attenuation effects due to clear air (both resonant and continuum absorption by water vapor and molecular oxygen) and non-precipitating clouds. From calculations and analysis of using this full forward model and an extensive match-up dataset a computationally efficient one-layer parameterized model has been developed for use in the physically-based Windsat retrieval algorithm. A performance assessment of forward models which utilize surface and atmospheric models, NWP model data assimilation profiles, and environmental measurements from other satellites, by comparison with WindSat measurements, is presented.

Advanced control of large-scale wind turbines: Structural load reduction and lifetime control

MODIS (Moderate Resolution Imaging Spectroradiometer) level-3 aerosol data, NCEP (National Centers for Environmental Prediction) reanalysis winds and QuikSCAT ocean surface winds were made use of to examine the role of atmospheric circulation in governing aerosol variations over the Bay of Bengal (BoB) during the first phase of the ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) campaign (March 18–April 12, 2006). An inter-comparison between MODIS level-3 aerosol optical depth (AOD) data and ship-borne MICROTOPS measurements showed good agreement with correlation 0.92 (p 850 hPa) indicated that direct transport of aerosols from central India was inhibited by the presence of a high pressure and a divergence over BoB in the lower altitudes. On the other hand, in the upper atmospheric levels, winds from central and northern India stretched south eastwards and converged over BoB with a negative vorticity indicative of a downdraft. These wind patterns pointed to the possibility of aerosol transport from central India to BoB by upper level winds. This mechanism was further confirmed by the significant correlations that AOD variations over BoB showed with aerosol flux convergence and flux vorticity at upper atmospheric levels (600–500 hPa). AOD in central and southern BoB away from continental influences displayed an exponential dependence on the QuikSCAT measured ocean surface wind speed. This study shows that particles transported from central and northern India by upper atmospheric circulations as well as the marine aerosols generated by ocean surface winds contributed to the AOD over the BoB during the first phase of ICARB.

Probability distributions of surface wind speeds (SWS) near coastal regions are needed for applications such as estimating offshore wind power and ocean surface fluxes and for offshore wind risk assessments. Ocean surface wind speed probability distribution (PDF) is characterized using three-year QuikSCAT and AIRS satellite observations in the southeast Pacific of marine stratus and stratocumulus (MSC) regions. Seasonal variation is removed from wind statistics. It was found that the observed SWS standard deviation has a linear positive relationship with its mean SWS; while the SWS skewness decreases with mean SWS in regimes of strong winds and increases with mean SWS in regimes of weak winds. A simple 1D conceptual model is developed near the Peruvian region, which successfully reproduces the observed relationship between higher moments of SWS and its mean value. The model based physical picture among ocean surface winds, SST, and marine boundary clouds are supported by three-year QuikSCAT surface wind observations and fifteen-year ERA40 re-analysis data. Model sensitive tests suggest that large-scale divergence, and strengths of momentum and cloud fluctuations have significant effects on the ocean SWS-PDF in marine stratus and stratocumulus regions.

The initial images and the corresponding evaluation of a MWIR polarization imaging system are discussed in this paper. This MWIR polarization imaging system consists of an objective, two folded mirrors, a polarimetric analyzer, a relay lens, a MCT 320×256 FPA with 30μm pitch and 3.7~4.8μm spectral region. The effective focal length of the re-imaging objective is 185mm, F-number is 4.0 and the polarimetric analyzer is located in the parallel light. When the analyzer is rotated to four different degrees, that of 0⁰, 45⁰, 90⁰ and 135⁰, a set of four original infrared intensity images is acquired, then the Stokes vector's elements of I, Q, U and the degree of linear polarization P of the incident radiation from remote scene can be solved. After that, several methods are taken to process the polarization information mentioned above, such as the I-P fusion, the IQU fusion, the IQU pseudocolor fusion and the IQU pseudocolor fusion by color adjusted. Respectively, the results of fused images are presented. The quantitative evaluations of the MWIR fused images shows that the mean of gray scales has been improved 35%, the standard difference of gray scales has been improved 61% and the gradient of fused images has been improved up to 255%. The results of this paper indicate that the MWIR fused polarization images are much clearer than that of the intensity images in profiles, edges and details of an object, and it can obviously increase the contrast between object interested and the background. The different types of polarimetric images can be displayed by means of image of fused images of different Stokes components gray scales or images of pseudocolor fusion. These results offer multiple choices to the displaying of polarimetric images. The different methods of polarimetric fused image can product different results. The results show polarimetric images contain new and useful information that is not obtainable from the intensity images.

The observation of surface water bodies in all weather conditions and better knowledge about inundation patterns are important for water resource management and flood early warning. Microwave radiometers at 37 GHz were applied to observe and study the inundation pattern in large subtropical floodplains in China, i.e. the Poyang Lake and Dongting Lake floodplains, due to the trade-off between the capability to penetrate hydrometeors and vegetation, revisiting time, and spatial coverage and resolution. Taking the shallow sensing depth at 37 GHz into account, open water, inundated area and water saturated soil surface all determine the surface emittance measured by the radiometer. Thus, Water Saturated Surface (WSS) is defined as the combination of these three land surface elements. In subtropical regions, seasonal changes in vegetation cover and various surface roughness conditions are the major challenges for the observation of surface water bodies with microwave radiometers. Atmospheric attenuation, observation gaps and errors in the microwave observations reduce the quality of daily radiometric observations. To deal with the attenuation due to vegetation and surface roughness, a two-step model was developed: the first step is to retrieve the polarization difference emissivity from Polarization Difference Brightness Temperature (PDBT) at 37 GHz with the simplified radiative transfer model and the vegetation optical thickness at 37 GHz parameterized from Normalized Difference Vegetation Index (NDVI) ; the second step is to retrieve the fractional area of WSS from the emissivity difference with a linear model, which can be parameterized according to the Qp surface roughness model. To remove the noise and extract the surface signal (including surface emittance and vegetation attenuation) from the daily PDBT time series, the Time Series Analysis Procedure (TSAP) was developed to identify the spectral features of noisy components in the frequency domain and remove them with a proper filter. The overall method combined the TSAP and the two-step model to derive daily observation of WSS area. The retrieved WSS area in the Poyang Lake floodplain was in a good agreement with the lake area observed from MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Synthetic Aperture Radar (ASAR). The observations and analysis of the inundation patterns in the Poyang Lake and Dongting Lake floodplains with this method illustrated the close relationship between inundated area, precipitation and stream flow. Furthermore, a lumped hydrological model, named the discrete rainfall-runoff model, was developed to fully use the retrieved WSS area and to study the role of inundated area in stream flow production. This model simulates stream flow as the integration of contributions of antecedent precipitation in a certain period. Three implementations of the model were developed with the help of ground water table depth and the retrieved WSS area. The case study in the Xiangjiang River basin (upstream catchment of the Dongting Lake floodplain), China, illustrated that: 1) the longest duration of antecedent precipitation is a key parameter to determine model performance; 2) long duration would increase the model uncertainty and lead to overfitting; 3) the application of the WSS area can reduce the duration required to achieve a reasonable accuracy. The model parameters indicated the interaction between stream flow and various water storages, and the calibration results of three implementations implied the recharge period of ground water.

Remote sensing is a fundamental tool to measure the dynamics of ice sheets and provides valuable information for ice sheet projections under a changing climate. There is, however, the potential to further reduce the uncertainties in these projections by developing innovative remote sensing methods. One of these remote sensing techniques, the polarimetric synthetic aperture radar interferometry (Pol-InSAR), is known since decades to have the potential to assess the geophysical properties below the surface of ice sheets, because of the penetration of microwave signals into dry snow, firn, and ice. Despite this, only very few studies have addressed this topic and the development of robust Pol-InSAR applications is at an early stage. Two potential Pol-InSAR applications are identified as the motivation for this thesis. First, the estimation and compensation of the penetration bias in digital elevation models derived with SAR interferometry. This bias can lead to errors of several meters or even tens of meters in surface elevation measurements. Second, the estimation of geophysical properties of the subsurface of glaciers and ice sheets using Pol-InSAR techniques. There is indeed potential to derive information about melt-refreeze processes within the firn, which are related to density and affect the mass balance. Such Pol-InSAR applications can be a valuable information source with the potential for monthly ice sheet wide coverage and high spatial resolution provided by the next generation of SAR satellites. However, the required models to link the Pol-InSAR measurements to the subsurface properties are not yet established. The aim of this thesis is to improve the modeling of the vertical backscattering distribution in the subsurface of ice sheets and its effect on polarimetric interferometric SAR measurements at different frequencies. In order to achieve this, polarimetric interferometric multi-baseline SAR data at different frequencies and from two different test sites on the Greenland ice sheet are investigated. This thesis contributes with three concepts to a better understanding and to a more accurate modeling of the vertical backscattering distribution in the subsurface of ice sheets. First, the integration of scattering from distinct subsurface layers. These are formed by refrozen melt water in the upper percolation zone and cause an interesting coherence undulation pattern, which cannot be explained with previously existing models. This represents a first link between Pol-InSAR data and geophysical subsurface properties. The second step is the improved modeling of the general vertical backscattering distribution of the subsurface volume. The advantages of more flexible volume models are demonstrated, but interestingly, the simple modification of a previously existing model with a vertical shift parameter lead to the best agreement between model and data. The third contribution is the model based compensation of the penetration bias, which is experimentally validated. At the investigated test sites, it becomes evident that the model based estimates of the surface elevations are more accurate than the interferometric phase center locations, which are conventionally used to derive surface elevations of ice sheets. This thesis therefore improves the state of the art of subsurface scattering modeling for Pol-InSAR applications, demonstrates the model-based penetration bias compensation, and makes a further research step towards the retrieval of geophysical subsurface information with Pol-InSAR.

The GNSS-R (Global Navigation Satellite System Reflectometry) technique has been envisaged as a promising low-cost complementary passive remote sensing tool to conventional active instruments for ocean, land and ice monitoring over the last decade. For the ocean surface wind retrieval of GNSS-R, the approach of fitting real measurements to a forward scattering model tuned with wind speed is used usually. To avoid the low accuracy and difficult acquirement of wind direction using this approach, the relationship between sea surface statistical parameters and wind speed, wind direction were researched. Moreover, the real sea surface statistical parameters were obtained through nonlinear optimization combining the aircraft data, accordingly the sea surface wind was retrieved. The results show good agreement with retrieved wind and other acquirable ways.

The MICAP(microwave imager combined active and passive), which has been selected to be a candidate payload for future Chinese ocean salinity mission, contains an L-band digital beam forming(DBF) scatterometer for the purpose of elimination of ocean surface roughness and wind retrieving. In this paper, data pre-processing flow for this scatterometer to obtain data ready for sea surface salinity and ocean surface wind retrieval was presented after corresponding data level definitions in this flow had been introduced. The processing flow was verified by processing data resulted from a data set simulated in a simple progress. Future work to perfect the pre-processing progress was also discussed.

The Special Sensor Microwave Imagers (SSM/I) flying on the DMSP polar orbiting derive relatively high resolution surface wind speeds over the oceans. The SSM/I is limited to ocean surface wind speed only. Recently, techniques to infer wind direction from passive microwave measurements have been developed. Polarimetric measurements are currently of interest. Active devices produce very high quality vector wind measurements. It is the goal of this paper to discuss a simple technique that could be used in conjunction with polarimetry and scatterometry to augment the complete description of boundary layer winds. Th authors discuss a simple technique first developed by T.S. Yu (1987), that yields a closed form solution to the horizontal momentum equations for the boundary layer vector wind field using in situ satellite measurements as a source of wind speed data, and surface pressure gradient data from state-of-the-art numerical weather prediction models. The SEASAT scatterometer and altimeter data were originally used as a proof of concept for this technique. The authors describe an implementation using the SSM/I as the source of wind speed data, and the Air Force Global Weather Central (AFGWC) High Resolution Analysis System (HIRAS) as the source of boundary layer pressure analysis. They calculate a wind field at the resolution of the SSM/I, with time and space interpolation of the model surface pressure analysis to match the satellite measurements. It should be noted that the HIRAS resolution is much coarser than the SSM/I data, therefore they are relying on the general smoothness ofthe surface pressure during the interpolation process.