The logarithmic cumulants (log-cumulants for short) of the second and third orders are widely used in the statistical analysis of polarimetric synthetic aperture radar (PolSAR) data. However, both the product model and the finite mixture model may produce the same values of these statistics, which means that the use of these log-cumulants is not enough to determine the statistical model of the data. In this letter, it is demonstrated that the log-cumulants of higher orders can help to distinguish the concept of texture from that of mixture, providing a physical insight into the data statistics. A tool called log-cumulant cube, which helps to visualize this difference, is proposed by considering texture distributions from the Pearson's family. Results on both simulated and real SAR data show that the use of higher order statistics is useful when it comes to the texture analysis of PolSAR data.
Global Navigation Satellite System-Reflectometry (GNSS-R) is a promising and innovative remote sensing technique. In this letter, an investigation on the polarimetric characteristics of GNSS-R signals is performed. The study is conducted by means of an ad hoc simulator based on a facet approach to approximate a 3-D time-domain generated sea surface. To best mimic a realistic configuration, the simulator uses both recorded GPS and Galileo signals as incident waves. The effect of the incoherent averaging in mitigating the speckle is shown. Finally, a relationship between the wind speed and the polarimetric ratio is analyzed.
Ceba, F.; Makhoul, E.; Broquetas, A.; Zhan, Y.; Beaton, A. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 10, p. 2145-2149 DOI: 10.1109/LGRS.2015.2453018 Data de publicació: 2015-10-01 Article en revista
This letter analyzes the problems of imaging small and fast boats with synthetic aperture radar (SAR), which may experience severe defocusing and/or smearing. This situation is encountered, inter alia, in ocean traffic monitoring and surveillance applications where the detection of small and fast vessels is of great interest. The target modeling and impact of moving vessels in SAR images are presented and discussed using simulated results. A matched filter bank is used before applying moving target indication techniques over multichannel SAR images, refocusing the target signal in the azimuth domain and enhancing the detectability of these types of vessels.
Corbella, I.; Duran, I.; Wu, L.; Torres, F.; Duffo, N.; Khazaal, A.; Martín, M. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 9, p. 1813-1817 DOI: 10.1109/LGRS.2015.2428653 Data de publicació: 2015-09-01 Article en revista
Land-sea contamination observed in Soil Moisture and Ocean Salinity (SMOS) brightness temperature images is found to have two main contributions: the floor error inherent of image reconstruction and a multiplicative error either in the antenna temperature or in the visibility samples measured by the correlator. The origin of this last one is traced down to SMOS calibration parameters to yield a simple correction scheme, which is validated against several geophysical scenarios. Autoconsistency rules in interferometric synthesis together with redundant and complementary calibration procedures provide a robust SMOS calibration scheme.
Konings, A.; McColl, K.; Piles, M.; Entekhabi, D. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 5, p. 1081-1085 DOI: 10.1109/LGRS.2014.2381641 Data de publicació: 2015-05-01 Article en revista
Remote sensing algorithms often invert multiple measurements simultaneously to retrieve a group of geophysical parameters. In order to create a robust retrieval algorithm, it is necessary to ensure that there are more unique measurements than parameters to be retrieved. If this is not the case, the inversion might have multiple solutions and be sensitive to noise. In this letter, we introduce a methodology to calculate the number of (possibly fractional) "degrees of information" in a set of measurements, representing the number of parameters that can be retrieved robustly from that set. Since different measurements may not be mutually independent, the amount of duplicate information is calculated using the information-theoretic concept of total correlation (a generalization of mutual information). The total correlation is sensitive to the full distribution of each measurement and therefore accounts for duplicate information even if multiple measurements are related only partially and nonlinearly. The method is illustrated using several examples, and applications to a variety of sensor types are discussed.
Remote sensing algorithms often invert multiple measurements simultaneously to retrieve a group of geophysical parameters. In order to create a robust retrieval algorithm, it is necessary to ensure that there are more unique measurements than parameters to be retrieved. If this is not the case, the inversion might have multiple solutions and be sensitive to noise. In this letter, we introduce a methodology to calculate the number of (possibly fractional)
Corbella, I.; Wu, L.; Torres, F.; Duffo, N.; Martín, M. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 3, p. 458-461 DOI: 10.1109/LGRS.2014.2345845 Data de publicació: 2015-03-01 Article en revista
Faraday rotation is dynamically computed from L-band radiometric data acquired by the Soil Moisture and Ocean Salinity satellite. Improved full polarimetric image reconstruction and smart spatiotemporal filtering are used to obtain good estimations of Faraday rotation, which compare very well with the values theoretically predicted from available ionosphere and geomagnetic field data.
Martin, F.; Camps, A.; Fabra, F.; Rius, A.; Martín, M.; d'Addio, S.; Alonso, A. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 2, p. 279-283 DOI: 10.1109/LGRS.2014.2335772 Data de publicació: 2015-02-01 Article en revista
In Global Navigation Satellite System Reflectometry (GNSS-R), power waveforms or Delay-Doppler Maps (DDM) are incoherently averaged to reduce the standard deviation of the fluctuations caused by the speckle and thermal noise. This letter proposes a simple and innovative processing concept based on the computation of the variance of the complex waveforms. By this approach, the coherent part of the signal is subtracted from the incoherent averaged waveform. It can be useful, for example, in scenarios where reflected signal is contaminated by the direct signal, as ground-based scenarios or in future experiments such as GEROS-ISS due to the nearby multipath of direct signals.
Iglesias, R.; Monells, D.; Lopez, C.; Mallorqui, J.J.; Fabregas, F.; Aguasca, A. IEEE Geoscience and Remote Sensing Letters Vol. 12, num. 1, p. 87-91 DOI: 10.1109/LGRS.2014.2326684 Data de publicació: 2015-01-01 Article en revista
The application of differential synthetic aperture radar interferometry (DInSAR) techniques has been traditionally limited to the single-polarimetric case. The launch of satellites with polarimetric capabilities has triggered the synergy between polarimetric and interferometric algorithms, leading to a significant improvement in final DInSAR products. During the last years, the different polarimetric optimization techniques available have been successfully applied to the so-called classical phase quality estimators, i.e., the coherence and the amplitude dispersion estimators. In this context, a new estimator to evaluate the pixels' phase quality, referred to as temporal sublook coherence (TSC), has recently demonstrated to provide promising results in DInSAR applications. The nature of this estimator, which is based on exploiting the spectral properties of pointlike scatterers through the coherence evaluation of different sublooks of the image spectrum, allows its adaptation to the existing polarimetric optimization methods. This letter presents the benefits of extending the TSC estimator to work with fully polarimetric data. For this purpose, a fully polarimetric data set consisting of ten X-band ground-based SAR (GB-SAR) images is employed. The final DInSAR results obtained by means of TSC polarimetric optimization are compared with the ones obtained with its classical single-polarimetric approach, achieving up to more than a twofold increase in the pixels' density.
Radio-frequency interference (RFI) seriously affects the retrieval of geophysical parameters from the measurements of microwave radiometers. An accurate geolocation of the RFI is crucial to effectively switch off illegal transmitters. In this letter, a new RFI localization method is proposed to improve the achievable angular resolution by using beamforming and direction-of- arrival (DOA) estimation techniques. The proposed RFI localization techniques can be employed in synthetic aperture interferometric radiometers, such as the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission. Two DOA estimation techniques are tested for the RFI localization: Capon and MUSIC. The feasibility of these methods is demonstrated with SMOS data. In the test results, the MUSIC beamforming shows a better performance of RFI localization than the SMOS Fourier imaging and the Capon, in terms of accuracy and resolution.
Lagunas, E.; Amin, Moeness G.; Ahmad, F.; Najar, M. IEEE Geoscience and Remote Sensing Letters Vol. 11, num. 12, p. 2193-2197 DOI: 10.1109/LGRS.2014.2324611 Data de publicació: 2014-12-01 Article en revista
Soil moisture (SM) is a key parameter in the climate studies at a global scale and a very important parameter in applications such as precision agriculture at a local scale. The Global Navigation Satellite Systems Interference Pattern Technique (IPT) has proven to be a useful technique for the determination of SM, based on observations at vertical polarization (V-Pol) due to the Brewster angle. The IPT can be applied at both V-Pol and horizontal polarization (H-Pol) at the same time, observing the Brewster angle only at V-Pol. This letter presents a measurement technique based on tracking the phase difference between V-Pol and H-Pol interference patterns to improve the accuracy of the Brewster angle determination and, consequently, that of the SM retrievals. This technique benefits from the different phase behavior of the reflection coefficients between H-Pol and V-Pol in the angular observation range. To be sensitive to the phase difference, the Rayleigh criterion for smooth surfaces must be accomplished. This technique is not sensitive to topography as it is intrinsically corrected. Experimental results are presented to validate the proposed algorithm.
Wu, L.; Torres, F.; Corbella, I.; Duffo, N.; Duran, I.; Vall-llossera, M.; Camps, A.; Delwart, S.; Martín, M. IEEE Geoscience and Remote Sensing Letters Vol. 10, num. 6, p. 1454-1458 DOI: 10.1109/LGRS.2013.2260128 Data de publicació: 2013-11 Article en revista
This work has been conducted in the framework of several projects devoted to assess the performance of the Soil Moisture and Ocean Salinity (SMOS) mission full-pol measurement mode. Since its launch in November 2009, SMOS is producing dual-polarization brightness temperature synthesized images that are yielding a high scientific return. However, these images are affected by a non-negligible spatial amplitude error, the so-called spatial bias (SB), that degrades geophysical parameter retrieval. This effect is particularly detrimental in SMOS polarimetric images where spatial bias is masking the polarimetric physical signature to a large extend. This paper presents a method to mitigate SMOS spatial bias by taking into account the co-and cross-polar antenna patterns in the image reconstruction algorithm through the, so called, full-pol G-matrix (FPG). The method is validated by producing spatial bias maps out of the comparison between SMOS full-pol images and an accurate polarimetric brightness temperature model of the ocean. This model has been provided to SMOS ESLs (Expert Support Laboratories) by LOCEAN (Laboratoire d'Océanographie et du Climat, France) as a test bench to validate and improve SMOS Level 1 (L1) data. Finally, a radiometric performance summary table comparing spatial bias and radiometric sensitivity between this new FPG approach and SMOS current co-polar G-matrix approach (CPG) is provided. This paper presents the best quality SMOS polarimetric images, which may lead a breakthrough in the science returns of the mission.
Synthetic aperture radar (SAR) systems are inherently band limited in both range and azimuth, and hence, the point spread function (PSF) has the shape of a bidimensional sinc function. In addition, all SAR images are slightly oversampled, and as a consequence, the contribution of a single target extends to more than a single cell. The main lobe and the side lobes of strong scatterers are sometimes clearly visible in the images. This characteristic of the SAR images must be considered when applying differential interferometric synthetic aperture radar (DInSAR) pixel selection algorithms. For persistent scatterers, the properties, for instance, the amplitude stability, are preserved in both redundant information around the main lobe and side lobes. For this reason, a cluster of pixels rather than just the pixel position corresponding to the exact location of the target will be detected. Spatially variant apodization (SVA) is a nonlinear filter based on cosine-on-pedestal weighting functions able to achieve a total side-lobe cancelation without degrading the original image resolution. When working with complex data under complex scattering scenarios, the PSF moves away from the ideal bidimensional sinc, and the SVA performance worsens. The amplitude and phase of the original images could be distorted by the SVA filtering compromising the pixel selection and the quality of the final DInSAR results. In this letter, SVA is used to method locate in the image the side lobes of high-power scatterers and generate a mask while preserving the amplitude and phase of the original images.
Park, H.; Valencia, E.; Camps, A.; Rius, A.; Ribó, S.; Martín, M. IEEE Geoscience and Remote Sensing Letters Vol. 10, num. 1, p. 57-61 DOI: 10.1109/LGRS.2012.2192255 Data de publicació: 2013-01 Article en revista
Corbella, I.; Martín, M.; Oliva, R.; Torres, F.; Duffo, N. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 5, p. 977-979 DOI: 10.1109/LGRS.2012.2188374 Data de publicació: 2012-09 Article en revista
Gourrion, J.; Sabia, R.; Portabella, M.; Tenerelli, J.; Guimbard, S.; Camps, A. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 4, p. 793-797 DOI: 10.1109/LGRS.2011.2181990 Data de publicació: 2012-07 Article en revista
The Soil Moisture and Ocean Salinity (SMOS) mission was launched on November 2nd, 2009 aiming at providing sea surface salinity (SSS) estimates over the oceans with frequent temporal coverage. The detection and mitigation of residual instrumental systematic errors in the measured brightness temperatures are key steps prior to the SSS retrieval. For such purpose, the so-called ocean target transformation (OTT) technique is currently used in the SMOS operational SSS processor. In this paper, an assessment of the OTT is performed. It is found that, to compute a consistent and robust OTT, a large ensemble of measurements is required. Moreover, several effects are reported to significantly impact the OTT computation, namely, the apparent instrument (temporal) drift, forward model imperfections, auxiliary data (used by forward model) uncertainty and external error sources, such as galactic noise and Sun effects (among others). These effects have to be properly mitigated or filtered during the OTT computation, so as to successfully retrieve SSS from SMOS measurements.
Rodriguez, N.; Bosch, X.; Camps, A.; Ramos, I.; Valencia, E.; Park, H.; Vall-llossera, M. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 2, p. 282-286 DOI: 10.1109/LGRS.2011.2166242 Data de publicació: 2012-03 Article en revista
Global Navigation Satellite Systems (GNSS) opportunity signals reflected at or near the Earth's surface have already shown their potential to perform retrievals of a number of geophysical parameters. Radio occultations using GNSS signals are also used for atmospheric sensing. This letter presents a GNSS technique to retrieve vegetation water content (VWC). This technique measures the received powers of the GPS signals in open sky and under the vegetation layer. From these two powers, the attenuation due to the vegetation is computed, which is related to the VWC. This letter presents the results obtained after deploying the instrument in a walnut-tree stand for 11 months.
Rodriguez-Alvarez, N.; Aguasca, A.; Valencia, E.; Bosch, X.; Camps, A.; Ramos, I.; Park, H.; Vall-llossera, M. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 6, p. 1109-1113 DOI: 10.1109/LGRS.2012.2190379 Data de publicació: 2012 Article en revista
Ramos, I.; Bosch, X.; Camps, A.; González, V.; Valencia, E.; Rodriguez-Alvarez, N.; Park, H.; Vall-llossera, M.; Forte, G. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 4, p. 774-777 DOI: 10.1109/LGRS.2011.2181483 Data de publicació: 2012 Article en revista
Interpolation strategies for calibration of the Soil
Moisture and Ocean Salinity (SMOS) mission of the European
Space Agency are tested and compared. Calibration strategy (how
and how often) is critical in achieving the required performance
of any instrument, but it is even more important in very complex
instruments such as the new family of synthetic aperture
interferometric radiometers and, in particular, in the Microwave
Imaging Radiometer by Aperture Synthesis instrument aboard
the SMOS mission. On one hand, frequent calibration reduces
the available observation time. On the other hand, the calibration
requirements for soil moisture applications are more relaxed than
those for ocean salinity, so the intercalibration time requirements
are very different. Since SMOS drifts are stationary, half-orbit
information is available to perform different interpolation strategies.
In this letter, these approaches are tested to estimate the
calibration parameters between consecutive calibrations. The average
root-mean-square phase error is then used to find the optimum
interpolation strategy and intercalibration time. On the
other side, in real-time instruments, the “future” calibration data
are not available at the time of taking the measurements, and
predictors are required to estimate the evolution of the calibration
parameters from past data only. For these systems, the extended
Kalman filter can be used. The intercalibration time in a real-time
instrument is evaluated, and the requirements and performances
are compared to offline instruments.
Torres, F.; Corbella, I.; Wu, L.; Duffo, N.; Gourrion, J.; Font, J.; Martín, M. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 1, p. 18-22 DOI: 10.1109/LGRS.2011.2158799 Data de publicació: 2012-01 Article en revista
Ruiz, J.; Broquetas, A.; Gonzalez, J.; Closa, J.; Labriola, M. IEEE Geoscience and Remote Sensing Letters Vol. 9, num. 2, p. 199-203 DOI: 10.1109/LGRS.2011.2162578 Data de publicació: 2011-09-15 Article en revista
In this letter, an optimized scanning in terrain observation by progressive scan synthetic aperture radar (TOPSAR)
mode is studied. A nonuniform steering rate of the radar array antenna in the along-track direction is proposed in order to obtain constant radiometric sensitivity and signal-to-noise ratio. This
is achieved owing to the longer integration time of the echoes received at both ends of the antenna azimuth sweep. By optimizing
iteratively the array discrete steering rate law, the radiometric impact of the array basic element pattern (subarray pattern) can
be accurately compensated. First, simulation results are presented to validate the nonuniform steering TOPSAR.
Valencia, E.; Camps, A.; Marchan, J.; Park, H.; Bosch, X.; Rodriguez, N.; Ramos, I. IEEE Geoscience and Remote Sensing Letters Vol. 8, num. 4, p. 750-754 DOI: 10.1109/LGRS.2011.2107500 Data de publicació: 2011-07 Article en revista
Global navigation satellite system reflectometry of signals is a promising technique to remotely sense a number of
Earth’s geophysical parameters, and it has been proposed for ocean monitoring applications such as mesoscale altimetry or sea state monitoring. So far, the following two main approaches have been considered to retrieve a sea state descriptor from measured
delay waveforms or delay–Doppler (DD) maps (DDMs): 1) fitting the measurements to a model tuned with the desired parameter
or 2) directly linking a property of the measurements with the parameter to be retrieved (e.g., volume of the normalized DDM). However, these approaches provide a single sea state descriptor related to the overall glistening zone where sea state conditions may not be homogeneous. In this letter, the relationship between the physical space and the DD domains is exploited, and a method to retrieve the bistatic scattering coefficient distribution over the observation ocean surface from measured DDMs is proposed.
Ferrer, P.; Lopez, C.; Aguasca, A.; Pipia, L.; Gonzalez, J.; Fabregas, F.; Romeu, J. IEEE Geoscience and Remote Sensing Letters Vol. 8, num. 4, p. 774-778 DOI: 10.1109/LGRS.2011.2113313 Data de publicació: 2011-07 Article en revista
The use of a low-profile, lightweight, and easy-to-fabricate transpolarizing surface placed on one side of a trihedral corner reflector (TCR) as a polarimetric calibrator is presented in this letter. The transpolarizing TCR presents a high
backscattered cross-polar response contrary to standard TCRs. The performance of this device has been tested at the X-band using
the Universitat Politecnica de Catalunya ground-based synthetic aperture radar.
The radiometric resolution of a motion-induced synthetic aperture radiometer (MISAR) is analytically obtained from the standard deviation of a baseline response, an observation scenario, and the imaging method of the MISAR. The intrinsic long integration time given by the whole dwell time on moving platforms improves the radiometric resolution compared with the
snapshot resolution of other nominal synthetic aperture radiometers.
In addition, it is illustrated that the MISAR imaging holds the tradeoff relationship between the radiometric and spatial resolutions.
This paper aims at introducing the recent theory of
compressive sensing to radar imaging systems in order to
retrieve the imaged scene with better resolution and a
reduced amount of collected samples. As a result of the
application of the alternative imaging technique proposed,
the use of matched filtering is avoided and the effect of its
sidelobes in the images is drastically diminished.
Furthermore, the amount of data to be stacked in the sensor
and then downlinked to the ground station is meaningfully
lower. This permits a more efficient management of
Prats, P.; Reigber, A.; Mallorqui, J.J.; Moreira A, R. IEEE Geoscience and Remote Sensing Letters Vol. 46, num. 4, p. 1065-1078 DOI: 10.1109/TGRS.2008.915758 Data de publicació: 2008-04 Article en revista
Sanz-Marcos, J.; López-Dekker, P.; Lopez, F.J.; Mallorqui, J.J.; Aguasca, A.; Prats, P. IEEE Geoscience and Remote Sensing Letters Vol. 4, num. 2, p. 307-311 DOI: 10.1109/LGRS.2007.894144 Data de publicació: 2007-04 Article en revista
Accurate measurement of the antenna voltage patterns of large-aperture synthesis radiometers is critical in order to achieve good radiometric accuracy, and a very time consuming and expensive task. Measurement requirements and a tradeoff study relating radiometric accuracy degradation and number of elements to be measured are presented.
The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission will be the first one using two-dimensional aperture synthesis radiometry for Earth observation. This study presents the formulation that relates instrument observables and brightness temperature maps including cross-polar antenna voltage patterns, which may be also different from element to element. Finally, the radiometric accuracy degradation if cross-polar patterns are neglected in the image reconstruction is studied.