IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
Global Navigation Satellite System Reflectometry (GNSS-R) is becoming a widely accepted technique... more Global Navigation Satellite System Reflectometry (GNSS-R) is becoming a widely accepted technique for Remote Sensing. The interferometric technique (iGNSS-R) correlates the direct signal received from a satellite and the same signal reflected on the Earth's surface, whereas the conventional technique (cGNSS-R) correlates the reflected signal with a locally generated replica of the transmitted code. As GNSS signals are received below the noise level, this technique is extremely sensitive to Radio Frequency Interference (RFI). The Distance Measurement Equipment (DME), and the TACtical Air Navigation (TACAN) systems are two radio navigation systems that transmit in the GPS L5, and Galileo E5 bands with powers up to 3.5 kW. This work studies in depth the impact of these systems on iGNSS-R, and cGNSS-R instruments. This study is then applied to an hypothetical reflectometer that will be placed in the International Space Station (ISS): the GEROS experiment. It is shown that the received power in space will be strong enough to degrade the system's performance by increasing the noise floor, but the sea altimetry precision will still be accurate enough for scientific studies.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Jul 1, 2017
Radio-Frequency Interference (RFI) signals are a well known threat for Microwave Radiometry (MWR)... more Radio-Frequency Interference (RFI) signals are a well known threat for Microwave Radiometry (MWR) applications. In order to alleviate this problem, different approaches for RFI detection and mitigation are currently under development. Since RFI signals are man-made, they tend to have their power more concentrated in the Time-Frequency (TF) space as compared to naturally emitted noise. The aim of this work is to perform an assessment of different TF RFI mitigation techniques in terms of probability of detection, resolution loss (RL), and mitigation performance. In this assessment, six different kinds of RFI signals have been considered: a glitch, a burst of pulses, a wide-band chirp, a narrow-band chirp, a continuous wave, and a wide-band modulation. The results show that the best performance occurs when the transform basis has a similar shape as compared to the RFI signal. For the best case performance, the maximum residual RFI temperature is 14.8 K, and the worst RL is 8.4%. Moreover, the Multiresolution Fourier Transform (MFT) technique appears as a good trade-off solution among all other techniques since it can mitigate all RFI signals under evaluation with a maximum residual RFI temperature of 21 K, and a worst RL of 26.3%. Although the obtained results are still far from an acceptable bias < 1 K for MWR applications, there is still work to do in a combined test using the information gathered simultaneously by all mitigation techniques which could improve the overall performance of RFI mitigation.
IEEE Transactions on Geoscience and Remote Sensing, Apr 1, 2016
SMOS brightness temperature images and calibrated visibilities are related by the so-called G-mat... more SMOS brightness temperature images and calibrated visibilities are related by the so-called G-matrix. Due to the incomplete sampling at some spatial frequencies, sharp transitions in the brightness temperature scenes generate a Gibbs-like contamination ringing and spread sidelobes. In the current SMOS image reconstruction strategy, a Blackman window is applied to the Fourier components of the brightness temperatures to diminish the amplitude of artifacts such as ripples, and other Gibbs-like effects. In this work, a novel image reconstruction algorithm focused on the reduction of Gibbs-like contamination in brightness temperature images is proposed. It is based on sampling the brightness temperature images at the nodal points, that is, at those points at which the oscillating interference causes the minimum distortion to the geophysical signal. Results show a significant reduction of ripples and sidelobes in strongly RFI-contaminated images. This technique has been throughly validated using snapshots over the ocean, by comparing brightness temperatures reconstructed in the standard way or using the nodal sampling with modeled brightness temperatures. Tests have revealed that the standard deviation of the difference between the measurement and the model is reduced around 1 K over clean and stable zones when using nodal sampling technique with respect to the SMOS image reconstruction baseline. The reduction is approximately 0.7 K when considering the global ocean. This represents a crucial improvement in brightness temperature quality, which will translate in an enhancement of the retrieved geophysical parameters, especially the sea surface salinity.
Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiom... more Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiometers. After the lessons learnt from SMOS, ESA is preparing the next-generation of L-band interferometric radiometers with RFI mitigation integrated in the cross-correlators. This work presents a preliminary design and results of a pre-correlation RFI mitigation algorithm tailored for interferometric radiometers. The results show that the correlation error introduced by the RFI is reduced on average to the half, with peaks of 20 dB of mitigation.
IEEE Transactions on Geoscience and Remote Sensing, 2022
Radio-frequency interference (RFI) is an increasing problem particularly for Earth observation us... more Radio-frequency interference (RFI) is an increasing problem particularly for Earth observation using microwave radiometry. RFI has been observed, for example, at L-band by the European Space Agency's (ESA's) soil moisture and ocean salinity (SMOS) Earth Explorer and by National Aero-nautics and Space Administration's (NASA's) soil moisture active passive (SMAP) and Aquarius missions, as well as at C-band by Advanced Microwave Scanning Radiometer (AMSR)-E and AMSR-2, and at 10.7 and 18.7 GHz by AMSR- , WindSat, and GPM Microwave Imager (GMI). Therefore, systems dedicated to interference detection and removal of contaminated measurements are nowadays a must in order to improve radiometric accuracy and reduce the loss of spatial coverage caused by interference. In this work, the feasibility of using the empirical mode decomposition (EMD) technique for RFI mitigation is explored. The EMD, also known as Hilbert-Huang transform (HHT), is an algorithm that decomposes the signal into intrinsic mode functions (IMFs). The achieved performance is analyzed, and the opportunities and caveats that this type of methods present are described. EMD is found to be a practical RFI mitigation method, albeit presenting some limitations and considerable complexity. Nevertheless, in some conditions, EMD exhibits a better performance than other commonly used methods (such as frequency binning). In particular, it has been found that EMD performs well for RFI affecting the <25% lower part of the intermediate frequency (IF) bandwidth.
This work demonstrates a methodology to detect sea ice presence over the Arctic and Antarctic reg... more This work demonstrates a methodology to detect sea ice presence over the Arctic and Antarctic regions using Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) data obtained with the UK TDS-1 satellite. The algorithm is based on estimating the degree of coherence of the received GNSS reflected waveform or Delay-Doppler Map (DDM). While at open ocean conditions, the scattered signal follows the diffuse scattering model, over flat sea ice it follows the coherent scattering model. In order to measure the degree of coherence of the received waveform or DDM, a correlation with the clean Woodward Ambiguity Function (WAF) is performed. The more similar the received signal is to the WAF, the more coherent is the scattering, and consequently, the more likely a flat sea ice surface is involved. In order to assess the performance of the proposed estimator a probabilistic study based on a Bayesian approach is performed, using the OSISAF Sea Ice Concentration (SIC) maps as ground truth. A probability of detection of 97%, a probability of false alarm of 2%, and a probability of error of 2.5% are the best results obtained for the Arctic region.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
This work presents a statistical analysis of the crosstalk phenomenon in interferometric Global N... more This work presents a statistical analysis of the crosstalk phenomenon in interferometric Global Navigation Satellite Systems Reflectometry (iGNSS-R). Cross-talk occurs when the Delay-Doppler Map (DDM) of a tracked satellite overlaps others from undesired satellites. This study is performed for groundbased and airborne receivers, and for a receiver on board the International Space Station (ISS) such as the upcoming GEROS-ISS experiment. Its impact on ocean altimetry retrievals is analyzed for different antenna arrays. Results show that for elevation angles higher than 60 degrees, cross-talk can be almost permanent from ground, up to 61% from airborne receivers at 2 km height, and up to ∼10% at the ISS. Cross-talk can only be mitigated using highly directive antennas with narrow beamwidths. Cross-talk impact using a 7-element hexagonal array still induces large errors on ground, but reduces to centimeter level on airborne receivers, and are negligible from the ISS.
IEEE Transactions on Geoscience and Remote Sensing, 2021
The calibration strategy for synthetic aperture interferometric radiometers based on noise inject... more The calibration strategy for synthetic aperture interferometric radiometers based on noise injection has been successfully demonstrated in the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA). With the emerging demands of increasing both array size and frequency bands, the internal calibration hardware becomes more difficult to implement. Although several external calibration methods have been proposed, the so-called phase ambiguity problem was not carefully considered. In this paper, a novel calibration strategy for both phase and amplitude calibration based on redundant space calibration is proposed, which utilizes the unique structure of equispaced circular array and rotational sampling strategy. The original irregular phase ambiguity degenerates to the fixed π-ambiguity, where only two possible solutions to phases are introduced. Both phase and amplitude of visibility can be simultaneously calibrated by the proposed strategy, and no cooperative calibration target is needed.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
This work presents the first ever dual-frequency multiconstellation Reflectometry using Global Na... more This work presents the first ever dual-frequency multiconstellation Reflectometry using Global Navigation Satellite Systems signals (GNSS-R) observations over boreal forests and lakes using GPS, GLONASS, and Galileo signals. The instrument used is the PYCARO reflectometer, which flew on-board a stratospheric balloon during the SNSB and ESA sponsored BEXUS 19 experiment. The coherent-to-incoherent scattering ratio over boreal forests is found to be as large as ∼1.5, while over lakes, it is as high as 16.5. The scatterers' height fluctuations measured using the phase of the peak of the reflected complex waveforms ranges from ±10 m, to the submetric level. Finally, reflectivity maps using the different GNSS codes are presented using the conventional GNSS-R for the open-access codes, and the reconstructed GNSS-R for the encrypted ones. The coherence of the reflected signal is found to be high enough to allow the PYCARO instrument to reconstruct the P(Y) code.
IEEE Transactions on Geoscience and Remote Sensing, Sep 1, 2017
A sea ice detection algorithm developed using UK TDS-1 Global Navigation Satellite Systems (GNSS)... more A sea ice detection algorithm developed using UK TDS-1 Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) data over the Arctic and Antarctic regions is presented. It is based on detecting the coherence of the received GNSS reflected waveform or Delay-Doppler Map (DDM). Over the open ocean the scattered signal has a diffusive, incoherent nature; it is described by the rough surface scattering model based on the Geometric optics and the Gaussian statistics for the ocean surface slopes. Over sea ice, and in particular newly formed sea ice, the scattering is mostly coherent which is typical for a mostly flat surface. In order to measure the degree of coherence of the received waveform or DDM three different estimators are presented: the normalized DDM average, the Trailing Edge Slope (TES), and the matched filter approach. Here we present a probabilistic study based on a Bayesian approach using two different and independent ground-truth datasets. This approach allow us thoroughly assessing the performance of the estimators. The best results are achieved for both the TES and the matched filter approach with a probability of detection of 97%, a probability of false alarm of ∼ 2%, and a probability of error of 2.5%. However, the matched filter approach is preferred due to its simplicity. The measurement of the sea ice concentration is also assessed in this work, but the nature of the UK TDS-1 data (lack of calibrated data) does not allow us to make any specific conclusions about the sea ice concentration.
Global warming effects are amplified in the Arctic, and sea ice has been declining with the annua... more Global warming effects are amplified in the Arctic, and sea ice has been declining with the annual minimum for September 2020 ending up being the second lowest in the 42-year satellite record (NSIDC, 2020). Sea ice governs heat transfer and influences atmospheric circulation, which is particularly important because low-and mid-latitude's climates are closely related to polar climate . Monitoring of both sea ice concentration (SIC), as the fraction of sea-ice cover within an observed cell, and sea ice thickness (SIT), are necessary for a consistent determination of sea ice dynamics. Microwave radiometry is independent of daylight and at lower microwave frequency it is mostly unaffected by atmospheric conditions. The emissivity in the microwave spectrum depends on the dielectric properties of sea ice, which are a function of its physical composition including salinity, density, surface temperature, and surface roughness. The collected signal is emitted from a radiating layer which depends on the penetration depth at sensor frequency. A snow layer on top of sea ice influences the radiative properties of sea ice and the energy received by the sensor. The contribution of snow to the emitted signal is less in the lower microwave spectrum, and the separability of surface properties, such as open water and sea ice including SIT, is-in theory-feasible.
IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium
Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content". A Working Group ha... more Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content". A Working Group has been assembled to draft this standard with the purpose of unifying and documenting GNSS-R measurements, calibration procedures, and product level definitions. The Working Group (ieee.org/community/technical-committees/standards-for-earthobservations/) includes members, collaborators, and contributors from academia, international space agencies, and private industry. In a recent face-to-face meeting held during the ARSI+KEO 2019 Conference, the need was recognized to develop a standard with a wide range of operations, providing procedure guidelines independently of constraints imposed by current limitations on geophysical parameters retrieval algorithms. As such, this effort aims to establish the fundamentals of a potential virtual network of satellites providing inter-comparable data to the scientific community.
The 3Cat-4 mission aims at demonstrating the capabilities of a CubeSat to perform Earth Observati... more The 3Cat-4 mission aims at demonstrating the capabilities of a CubeSat to perform Earth Observation (EO) by integrating a combined GNSS-R and Microwave Radiometer payload into a 1-Unit CubeSat. One of the greatest challenges is the design of an antenna that respects the 1-Unit CubeSat envelope while operating at the different frequency bands: Global Positioning System (GPS) L1 and Galileo E1 band (1575 MHz), GPS L2 band (1227 MHz), and the microwave radiometry band (1400–1427 MHz). Moreover, it requires between 8 and 12 dB of directivity depending on the band whilst providing at least 10 dB of front-to-back lobe ratio in L1 and L2 GPS bands. After a trade-off analysis on the type of antenna that could be used, a helix antenna was found to be the most suitable option to comply with the requirements, since it can be stowed during launch and deployed once in orbit. This article presents the antenna design from a radiation performance point of view starting with a theoretical analysis, ...
IEEE Transactions on Geoscience and Remote Sensing, 2022
The calibration strategy for synthetic aperture interferometric radiometers based on noise inject... more The calibration strategy for synthetic aperture interferometric radiometers based on noise injection has been successfully demonstrated in the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA). With the emerging demands of increasing both array size and frequency bands, the internal calibration hardware becomes more difficult to implement. Although several external calibration methods have been proposed, the so-called phase ambiguity problem was not carefully considered. In this paper, a novel calibration strategy for both phase and amplitude calibration based on redundant space calibration is proposed, which utilizes the unique structure of equispaced circular array and rotational sampling strategy. The original irregular phase ambiguity degenerates to the fixed π-ambiguity, where only two possible solutions to phases are introduced. Both phase and amplitude of visibility can be simultaneously calibrated by the proposed strategy, and no cooperative calibration target is needed.
IEEE Transactions on Geoscience and Remote Sensing, 2021
For the first time, rain effects on the polarimetric observations of the Global Navigation Satell... more For the first time, rain effects on the polarimetric observations of the Global Navigation Satellite System Reflectometry (GNSS-R) are investigated. The physical feasibility of tracking the modifications in the surface roughness by rain splash and the surface salinity by the accumulation of freshwater is theoretically discussed. An empirical analysis is carried out using measurements of a coastal GNSS-R station with two side-looking antennas in right and left-handed circular polarization (RHCP and LHCP). Discernible drops in RHCP and LHCP powers are observed during rain over a calm sea. The power drop becomes larger in size at higher elevation angles. The average LHCP power drops by ≈ 5 dB at an elevation angle of 45 • . The amplitude of the correlation sum shows a dampening, responding to rain rate systematically. The LHCP observations show higher sensitivity to rainfall compared to RHCP observations. The retrieved standard deviation of surface heights shows a steady increase over the rain rate. The derived surface salinity shows a decrease at rains higher than 10 mm/h. This study confirms the potential under environmental conditions of the GNSS-R groundbased station, e.g. with salinity mostly lower than 30 psu, over a calm sea, being a starting point for future investigations.
IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium, 2020
ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is st... more ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is still in good health, providing valuable L-band observations of the Earth surface . A number of products are obtained from these, including thin sea ice [2], frost/thaw soils [3], high winds [4], ocean surface wind and Sun brightness temperature [6], besides the main mission measurements of soil moisture and sea surface salinity [7][8]. This paper deals with the description and early results of some technology activities conducted by ESA applying the lessons learnt by SMOS and in preparation of an advanced L-band radiometer mission.
2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021
Vegetation Optical Depth (VOD) is highly sensitive to Above-ground Biomass (AGB), particularly at... more Vegetation Optical Depth (VOD) is highly sensitive to Above-ground Biomass (AGB), particularly at L-band, as this band is more sensitive to the canopy layer and has greater capacity to penetrate the vegetation. However, higher frequencies, such as C-and X-bands are more related to smaller vegetation structures. Then a combination of these three bands is expected to be relevant to accurate AGB retrievals. This study presents a comparison of three VOD products at three bands and evaluates the performance of a multi-frequency VOD combination by applying a Principal Component Regression (PCR) to Forest biomass. Results show that L-band VOD captures the 84% of the biomass and the PCR is built, mainly, for this band meaning that higher frequencies do not contribute substantially to the AGB retrieval in this study, still they do not affect the retrievals negatively.
2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021
Incidence angle diversity of space-borne L-band radiometers needs to be taken into account for a ... more Incidence angle diversity of space-borne L-band radiometers needs to be taken into account for a consistent estimation of surface soil moisture (SM). In this study, the Land Parameter Retrieval Model (LPRM) is applied to SMOS brightness temperatures to calibrate the effective scattering albedo (w) and the soil roughness (h 1 ) parameter against ERA5-land SM. The analysis is carried out for SMOS data at three different incidence angles (32.5±5°, 42.5±5° and 52.5±5°) focusing in 2016 on the three main land cover types of the Iberian Peninsula according to the Climate Change Initiative (agricultural, forest and grassland). The parameterization shows an increasing trend of w and h 1 with rise of incidence angle. The SM retrieval have been evaluated with in situ SM measurements of the REMEDHUS network on rainfed crop fields. Both compare well at the three incidence angles, obtaining high correlations (0.81-0.85), an ubRMSE around 0.04 m 3 m -3 and low bias (0-0.015 m 3 m -3 ).
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
Global Navigation Satellite System Reflectometry (GNSS-R) is becoming a widely accepted technique... more Global Navigation Satellite System Reflectometry (GNSS-R) is becoming a widely accepted technique for Remote Sensing. The interferometric technique (iGNSS-R) correlates the direct signal received from a satellite and the same signal reflected on the Earth's surface, whereas the conventional technique (cGNSS-R) correlates the reflected signal with a locally generated replica of the transmitted code. As GNSS signals are received below the noise level, this technique is extremely sensitive to Radio Frequency Interference (RFI). The Distance Measurement Equipment (DME), and the TACtical Air Navigation (TACAN) systems are two radio navigation systems that transmit in the GPS L5, and Galileo E5 bands with powers up to 3.5 kW. This work studies in depth the impact of these systems on iGNSS-R, and cGNSS-R instruments. This study is then applied to an hypothetical reflectometer that will be placed in the International Space Station (ISS): the GEROS experiment. It is shown that the received power in space will be strong enough to degrade the system's performance by increasing the noise floor, but the sea altimetry precision will still be accurate enough for scientific studies.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Jul 1, 2017
Radio-Frequency Interference (RFI) signals are a well known threat for Microwave Radiometry (MWR)... more Radio-Frequency Interference (RFI) signals are a well known threat for Microwave Radiometry (MWR) applications. In order to alleviate this problem, different approaches for RFI detection and mitigation are currently under development. Since RFI signals are man-made, they tend to have their power more concentrated in the Time-Frequency (TF) space as compared to naturally emitted noise. The aim of this work is to perform an assessment of different TF RFI mitigation techniques in terms of probability of detection, resolution loss (RL), and mitigation performance. In this assessment, six different kinds of RFI signals have been considered: a glitch, a burst of pulses, a wide-band chirp, a narrow-band chirp, a continuous wave, and a wide-band modulation. The results show that the best performance occurs when the transform basis has a similar shape as compared to the RFI signal. For the best case performance, the maximum residual RFI temperature is 14.8 K, and the worst RL is 8.4%. Moreover, the Multiresolution Fourier Transform (MFT) technique appears as a good trade-off solution among all other techniques since it can mitigate all RFI signals under evaluation with a maximum residual RFI temperature of 21 K, and a worst RL of 26.3%. Although the obtained results are still far from an acceptable bias < 1 K for MWR applications, there is still work to do in a combined test using the information gathered simultaneously by all mitigation techniques which could improve the overall performance of RFI mitigation.
IEEE Transactions on Geoscience and Remote Sensing, Apr 1, 2016
SMOS brightness temperature images and calibrated visibilities are related by the so-called G-mat... more SMOS brightness temperature images and calibrated visibilities are related by the so-called G-matrix. Due to the incomplete sampling at some spatial frequencies, sharp transitions in the brightness temperature scenes generate a Gibbs-like contamination ringing and spread sidelobes. In the current SMOS image reconstruction strategy, a Blackman window is applied to the Fourier components of the brightness temperatures to diminish the amplitude of artifacts such as ripples, and other Gibbs-like effects. In this work, a novel image reconstruction algorithm focused on the reduction of Gibbs-like contamination in brightness temperature images is proposed. It is based on sampling the brightness temperature images at the nodal points, that is, at those points at which the oscillating interference causes the minimum distortion to the geophysical signal. Results show a significant reduction of ripples and sidelobes in strongly RFI-contaminated images. This technique has been throughly validated using snapshots over the ocean, by comparing brightness temperatures reconstructed in the standard way or using the nodal sampling with modeled brightness temperatures. Tests have revealed that the standard deviation of the difference between the measurement and the model is reduced around 1 K over clean and stable zones when using nodal sampling technique with respect to the SMOS image reconstruction baseline. The reduction is approximately 0.7 K when considering the global ocean. This represents a crucial improvement in brightness temperature quality, which will translate in an enhancement of the retrieved geophysical parameters, especially the sea surface salinity.
Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiom... more Radio Frequency Interference (RFI) is a major concern for both real and synthetic aperture radiometers. After the lessons learnt from SMOS, ESA is preparing the next-generation of L-band interferometric radiometers with RFI mitigation integrated in the cross-correlators. This work presents a preliminary design and results of a pre-correlation RFI mitigation algorithm tailored for interferometric radiometers. The results show that the correlation error introduced by the RFI is reduced on average to the half, with peaks of 20 dB of mitigation.
IEEE Transactions on Geoscience and Remote Sensing, 2022
Radio-frequency interference (RFI) is an increasing problem particularly for Earth observation us... more Radio-frequency interference (RFI) is an increasing problem particularly for Earth observation using microwave radiometry. RFI has been observed, for example, at L-band by the European Space Agency's (ESA's) soil moisture and ocean salinity (SMOS) Earth Explorer and by National Aero-nautics and Space Administration's (NASA's) soil moisture active passive (SMAP) and Aquarius missions, as well as at C-band by Advanced Microwave Scanning Radiometer (AMSR)-E and AMSR-2, and at 10.7 and 18.7 GHz by AMSR- , WindSat, and GPM Microwave Imager (GMI). Therefore, systems dedicated to interference detection and removal of contaminated measurements are nowadays a must in order to improve radiometric accuracy and reduce the loss of spatial coverage caused by interference. In this work, the feasibility of using the empirical mode decomposition (EMD) technique for RFI mitigation is explored. The EMD, also known as Hilbert-Huang transform (HHT), is an algorithm that decomposes the signal into intrinsic mode functions (IMFs). The achieved performance is analyzed, and the opportunities and caveats that this type of methods present are described. EMD is found to be a practical RFI mitigation method, albeit presenting some limitations and considerable complexity. Nevertheless, in some conditions, EMD exhibits a better performance than other commonly used methods (such as frequency binning). In particular, it has been found that EMD performs well for RFI affecting the <25% lower part of the intermediate frequency (IF) bandwidth.
This work demonstrates a methodology to detect sea ice presence over the Arctic and Antarctic reg... more This work demonstrates a methodology to detect sea ice presence over the Arctic and Antarctic regions using Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) data obtained with the UK TDS-1 satellite. The algorithm is based on estimating the degree of coherence of the received GNSS reflected waveform or Delay-Doppler Map (DDM). While at open ocean conditions, the scattered signal follows the diffuse scattering model, over flat sea ice it follows the coherent scattering model. In order to measure the degree of coherence of the received waveform or DDM, a correlation with the clean Woodward Ambiguity Function (WAF) is performed. The more similar the received signal is to the WAF, the more coherent is the scattering, and consequently, the more likely a flat sea ice surface is involved. In order to assess the performance of the proposed estimator a probabilistic study based on a Bayesian approach is performed, using the OSISAF Sea Ice Concentration (SIC) maps as ground truth. A probability of detection of 97%, a probability of false alarm of 2%, and a probability of error of 2.5% are the best results obtained for the Arctic region.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
This work presents a statistical analysis of the crosstalk phenomenon in interferometric Global N... more This work presents a statistical analysis of the crosstalk phenomenon in interferometric Global Navigation Satellite Systems Reflectometry (iGNSS-R). Cross-talk occurs when the Delay-Doppler Map (DDM) of a tracked satellite overlaps others from undesired satellites. This study is performed for groundbased and airborne receivers, and for a receiver on board the International Space Station (ISS) such as the upcoming GEROS-ISS experiment. Its impact on ocean altimetry retrievals is analyzed for different antenna arrays. Results show that for elevation angles higher than 60 degrees, cross-talk can be almost permanent from ground, up to 61% from airborne receivers at 2 km height, and up to ∼10% at the ISS. Cross-talk can only be mitigated using highly directive antennas with narrow beamwidths. Cross-talk impact using a 7-element hexagonal array still induces large errors on ground, but reduces to centimeter level on airborne receivers, and are negligible from the ISS.
IEEE Transactions on Geoscience and Remote Sensing, 2021
The calibration strategy for synthetic aperture interferometric radiometers based on noise inject... more The calibration strategy for synthetic aperture interferometric radiometers based on noise injection has been successfully demonstrated in the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA). With the emerging demands of increasing both array size and frequency bands, the internal calibration hardware becomes more difficult to implement. Although several external calibration methods have been proposed, the so-called phase ambiguity problem was not carefully considered. In this paper, a novel calibration strategy for both phase and amplitude calibration based on redundant space calibration is proposed, which utilizes the unique structure of equispaced circular array and rotational sampling strategy. The original irregular phase ambiguity degenerates to the fixed π-ambiguity, where only two possible solutions to phases are introduced. Both phase and amplitude of visibility can be simultaneously calibrated by the proposed strategy, and no cooperative calibration target is needed.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Oct 1, 2016
This work presents the first ever dual-frequency multiconstellation Reflectometry using Global Na... more This work presents the first ever dual-frequency multiconstellation Reflectometry using Global Navigation Satellite Systems signals (GNSS-R) observations over boreal forests and lakes using GPS, GLONASS, and Galileo signals. The instrument used is the PYCARO reflectometer, which flew on-board a stratospheric balloon during the SNSB and ESA sponsored BEXUS 19 experiment. The coherent-to-incoherent scattering ratio over boreal forests is found to be as large as ∼1.5, while over lakes, it is as high as 16.5. The scatterers' height fluctuations measured using the phase of the peak of the reflected complex waveforms ranges from ±10 m, to the submetric level. Finally, reflectivity maps using the different GNSS codes are presented using the conventional GNSS-R for the open-access codes, and the reconstructed GNSS-R for the encrypted ones. The coherence of the reflected signal is found to be high enough to allow the PYCARO instrument to reconstruct the P(Y) code.
IEEE Transactions on Geoscience and Remote Sensing, Sep 1, 2017
A sea ice detection algorithm developed using UK TDS-1 Global Navigation Satellite Systems (GNSS)... more A sea ice detection algorithm developed using UK TDS-1 Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) data over the Arctic and Antarctic regions is presented. It is based on detecting the coherence of the received GNSS reflected waveform or Delay-Doppler Map (DDM). Over the open ocean the scattered signal has a diffusive, incoherent nature; it is described by the rough surface scattering model based on the Geometric optics and the Gaussian statistics for the ocean surface slopes. Over sea ice, and in particular newly formed sea ice, the scattering is mostly coherent which is typical for a mostly flat surface. In order to measure the degree of coherence of the received waveform or DDM three different estimators are presented: the normalized DDM average, the Trailing Edge Slope (TES), and the matched filter approach. Here we present a probabilistic study based on a Bayesian approach using two different and independent ground-truth datasets. This approach allow us thoroughly assessing the performance of the estimators. The best results are achieved for both the TES and the matched filter approach with a probability of detection of 97%, a probability of false alarm of ∼ 2%, and a probability of error of 2.5%. However, the matched filter approach is preferred due to its simplicity. The measurement of the sea ice concentration is also assessed in this work, but the nature of the UK TDS-1 data (lack of calibrated data) does not allow us to make any specific conclusions about the sea ice concentration.
Global warming effects are amplified in the Arctic, and sea ice has been declining with the annua... more Global warming effects are amplified in the Arctic, and sea ice has been declining with the annual minimum for September 2020 ending up being the second lowest in the 42-year satellite record (NSIDC, 2020). Sea ice governs heat transfer and influences atmospheric circulation, which is particularly important because low-and mid-latitude's climates are closely related to polar climate . Monitoring of both sea ice concentration (SIC), as the fraction of sea-ice cover within an observed cell, and sea ice thickness (SIT), are necessary for a consistent determination of sea ice dynamics. Microwave radiometry is independent of daylight and at lower microwave frequency it is mostly unaffected by atmospheric conditions. The emissivity in the microwave spectrum depends on the dielectric properties of sea ice, which are a function of its physical composition including salinity, density, surface temperature, and surface roughness. The collected signal is emitted from a radiating layer which depends on the penetration depth at sensor frequency. A snow layer on top of sea ice influences the radiative properties of sea ice and the energy received by the sensor. The contribution of snow to the emitted signal is less in the lower microwave spectrum, and the separability of surface properties, such as open water and sea ice including SIT, is-in theory-feasible.
IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium
Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content". A Working Group ha... more Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content". A Working Group has been assembled to draft this standard with the purpose of unifying and documenting GNSS-R measurements, calibration procedures, and product level definitions. The Working Group (ieee.org/community/technical-committees/standards-for-earthobservations/) includes members, collaborators, and contributors from academia, international space agencies, and private industry. In a recent face-to-face meeting held during the ARSI+KEO 2019 Conference, the need was recognized to develop a standard with a wide range of operations, providing procedure guidelines independently of constraints imposed by current limitations on geophysical parameters retrieval algorithms. As such, this effort aims to establish the fundamentals of a potential virtual network of satellites providing inter-comparable data to the scientific community.
The 3Cat-4 mission aims at demonstrating the capabilities of a CubeSat to perform Earth Observati... more The 3Cat-4 mission aims at demonstrating the capabilities of a CubeSat to perform Earth Observation (EO) by integrating a combined GNSS-R and Microwave Radiometer payload into a 1-Unit CubeSat. One of the greatest challenges is the design of an antenna that respects the 1-Unit CubeSat envelope while operating at the different frequency bands: Global Positioning System (GPS) L1 and Galileo E1 band (1575 MHz), GPS L2 band (1227 MHz), and the microwave radiometry band (1400–1427 MHz). Moreover, it requires between 8 and 12 dB of directivity depending on the band whilst providing at least 10 dB of front-to-back lobe ratio in L1 and L2 GPS bands. After a trade-off analysis on the type of antenna that could be used, a helix antenna was found to be the most suitable option to comply with the requirements, since it can be stowed during launch and deployed once in orbit. This article presents the antenna design from a radiation performance point of view starting with a theoretical analysis, ...
IEEE Transactions on Geoscience and Remote Sensing, 2022
The calibration strategy for synthetic aperture interferometric radiometers based on noise inject... more The calibration strategy for synthetic aperture interferometric radiometers based on noise injection has been successfully demonstrated in the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA). With the emerging demands of increasing both array size and frequency bands, the internal calibration hardware becomes more difficult to implement. Although several external calibration methods have been proposed, the so-called phase ambiguity problem was not carefully considered. In this paper, a novel calibration strategy for both phase and amplitude calibration based on redundant space calibration is proposed, which utilizes the unique structure of equispaced circular array and rotational sampling strategy. The original irregular phase ambiguity degenerates to the fixed π-ambiguity, where only two possible solutions to phases are introduced. Both phase and amplitude of visibility can be simultaneously calibrated by the proposed strategy, and no cooperative calibration target is needed.
IEEE Transactions on Geoscience and Remote Sensing, 2021
For the first time, rain effects on the polarimetric observations of the Global Navigation Satell... more For the first time, rain effects on the polarimetric observations of the Global Navigation Satellite System Reflectometry (GNSS-R) are investigated. The physical feasibility of tracking the modifications in the surface roughness by rain splash and the surface salinity by the accumulation of freshwater is theoretically discussed. An empirical analysis is carried out using measurements of a coastal GNSS-R station with two side-looking antennas in right and left-handed circular polarization (RHCP and LHCP). Discernible drops in RHCP and LHCP powers are observed during rain over a calm sea. The power drop becomes larger in size at higher elevation angles. The average LHCP power drops by ≈ 5 dB at an elevation angle of 45 • . The amplitude of the correlation sum shows a dampening, responding to rain rate systematically. The LHCP observations show higher sensitivity to rainfall compared to RHCP observations. The retrieved standard deviation of surface heights shows a steady increase over the rain rate. The derived surface salinity shows a decrease at rains higher than 10 mm/h. This study confirms the potential under environmental conditions of the GNSS-R groundbased station, e.g. with salinity mostly lower than 30 psu, over a calm sea, being a starting point for future investigations.
IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium, 2020
ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is st... more ESA's Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 and, to date, is still in good health, providing valuable L-band observations of the Earth surface . A number of products are obtained from these, including thin sea ice [2], frost/thaw soils [3], high winds [4], ocean surface wind and Sun brightness temperature [6], besides the main mission measurements of soil moisture and sea surface salinity [7][8]. This paper deals with the description and early results of some technology activities conducted by ESA applying the lessons learnt by SMOS and in preparation of an advanced L-band radiometer mission.
2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021
Vegetation Optical Depth (VOD) is highly sensitive to Above-ground Biomass (AGB), particularly at... more Vegetation Optical Depth (VOD) is highly sensitive to Above-ground Biomass (AGB), particularly at L-band, as this band is more sensitive to the canopy layer and has greater capacity to penetrate the vegetation. However, higher frequencies, such as C-and X-bands are more related to smaller vegetation structures. Then a combination of these three bands is expected to be relevant to accurate AGB retrievals. This study presents a comparison of three VOD products at three bands and evaluates the performance of a multi-frequency VOD combination by applying a Principal Component Regression (PCR) to Forest biomass. Results show that L-band VOD captures the 84% of the biomass and the PCR is built, mainly, for this band meaning that higher frequencies do not contribute substantially to the AGB retrieval in this study, still they do not affect the retrievals negatively.
2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021
Incidence angle diversity of space-borne L-band radiometers needs to be taken into account for a ... more Incidence angle diversity of space-borne L-band radiometers needs to be taken into account for a consistent estimation of surface soil moisture (SM). In this study, the Land Parameter Retrieval Model (LPRM) is applied to SMOS brightness temperatures to calibrate the effective scattering albedo (w) and the soil roughness (h 1 ) parameter against ERA5-land SM. The analysis is carried out for SMOS data at three different incidence angles (32.5±5°, 42.5±5° and 52.5±5°) focusing in 2016 on the three main land cover types of the Iberian Peninsula according to the Climate Change Initiative (agricultural, forest and grassland). The parameterization shows an increasing trend of w and h 1 with rise of incidence angle. The SM retrieval have been evaluated with in situ SM measurements of the REMEDHUS network on rainfed crop fields. Both compare well at the three incidence angles, obtaining high correlations (0.81-0.85), an ubRMSE around 0.04 m 3 m -3 and low bias (0-0.015 m 3 m -3 ).
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Papers by Adriano Camps