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C S Ruf - One of the best experts on this subject based on the ideXlab platform.

  • inter calibration of Microwave Radiometers using the vicarious cold calibration double difference method
    IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2012
    Co-Authors: Rachael Kroodsma, Darren Mckague, C S Ruf
    Abstract:

    The double difference method of inter-calibration between spaceborne Microwave Radiometers is combined with the vicarious cold calibration method for calibrating an individual radiometer. Vicarious cold calibration minimizes the effects of geophysical variability on radiative transfer models (RTMs) of the brightness temperature (TB) data and it accounts for frequency and incidence angle dissimilarity between Radiometers. Double differencing reduces the sensitivity of the inter-calibration to RTM error and improper accounting for geophysical variables in the RTM. When combined together, the two methods significantly improve the confidence with which calibration differences can be identified and characterized. This paper analyzes the performance of the vicarious cold calibration double difference method for conical scanning Microwave Radiometers and quantifies the improvement this method provides compared to performing a simpler inter-calibration by direct comparison of radiometer measurements.

  • beam spoiling correction for spaceborne Microwave Radiometers using the two point vicarious calibration method
    IEEE Transactions on Geoscience and Remote Sensing, 2011
    Co-Authors: Darren Mckague, C S Ruf, John Puckett
    Abstract:

    The vicarious warm and cold calibration techniques are combined to provide an end-to-end two point calibration method for spaceborne Microwave Radiometers. The method uses stable external calibration sources to permit an end-to-end calibration of the complete radiometer, including its primary antenna. Both gain and offset corrections to the radiometer calibration can be computed since vicarious reference points at both the cold and warm ends of the measurement range are available. The method is demonstrated using the WindSat radiometer. Calibration errors are found which vary with azimuthal scan position in a manner that suggests that the cause is beam spoiling from on-board spacecraft obstructions. The impact on gain and offset calibration errors of the on-board obstructions can be determined from the vicarious calibration. This information is used to characterize the beam spoiling-specifically to determine the decrease in the antenna's beam efficiency and the mean brightness temperature entering the far sidelobes of the antenna, both as functions of azimuthal scan position. With this characterization available, a calibration correction algorithm can be constructed that is based on the root cause of the problem.

  • calibration method for fully polarimetric Microwave Radiometers using the correlated noise calibration standard
    International Geoscience and Remote Sensing Symposium, 2008
    Co-Authors: Jinzheng Peng, C S Ruf
    Abstract:

    In this paper, a new and improved L-band version of the correlated noise calibration standard (CNCS) has been developed to aid in the characterization of polarimetric Microwave Radiometers. The CNCS generates a series of known polarimetric test signals using a two-channel commercial arbitrary waveform generator (AWG) and a pair of frequency-upconversion modules. When it is inserted in place of the antenna used by a radiometer-under-test (RUT), it can fully characterize the polarimetric response of the receiver portion of the RUT. Both hardware and software improvements have been made over a previous version of the CNCS. The frequency upconverters now include integral warm and cold calibration-reference targets to automatically compensate for small drifts in AWG output-signal strength. The procedure used to calibrate the RUT has been generalized to correct for nonideal characteristics of the CNCS itself. Moreover, the effects on the derived polarimetric gain matrix of impedance mismatches between the RUT, and either the CNCS or the antenna have been determined. Details of the CNCS improvements are presented. The results of an experimental demonstration of its use and of a series of validation tests of its performance are also presented.

  • vicarious calibration of global precipitation measurement Microwave Radiometers
    International Geoscience and Remote Sensing Symposium, 2008
    Co-Authors: Darren Mckague, C S Ruf, John Puckett
    Abstract:

    The vicarious cold calibration method of Ruf has been used to assess the calibration of the TMI, WindSat, SSM/I F13 and SSM/I F14 Microwave Radiometers using data from the GPM Inter-Calibration Working Group. Significant scan position dependent biases are seen for TMI (as large as 1 K) and for WindSat (as large as 5 K)-scan position dependent biases in SSM/I data were removed prior to processing. These biases are thought to be due to obstructions in the edge of scan field of view from the given instrument and its spacecraft. WindSat vertically polarized data also show a linear decrease in vicarious cold calibration brightness temperatures with scan position. SSM/I F13 and F14 vicarious cold brightness temperatures differ by an amount consistent with a ~0.2deg offset in their relative Earth incidence angles.

  • detection of calibration drifts in spaceborne Microwave Radiometers using a vicarious cold reference
    IEEE Transactions on Geoscience and Remote Sensing, 2000
    Co-Authors: C S Ruf
    Abstract:

    The coldest possible brightness temperatures observed by a downward-looking Microwave radiometer from space are often produced by calm oceans under cloud-free skies and very low humidity. This set of conditions tends to occur with sufficient regularity that an orbiting radiometer will accumulate a useful number of observations within a period of a few days to weeks. Histograms of the radiometer's coldest measurements provide an anchor point against which very small drifts in absolute calibration can be detected. This technique is applied to the TOPEX Microwave radiometer (TMR), and a statistically significant drift of several tenths of a Kelvin per year is clearly detected in one of the channels. TMR housekeeping calibration data indicates a likely cause for the drift, as small changes in the isolation of latching ferrite circulators that are used in the onboard calibration-switch assembly. This method can easily be adapted to other Microwave Radiometers, especially imagers operating at frequencies in the atmospheric windows. In addition to detecting long-term instrument drifts with high precision, the method also provides a means for cross-calibrating different instruments. The cold reference provides a common tie point, even between sensors operating at different polarizations and/or incidence angles.

Thomas J. Jackson - One of the best experts on this subject based on the ideXlab platform.

  • soil water infiltration observation with Microwave Radiometers
    IEEE Transactions on Geoscience and Remote Sensing, 1998
    Co-Authors: Thomas J. Jackson, T J Schmugge, P E Oneill, Marc B Parlange
    Abstract:

    Experiments were conducted using truck-based Microwave Radiometers operating at 1.41- (L-band) and 2.65-GHz (S-band) horizontal polarization to observe small plots during and following sprinkler irrigation. These experiments were conducted on a sandy loam soil in 1993 and a silt loam in 1995. Sandy loam soils typically have higher infiltration capabilities than clays, and in the authors' studies, they were not able to exceed this with the irrigation system. The observed brightness temperature (T/sub B/) quickly reached a nominally constant value during irrigation. When the irrigation was stopped, the T/sub B/ began to increase as drainage took place. Contributing depth-related differences were observed for L- and S-band as expected. The irrigation rates in 1995 with the silt loam soil exceeded the saturated conductivity of the soil. During irrigation, the T/sub B/ values exhibited a phenomenon that had not been previously observed and identified and is associated with coherent interference. The Land S-band exhibited similar patterns but were not identical due to contributing depth. These results suggested the existence of a sharp dielectric boundary (wet over dry soil) that was increasing in depth with time. The temporal description of the wetting front boundary was used with a coherent radiative transfer model to predict T/sub B/ for L- and S-band.

  • Multifrequency ground-based Microwave remote sensing of soil moisture
    IGARSS '98. Sensing and Managing the Environment. 1998 IEEE International Geoscience and Remote Sensing. Symposium Proceedings. (Cat. No.98CH36174), 1998
    Co-Authors: Charles A. Laymon, William L. Crosson, V.v. Soman, Thomas J. Jackson, A. Manu, Teferi D. Tsegaye
    Abstract:

    A multifrequency passive Microwave remote sensing system was deployed during a ground-based experiment. Conducted near Huntsville, Alabama in 1996. The Sand L-band Microwave Radiometers were able to detect both the large scale moisture changes due to the irrigation and rainfall events and also diurnal fluctuations in surface soil moisture content. Using a two frequency system provides much more information about near-surface soil water dynamics than does a single frequency.

  • soil water infiltration observation with Microwave Radiometers
    International Geoscience and Remote Sensing Symposium, 1998
    Co-Authors: Thomas J. Jackson, T J Schmugge, P E Oneill, Marc B Parlange
    Abstract:

    Experiments were conducted using truck-based Microwave Radiometers operating at 1.41- (L-band) and 2.65-GHz (S-band) horizontal polarization to observe small plots during and following sprinkler irrigation. These experiments were conducted on a sandy loam soil in 1994 and a silt loam in 1995. Sandy loam soils typically have higher infiltration capabilities than clays, and in our studies, we were not able to exceed this with the irrigation system. The observed brightness temperature (T B ) quickly reached a nominally constant value during irrigation. When the irrigation was stopped, the T B began to increase as drainage took place. Contributing depth-related differences were observed for L- and S-band as expected. The irrigation rates in 1995 with the silt loam soil exceeded the saturated conductivity of the soil. During irrigation, the T B values exhibited a phenomenon that had not been previously observed and identified and is associated with coherent interference. The L- and S-band exhibited similar patterns but were not identical due to contributing depth. These results suggested the existence of a sharp dielectric boundary (wet over dry soil) that was increasing in depth with time. The temporal description of the wetting front boundary was used with a coherent radiative transfer model to predict T B for L- and S-band.

  • mapping surface soil moisture with Microwave Radiometers
    Meteorology and Atmospheric Physics, 1994
    Co-Authors: T J Schmugge, Thomas J. Jackson
    Abstract:

    Water stored in the soil serves as a reservoir for the evapotranspiration (ET) process on land surfaces, therefore knowledge of the soil moisture content is important for partitioning the incoming solar radiation into latent and sensible heat components. There is no remote sensing technique which directly observes the amount of water in this reservoir, however Microwave remote sensing at long wavelengths (λ>10 cm) can give estimates of the moisture stored in the surface 5-cm layer of the soil. This approach is based on the large dielectric contrast between water and dry soil, resulting in emissivity changes from 0.96 for a dry smooth soil to less than 0.6.

Frank J Wentz - One of the best experts on this subject based on the ideXlab platform.

  • wind vector retrievals under rain with passive satellite Microwave Radiometers
    IEEE Transactions on Geoscience and Remote Sensing, 2009
    Co-Authors: Thomas Meissner, Frank J Wentz
    Abstract:

    We have developed algorithms that retrieve ocean-surface wind speed and direction under rain using brightness-temperature (TB) measurements from passive satellite Microwave Radiometers. For accurate radiometer retrievals of wind speeds in the rain, it is essential to use TB signals at different frequencies, whose spectral signature makes it possible to find channel combinations that are sufficiently sensitive to wind speed but little or not sensitive to rain. The wind-speed retrieval accuracy of an algorithm that utilizes C-band frequencies and is trained for tropical cyclones ranges from 2.0 m/s in light rain to 4.0 m/s in heavy rain. We have also trained and tested global algorithms that are less accurate in tropical storms but can be applied under all conditions. The wind-direction retrieval accuracy degrades from about 10deg in light rain to 30deg at the onset of heavy rain. We compare the performance of wind-vector retrievals under rain from Microwave Radiometers with those from scatterometers and discuss advantages and shortcomings of both instruments. We have also analyzed the wind-induced sea-surface emissivity, including its wind-direction dependence for wind speeds up to 45 m/s.

  • wind vector retrievals under rain with passive satellite Microwave Radiometers
    Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment - MicroRad'08, 2009
    Co-Authors: Thomas Meissner, Frank J Wentz
    Abstract:

    We have developed algorithms that retrieve ocean-surface wind speed and direction under rain using brightness-temperature (TB) measurements from passive satellite Microwave Radiometers. For accurate radiometer retrievals of wind speeds in the rain, it is essential to use TB signals at different frequencies, whose spectral signature makes it possible to find channel combinations that are sufficiently sensitive to wind speed but little or not sensitive to rain. The wind-speed retrieval accuracy of an algorithm that utilizes C-band frequencies and is trained for tropical cyclones ranges from 2.0 m/s in light rain to 4.0 m/s in heavy rain. We have also trained and tested global algorithms that are less accurate in tropical storms but can be applied under all conditions. The wind-direction retrieval accuracy degrades from about 10° in light rain to 30° at the onset of heavy rain. We compare the performance of wind-vector retrievals under rain from Microwave Radiometers with those from scatterometers and discuss advantages and shortcomings of both instruments. We have also analyzed the wind-induced sea-surface emissivity, including its wind-direction dependence for wind speeds up to 45 m/s.

  • measurement of oceanic wind vector using satellite Microwave Radiometers
    IEEE Transactions on Geoscience and Remote Sensing, 1992
    Co-Authors: Frank J Wentz
    Abstract:

    The possibility of retrieving both wind speed and direction from Microwave radiometer measurements of the ocean is studied using Special Sensor Microwave/Imager (SSM/I) measurements collocated with buoy reports from the National Data Buoy Center (NDBC). A physically based algorithm is used to retrieve the wind speed. The RMS difference between the SSM/I and buoy wind speed is 1.6 m/s for 3321 comparisons. It is found that the SSM/I minus buoy wind speed difference is correlated with wind direction. When this wind direction signal is removed, the RMS difference between the SSM/I and buoy winds reduces to 1.3 m/s. The wind direction signal is used to make global, low-resolution maps of the monthly mean oceanic vector. The wind direction sensing capability of a prospective two-look satellite radiometer is also processed. >

Marc B Parlange - One of the best experts on this subject based on the ideXlab platform.

  • soil water infiltration observation with Microwave Radiometers
    IEEE Transactions on Geoscience and Remote Sensing, 1998
    Co-Authors: Thomas J. Jackson, T J Schmugge, P E Oneill, Marc B Parlange
    Abstract:

    Experiments were conducted using truck-based Microwave Radiometers operating at 1.41- (L-band) and 2.65-GHz (S-band) horizontal polarization to observe small plots during and following sprinkler irrigation. These experiments were conducted on a sandy loam soil in 1993 and a silt loam in 1995. Sandy loam soils typically have higher infiltration capabilities than clays, and in the authors' studies, they were not able to exceed this with the irrigation system. The observed brightness temperature (T/sub B/) quickly reached a nominally constant value during irrigation. When the irrigation was stopped, the T/sub B/ began to increase as drainage took place. Contributing depth-related differences were observed for L- and S-band as expected. The irrigation rates in 1995 with the silt loam soil exceeded the saturated conductivity of the soil. During irrigation, the T/sub B/ values exhibited a phenomenon that had not been previously observed and identified and is associated with coherent interference. The Land S-band exhibited similar patterns but were not identical due to contributing depth. These results suggested the existence of a sharp dielectric boundary (wet over dry soil) that was increasing in depth with time. The temporal description of the wetting front boundary was used with a coherent radiative transfer model to predict T/sub B/ for L- and S-band.

  • soil water infiltration observation with Microwave Radiometers
    International Geoscience and Remote Sensing Symposium, 1998
    Co-Authors: Thomas J. Jackson, T J Schmugge, P E Oneill, Marc B Parlange
    Abstract:

    Experiments were conducted using truck-based Microwave Radiometers operating at 1.41- (L-band) and 2.65-GHz (S-band) horizontal polarization to observe small plots during and following sprinkler irrigation. These experiments were conducted on a sandy loam soil in 1994 and a silt loam in 1995. Sandy loam soils typically have higher infiltration capabilities than clays, and in our studies, we were not able to exceed this with the irrigation system. The observed brightness temperature (T B ) quickly reached a nominally constant value during irrigation. When the irrigation was stopped, the T B began to increase as drainage took place. Contributing depth-related differences were observed for L- and S-band as expected. The irrigation rates in 1995 with the silt loam soil exceeded the saturated conductivity of the soil. During irrigation, the T B values exhibited a phenomenon that had not been previously observed and identified and is associated with coherent interference. The L- and S-band exhibited similar patterns but were not identical due to contributing depth. These results suggested the existence of a sharp dielectric boundary (wet over dry soil) that was increasing in depth with time. The temporal description of the wetting front boundary was used with a coherent radiative transfer model to predict T B for L- and S-band.

Darren Mckague - One of the best experts on this subject based on the ideXlab platform.

  • inter calibration of Microwave Radiometers using the vicarious cold calibration double difference method
    IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2012
    Co-Authors: Rachael Kroodsma, Darren Mckague, C S Ruf
    Abstract:

    The double difference method of inter-calibration between spaceborne Microwave Radiometers is combined with the vicarious cold calibration method for calibrating an individual radiometer. Vicarious cold calibration minimizes the effects of geophysical variability on radiative transfer models (RTMs) of the brightness temperature (TB) data and it accounts for frequency and incidence angle dissimilarity between Radiometers. Double differencing reduces the sensitivity of the inter-calibration to RTM error and improper accounting for geophysical variables in the RTM. When combined together, the two methods significantly improve the confidence with which calibration differences can be identified and characterized. This paper analyzes the performance of the vicarious cold calibration double difference method for conical scanning Microwave Radiometers and quantifies the improvement this method provides compared to performing a simpler inter-calibration by direct comparison of radiometer measurements.

  • beam spoiling correction for spaceborne Microwave Radiometers using the two point vicarious calibration method
    IEEE Transactions on Geoscience and Remote Sensing, 2011
    Co-Authors: Darren Mckague, C S Ruf, John Puckett
    Abstract:

    The vicarious warm and cold calibration techniques are combined to provide an end-to-end two point calibration method for spaceborne Microwave Radiometers. The method uses stable external calibration sources to permit an end-to-end calibration of the complete radiometer, including its primary antenna. Both gain and offset corrections to the radiometer calibration can be computed since vicarious reference points at both the cold and warm ends of the measurement range are available. The method is demonstrated using the WindSat radiometer. Calibration errors are found which vary with azimuthal scan position in a manner that suggests that the cause is beam spoiling from on-board spacecraft obstructions. The impact on gain and offset calibration errors of the on-board obstructions can be determined from the vicarious calibration. This information is used to characterize the beam spoiling-specifically to determine the decrease in the antenna's beam efficiency and the mean brightness temperature entering the far sidelobes of the antenna, both as functions of azimuthal scan position. With this characterization available, a calibration correction algorithm can be constructed that is based on the root cause of the problem.

  • vicarious calibration of global precipitation measurement Microwave Radiometers
    International Geoscience and Remote Sensing Symposium, 2008
    Co-Authors: Darren Mckague, C S Ruf, John Puckett
    Abstract:

    The vicarious cold calibration method of Ruf has been used to assess the calibration of the TMI, WindSat, SSM/I F13 and SSM/I F14 Microwave Radiometers using data from the GPM Inter-Calibration Working Group. Significant scan position dependent biases are seen for TMI (as large as 1 K) and for WindSat (as large as 5 K)-scan position dependent biases in SSM/I data were removed prior to processing. These biases are thought to be due to obstructions in the edge of scan field of view from the given instrument and its spacecraft. WindSat vertically polarized data also show a linear decrease in vicarious cold calibration brightness temperatures with scan position. SSM/I F13 and F14 vicarious cold brightness temperatures differ by an amount consistent with a ~0.2deg offset in their relative Earth incidence angles.