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B W Lites - One of the best experts on this subject based on the ideXlab platform.
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weak field magnetogram Calibration using advanced stokes polarimeter flux density maps i solar optical universal polarimeter Calibration
Solar Physics, 2002Co-Authors: T E Berger, B W LitesAbstract:Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data.
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Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration
Solar Physics, 2002Co-Authors: T E Berger, B W LitesAbstract:Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V / I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm^−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. ^*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. ^†The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Akihiro Yamazaki - One of the best experts on this subject based on the ideXlab platform.
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The instrument Constant of sky radiometers (POM-02) – Part 1: Calibration Constant
Atmospheric Measurement Techniques, 2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of the optical properties of aerosols using radiometers. In this study, the precision of the Calibration Constant ( V0 ) for the sky radiometer (POM-02) that is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba (36.05 ∘ N, 140.13 ∘ E), with a monthly mean temperature range of 2.7 to 25.5 ∘ C. In the other channels, the correction factors were less than 0.5 %. The coefficient of variation (CV, standard deviation/mean) of V0 from the normal Langley method, based on the data measured at the NOAA Mauna Loa Observatory, is between 0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for wavelengths shorter than 400 nm ( − 10 % to − 4 % per year) was larger than that for wavelengths longer than 500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from the reference POM-02 was 0.1 % to 0.5 %. Here, the data were simultaneously taken at 1 min intervals on a fine day, and data when the air mass was less than 2.5 were compared. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 % to 3 %. The root mean square error (RMSE) from the IML method was about 0.6 % to 2.5 %, and in some cases the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. Furthermore, the Calibration Constants determined by the IML method had much higher noise than those transferred from the reference. The modified Langley method was used to calibrate the 940 nm channel with on-site measurement data. The V0 obtained with the modified Langley method compared to the Langley method was 1 % more accurate on stable and fine days. The general method was also used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm) with on-site measurement data; the V0 obtained with the general method differed from that obtained with the Langley method of V0 by 0.8 %, 0.4 %, and 0.1 % in December 2015, respectively.
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the instrument Constant of sky radiometers pom 02 part 1 Calibration Constant
Atmospheric Measurement Techniques, 2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of the optical properties of aerosols using radiometers. In this study, the precision of the Calibration Constant ( V0 ) for the sky radiometer (POM-02) that is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba (36.05 ∘ N, 140.13 ∘ E), with a monthly mean temperature range of 2.7 to 25.5 ∘ C. In the other channels, the correction factors were less than 0.5 %. The coefficient of variation (CV, standard deviation/mean) of V0 from the normal Langley method, based on the data measured at the NOAA Mauna Loa Observatory, is between 0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for wavelengths shorter than 400 nm ( − 10 % to − 4 % per year) was larger than that for wavelengths longer than 500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from the reference POM-02 was 0.1 % to 0.5 %. Here, the data were simultaneously taken at 1 min intervals on a fine day, and data when the air mass was less than 2.5 were compared. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 % to 3 %. The root mean square error (RMSE) from the IML method was about 0.6 % to 2.5 %, and in some cases the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. Furthermore, the Calibration Constants determined by the IML method had much higher noise than those transferred from the reference. The modified Langley method was used to calibrate the 940 nm channel with on-site measurement data. The V0 obtained with the modified Langley method compared to the Langley method was 1 % more accurate on stable and fine days. The general method was also used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm) with on-site measurement data; the V0 obtained with the general method differed from that obtained with the Langley method of V0 by 0.8 %, 0.4 %, and 0.1 % in December 2015, respectively.
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The instrument Constant of sky radiometer (POM-02), Part I: Calibration Constant
2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of aerosols using radiometers. In this study, the accuracy of the Calibration Constant (V0) for the sky radiometer (POM-02) which is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels, and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba. In the other channels, the correction factors were less than 0.5 %. The accuracy of V0 from the normal Langley method is between 0.2 and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for shorter wavelengths was larger than that for longer wavelengths. The accuracy of V0 estimated from the side-by-side measurements was 0.1 to 0.5 %. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 to 3 %. The RMS error from the IML method was about 0.6 to 2.5 %, and in some cases, the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. The Calibration method for water vapor in the 940 nm channel was developed using an empirical formula for transmittance. The accuracy of V0 was better than 1 % on relatively stable and fine days. A Calibration method for the near-infrared channels, 1225, 1627, and 2200 nm, was also developed. The logarithm of the ratio of the sensor output can be written as a linear function of the airmass, by assuming that the ratio of the optical thicknesses between the two channels is Constant. The accuracy of V0 was better than 1 % on days with good conditions.
T E Berger - One of the best experts on this subject based on the ideXlab platform.
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weak field magnetogram Calibration using advanced stokes polarimeter flux density maps i solar optical universal polarimeter Calibration
Solar Physics, 2002Co-Authors: T E Berger, B W LitesAbstract:Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data.
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Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration
Solar Physics, 2002Co-Authors: T E Berger, B W LitesAbstract:Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V / I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear Calibration Constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP Calibration Constant is 17 000 ± 550 Mx cm^−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. ^*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. ^†The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Akihiro Uchiyama - One of the best experts on this subject based on the ideXlab platform.
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Development of on-site self-Calibration and retrieval methods for sky-radiometer observations of precipitable water vapor
Atmospheric Measurement Techniques, 2020Co-Authors: Masahiro Momoi, Akihiro Uchiyama, Rei Kudo, Kazuma Aoki, Tatsuhiro Mori, Kazuhiko Miura, Hiroshi Okamoto, Hitoshi Irie, Yoshinori Shoji, Osamu IjimaAbstract:Abstract. The Prede sky radiometer measures direct solar irradiance and the angular distribution of diffuse radiances at the ultraviolet, visible, and near-infrared wavelengths. These data are utilized for the remote sensing of aerosols, water vapor, ozone, and clouds, but the Calibration Constant, which is the sensor output current of the extraterrestrial solar irradiance at the mean distance between Earth and the Sun, is needed. The aerosol channels, which are the weak gas absorption wavelengths of 340, 380, 400, 500, 675, 870, and 1020 nm, can be calibrated by an on-site self-Calibration method, the Improved Langley method. This on-site self-Calibration method is useful for the continuous long-term observation of aerosol properties. However, the continuous long-term observation of precipitable water vapor (PWV) by the sky radiometer remains challenging because calibrating the water vapor absorption channel of 940 nm generally relies on the standard Langley (SL) method at limited observation sites (e.g., the Mauna Loa Observatory) and the transfer of the Calibration Constant by a side-by-side comparison with the reference sky radiometer calibrated by the SL method. In this study, we developed the SKYMAP algorithm, a new on-site method of self-calibrating the water vapor channel of the sky radiometer using diffuse radiances normalized by direct solar irradiance (normalized radiances). Because the sky radiometer measures direct solar irradiance and diffuse radiance using the same sensor, the normalization cancels the Calibration Constant included in the measurements. The SKYMAP algorithm consists of three steps. First, aerosol optical and microphysical properties are retrieved using direct solar irradiances and normalized radiances at aerosol channels. The aerosol optical properties at the water vapor channel are interpolated from those at aerosol channels. Second, PWV is retrieved using the angular distribution of the normalized radiances at the water vapor channel. Third, the Calibration Constant at the water vapor channel is estimated from the transmittance of PWV and aerosol optical properties. Intensive sensitivity tests of the SKYMAP algorithm using simulated data of the sky radiometer showed that the Calibration Constant is retrieved reasonably well for PWV cm, which indicates that the SKYMAP algorithm can calibrate the water vapor channel on-site in dry conditions. Next, the SKYMAP algorithm was applied to actual measurements under the clear-sky and low-PWV ( cm) conditions at two sites, Tsukuba and Chiba, Japan, and the annual mean Calibration Constants at the two sites were determined. The SKYMAP-derived Calibration Constants were 10.1 % and 3.2 % lower, respectively, than those determined by a side-by-side comparison with the reference sky radiometer. After determining the Calibration Constant, we obtained PWV from the direct solar irradiances in both the dry and wet seasons. The retrieved PWV values corresponded well to those derived from a global-navigation-satellite-system–global-positioning-system receiver, a microwave radiometer, and an AERONET (Aerosol Robotic Network) sun–sky radiometer at both sites. The correlation coefficients were greater than 0.96. We calculated the bias errors and the root mean square errors by comparing PWV between the DSRAD (direct solar irradiance) algorithm and other instruments. The magnitude of the bias error and the root mean square error were and cm for PWV cm, respectively. However, our method tended to underestimate PWV in the wet conditions, and the magnitude of the bias error and the root mean square error became large, and cm for PWV>3 cm, respectively. This problem was mainly due to the overestimation of the aerosol optical thickness before the retrieval of PWV. These results show that the SKYMAP algorithm enables us to observe PWV over the long term, based on its unique on-site self-Calibration method.
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The instrument Constant of sky radiometers (POM-02) – Part 1: Calibration Constant
Atmospheric Measurement Techniques, 2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of the optical properties of aerosols using radiometers. In this study, the precision of the Calibration Constant ( V0 ) for the sky radiometer (POM-02) that is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba (36.05 ∘ N, 140.13 ∘ E), with a monthly mean temperature range of 2.7 to 25.5 ∘ C. In the other channels, the correction factors were less than 0.5 %. The coefficient of variation (CV, standard deviation/mean) of V0 from the normal Langley method, based on the data measured at the NOAA Mauna Loa Observatory, is between 0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for wavelengths shorter than 400 nm ( − 10 % to − 4 % per year) was larger than that for wavelengths longer than 500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from the reference POM-02 was 0.1 % to 0.5 %. Here, the data were simultaneously taken at 1 min intervals on a fine day, and data when the air mass was less than 2.5 were compared. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 % to 3 %. The root mean square error (RMSE) from the IML method was about 0.6 % to 2.5 %, and in some cases the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. Furthermore, the Calibration Constants determined by the IML method had much higher noise than those transferred from the reference. The modified Langley method was used to calibrate the 940 nm channel with on-site measurement data. The V0 obtained with the modified Langley method compared to the Langley method was 1 % more accurate on stable and fine days. The general method was also used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm) with on-site measurement data; the V0 obtained with the general method differed from that obtained with the Langley method of V0 by 0.8 %, 0.4 %, and 0.1 % in December 2015, respectively.
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the instrument Constant of sky radiometers pom 02 part 1 Calibration Constant
Atmospheric Measurement Techniques, 2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of the optical properties of aerosols using radiometers. In this study, the precision of the Calibration Constant ( V0 ) for the sky radiometer (POM-02) that is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba (36.05 ∘ N, 140.13 ∘ E), with a monthly mean temperature range of 2.7 to 25.5 ∘ C. In the other channels, the correction factors were less than 0.5 %. The coefficient of variation (CV, standard deviation/mean) of V0 from the normal Langley method, based on the data measured at the NOAA Mauna Loa Observatory, is between 0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for wavelengths shorter than 400 nm ( − 10 % to − 4 % per year) was larger than that for wavelengths longer than 500 nm ( − 1 to nearly 0 % per year). The CV of V0 transferred from the reference POM-02 was 0.1 % to 0.5 %. Here, the data were simultaneously taken at 1 min intervals on a fine day, and data when the air mass was less than 2.5 were compared. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 % to 3 %. The root mean square error (RMSE) from the IML method was about 0.6 % to 2.5 %, and in some cases the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. Furthermore, the Calibration Constants determined by the IML method had much higher noise than those transferred from the reference. The modified Langley method was used to calibrate the 940 nm channel with on-site measurement data. The V0 obtained with the modified Langley method compared to the Langley method was 1 % more accurate on stable and fine days. The general method was also used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm) with on-site measurement data; the V0 obtained with the general method differed from that obtained with the Langley method of V0 by 0.8 %, 0.4 %, and 0.1 % in December 2015, respectively.
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The instrument Constant of sky radiometer (POM-02), Part I: Calibration Constant
2018Co-Authors: Akihiro Uchiyama, Tsuneo Matsunaga, Akihiro YamazakiAbstract:Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of aerosols using radiometers. In this study, the accuracy of the Calibration Constant (V0) for the sky radiometer (POM-02) which is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels, and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba. In the other channels, the correction factors were less than 0.5 %. The accuracy of V0 from the normal Langley method is between 0.2 and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for shorter wavelengths was larger than that for longer wavelengths. The accuracy of V0 estimated from the side-by-side measurements was 0.1 to 0.5 %. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 to 3 %. The RMS error from the IML method was about 0.6 to 2.5 %, and in some cases, the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. The Calibration method for water vapor in the 940 nm channel was developed using an empirical formula for transmittance. The accuracy of V0 was better than 1 % on relatively stable and fine days. A Calibration method for the near-infrared channels, 1225, 1627, and 2200 nm, was also developed. The logarithm of the ratio of the sensor output can be written as a linear function of the airmass, by assuming that the ratio of the optical thicknesses between the two channels is Constant. The accuracy of V0 was better than 1 % on days with good conditions.
Weiwei Li - One of the best experts on this subject based on the ideXlab platform.
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Calibration of the Copolarized Backscattering Measurements From Gaofen-3 Synthetic Aperture Radar Wave Mode Imagery
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2019Co-Authors: He Wang, Huimin Li, Jing Wang, Weiwei LiAbstract:Accurate absolute radiometric Calibration of spaceborne synthetic aperture radar (SAR) sensor is of great importance in quantitative oceanic monitoring. Traditionally, the Calibration Constant is determined by analyzing measurements of man-made calibrators deployed on land at a high expense. In this paper, a technique called numerical weather prediction (NWP)-based ocean Calibration method for estimating the Calibration Constant on the basis of SAR observations over ocean is applied to the first Chinese C-band SAR satellite Gaofen-3 (GF-3). The ocean Calibration is performed on GF-3 wave mode SAR images at VV and HH polarizations over a period of one year from September 2017 to August 2018. Verification against the independent scatterometer-derived winds shows that after NWP-based ocean reCalibration, the accuracy of wind speed retrieval from GF-3 wave mode imagery could reach the wind speed estimation performance using the state-of-the-art of SAR data. These results indicate that GF-3 wind retrievals are promising for operational oceanic products and applications if the SAR data are appropriately calibrated by the proposed NWP-based ocean Calibration approach.
