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Papers by David Aaron

ABSTRACT The presentation gives a general description of the verification and validation capabili... more ABSTRACT The presentation gives a general description of the verification and validation capabilities available at the John C. Stennis Space Center in southern Mississippi.

... by Mary Pagnutti, David Carver, Kara Holekamp, Kelly Knowlton, Robert Ryan, Vicki Zanoni, Kur... more ... by Mary Pagnutti, David Carver, Kara Holekamp, Kelly Knowlton, Robert Ryan, Vicki Zanoni, Kurtis Thome, David Aaron. ... Radiometric Use of QuickBird Imagery. Keith Krause in World (2005). 6 readers Save reference to library · Related research. Radiometric characterization of ...

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), 1989

An assessment of the Advanced Wide Field Sensor (AWiFS) is presented. The contents include: 1) Ov... more An assessment of the Advanced Wide Field Sensor (AWiFS) is presented. The contents include: 1) Overview of AWiFS sensor; 2) Description of University of Arizona approach; 3) Description of South Dakota State approach and results; 4) Description of Stennis Space Center approach and results; 5) Summary of results for all groups.

IEEE Transactions on Geoscience and Remote Sensing, Mar 1, 2013

ABSTRACT To monitor land surface processes over a wide range of temporal and spatial scales, it i... more ABSTRACT To monitor land surface processes over a wide range of temporal and spatial scales, it is critical to have coordinated observations of the Earth's surface acquired from multiple spaceborne imaging sensors. However, an integrated global observation framework requires an understanding of how land surface processes are seen differently by various sensors. This is particularly true for sensors acquiring data in spectral bands whose relative spectral responses (RSRs) are not similar and thus may produce different results while observing the same target. The intrinsic offsets between two sensors caused by RSR mismatches can be compensated by using a spectral band adjustment factor (SBAF), which takes into account the spectral profile of the target and the RSR of the two sensors. The motivation of this work comes from the need to compensate the spectral response differences of multispectral sensors in order to provide a more accurate cross-calibration between the sensors. In this paper, radiometric cross-calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors was performed using near-simultaneous observations over the Libya 4 pseudoinvariant calibration site in the visible and near-infrared spectral range. The RSR differences of the analogous ETM+ and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectances between the two sensors over their lifetimes. The average percent differences in the long-term trends ranged from -5% to +6%. The RSR compensated ETM+ TOA reflectance (ETM+*) measurements were then found to agree with MODIS TOA reflectance to within 5% for all bands when Earth Observing-1 Hyperion hyperspectral data were used to produce the SBAFs. These differences were later reduced to with- n 1% for all bands (except band 2) by using Environmental Satellite Scanning Imaging Absorption Spectrometer for Atmospheric Cartography hyperspectral data to produce the SBAFs.

AGU Fall Meeting Abstracts, Dec 8, 2020

IEEE Transactions on Geoscience and Remote Sensing, Nov 1, 2019

The estimated yearly drift in these bands ranges between -0.136% and -0.049%. After correction ac... more The estimated yearly drift in these bands ranges between -0.136% and -0.049%. After correction accounting for the estimated drift, the absolute radiometric calibration of the sensor was evaluated through vicarious reflectance-based calibrations performed at the South Dakota State University (SDSU) test site and the Radiometric Calibration Network (RadCalNet) Railroad Valley site using data from 2002 to 2015. Calibration correction coefficients including gain as much as 1.40 and bias as large as 0.132 were found except absorption bands (890-980, 1090-1180, 1305-1520, and 1750-2050 nm). Finally, the yearly drift and calibration correction coefficients were validated by comparison of the banded multispectral Hyperion data after any significant drift and calibration coefficient correction with Landsat 7 data. The validation showed that after drift and calibration coefficient correction, there is no significant gain and bias using different test sites at different signal levels. The drift and calibration correction coefficients of each band are provided in this paper.

Remote Sensing, May 11, 2016

Brazil and China have a long-term joint space based sensor program called China-Brazil Earth Reso... more Brazil and China have a long-term joint space based sensor program called China-Brazil Earth Resources Satellite (CBERS). The most recent satellite of this program (CBERS-4) was successfully launched on 7 December 2014. This work describes a complete procedure, along with the associated uncertainties, used to calculate the in-flight absolute calibration coefficients for the sensors Multispectral Camera (MUX) and Wide-Field Imager (WFI) on-board CBERS-4. Two absolute radiometric calibration techniques were applied: (i) reflectance-based approach and (ii) cross-calibration method. A specific site at Algodones Dunes region located in the southwestern portion of the United States of America was used as a reference surface. Radiometric ground and atmospheric measurements were carried out on 9 March 2015, when CBERS-4 passed over the region. In addition, a cross-calibration between both MUX and WFI on-board CBERS-4 and the Operational Land Imager (OLI) on-board Landsat-8 was performed using the Libya-4 Pseudo Invariant Calibration Site. The gain coefficients are now available: 1.68, 1.62, 1.59 and 1.42 for MUX and 0.379, 0.498, 0.360 and 0.351 for WFI spectral bands blue, green, red and NIR, respectively, in units of (W/(m 2¨s r¨µm))/DN. These coefficients were determined with uncertainties lower than 3.5%. As a validation of these radiometric coefficients, cross-calibration was also undertaken. An evaluation of radiometric consistency was performed between the two instruments (MUX and WFI) on-board CBERS-4 and with the well calibrated Landsat-7 ETM+. Results show that the reflectance values match in all the analogous spectral bands within the specified calibration uncertainties.

Remote Sensing Letters, Jun 3, 2016

ABSTRACT Cross-calibration is one of the various methods applied for Earth Observation Satellites... more ABSTRACT Cross-calibration is one of the various methods applied for Earth Observation Satellites sensor calibration. In the cross-calibration procedure, one sensor is calibrated against another sensor, in which the radiometric calibration is better known, via near-simultaneous imaging of a common ground target. One of the most important steps during the cross-calibration is the Spectral Band Adjustment Factor (SBAF) assessment. The SBAF is used to compensate the differences in the spectral responses of the sensors, avoiding large uncertainties in cross-calibration results. The investigation described in this work focussed on the evaluation of the SBAF’s inherent uncertainties using Monte Carlo Simulation method. Basically, the Monte Carlo approach is based on calculating multiple integral by random sampling. The SBAFs were developed for analogous Landsat 8 Operational Land Imager and CBERS 4 Multispectral Camera spectral bands. The Hyperion hyperspectral sensor on-board Earth Observing-1 was utilized to understand the spectral profile of the target and to derive the SBAF. This study was performed over two pseudo invariant calibration sites: Algodones Dunes and Libya-4. The spectral uncertainty of the SBAFs using Monte Carlo was found to be within 0.01–1.79%. The results suggested that the uncertainty of the SBAFs is dependent on the correlation between the input variables: the higher the correlation, the lowest is the SBAF uncertainty.

The high sensitivity of the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) high... more The high sensitivity of the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) high gain stage (HGS) cannot directly use on-board calibration sources such as solar di users, so other calibration approaches are needed. A fully automated, NIST-Traceable active point source called the Accurate Active Light Source (AALS) based on a 1-meter diameter integrating sphere illuminated with High Pressure Sodium (HPS) lamps has been developed to calibrate the VIIRS DNB HGS mode. This poster provides the theory and operation of the AALS. The AALS is housed in an environmentally controlled, moveable, enclosed trailer that protects the sphere and is being deployed in South Dakota during the summer and Mississippi during the winter. AALS Lambertian source radiance is measured by a NIST-traceable spectrometer. MODTRAN-based top of atmosphere (TOA) radiance is calculated using nighttime aerosol estimates and point source radiative transfer techniques that account for the VIIRS nite Point Spread Function (PSF). This new active point source complements current vicarious calibration techniques, which are based on lunar illumination of extended sources (clouds, playas, etc.) and helps improve our understanding of the point source radiative transfer needed to develop nighttime arti cial light source applications. An uncertainty budget for TOA radiance indicated that accuracy under clear, moonless conditions should be comparable to daytime vicarious calibration methods. This poster shows that the major uncertainty is the ground-to-space atmospeheric transmission and discusses approaches to reduce this error. Initial assessment of the VIIRS DNB HGS on the Suomi-NPP and NOAA-20 satellites are presented.

Remote Sensing, Mar 24, 2019

A fully automated, National Institute of Standards and Technology (NIST)-traceable artificial lig... more A fully automated, National Institute of Standards and Technology (NIST)-traceable artificial light source called Terra Vega has been developed to radiometrically calibrate the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) working in high gain stage (HGS) mode. The Terra Vega active point source is a calibrated integrating sphere that is only a fraction in size of a VIIRS DNB pixel. As such, it can be considered analogous to a ground-based photometric reference star. Vicarious calibrations that employ active point sources are different than those that make use of traditional extended sources and can be applyed to quantify the brightness of artificial light sources. The active source is successfully fielded, and early results indicate that it can be used to augment and validate the radiometric calibration of the VIIRS DNB HGS sensor on both the Suomi National Polar-orbiting Partnership (NPP) and NOAA-20 satellites. The VIIRS DNB HGS sensor can benefit from this technology as on-board calibration is challenging and hinges on transferring low gain stage (LGS) calibration using a solar diffuser to the medium gain stage (MGS) and HGS via regions of overlap. Current vicarious calibration methods that use a lunar-illuminated extended source estimate the HGS radiometric accuracy to within 8-15%. By comparison, early results and analysis showed that Terra Vega is stable to about 1%. Under clear dark night conditions, predicted top-of-atmosphere radiance from Terra Vega ranged between 1-11% of VIIRS measured values. Terra Vega's excellent stability opens up new opportunities to validate and develop nighttime imaging applications based on point sources.

IEEE Transactions on Geoscience and Remote Sensing, Dec 1, 2004

The reflectance-based method of vicarious calibration has been used for the absolute radiometric ... more The reflectance-based method of vicarious calibration has been used for the absolute radiometric calibration of the Landsat series of sensors since the launch of Landsat-4. The reflectance-based method relies on ground-based measurements of the surface reflectance and atmospheric conditions at a selected test site nearly coincident with the imaging of that site by the sensor of interest. The results of this approach are presented here for Landsat-5 Thematic Mapper (TM) and Landsat-7 Enhanced Thematic Mapper Plus (ETM+).

IEEE Transactions on Geoscience and Remote Sensing, Mar 1, 2013

Cross-calibration of satellite sensors permits the quantitative comparison of measurements obtain... more Cross-calibration of satellite sensors permits the quantitative comparison of measurements obtained from different Earth Observing (EO) systems. Cross-calibration studies usually use simultaneous or near-simultaneous observations from several spaceborne sensors to develop band-by-band relationships through regression analysis. The investigation described in this paper focuses on evaluation of the uncertainties inherent in the cross-calibration process, including contributions due to different spectral responses, spectral resolution, spectral filter shift, geometric misregistrations, and spatial resolutions. The hyperspectral data from the Environmental Satellite SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY and the EO-1 Hyperion, along with the relative spectral responses (RSRs) from the Landsat 7 Enhanced Thematic Mapper (TM) Plus and the Terra Moderate Resolution Imaging Spectroradiometer sensors, were used for the spectral uncertainty study. The data from Landsat 5 TM over five representative land cover types (desert, rangeland, grassland, deciduous forest, and coniferous forest) were used for the geometric misregistrations and spatial-resolution study. The spectral resolution uncertainty was found to be within 0.25%, spectral filter shift within 2.5%, geometric misregistrations within 0.35%, and spatial-resolution effects within 0.1% for the Libya 4 site. The one-sigma uncertainties presented in this paper are uncorrelated, and therefore, the uncertainties can be summed orthogonally. Furthermore, an overall total uncertainty was developed. In general, the results suggested that the spectral uncertainty is more dominant compared to other uncertainties presented in this paper. Therefore, the effect of the sensor RSR differences needs to be quantified and compensated to avoid large uncertainties in cross-calibration results.

ABSTRACT The presentation gives a general description of the verification and validation capabili... more ABSTRACT The presentation gives a general description of the verification and validation capabilities available at the John C. Stennis Space Center in southern Mississippi.

... by Mary Pagnutti, David Carver, Kara Holekamp, Kelly Knowlton, Robert Ryan, Vicki Zanoni, Kur... more ... by Mary Pagnutti, David Carver, Kara Holekamp, Kelly Knowlton, Robert Ryan, Vicki Zanoni, Kurtis Thome, David Aaron. ... Radiometric Use of QuickBird Imagery. Keith Krause in World (2005). 6 readers Save reference to library · Related research. Radiometric characterization of ...

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), 1989

An assessment of the Advanced Wide Field Sensor (AWiFS) is presented. The contents include: 1) Ov... more An assessment of the Advanced Wide Field Sensor (AWiFS) is presented. The contents include: 1) Overview of AWiFS sensor; 2) Description of University of Arizona approach; 3) Description of South Dakota State approach and results; 4) Description of Stennis Space Center approach and results; 5) Summary of results for all groups.

IEEE Transactions on Geoscience and Remote Sensing, Mar 1, 2013

ABSTRACT To monitor land surface processes over a wide range of temporal and spatial scales, it i... more ABSTRACT To monitor land surface processes over a wide range of temporal and spatial scales, it is critical to have coordinated observations of the Earth's surface acquired from multiple spaceborne imaging sensors. However, an integrated global observation framework requires an understanding of how land surface processes are seen differently by various sensors. This is particularly true for sensors acquiring data in spectral bands whose relative spectral responses (RSRs) are not similar and thus may produce different results while observing the same target. The intrinsic offsets between two sensors caused by RSR mismatches can be compensated by using a spectral band adjustment factor (SBAF), which takes into account the spectral profile of the target and the RSR of the two sensors. The motivation of this work comes from the need to compensate the spectral response differences of multispectral sensors in order to provide a more accurate cross-calibration between the sensors. In this paper, radiometric cross-calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors was performed using near-simultaneous observations over the Libya 4 pseudoinvariant calibration site in the visible and near-infrared spectral range. The RSR differences of the analogous ETM+ and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectances between the two sensors over their lifetimes. The average percent differences in the long-term trends ranged from -5% to +6%. The RSR compensated ETM+ TOA reflectance (ETM+*) measurements were then found to agree with MODIS TOA reflectance to within 5% for all bands when Earth Observing-1 Hyperion hyperspectral data were used to produce the SBAFs. These differences were later reduced to with- n 1% for all bands (except band 2) by using Environmental Satellite Scanning Imaging Absorption Spectrometer for Atmospheric Cartography hyperspectral data to produce the SBAFs.

AGU Fall Meeting Abstracts, Dec 8, 2020

IEEE Transactions on Geoscience and Remote Sensing, Nov 1, 2019

The estimated yearly drift in these bands ranges between -0.136% and -0.049%. After correction ac... more The estimated yearly drift in these bands ranges between -0.136% and -0.049%. After correction accounting for the estimated drift, the absolute radiometric calibration of the sensor was evaluated through vicarious reflectance-based calibrations performed at the South Dakota State University (SDSU) test site and the Radiometric Calibration Network (RadCalNet) Railroad Valley site using data from 2002 to 2015. Calibration correction coefficients including gain as much as 1.40 and bias as large as 0.132 were found except absorption bands (890-980, 1090-1180, 1305-1520, and 1750-2050 nm). Finally, the yearly drift and calibration correction coefficients were validated by comparison of the banded multispectral Hyperion data after any significant drift and calibration coefficient correction with Landsat 7 data. The validation showed that after drift and calibration coefficient correction, there is no significant gain and bias using different test sites at different signal levels. The drift and calibration correction coefficients of each band are provided in this paper.

Remote Sensing, May 11, 2016

Brazil and China have a long-term joint space based sensor program called China-Brazil Earth Reso... more Brazil and China have a long-term joint space based sensor program called China-Brazil Earth Resources Satellite (CBERS). The most recent satellite of this program (CBERS-4) was successfully launched on 7 December 2014. This work describes a complete procedure, along with the associated uncertainties, used to calculate the in-flight absolute calibration coefficients for the sensors Multispectral Camera (MUX) and Wide-Field Imager (WFI) on-board CBERS-4. Two absolute radiometric calibration techniques were applied: (i) reflectance-based approach and (ii) cross-calibration method. A specific site at Algodones Dunes region located in the southwestern portion of the United States of America was used as a reference surface. Radiometric ground and atmospheric measurements were carried out on 9 March 2015, when CBERS-4 passed over the region. In addition, a cross-calibration between both MUX and WFI on-board CBERS-4 and the Operational Land Imager (OLI) on-board Landsat-8 was performed using the Libya-4 Pseudo Invariant Calibration Site. The gain coefficients are now available: 1.68, 1.62, 1.59 and 1.42 for MUX and 0.379, 0.498, 0.360 and 0.351 for WFI spectral bands blue, green, red and NIR, respectively, in units of (W/(m 2¨s r¨µm))/DN. These coefficients were determined with uncertainties lower than 3.5%. As a validation of these radiometric coefficients, cross-calibration was also undertaken. An evaluation of radiometric consistency was performed between the two instruments (MUX and WFI) on-board CBERS-4 and with the well calibrated Landsat-7 ETM+. Results show that the reflectance values match in all the analogous spectral bands within the specified calibration uncertainties.

Remote Sensing Letters, Jun 3, 2016

ABSTRACT Cross-calibration is one of the various methods applied for Earth Observation Satellites... more ABSTRACT Cross-calibration is one of the various methods applied for Earth Observation Satellites sensor calibration. In the cross-calibration procedure, one sensor is calibrated against another sensor, in which the radiometric calibration is better known, via near-simultaneous imaging of a common ground target. One of the most important steps during the cross-calibration is the Spectral Band Adjustment Factor (SBAF) assessment. The SBAF is used to compensate the differences in the spectral responses of the sensors, avoiding large uncertainties in cross-calibration results. The investigation described in this work focussed on the evaluation of the SBAF’s inherent uncertainties using Monte Carlo Simulation method. Basically, the Monte Carlo approach is based on calculating multiple integral by random sampling. The SBAFs were developed for analogous Landsat 8 Operational Land Imager and CBERS 4 Multispectral Camera spectral bands. The Hyperion hyperspectral sensor on-board Earth Observing-1 was utilized to understand the spectral profile of the target and to derive the SBAF. This study was performed over two pseudo invariant calibration sites: Algodones Dunes and Libya-4. The spectral uncertainty of the SBAFs using Monte Carlo was found to be within 0.01–1.79%. The results suggested that the uncertainty of the SBAFs is dependent on the correlation between the input variables: the higher the correlation, the lowest is the SBAF uncertainty.

The high sensitivity of the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) high... more The high sensitivity of the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) high gain stage (HGS) cannot directly use on-board calibration sources such as solar di users, so other calibration approaches are needed. A fully automated, NIST-Traceable active point source called the Accurate Active Light Source (AALS) based on a 1-meter diameter integrating sphere illuminated with High Pressure Sodium (HPS) lamps has been developed to calibrate the VIIRS DNB HGS mode. This poster provides the theory and operation of the AALS. The AALS is housed in an environmentally controlled, moveable, enclosed trailer that protects the sphere and is being deployed in South Dakota during the summer and Mississippi during the winter. AALS Lambertian source radiance is measured by a NIST-traceable spectrometer. MODTRAN-based top of atmosphere (TOA) radiance is calculated using nighttime aerosol estimates and point source radiative transfer techniques that account for the VIIRS nite Point Spread Function (PSF). This new active point source complements current vicarious calibration techniques, which are based on lunar illumination of extended sources (clouds, playas, etc.) and helps improve our understanding of the point source radiative transfer needed to develop nighttime arti cial light source applications. An uncertainty budget for TOA radiance indicated that accuracy under clear, moonless conditions should be comparable to daytime vicarious calibration methods. This poster shows that the major uncertainty is the ground-to-space atmospeheric transmission and discusses approaches to reduce this error. Initial assessment of the VIIRS DNB HGS on the Suomi-NPP and NOAA-20 satellites are presented.

Remote Sensing, Mar 24, 2019

A fully automated, National Institute of Standards and Technology (NIST)-traceable artificial lig... more A fully automated, National Institute of Standards and Technology (NIST)-traceable artificial light source called Terra Vega has been developed to radiometrically calibrate the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) working in high gain stage (HGS) mode. The Terra Vega active point source is a calibrated integrating sphere that is only a fraction in size of a VIIRS DNB pixel. As such, it can be considered analogous to a ground-based photometric reference star. Vicarious calibrations that employ active point sources are different than those that make use of traditional extended sources and can be applyed to quantify the brightness of artificial light sources. The active source is successfully fielded, and early results indicate that it can be used to augment and validate the radiometric calibration of the VIIRS DNB HGS sensor on both the Suomi National Polar-orbiting Partnership (NPP) and NOAA-20 satellites. The VIIRS DNB HGS sensor can benefit from this technology as on-board calibration is challenging and hinges on transferring low gain stage (LGS) calibration using a solar diffuser to the medium gain stage (MGS) and HGS via regions of overlap. Current vicarious calibration methods that use a lunar-illuminated extended source estimate the HGS radiometric accuracy to within 8-15%. By comparison, early results and analysis showed that Terra Vega is stable to about 1%. Under clear dark night conditions, predicted top-of-atmosphere radiance from Terra Vega ranged between 1-11% of VIIRS measured values. Terra Vega's excellent stability opens up new opportunities to validate and develop nighttime imaging applications based on point sources.

IEEE Transactions on Geoscience and Remote Sensing, Dec 1, 2004

The reflectance-based method of vicarious calibration has been used for the absolute radiometric ... more The reflectance-based method of vicarious calibration has been used for the absolute radiometric calibration of the Landsat series of sensors since the launch of Landsat-4. The reflectance-based method relies on ground-based measurements of the surface reflectance and atmospheric conditions at a selected test site nearly coincident with the imaging of that site by the sensor of interest. The results of this approach are presented here for Landsat-5 Thematic Mapper (TM) and Landsat-7 Enhanced Thematic Mapper Plus (ETM+).

IEEE Transactions on Geoscience and Remote Sensing, Mar 1, 2013

Cross-calibration of satellite sensors permits the quantitative comparison of measurements obtain... more Cross-calibration of satellite sensors permits the quantitative comparison of measurements obtained from different Earth Observing (EO) systems. Cross-calibration studies usually use simultaneous or near-simultaneous observations from several spaceborne sensors to develop band-by-band relationships through regression analysis. The investigation described in this paper focuses on evaluation of the uncertainties inherent in the cross-calibration process, including contributions due to different spectral responses, spectral resolution, spectral filter shift, geometric misregistrations, and spatial resolutions. The hyperspectral data from the Environmental Satellite SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY and the EO-1 Hyperion, along with the relative spectral responses (RSRs) from the Landsat 7 Enhanced Thematic Mapper (TM) Plus and the Terra Moderate Resolution Imaging Spectroradiometer sensors, were used for the spectral uncertainty study. The data from Landsat 5 TM over five representative land cover types (desert, rangeland, grassland, deciduous forest, and coniferous forest) were used for the geometric misregistrations and spatial-resolution study. The spectral resolution uncertainty was found to be within 0.25%, spectral filter shift within 2.5%, geometric misregistrations within 0.35%, and spatial-resolution effects within 0.1% for the Libya 4 site. The one-sigma uncertainties presented in this paper are uncorrelated, and therefore, the uncertainties can be summed orthogonally. Furthermore, an overall total uncertainty was developed. In general, the results suggested that the spectral uncertainty is more dominant compared to other uncertainties presented in this paper. Therefore, the effect of the sensor RSR differences needs to be quantified and compensated to avoid large uncertainties in cross-calibration results.