A cross-calibration methodology has been developed using coincident image pairs from the Terra Moderate
Resolution Imaging Spectroradiometer (MODIS), the Landsat 7 (L7) Enhanced Thematic Mapper Plus
(ETM+) and the Earth Observing EO-1 Advanced Land Imager (ALI) to verify the absolute radiometric
calibration accuracy of these sensors with respect to each other. To quantify the effects due to different
spectral responses, the Relative Spectral Responses (RSR) of these sensors were studied and compared by
developing a set of "figures-of-merit." Seven cloud-free scenes collected over the Railroad Valley Playa,
Nevada (RVPN), test site were used to conduct the cross-calibration study. This cross-calibration approach
was based on image statistics from near-simultaneous observations made by different satellite sensors.
Homogeneous regions of interest (ROI) were selected in the image pairs, and the mean target statistics were
converted to absolute units of at-sensor reflectance. Using these reflectances, a set of cross-calibration
equations were developed giving a relative gain and bias between the sensor pair.
Data from multiple sensors must be used together to gain a more complete understanding of land surface
processes at a variety of scales. Although higher-level products derived from different sensors (e.g.,
vegetation cover, albedo, surface temperature) can be validated independently, the degree to which these
sensors and their products can be compared to one another is vastly improved if their relative spectroradiometric
responses are known. Most often, sensors are directly calibrated to diffuse solar irradiation or
vicariously to ground targets. However, space-based targets are not traceable to metrological standards, and
vicarious calibrations are expensive and provide a poor sampling of a sensor's full dynamic range. Crosscalibration
of two sensors can augment these methods if certain conditions can be met: (1) the spectral
responses are similar, (2) the observations are reasonably concurrent (similar atmospheric & solar
illumination conditions), (3) errors due to misregistrations of inhomogeneous surfaces can be minimized
(including scale differences), and (4) the viewing geometry is similar (or, some reasonable knowledge of
surface bi-directional reflectance distribution functions is available).
This study extends on a previous study of Terra/MODIS and Landsat/ETM+ cross calibration by including the
Terra/ASTER and EO-1/ALI sensors, exploring the impacts of cross-calibrating sensors when conditions
described above are met to some degree but not perfectly. Measures for spectral response differences and
methods for cross calibrating such sensors are provided in this study. These instruments are cross calibrated
using the Railroad Valley playa in Nevada. Best fit linear coefficients (slope and offset) are provided for ALIto-
MODIS and ETM+-to-MODIS cross calibrations, and root-mean-squared errors (RMSEs) and correlation
coefficients are provided to quantify the uncertainty in these relationships. Due to problems with direct
calibration of ASTER data, linear fits were developed between ASTER and ETM+ to assess the impacts of
spectral bandpass differences between the two systems. In theory, the linear fits and uncertainties can be
used to compare radiance and reflectance products derived from each instrument.
Increasingly, data from multiple sensors are used to gain a more complete understanding of land surface
processes at a variety of scales. Although higher-level products (e.g., vegetation cover, albedo, surface
temperature) derived from different sensors can be validated independently, the degree to which these
sensors and their products can be compared to one another is vastly improved if their relative spectroradiometric
responses are known. Most often, sensors are directly calibrated to diffuse solar irradiation or
vicariously to ground targets. However, space-based targets are not traceable to metrological standards, and
vicarious calibrations are expensive and provide a poor sampling of a sensor's full dynamic range. Cross-calibration
of two sensors can augment these methods if certain conditions can be met: (1) the spectral
responses are similar, (2) the observations are reasonably concurrent (similar atmospheric & solar
illumination conditions), (3) errors due to misregistrations of inhomogeneous surfaces can be minimized
(including scale differences), and (4) the viewing geometry is similar (or, some reasonable knowledge of
surface bi-directional reflectance distribution functions is available).
This study explores the impacts of cross-calibrating sensors when such conditions are met to some degree
but not perfectly. In order to constrain the range of conditions at some level, the analysis is limited to sensors
where cross-calibration studies have been conducted (Enhanced Thematic Mapper Plus (ETM+) on Landsat-
7 (L7), Advance Land Imager (ALI) and Hyperion on Earth Observer-1 (EO-1)) and including systems having
somewhat dissimilar geometry, spatial resolution & spectral response characteristics but are still part of the
so-called "A.M. constellation" (Moderate Resolution Imaging Spectrometer (MODIS) aboard the Terra
platform). Measures for spectral response differences and methods for cross calibrating such sensors are
provided in this study. These instruments are cross calibrated using the Railroad Valley playa in Nevada.
Best fit linear coefficients (slope and offset) are provided for ALI-to-MODIS and ETM+-to-MODIS cross
calibrations, and root-mean-squared errors (RMSEs) and correlation coefficients are provided to quantify the
uncertainty in these relationships. In theory, the linear fits and uncertainties can be used to compare radiance
and reflectance products derived from each instrument.
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