24. AUSTRALIAN SITES FOR THE VALIDATION OF SATELLITE
RETRIEVALS OF THE RADIATIVE PROPERTIES OF LAND
SURFACES
239
5.
VALIDATION OF ATSR-2 SHORTWAVE
CHANNELS
The utility of the site for validating satellite measurements is illustrated
by comparing measurements of the spectral radiance of the site made at the
top of the atmosphere by ATSR-2 and at the ground during a collaborative
campaign in April 1997 between CSIRO and the University of Nottingham.
During the ATSR-2 atmospheric Correction Experiment (ACEX) the
directional surface reflectance was measured during one week (three ATSR2 overpasses) at each site. The atmospheric water vapor was characterized
by radiosonde profiles taken at the site at the overpass times and the aerosol
optical depth was measured by a shadowband radiometer. The CSIRO and
University of Nottingham teams each took measurements with different
instruments and analyzed them with different radiative transfer codes.
1.
2.
3.
4.
The validation procedure consists of
measuring the surface directional spectral reflectance at the validation
site: in the view direction used by ATSR-2 at the time of overpasses, and
by interpolating measurements over the whole hemisphere made at other
times;
applying an atmospheric correction to the surface measurements, using
the measured water vapor profile and aerosol amount - CSIRO used the
MODTRAN3B radiation code (Kneizys et al., 1988), and the University
of Nottingham used a scheme (Mackay et al., 1994) built around the 5S
code (Tanré et al., 1990);
integrating the top-of-atmosphere (TOA) spectral radiance with the filter
response function over each ATSR-2 band;
comparing the result with the calibrated ATSR-2 TOA radiances.
Figure 7 shows preliminary validation results using the
CSIRO/University of Nottingham measurements and MODTRAN3B
modeling. The results suggest good agreement to within a few percent with
the existing calibration of the ATSR-2. Understanding of the differences in
detail requires further investigation.
RETRIEVALS OF THE RADIATIVE PROPERTIES OF LAND
SURFACES
239
5.
VALIDATION OF ATSR-2 SHORTWAVE
CHANNELS
The utility of the site for validating satellite measurements is illustrated
by comparing measurements of the spectral radiance of the site made at the
top of the atmosphere by ATSR-2 and at the ground during a collaborative
campaign in April 1997 between CSIRO and the University of Nottingham.
During the ATSR-2 atmospheric Correction Experiment (ACEX) the
directional surface reflectance was measured during one week (three ATSR2 overpasses) at each site. The atmospheric water vapor was characterized
by radiosonde profiles taken at the site at the overpass times and the aerosol
optical depth was measured by a shadowband radiometer. The CSIRO and
University of Nottingham teams each took measurements with different
instruments and analyzed them with different radiative transfer codes.
1.
2.
3.
4.
The validation procedure consists of
measuring the surface directional spectral reflectance at the validation
site: in the view direction used by ATSR-2 at the time of overpasses, and
by interpolating measurements over the whole hemisphere made at other
times;
applying an atmospheric correction to the surface measurements, using
the measured water vapor profile and aerosol amount - CSIRO used the
MODTRAN3B radiation code (Kneizys et al., 1988), and the University
of Nottingham used a scheme (Mackay et al., 1994) built around the 5S
code (Tanré et al., 1990);
integrating the top-of-atmosphere (TOA) spectral radiance with the filter
response function over each ATSR-2 band;
comparing the result with the calibrated ATSR-2 TOA radiances.
Figure 7 shows preliminary validation results using the
CSIRO/University of Nottingham measurements and MODTRAN3B
modeling. The results suggest good agreement to within a few percent with
the existing calibration of the ATSR-2. Understanding of the differences in
detail requires further investigation.