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. 2017 Apr;55(4):2288-2298.
doi: 10.1109/TGRS.2016.2641258. Epub 2017 Jan 16.

Aqua and Terra MODIS RSB calibration comparison using BRDF modeled reflectance

Tiejun Chang  1 , Xiaoxiong Xiong  2 , Amit Angal  1 , Aisheng Wu  1 , Xu Geng  1
Affiliations

Aqua and Terra MODIS RSB calibration comparison using BRDF modeled reflectance

Tiejun Chang et al. IEEE Trans Geosci Remote Sens. 2017 Apr.

Abstract

The inter-comparison of MODIS reflective solar bands onboard Aqua and Terra is very important for assessment of each instrument's calibration. One of the limitations is the lack of simultaneous nadir overpasses. Their measurements over a selected Earth view target have significant differences in solar and view angles, which magnify the effects of atmospheric scattering and Bidirectional Reflectance Distribution Function (BRDF). In this work, an inter-comparison technique is formulated after correction for site's BRDF and atmospheric effects. The reflectance measurements over Libya desert sites 1, 2, and 4 from both the Aqua and Terra MODIS are regressed to a BRDF model with an adjustable coefficient accounting for calibration difference. The ratio between Aqua and Terra reflectance measurements are derived for bands 1 to 9 and the results from different sites show good agreement. For year 2003, the ratios are in the range of 0.985 to 1.010 for band 1 to 9. Band 3 shows the lowest ratio 0.985 and band 1shows the highest ratio 1.010. For the year 2014, the ratio ranges from approximately 0.983 for bands 2 and 1.012 for band 8. The BRDF corrected reflectance for the two instruments are also derived for every year from 2003 to 2014 for stability assessment. Bands 1 and 2 show greater than 1% differences between the two instruments. Aqua bands 1 and 2 show downward trends while Terra bands 1 and 2 show upward trends. Bands 8 and 9 of both Aqua and Terra show large variations of reflectance measurement over time.

Keywords: Atmospheric correction; BRDF; Inter-comparison; MODIS; Radiometric calibration.

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Figures

Figure 1.
Figure 1.
The correlation of the relative azimuth angle with sensor zenith angle (left), solar zenith angle with sensor zenith angle (right), for Libya Desert 4 in year 2003. Each symbol presents the average of reflectance measurement of the pixels in one granule. The black diamonds are for Aqua and the red squares are for Terra.
Figure 2.
Figure 2.
The correlation of the relative azimuth angle with day of the year (left) and solar zenith angle with day of the year (right) for Libya Desert 4 in year 2003. Each symbol presents the average of reflectance measurement of the pixels in one granule. The black diamonds are for Aqua and the red squares are for Terra.
Figure 3.
Figure 3.
The regressions of Libya desert 4 reflectance measurement of band 3 and 8 in year 2003 using semi-empirical model. The plots are the measurements from both Aqua and Terra RA⊕αT with optimized ratio against the modeled values k0 + k1f1A⊕T + k2f2A⊕T. Each symbol presents the average of reflectance measurement of the pixels in one granule. The black diamonds are Aqua measurements and the red squares are Terra measurements adjusted by the optimized ratio factor. The blue lines represent the model fitting.
Figure 4.
Figure 4.
BRDF effect corrected reflectance for Aqua from year 2003 to 2014 derived from the averaged results of three desert sites. The corrected reflectance are normalized to 2003 reflectance. The black diamonds are the corrected reflectance for Aqua and red triangles are for Terra.
Figure 5.
Figure 5.
Aqua/Terra ratio for band 1 to 9 for year 2003 (Top) and for 2014 (Bottom). The ratios are from the regression of the reflectance measurement over Libya Desert 1, 2, and 4 using semi-empirical BRDF model (labeled as model 1) and verified using pure empirical BRDF model (model 2).
Figure 6.
Figure 6.
Aqua/Terra ratio from year 2003 to 2014. The ratio is the averaged results of three desert sites. The results using semi-empirical BRDF model are black diamonds and results using empirical model are red triangles.
Figure 7.
Figure 7.
The ratio difference between the results using two BRDF models from year 2003 to 2014.
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