Aqua and Terra MODIS RSB calibration comparison using BRDF modeled reflectance

Area of Science:

• Earth Observation
• Remote Sensing Calibration
• Instrument Inter-comparison

Background:

• Accurate calibration of Earth-observing instruments like MODIS is crucial for reliable climate data.
• Differences in solar and view angles during non-simultaneous overpasses complicate direct instrument comparisons.
• Atmospheric scattering and Bidirectional Reflectance Distribution Function (BRDF) effects introduce significant variability in reflectance measurements.

Purpose of the Study:

• To inter-compare the calibration of MODIS instruments onboard Aqua and Terra satellites.
• To develop and apply a technique for comparing measurements corrected for BRDF and atmospheric effects.
• To assess the long-term stability of MODIS bands 1-9 using inter-comparison data.

Main Methods:

• Utilized Libya desert sites (1, 2, and 4) for inter-comparison due to their stable surface reflectance.
• Applied a BRDF model with an adjustable coefficient to correct for angular effects and account for calibration differences.
• Regressed reflectance measurements from Aqua and Terra MODIS against the BRDF model.
• Calculated reflectance ratios between Aqua and Terra for bands 1-9 for the years 2003-2014.

Main Results:

• Reflectance ratios between Aqua and Terra MODIS for bands 1-9 showed good agreement across different sites and years.
• In 2003, ratios ranged from 0.985 (band 3) to 1.010 (band 1). In 2014, ratios ranged from 0.983 (band 2) to 1.012 (band 8).
• Significant inter-instrument differences (>1%) were observed for bands 1 and 2, with Aqua showing downward trends and Terra upward trends. Bands 8 and 9 exhibited high temporal variability.

Conclusions:

• The developed BRDF-corrected inter-comparison technique provides a reliable method for assessing MODIS calibration.
• While overall agreement is good, specific bands (1 and 2) indicate potential calibration drifts or differences between Aqua and Terra MODIS.
• Long-term stability assessment reveals contrasting trends in bands 1 and 2, highlighting the need for continued monitoring and potential recalibration.