Investigating the Consistency between VIIRS and MODIS over the Oceans: The Sensor/Environmental Data Records

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the National Preparatory Project (NPP) was launched in October 2011 to continue monitoring the globe in a similar fashion as the heritage sensors, such as the MODerate resolution Imaging Spectroradiometer (MODIS). One of the primary missions of VIIRS is the continuity in providing the science community with the Environmental Data Records (EDRs) including global ocean color products and climatology (eg. chlorophyll-a, inherent optical properties (IOPs)). In order to deliver observations consistent with the existing ocean color products, it is crucial to characterize the in-orbit calibration status of the VIIRS instrument. Such efforts are independent of the pre-flight calibration/characterization and ensure consistency in the data/product quality. In this paper, the MODIS L1B (Aqua) top-of-atmosphere (TOA) reflectances are utilized as references to evaluate the relative calibration of the VIIRS instrument over oceanic/coastal waters (>10 km offshore). The MODIS scenes (collection 6) over open waters (located in low/high latitude regions) are compared against the TOA VIIRS sensor data records (SDRs). The VIIRS data obtained from NOAA-CLASS are used to conduct a system-level inter-comparison between the two sensors. In addition to using the low-latitude simultaneous nadir orbits (SNOs), high latitude scene pairs acquired from near-identical/simultaneous orbits are also considered for the cross-comparison study. While the SNOs represent <10 minutes time difference, the high-latitude scenes were < 20 minutes apart. Various criteria were defined to isolate homogenous water bodies, select identical atmospheric conditions (aerosol distributions), and, subsequently, locate suitable areas (5*5 pixel windows). Due to the cloud coverage over all of the SNOs, pseudo-SNOs, which represent areas imaged at near-nadir viewing angles, are applied instead. The small differences in the scan angles (<5 degrees) and in the relative spectral responses (RSRs) of the two sensors are modeled through radiative transfer simulations to enable realistic comparisons in the TOA reflectance domain. It should be noted that surface-leaving albedo (needed for simulations) is held identical for all the simulations. Approximately, 23 scene pairs were used in this study from March – August 2012. Preliminary results show that the VIIRS responses at channels M2, M3, and M6 of VIIRS are very consistent with the corresponding MODIS bands, whereas M1, M4, M5, and M7 exhibit higher signals than those of MODIS. Figure 1 illustrates average percent differences, i.e., (VIIRS-MODIS)/MODIS, expressed in percentage, found for the scene pairs (n=23) from March to August 2012. In general, the differences range from -2% to +6%. The error bars indicate standard deviations (uncertainties) obtained from mean differences for all of the scene pairs. The largest uncertainty associated with the mean differences corresponds to M7. This is due to the slight inconsistencies in the atmospheric conditions between the overpasses, which have the greatest effects on M7. The temporal analysis (shown in Fig. 2) for all the bands shows that the calibration differences remain relatively stable over this timeframe. However, there is a gradual (linear) decrease in the disparity for M6 (746 nm), i.e., slope: +0.15 and intercept: -2.7. It should be noted that the analyses presented are detector-independent as the differences were commonly computed for various areas in each scene. In order to investigate how uncertainties in the TOA reflectance domain influence the EDR, the remote sensing reflectance (Rrs) derived through standard atmospheric corrections over the nearly stable Gyres (Atlantic/Pacific oceans) are examined. The inter-band disparities in the Rrs domain are in agreement with the findings in the TOA reflectance domain, i.e., > 20% larger signals at M4, M5, and M7. Further evaluations of the EDR products, such as Rrs, IOPs, aerosol optical thickness (AOT), and chlorophyll-a, are currently in progress for the scenes that were employed in the cross-comparison study. This preliminary analysis will demonstrate that the VIIRS products are consistent with those from MODIS and identify areas the community would focus on for improvements.