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Extended Simultaneous Nadir Overpass (SNO-x) in low latitudes is a recently introduced approach (inherited from traditional SNO approach) that extends SNO orbits to low latitudes for inter-comparing sensors in tropical region over a wide dynamic range such as over ocean surface, desert targets, green vegetation etc.
In addition to SNO events at high latitude polar region, there exists SNO events between Suomi NPP VIIRS and AQUA MODIS at low latitudes, every 2-3 days apart but with larger time differences of more than 8 minutes.
The two sensors are compared at overlapping regions of extended SNO orbits at North African deserts whereas the inter-comparison is done at both the exact orbital intersection and extended overlapping orbits over ocean.
VIIRS moderate resolution channels (M-1 through M-8) are compared to MODIS equivalent channels to assess radiometric bias.
The observed bias is target dependent due to a) spectral differences of targets and b) differences in spectral response functions of matching VIIRS and MODIS channels c) BRDF due to time difference between MODIS and VIIRS observation, atmospheric variability etc. Expected bias due to spectral differences is quantified using instruments such as Hyperion and AVIRIS and using radiative transfer models such as MODTRAN
Stable earth targets such as Antarctica Dome C site is also used to evaluate the temporal radiometric calibration stability of VIIRS and verify the radiometric bias of VIIRS bands.
Radiometric Comparison between Suomi NPP VIIRS and AQUA MODIS using Extended Simultaneous Nadir Overpass in the Low LatitudesSirish UpretyaChangyong CaobSlawomir BlonskicXi ShaodaCIRA, Colorado State University, College Park, MD, bNOAA/NESDIS/STAR, College Park, MD, c, dCICS, University of Maryland, College Park, MD
Figure 4. Reflectance spectra at desert, ocean and Dome C along with VIIRS and MODIS matching RSRs
Figure 5.Residual Bias trends (linear trends from Table 2 and 3 after subtracting expected bias) left: Desert and right: Ocean
Figure 6. VIIRS and MODIS TOA reflectance time series at Antarctica Dome C
Table 3. VIIRS radiometric bias at Antarctica Dome C
Identify low latitude SNO events and collect VIIRS and MODIS data for extended exact and extended SNO orbits
Map VIIRS into MODIS grid using fast geospatial matching (GSM) algorithm
Convolve Hyperion/AVIRIS/MODTRAN reflectance with RSR of VIIRS and MODIS
Calculate Spectrally induced bias
Figure 3. VIIRS radiometric bias time series over Left: Desert; and Right: Ocean surface; for matching bands, Bias=(V-M)×100%/M
Table 1. VIIRS observed radiometric bias at ocean and desert
Figure 1. Flowchart: Computing observed bias using SNO-x and expected bias
 Cao, C, Weinreb, M, Xu, H: Predicting simultaneous nadir overpasses among polar-orbiting meteorological satellites for the intersatellite calibration of radiometers. Journal of Atmospheric Technology 21(4), 537-542 (2004)
 Heidinger, A. K., C. Cao, and J. T. Sullivan: Using Moderate Resolution Imaging Spectrometer (MODIS) to calibrate advanced very high resolution radiometer reflectance channels, Journal of Geophysical Research, vol. 107, no. 0, XXXX, doi:10.1029/2001JD002035. (2002)
 Cao, C., F. Deluccia, X. Xiong, R. Wolfe, and F. Weng: Early on orbit performance of VIIRS, submitted to TGRS, (2013).
 Uprety. S. and C. Cao: Radiometric and spectral characterization and comparison of the Antarctic Dome C and Sonoran Desert sites for the calibration and validation of visible and near-infrared radiometers, J. Appl. Remote Sens., (2012)
Table 2. VIIRS expected bias at ocean and desert
This study is partially funded by the Joint Polar Satellite System (JPSS) program.
Figure 2. b) Orbits showing Low latitude SNO events i) Extended SNOs to desert ii) SNOs over ocean
Figure 2. a) MODIS and b) VIIRS image mapped to MODIS latitude/longitude