1. Sirish Uprety a and Changyong Cao b a Perot Systems Government Services, Fairfax, VA, 22031 b NOAA/NESDIS/STAR, Camp Springs, MD, 20746 August 1, 2010

2. Objective Background Radiometric Test Sites Instruments used in the study Methodology Results and Discussion Radiometric Spectral Instrument Bias Summary Future Work 2

3. Stable earth targets are used as an independent source of radiometric calibration and validation of satellite instruments either with or without onboard calibration devices (AVHRR calibration by Rao & Chen 1993/1995/2002, Wu 2005, etc). These sites can be used to characterize the sensor degradation, validate the instrument performance over time, perform the inter-comparison between sensors etc. This study uses the Antarctic Dome C (-75.102°, 123.395°) and the Sonoran Desert (32.0°, -114.39°) sites. Dome C is studied and identified as one of the potential cal/val landnet sites by CEOS. Sonoran Desert is one of the most stable (Morstead & Helder, 2008) and largest desert site in North America that is visible from both POES and GOES/GOES-R. Several studies were done in the past using Sonoran desert for cal/val and inter-calibration of the environmental satellites (Rao & Chen 2003, Angal et. al. 2010, personal communication with Ping Jing etc). 3

4. To perform radiometric and spectral characterization and comparison of Dome C and Sonoran Desert over visible (0.64 µm) and near-infrared (0.86 µm) bands..  Analyze the radiometric stability using nearly 8 years of AQUA/MODIS data and use the METOP-A/AVHRR to quantify water vapor absorption variability.  Analyze the radiometric bias of AVHRR using MODIS.  Use EO-1/Hyperion to analyze spectral characteristics of the test sites and use measured reflectance along with MODIS and AVHRR SRF to model the radiometric bias between these instruments due to spectral differences. 4

5. Dome C Large homogenous permanent snow field in Antarctica at an altitude of 3.25 km Unique characteristics like, high altitude; high reflectance; > 75% of cloud-free time; low water vapor content; very cold and dry climate; low aerosol and dust etc. Since the orbits converge in polar region, POES instruments can view the site more frequently. Limitations include accessibility, availability of data only during austral summer, high BRDF, not visible by GOES/GOES-R instruments etc. Figure 1. Dome C Site 5

6. Sonoran Desert Largest desert site in north America and used more often by Landsat and GOES communities. Mild vegetation with precipitaion ranging from 50 to 125 mm/year. ROI (A, ~ 80 m altitude) has some sand dunes. Several studies in the past used other ROIs around yellow and blue marks for GOES and POES. (Rao et. al. 2003, Angal et. al. 2010 etc). The comparison performed over all marked areas revealed that the ROI (A) selected in this study has best temporal stability but less spatial uniformity compared to other 3 marked areas. ROI surface texture Figure 2. Sonoran Desert Site 6 A

7. All the instruments used in this study are polar orbiting and sun-synchronous. MODIS  Whiskbroom scanner launched aboard Terra(1999) and Aqua(2002) satellites.  36 spectral bands with wavelengths ranging from 0.4 um to 14.4 um.  1 st two bands (0.64 um and 0.86 um) are used in this study.  has onboard calibration system (solar diffuser) AVHRR  Also whiskbroom sensor launched aboard NOAA and MetOp series of meteorological satellites.  first 2 bands of the AVHRR closely match with the MODIS two reflective bands  solar reflective bands do not have onboard calibrators  Uses earth target, Libyan Desert (21°- 23° N, 28°- 29° E) for post launch on-orbit calibration HYPERION  Hyper-spectral sensor which images the earth in 220 spectral bands with wavelengths ranging from 0.4 µm to 2.5 µm.  Push broom sensor with footprint size of 30m.  The radiometric calibration is monitored using solar, lunar and in-flight calibration sources 7

8. 8 Collect L1B Data Radiometric Stability and Bias Remove BRDF Extract Reflectance Remove cloud contaminated pixels Select ROI 30*50 km for Dome C, 20*20 km for Sonora cloud mask for Sonoran, spatial uniformity test for Dome C nadir view data for each pixel over an ROI Warren-1998 for Dome C Sol. Zen. angle for Sonora MODIS/AVHRR over Dome C/Sonora Spectral characteristics and Bias MODIS/AVHRR Collect L1Gst Data Hyperion Extract Reflectance for ROI over Dome C/Sonora 3*3 km

9. Radiometric Stability at Dome C Extremely homogenous with spatial uniformity less than 1%. A periodic data gap exists (figures 3 a & b) The annual seasonal variation is greater than 5% and almost disappear after BRDF normalization (figure 3b). No significant trends and the radiometric stability over 8 years for MODIS channel 1 is 1.68% and that for channel 2 is 1.56% (similar to findings in Cao. et. al. 2010). AVHRR shows 1.91% stability for band 1 and 2.68% for band 2. Results and Discussion Figure 3. TOA reflectance time series at Dome C before and after BRDF normalization 9 Band 1 before BRDF norm. Band 1 after BRDF norm.

10. Seasonal variation ~ 2%. No significant trends and the radiometric stability of the site using MODIS is 2.01% and 1.91% for bands 1 and 2. The radiometric stability are comparable for both sites using MODIS channels ( < 2%). Data anomaly during the year 2005 due to very high precipitation over the site. Stability of AVHRR channel 1 is 2.06% and channel 2 is 6.26% (strong water vapor variability). Show residual periodic pattern after solar zenith angle normalization. Radiometric Stability at Sonoran Desert Figure 4. TOA reflectance time series at Sonoran Desert before and after SZA normalization 10 Band 1 before sol_zen norm. Band 1 after sol_zen norm. Band 2

11. Spectral Characterization Dome C is spectrally more flat over the VNIR band (0.4 - 0.9 um) whereas monotonously increasing for Sonoron Desert. Atmospheric water vapor absorption magnitude at around 0.94 µm is very small at Dome C compared to the Sonoran Desert Sonoran Desert and Dome C sites complement each other spectrally. 11 Figure 5. Hyperion spectral response

12. TOA reflectance time series for MODIS and AVHRR over Dome C and Sonoran Desert exhibits radiometric bias between the instruments (Figure 6). Bias exists due to differences in spectral response functions of instruments (figure 7), calibration traceability issues with AVHRR etc. Observed Bias ( Using MODIS and AVHRR observation)  (Instrument observation at 63 ⁰ for Dome C / at 30 ⁰ for Sonoran Desert), figure 6. Modeled (Theoretical) Bias (using figure 7)  S where Figure 7. Hyperion reflectance over Dome C along with MODIS and AVHRR SRFs. 12 Band 1 Band 2 Figure 6. Ref. vs. Sol. Zen.

13. The actual bias (observed – modeled) for AVHRR band 1 is -9.53% +/- 1.44% at Dome C and -8.53% +/- 0.56% at Sonoran Desert. For band 2, the difference is -1.45% +/- 1.9% at Dome C and -7.34% +/- 1.9% at Sonora. The bias difference at two sites are comparable for band 1 whereas the difference is significant for band 2 (~ -6%) indicating very high water vapor absorption variability at Sonoran Desert. The large difference in bias also indicates the complexities and challenges in accurately calibrating water vapor absorption bands. 13 Figure 8. Observed and modeled bias at Dome C and Sonora for channels 1 & 2.

14. The radiometric stability of VNIR bands using MODIS suggest both Dome C and Sonoran Desert to be equally useful to study sensor performance with uncertainty less than 2%. For AVHRR band 2, Sonoran Desert is highly unstable with uncertainty greater than 6% compared to nearly 2.5% at Dome C mainly due to large variability in atmospheric water vapor absorption. Dome C site is extremely homogenous with spatial uniformity less than 1% compared to more than 5% over Sonoran Desert. Bias of AVHRR is ~ -9% for band 1 at both sites primarily due to calibration traceability issues (similar to Cao. et. al. 2008). For band 2, the actual bias varies from ~ -1.45 % at Dome C to -7.5% at Sonoran Desert due to water vapor absorption variability. Dome C and Sonoran Desert complements each other spectrally and geo-location wise i.e. Dome C has accessibility issue (Antarctica) and visibility issue (only for POES). 14

15. Cloud screening over Dome C needs to be further investigated. BRDF model for Sonoran Desert needs to be developed. The large bias for band 1 (observed - modeled ~ 9 %) of AVHRR needs to be resolved. Analyze other vicarious calibration sites and compare the results. 15