Sirish Uprety

Radiometric Intercomparison between Suomi-NPP VIIRS and Aqua MODIS Reflective Solar Bands Using Simultaneous Nadir Overpass in the Low Latitudes

AbstractOn-orbit radiometric performance of the Suomi National Polar-Orbiting Partnership (Suomi-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) is studied using the extended simultaneous nadir overpass (SNO-x) approach. Unlike the traditional SNO analysis of data in the high latitudes, this study extends the analysis to the low latitudes—in particular, over desert and ocean sites with relatively stable and homogeneous radiometric properties—for intersatellite comparisons. This approach utilizes a pixel-by-pixel match with an efficient geospatial matching algorithm to map VIIRS data into the Moderate Resolution Imaging Spectroradiometer (MODIS). VIIRS moderate-resolution bands M-1 through M-8 are compared with Aqua MODIS equivalent bands to quantify radiometric bias over the North African desert and over the ocean. Biases exist between VIIRS and MODIS in several bands, primarily because of spectral differences as well as possible calibration uncertainties, residual cloud contamination, and bidirect...

Establishing the Antarctic Dome C community reference standard site towards consistent measurements from Earth observation satellites

Establishing satellite measurement consistency by using common desert sites has become increasingly more important not only for climate change detection but also for quantitative retrievals of geophysical variables in satellite applications. Using the Antarctic Dome C site (75°06′S, 123°21′E, elevation 3.2 km) for satellite radiometric calibration and validation (Cal/Val) is of great interest owing to its unique location and characteristics. The site surface is covered with uniformly distributed permanent snow, and the atmospheric effect is small and relatively constant. In this study, the long-term stability and spectral characteristics of this site are evaluated using well-calibrated satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Preliminary results show that despite a few limitations, the site in general is stable in the long term, the bidirectional reflectance distribution function (BRDF) model works well, and the site is most suitable for the Cal/Val of reflective solar bands in the 0.4–1.0 µm range. It was found that for the past decade, the reflectivity change of the site is within 1.35% at 0.64 µm, and interannual variability is within 2%. The site is able to resolve calibration biases between instruments at a level of ∼1%. The usefulness of the site is demonstrated by comparing observations from seven satellite instruments involving four space agencies, including OrbView-2–SeaWiFS, Terra–Aqua MODIS, Earth Observing 1 (EO-1) – Hyperion, Meteorological Operational satellite programme (MetOp) – Advanced Very High Resolution Radiometer (AVHRR), Envisat Medium Resolution Imaging Spectrometer (MERIS) – dvanced Along-Track Scanning Radiometer (AATSR), and Landsat 7 Enhanced Thematic Mapper Plus (ETM+). Dome C is a promising candidate site for climate quality calibration of satellite radiometers towards more consistent satellite measurements, as part of the framework for climate change detection and data quality assurance for the Global Earth Observation System of Systems (GEOSS).

Detecting Light Outages After Severe Storms Using the S-NPP/VIIRS Day/Night Band Radiances

Power outages after a major storm affect the lives of millions of people and cause massive light outages. The launch of the Suomi National Polar-orbiting Partnership satellite with the Visible Infrared Imaging Radiometer Suite (VIIRS) significantly enhances our capability to monitor and detect light outages with the well-calibrated day/night band (DNB) and to use light loss signatures as indication of regional power outages. This study explores the use of the DNB in quantifying light outages due to the derecho storm in the Washington DC metropolitan area in June 2012 and Hurricane Sandy at the end of October 2012 on the East Coast of U.S. The results show that the DNB data are very useful in detecting power outages by quantifying light loss, but it also has some challenges due to clouds, lunar illumination, and straylight effect. Comparison of light outage and recovery trend determined from DNB data with power company survey shows reasonable agreement, demonstrating the usefulness of DNB in independently verifying and complementing the statistics from power companies.

Vicarious calibration of S-NPP/VIIRS day-night band

The Day Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite provides imagery of clouds and other Earth features over illumination levels ranging from full sunlight to quarter moon. In order to cover this extremely broad measurement range, the DNB employs four imaging arrays that comprise three gain stages. The low gain stage (LGS) gain values are determined by solar diffuser data. In operation, the medium and high gain stage values are determined by multiplying the LGS gains by the medium gain stage (MGS)/LGS and high gain stage (HGS)/LGS gain ratios, respectively. This paper demonstrates a scheme of using DNB observation of ground vicarious sites under lunar illumination at night to independently verify the radiometric accuracy of HGS of DNB. We performed vicarious calibration of DNB when S-NPP flies above the vicarious site such as Dome C in Antarctic and Greenland in northern hemisphere at night and the moon illuminates the site with lunar phase being more than half moon. Lunar spectral irradiance model as a function of Sun-Earth-Moon distances and lunar phase is used to assist the determination of top-of-atmosphere reflectance at the vicarious site. Analysis of the vicariously-derived reflectance from DNB observations show agreement with the reflectance derived from Hyperion observations of the vicarious sites.

Using the Dome C site to characterize AVHRR near-infrared channel for consistent radiometric calibration

AVHRR is a heritage instrument on NOAAs polar orbiting satellites with more than 30 years of global earth observation. Due to absence of onboard calibrator for the visible and near-infrared channels, AVHRR sensors rely on desert sites for relative calibration with uncertainties primarily due to lack of rigorous site characterization and atmospheric effects. This study aims at quantifying the long term degradation of the near-infrared channel (0.86 μm) of AVHRR using the Antarctic Dome C site which has very small atmospheric effects. All afternoon-orbit NOAA series AVHRR instruments are included in this study. Though the TOA reflectance data exists only during austral summer for Dome C, the degradation estimated using TOA reflectance time series for the respective instruments is comparable to those from the previous studies. The degradation estimated suggests that NOAA-7 and -9 have the largest calibration drift (greater than -3% per year) compared to the other instruments which have less than -1.5% drift per year. The AVHRR channel 2 (0.86 μm) calibration using desert sites has always been challenging due to high uncertainty mainly introduced by the presence of water vapor absorption at this wavelength. The study shows that, due to the extremely cold and dry climate of Dome C, the water vapor absorption effect is negligible and thus it is possible to calibrate nearinfrared channel (0.86 μm) with calibration uncertainty less than 1%.

A comparison of the Antarctic Dome C and Sonoran Desert sites for the cal/val of visible and near infrared radiometers

Stable earth sites are essential for comparing the measurements from different satellite instruments in the visible and near-infrared in order to maintain the consistency of radiometric calibration, and for quantifying the sensor degradation over time. This study focuses on the radiometric and spectral characterization and comparison between two potential calibration sites, Dome C and Sonoran Desert. The long-term TOA reflectance time series analysis using MODIS observation shows that the radiometric stability of Dome C and Sonoran Desert is better than 2% over the period of 8 years. The study also shows that Dome C is much affected by seasonal variation due to bi-directional reflection, compared to the Sonoran Desert, although the BRDF normalization reduced the uncertainty of Dome C observations to less than 2% for both the visible and NIR band. For AVHRR band 2, at Sonoran Desert, a large variability (>6%) is observed compared to that of MODIS (<2%) due to water vapor absorption. The spectral characteristics of these sites studied using EO-1 Hyperion sensor further show the water vapor absorption differences at the two sites. Nevertheless, the operationally calibrated AVHRR TOA reflectance at both Dome C and Sonoran Desert are significantly lower than that of MODIS, primarily due to calibration traceability issues. The study suggests that, both Dome C and Sonoran Desert sites can be used for postlaunch calibration/validation of the visible/near-infrared bands with uncertainty less than 2% excluding channels affected by water vapor.

VIIRS reflective solar bands calibration changes and potential impacts on ocean color applications

The VIIRS (Visible-Infrared Imaging Radiometer Suite) instrument onboard the Suomi NPP (National Polar-orbiting Partnership) spacecraft started acquiring Earth observations in November 2011. Since then, radiometric calibration applied to the VIIRS RSB (Reflective Solar Band) measurements for the SDR (Sensor Data Record) production has been improved several times. In this paper, timeline of the main upgrades to the calibration software and parameters is compared with the changes of the radiometric coefficients applied in the operational production of the VIIRS SDR. Initially, radiometric calibration coefficients were updated once per week to correct for the responsivity degradation that occurs for some of the sensor’s spectral bands due to contamination of the VIIRS telescope’s mirrors. Despite the frequent updates, discontinuities in the radiometric calibration could still affect ocean color time series. In August 2012, magnitude of the radiometric coefficient changes was greatly reduced by implementing a procedure that predicts (about a week ahead) values of the calibration coefficients for each Earth scan until a subsequent update. The updates have been continued with the weekly frequency, and the coefficient prediction errors were monitored by comparisons with the initial invariant coefficients from the following week. The predicted coefficients were also compared with the coefficients derived once per orbit from the onboard solar diffuser measurements by an automated procedure implemented in the VIIRS data operational processing software. The paper evaluates the changes in the VIIRS RSB coefficient updates for bands M1 to M7 and potential impacts of these changes on ocean color applications.

On-orbit radiometric performance characterization of S-NPP VIIRS reflective solar bands

It has been nearly four years that the S-NPP was launched. In an effort to improve the VIIRS calibration, VIIRS has undergone a number of major look up table updates during this period. RSB bands such as M1 through M3 suggested higher solar diffuser degradation rate. Similarly, for higher wavelengths, even though the solar diffuser degradation is much smaller and even negligible for SWIR bands, bands such as M7 suffer from major degradation due to RTA throughput degradation. Even though the solar diffuser and mirror degradation is well characterized, the data quality needs to be independently validated to ensure that data are well within the specification. We have used on-orbit calibration/validation techniques such as extended SNOs to estimate the bias of these bands and quantify the radiometric performance since launch. Assuming MODIS as a standard reference, intercomparison was performed to analyze the VIIRS radiometric performance. It was observed that some of the VIIRS bands such as M5 and M7 suggest bias on the order of 1.5% or more for most of the time period since early launch. VIIRS bias trends keep changing over time which can be mainly correlated to calibration updates and instrument anomalies. Results on VIIRS on-orbit calibration performance and its bias since early launch will be presented during meeting to help users better understand the data quality and its impacts on broader scientific research and applications.

Comparing Hyperion Lunar Observation with model calculations in support of GOES-R Advanced Baseline Imager (ABI) calibration

Radiometric stability of the lunar surface and its smooth reflectance spectrum makes the moon an attractive candidate for calibrating satellite-based hyper/multi-band visible and infrared imagers. Long-term performance monitoring of satellite instrument using Moon can reveal the degradation of instruments. In this paper, analysis of Hyperion lunar observations and comparison with lunar model are performed in support of Cal/Val activities for satellite photometric imager such as GOES-R Advanced Baseline Imager (ABI) instrument. Hyperion makes hyper-spectral observations of the moon regularly with moon phase mostly at 7 degree and it covers visible and shirt-wavelength infrared (SWIR) channels with 10 nm spectral resolution. Five Hyperion lunar observations are analyzed. Lunar reflectance is derived from Hyperion observation and the mean absolute lunar spectral reflectance difference between Hyperion derivation and lunar model is 4.0 ± 2.62%. Through reflectance comparison, over-compensation of two strong atmospheric water absorption bands in Hyperion calibration is identified. The radiometric variance and degradation of Hyperion are assessed. To support the calibration of GOES-R ABI, hyper-spectral data of Hyperion lunar observation is convoluted with ABI spectral response functions for reflective solar bands to synthesize predicted lunar images to be observed by ABI. Lunar irradiances are derived from these synthesized lunar images for ABI and compared with lunar model predictions to quantify spectral biases. Long-term lunar imaging window of opportunities for GOES-R ABI are also assessed. The ability of using lunar model and Hyperion observation to calibrate satellite VNIR/SWIR sensors and reduce the measurement uncertainties is essential to support post-launch Cal/Val activities of GOES-R ABI.

Developing an automated global validation site time series system for VIIRS

Long-term top of atmosphere (TOA) reflectance, brightness temperature, and band ratio time series over well-established validation sites provide important information for post-launch calibration stability monitoring and analysis. In this study, we present an automated and highly extensible global validation site time series system for the Visible and Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite. The system includes 30 globally distributed sites. Validation site database and quality control parameters for each site can be easily modified without software code modification. VIIRS sensor data records over each validation site are automatically identified from NPP product archives. Validation time series plots for all VIIRS Reflective Solar Bands and Thermal Emissive Bands are updated daily and available online. Bands I2/M7 and I3/M10 inter-channel consistency analysis using the validation time series developed in this study is also included.