Xi Shao

Comparison of the Calibration Algorithms and SI Traceability of MODIS, VIIRS, GOES, and GOES-R ABI Sensors

The radiometric calibration equations for the thermal emissive bands (TEB) and the reflective solar bands (RSB) measurements of the earth scenes by the polar satellite sensors, (Terra and Aqua) MODIS and Suomi NPP (VIIRS), and geostationary sensors, GOES Imager and the GOES-R Advanced Baseline Imager (ABI) are analyzed towards calibration algorithm harmonization on the basis of SI traceability which is one of the goals of the NOAA National Calibration Center (NCC). One of the overarching goals of NCC is to provide knowledge base on the NOAA operational satellite sensors and recommend best practices for achieving SI traceability for the radiance measurements on-orbit. As such, the calibration methodologies of these satellite optical sensors are reviewed in light of the recommended practice for radiometric calibration at the National Institute of Standards and Technology (NIST). The equivalence of some of the spectral bands in these sensors for their end products is presented. The operational and calibration features of the sensors for on-orbit observation of radiance are also compared in tabular form. This review is also to serve as a quick cross reference to researchers and analysts on how the observed signals from these sensors in space are converted to radiances.

Spectral Dependent Degradation of the Solar Diffuser on Suomi-NPP VIIRS Due to Surface Roughness-Induced Rayleigh Scattering

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard Suomi National Polar Orbiting Partnership (SNPP) uses a solar diffuser (SD) as its radiometric calibrator for the reflective solar band calibration. The SD is made of Spectralon™ (one type of fluoropolymer) and was chosen because of its controlled reflectance in the Visible/Near-Infrared/Shortwave-Infrared region and its near-Lambertian reflectance property. On-orbit changes in VIIRS SD reflectance as monitored by the Solar Diffuser Stability Monitor showed faster degradation of SD reflectance for 0.4 to 0.6 µm channels than the longer wavelength channels. Analysis of VIIRS SD reflectance data show that the spectral dependent degradation of SD reflectance in short wavelength can be explained with a SD Surface Roughness (length scale << wavelength) based Rayleigh Scattering (SRRS) model due to exposure to solar UV radiation and energetic particles. The characteristic length parameter of the SD surface roughness is derived from the long term reflectance data of the VIIRS SD and it changes at approximately the tens of nanometers level over the operational period of VIIRS. This estimated roughness length scale is consistent with the experimental result from radiation exposure of a fluoropolymer sample and validates the applicability of the Rayleigh scattering-based model. The model is also applicable to explaining the spectral dependent degradation of the SDs on other satellites. This novel approach allows us to better understand the physical processes of the SD degradation, and is complementary to previous mathematics based models.

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.

Radiometric calibration of DMSP-OLS sensor using VIIRS day/night band

Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) has been collecting global night light imaging data for more than 40 years. With the launch of Suomi-NPP satellite in 2011, the Day/Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) represents a major advancement in night time imaging capabilities because it surpasses DMSP-OLS in having broader radiometric measurement range, more accurate radiometric calibration, finer spatial resolution, and better geometric quality. DMSP-OLS sensor does not have on-board calibration and data is recorded as digital number (DN). Therefore, VIIRS-DNB provides opportunities to perform quantitative radiometric calibration of DMSP-OLS sensor. In this paper, vicarious radiometric calibration of DMSP-OLS at night under lunar illumination is performed. Events were selected when satellite flies above Dome C in Antarctic at night and the moon illuminates the site with lunar phase being more than quarter moon. Additional event selection criteria to limit solar and lunar zenith angle range have been applied to ensure no influence of stray light effects and adequate lunar illumination. The data from DMSP-OLS and VIIRS-DNB were analyzed to derive the characteristic radiance or DN for the region of interest. The scaling coefficient for converting DMSP-OLS DN values into radiance is determined to optimally merge the observation of DMSP-OLS into VIIRS-DNB radiance data as a function of lunar phases. Calibrating the nighttime light data collected by the DMSP-OLS sensors into radiance unit can enable applications of using both sensor data and advance the applications of night time imagery data.

Evaluation of Himawari-8 AHI geospatial calibration accuracy using SNPP VIIRS SNO data

Japan Meteorological Agency (JMA) Himawari-8 Advanced Himawari Imager (AHI) is the first in the series of next-generation geostationary (GEO) weather instruments. It has 16 spectral solar reflective and emissive bands located in three focal plane modules (FPM): one visible and near-infrared (VNIR) FPM, one midwave infrared (MWIR) FPM, and one longwave infrared (LWIR) FPM. All the AHI bands are geo-spatially calibrated to provide continuous environmental measurements from the Asian-Pacific area for the weather forecasting, disaster monitoring, and longterm climatic change studies. This study is to evaluate the navigation and co-registration accuracies of three AHI bands at the three FPMs using the simultaneous nadir observations (SNO) of Suomi National Polar-orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) I-band images. The preliminary results showed that the mean navigation difference between the two instruments is within 0.5 AHI pixels at 2km spatial resolution. The AHI band-to-band co-registration (BBR) difference between the two thermal bands is generally less than 0.2 pixels, better than the BBR between the VNIR and IR bands which ranges between 0.2 to 0.7 pixels during the study period.

Initial design and performance of the near surface unmanned aircraft system sensor suite in support of the GOES-R field campaign

One of the main objectives of the Geostationary Operational Environmental Satellite R-Series (GOES-R) field campaign is to validate the SI traceability of the Advanced Baseline Imager. The campaign plans include a feasibility demonstration study for new near surface unmanned aircraft system (UAS) measurement capability that is being developed to meet the challenges of validating geostationary sensors. We report our progress in developing our initial systems by presenting the design and preliminary characterization results of the sensor suite. The design takes advantage of off-the-shelf technologies and fiber-based optical components to make hemispheric directional measurements from a UAS. The characterization results -- including laboratory measurements of temperature effects and polarization sensitivity -- are used to refine the radiometric uncertainty budget towards meeting the validation objectives for the campaign. These systems will foster improved validation capabilities for the GOES-R field campaign and other next generation satellite systems.

Overview of Suomi NPP VIIRS performance in the last 2.5 years

Since the successful launch of the Suomi NPP on October 28, 2011, the VIIRS instrument has performed well in general. This paper provides an overview of the evolution of the VIIRS instrument performance, major events experienced in the nearly three years since launch, and the ground processing system changes to account for various effects and discrepancies. The mirror degradation in the near-infrared bands due to prelaunch mirror contamination has been gradually leveling off, although the degradation in the solar diffuser continues. In the ground processing, many changes have been implemented in the operational code. This includes the stray-light correction for the Day/Night band, the automatic calibration for the reflective solar band, and corrections for several errors in the code, and resolving various discrepancies in the calibration equations and coefficients. The scientific community is generally satisfied with the quality of the VIIRS SDR data. However, there are remaining issues to be resolved through further research and development. These issues include meeting the more stringent requirement and desire for ocean color applications, better understanding of the polarization effects especially off-nadir, understanding and resolving inconsistencies between solar and lunar calibration. The Suomi NPP VIIRS SDR has been used for generating a variety of products with great success by worldwide users. Together with the follow-on instruments J1 and J2, VIIRS will be the primary data source for moderate resolution satellite observations in the next decades.

Stability Monitoring of the VIIRS Day/Night Band over Dome C with a Lunar Irradiance Model and BRDF Correction

The unique feature of the Visible Infrared Imager Radiometer Suite (VIIRS) day/night band (DNB) is its ability to take quantitative measurements of low-light scenes at night. In order to monitor the stability of the high gain stage (HGS) of the DNB, nighttime observations over the Dome C site under moonlight are analyzed in this study. The Miller and Turner 2009 (MT2009) lunar irradiance model has been used to simulate lunar illumination over Dome C. However, the MT2009 model does not differentiate the waxing and waning lunar phases. In this paper, the MT-SWC (SeaWiFS Corrected) lunar irradiance model differentiating the waxing and waning lunar phases is derived by correcting the MT2009 model using lunar observations made by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS). In addition, a top of atmosphere (TOA) bi-directional reflectance distribution function (BRDF) model during nighttime over Dome C is developed to remove the angular dependence from the nighttime TOA reflectance. The long-term stability monitoring of the DNB high-gain stage (HGS) reveals a lower reflectance factor in 2012 in comparison to the following years, which can be traced back to the change in relative spectral response (RSR) of National Oceanic & Atmospheric Administration’s (NOAA’s) Interface Data Processing Segment (IDPS) VIIRS DNB in April 2013. It also shows the radiometric stability of DNB data, with long-term stability of less than 1.58% over the periods from 2013 to 2016. This method can be used to monitor the radiometric stability of other low-light observing sensors using vicarious calibration sites under moonlight illumination.

Validation of GOES-16 ABI infrared spatial response uniformity.

GOES-16, the first new generation of NOAA’s geostationary satellite, was launched on November 19, 2016. The Advanced Baseline Imager (ABI) is the key payload of the mission. The instrument performance and satellite intercalibration results show that infrared (IR) radiances are well calibrated and very stable. Yet during its early post-launch tests (PLT) and post-launch product tests (PLPT) period, several calibration anomalies were identified with the IR bands: 1) the IR measurements of the Continental United States (CONUS) and mesoscale (MESO) images demonstrated an artificial periodicity of 15 minutes - Periodic Infrared Calibration Anomaly (PICA), in line with the Mode-3 timeline; and 2) the calibration coefficients displayed small discontinuities twice a day around satellite noon and midnight, which resulted in slight detectable diurnal calibration variations. This work is to report our investigation to the root causes of these anomalies, validation of the anomaly corrections, and assessment of the impacts of the corrections on the radiance quality. By examining the radiometrically calibrated space-swath radiance collected from the moon chasing events, it was found that these anomalies were attributed to the residuals of the spatial uniformity corrections for the scan mirrors. A new set of scan mirror emissivity correction Look-Up Tables (LUTs) were later delivered by the Vendor and implemented operationally. Further analyses showed that the new emissivity LUTs significantly reduced the periodic radiometric variation and diurnal variations. The same method will be applied to validate the IR spatial uniformity for the future GOES-R series ABI instruments.

Characterization of Himawari-8 AHI 3.9-um channel stray light

The Advanced Himawari Imager (AHI) is the primary instrument aboard Himawari-8 and has 16 multispectral channels, including six visible and near infrared and 10 thermal emissive bands. The 3.9-μm channel imagery of AHI has spatial resolution of 2 km and performs routine full-disk imaging every 10 minutes. There have been stray light observed in the full disk imagery of the AHI 3.9-μm channel over a few weeks around February and October-November when the line of sight of the sun is at ~10 to ~20 degrees south of the nadir of the Himawari-8. In this paper, difference data between consecutive AHI 3.9-μm images have been processed to quantitatively characterize and monitor the AHI stray light. Stray light indices are also developed to trend the occurrence, position and magnitude of the stray light in the AHI 3.9- μm imageries. It is also found that the stray light is the greatest in the AHI 3.9-μm band but also is detectable in other Mid-Wavelength IR channels. Analysis of the ratio of stray light magnitude between AHI 3.9-μm and 6.2-μm band indicates that it is consistent with the ratio of solar radiance for these two bands. This suggests that the stray light is mainly due to direct illumination of the attenuated solar radiation on the AHI detector rather than from onboard thermal body emission due to heating. The upcoming Advanced Baseline Imager (ABI) onboard the GOES-R satellite has very similar spectral and spatial characteristics as AHI. Therefore, characterizing the stray light in the 3.9-μm channel of AHI helps support post-launch calibration activities of ABI.