Fangfang Yu

The Global Space-Based Inter-Calibration System

The Global Space-based Inter-Calibration System (GSICS) is a new international program to assure the comparability of satellite measurements taken at different times and locations by different instruments operated by different satellite agencies. Sponsored by the World Meteorological Organization and the Coordination Group for Meteorological Satellites, GSICS will intercalibrate the instruments of the international constellation of operational low-earth-orbiting (LEO) and geostationary earth-orbiting (GEO) environmental satellites and tie these to common reference standards. The intercomparability of the observations will result in more accurate measurements for assimilation in numerical weather prediction models, construction of more reliable climate data records, and progress toward achieving the societal goals of the Global Earth Observation System of Systems. GSICS includes globally coordinated activities for prelaunch instrument characterization, onboard routine calibration, sensor intercomparison of...

Diurnal and Scan Angle Variations in the Calibration of GOES Imager Infrared Channels

The current Geostationary Operational Environmental Satellite (GOES) Imager infrared (IR) channels experience a midnight effect that can result in erroneous instrument responsivity around satellite midnight. An empirical method named the Midnight Blackbody Calibration Correction (MBCC) was developed and implemented in the GOES Imager IR operational calibration, aiming to correct the midnight calibration errors. The main objective of this study is to evaluate the MBCC performance for the GOES-11/-12 Imager IR channels by examining the diurnal variation of the mean brightness temperature (Tb) bias with respect to reference instruments. Two well-calibrated hyperspectral radiometers on low Earth orbits (LEOs), the Atmospheric Infrared Sounder on the Aqua satellite and the Infrared Atmospheric Sounding Interferometer (IASI) on the Metop-A satellite, are used as the reference instruments in this study. However, as the timing of the collocated geostationary-LEO intercalibration data is related to the GOES scan angle, it is then necessary to assess the GOES scan angle calibration variations, which becomes the second objective of this study. Our results show that the applications and performance of the MBCC method varies greatly between the different channels and different times. While it is usually applied with high frequency for about 8 h around satellite midnight for the short-wave channels (Ch2), it may only be intensively used right after satellite midnight or even barely used for the other IR channels. The MBCC method, if applied with high frequency, can reduce the mean day/night calibration difference to less than 0.15 K in almost all the GOES IR channels studied in this paper except for Ch4 (10.7 μm). The uncertainty of the nighttime GOES and IASI Tb difference for different scan angles is less than 0.1 K in each IR channel, indicating that there is no apparent systematic variation with the scan angle, and therefore, the estimated diurnal cycles of GOES Imager calibration is not prone to the systematic effects due to scan angle.

Intercalibration of GOES Imager visible channels over the Sonoran Desert

The Geostationary Operational Environmental Satellites (GOES) have been observing the Western Hemisphere since the late 1970s, providing valuable information for weather forecast and climate change studies. Due to the lack of an onboard calibration device for the visible channel, accurate reflectance of the visible channel data depends on vicarious calibration methods to provide postlaunch calibration coefficients to compensate for the degraded responsivity. In this study, the Sonoran Desert, which can be viewed by both GOES-East and GOES-West satellites, is used to intercalibrate the visible channels on board the three-axis stabilized GOES satellite Imagers traceable to the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 (C6) calibration standard. It was found that when the anomalous reflectance in 2004 and 2005 are excluded, the Sonoran Desert is radiometrically, spatially, and spectrally stable at the GOES viewing geometries and thus can be considered as a pseudo-invariant calibration site to develop long-term GOES Imager visible data set. To characterize the desert target reflectance with the MODIS data, GOES observations over 1 year period are used to convert the MODIS reflectance to the GOES viewing and solar illumination geometries. The spectral band adjustment factor for each GOES Imager visible channel is generated with a set of clear-sky Hyperion measurements. A trending algorithm, which consists of a polynomial function for the description of instrument degradation performance and two sine terms for the impacts of the seasonal variations of the solar zenith angle and atmospheric components, is applied to fit the time series of prelaunch calibrated reflectance. The combined calibration uncertainty of the desert calibration method is less than 4% at the Aqua MODIS C6 calibration standard. The difference of the postlaunch calibration coefficients between the desert calibration and the current GOES visible operational calibration methods is mainly within 5%.

On-orbit characterization of the GOES Imager channel-to-channel co-registration

The channel-to-channel co-registration of a satellite imaging system is an important performance metric that has a direct impact upon the reliability of the imager’s quantitatively-derived products. In this work, standard full-disk image data is used to measure the on-orbit channel-to-channel co-registration of the infrared channels of several GOES Imagers at a sub-pixel level. This is accomplished with two separate methods, one of which furthers preliminary research by Wu et. al.1 using GOES-8 and 9 spatial-spectral brightness temperature gradients, the other of which uses a statistical approach. The diurnal, seasonal, and long-term co-registration behavior is analyzed.

Assessment of MetOp-A Advanced Very High Resolution Radiometer AVHRR short-wave infrared channel measurements using Infrared Atmospheric Sounding Interferometer IASI observations and line-by-line radiative transfer model simulations

MetOp-A satellite-based hyper-spectral Infrared Atmospheric Sounding Interferometer (IASI) observations are used to evaluate the accuracy of the broadband short-wave infrared (SWIR) atmospheric window channel (channel 3B) centred at 3.74 μm of the Advanced Very High Resolution Radiometer (AVHRR) carried on the same platform. To complement the partial spectral coverage of IASI, line-by-line radiative transfer model (LBLRTM)-simulated IASI spectra are used. The comparisons result in significant negative AVHRR minus IASI bias in radiance (∼–0.04 mW m–2 sr–1 cm–1) with scene temperature dependency in which the absolute value of the bias linearly increases with increasing temperature. It is demonstrated that the negative bias and the scene temperature dependency of the bias are the results of significant absorption in the portion of AVHRR spectral band not seen by IASI, leading to the conclusion that MetOp-A AVHRR channel 3B is not purely an ‘atmospheric window’ channel.

Recent operational status of GSICS GEO-LEO and GEO-GEO Inter-Calibrations at NOAA/NESDIS

The Global Space-based Inter-Calibration System (GSICS), initiated by World Meteorological Organization (WMO) and Coordination Group for Meteorological Satellites (CGMS) in 2005, is an international collaborative effect with the mission to produce consistent and accurate measurements from the constellation of operational meteorological satellites by inter-calibration between a variety of different instruments. NOAA GSICS Processing and Research Center (GPRC), located at NOAA/NESDIS, has been conducted the GSICS-related processing and research to ensure the quality of radiance measured with NOAA satellites, as well as the international satellite instruments. In this paper, we summarized most recent activities conducted at NOAA GSICS GPRC in monitoring and improving the Geostationary (GEO) infrared (IR) calibration accuracy, especially the GOES Imager and Sounder instruments.

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.

In-orbit response versus scan-angle (RVS) validation for the GOES-16 ABI solar reflective bands.

The weather instrument of Advanced Baseline Imager (ABI) is the mission critical instrument on-board the GOES-16 satellite. Compared to the predecessor GOES Imager, GOES-16 ABI has many new advanced technical devices and algorithms to improve the data quality, including the double scan-mirror system. To validate the in-orbit response versus scan-angle (RVS), the Moon is used as a reference target for this purpose. During the post-launch test (PLT) and post-launch product test (PLPT) period, a series of special scans were conducted to chase and collect the lunar images at optimal phase angle range when it transited across the space within the ABI Field of Regard (FOR) from West to East. Analyses of the chasing events above and below the Earth indicated that the RVS variations at the East-West (EW) direction are generally less than 1% for all the six solar reflective bands. Same method is being applied to validate the GOES-17 ABI spatial uniformity for the visible and near-infrared (VNIR) bands.

Radiometric quality assessment of GOES-16 ABI L1b images.

The Advanced Baseline Imager (ABI) onboard NOAA’s GOES-16 satellite has been operational as GOES-East since December 18th, 2017. It is a multi-channel passive imaging radiometer with 16 spectral bands covering the visible, near infrared and infrared (IR) spectra, to captured variable area imagery and radiometric information of the Earth’s surface, atmosphere and cloud cover. The Level 1B (L1b) radiance images of these channels are geometrically and radiometrically corrected to provide high quality input data to the user communities. Three series of tests are undertaken to validate the product maturity levels: Post-launch Test (PLT), Post-launch Product Test (PLPT) and Extended Validation (EV). Engineering-focused metrics reflecting the radiometric quality of ABI L1b radiance image are assessed in these tests, such as signal-to-noise ratio (SNR)/noise-equivalent-differential temperature (NEdT), background coherent noise pattern, detector dynamic range, detector linearity, etc. Direct Earth view image analysis using image processing tool such as Fourier transform can also reveal information about its quality. In this presentation, initial results of selected PLPTs undertaken by GOES-R Calibration Working Group (CWG) are provided with the focus for IR bands. The results show that the general criterion for product maturity have been largely met. Occasional artifacts still existing at smaller scale are reported. There has been continuous effort to monitor, analyze and resolve these artifacts to further improve the L1b image quality.

Validation of GOES-16 ABI reflective solar band calibration through reanalysis and comparison with field campaign data.

The Advanced Baseline Imager (ABI) is a critical instrument onboard GOES-16 which provides high quality Reflective Solar Bands (RSB) data though radiometric calibration using onboard solar diffuser. Intensive field campaign for post-launch validation of the ABI L1B spectral radiance observations was carried out during March-May, 2017 to ensure the SI traceability of ABI. In this paper, radiometric calibrations of the five RSBs of ABI are evaluated with the measurements by Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG) onboard the high-altitude aircraft ER2. The ABI MESO data processed by the vendor with ray-matching to AVIRIS-NG during the field campaign was compared with the AVIRIS-NG measurements for radiometric bias evaluation. Furthermore, there were several implementations and updates in the solar calibration of ABI RSBs which resulted in different versions of detector gains and nonlinear calibration factors. These calibrations included the calibration by the operational ground processing system, by vendor and the calibration with updated nonlinear calibration factor table for striping mitigation and accounting for the integration time difference between solar calibration and Earth view. The North-South Scan (NSS) field campaign data of ABI were re-processed with these calibration coefficients to quantitatively evaluate the detector uniformity change. The detector uniformity difference are traced back to the difference in the implementation of the solar calibration.