MODIS Characterization Support Team Research Articles

Terra and Aqua MODIS collection 7 level 1B algorithm

Moderate resolution imaging spectroradiometer (MODIS) continues to be an important instrument for NASA’s earth observing system. Terra and Aqua MODIS have produced more than 22 and 20 years of global datasets that have significantly helped scientists better understand the Earth’s systems, respectively. The MODIS level-1B (L1B) algorithms use the uncalibrated, geolocated Earth scene observations as input and convert the instrument response into calibrated reflectance and radiance, which are used to generate the downstream science products. The sustained calibration and characterization activities undertaken by the MODIS characterization support team have resulted in several upgrades to the L1B algorithms to maintain accurate calibration in the data products. We present an overview of the L1B algorithm designated as Collection 7. Various algorithm enhancements both in the reflective bands and thermal bands characterization are currently under science testing and evaluation. Once applied in data processing (projected in early 2023), they are expected to manifest in improved science products, both in terms of radiometric accuracy and long-term stability.

On-Orbit Calibration of Terra MODIS VIS Bands Using Polarization-Corrected Desert Observations

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra spacecraft is completing two decades of successful Earth observations, providing the scientific community with numerous products and supporting applications, such as land surface cover, sea surface temperature, aerosol properties, and vegetation. The 20 reflective solar bands (RSBs) cover a wavelength range from 0.41 to 2.1 $\mu \text{m}$ and are calibrated primarily using a solar diffuser (SD), with lunar measurements and Earth-view (EV) response trends from desert sites used for the response versus scan-angle (RVS) characterization. Prelaunch analysis showed that a few short-wavelength RSBs of Terra MODIS are particularly sensitive to the polarization of the incident light, with on-orbit results further indicating that the polarization sensitivity has experienced changes that are wavelength, mirror side (MS), and scan-angle dependent. Although the primary calibrator, the SD, should provide an unpolarized light, supplemental inputs from the EV response trends used in the RVS characterization are impacted by the Earth scene polarization. The EV trends, uncorrected for polarization, pose a significant challenge in the RVS characterization for the short wavelength bands (3, 8, 9, and 10), therefore impacting the long-term trends and uncertainty characterization. Previous studies from the MODIS Characterization Support Team (MCST) and the National Aeronautics and Space Administration (NASA) Ocean Biology Processing Group (OBPG) have independently estimated the polarization correction to be applied on the calibrated products to mitigate the impacts to some extent. In this article, the RVS is characterized using the EV response trends after correcting for the polarization effects. Results indicate a significant improvement in the long-term trending, reduced uncertainties in the forward prediction, and a more accurate per-pixel uncertainty provided by the uncertainty index (UI) in the MODIS L1B. This enhanced approach will, therefore, be considered for implementation in the future L1B reprocessing efforts. The enhanced approach also vastly simplifies the process of adding long-term trend corrections on top of the MODIS L1B, which should streamline the science data production in the future.

Optimization of a Deep Convective Cloud Technique in Evaluating the Long-Term Radiometric Stability of MODIS Reflective Solar Bands

MODIS reflective solar bands are calibrated on-orbit using a solar diffuser and near-monthly lunar observations. To monitor the performance and effectiveness of the on-orbit calibrations, pseudo-invariant targets such as deep convective clouds (DCCs), Libya-4, and Dome-C are used to track the long-term stability of MODIS Level 1B product. However, the current MODIS operational DCC technique (DCCT) simply uses the criteria set for the 0.65-µm band. We optimize several critical DCCT parameters including the 11-µm IR-band Brightness Temperature (BT11) threshold for DCC identification, DCC core size and uniformity to help locate DCCs at convection centers, data collection time interval, and probability distribution function (PDF) bin increment for each channel. The mode reflectances corresponding to the PDF peaks are utilized as the DCC reflectances. Results show that the BT11 threshold and time interval are most critical for the Short Wave Infrared (SWIR) bands. The Bidirectional Reflectance Distribution Function model is most effective in reducing the DCC anisotropy for the visible channels. The uniformity filters and PDF bin size have minimal impacts on the visible channels and a larger impact on the SWIR bands. The newly optimized DCCT will be used for future evaluation of MODIS on-orbit calibration by MODIS Characterization Support Team.

Alternative method of on-orbit response-versus-scan-angle characterization for MODIS reflective solar bands

The moderate resolution imaging spectroradiometer (MODIS) has 20 reflective solar bands (RSB), covering a spectral range from 0.41 to 2.2 μm, which are calibrated on-orbit using its onboard calibrators, which include a solar diffuser, a solar diffuser stability monitor, and a spectroradiometric calibration assembly. A space view (SV) port is used to provide a background reference and also facilitates near-monthly lunar observations through a spacecraft roll. In every scan, the Earth’s surface, SV, and onboard calibrators are viewed via a two-sided scan mirror, the reflectance of which depends on the angle of incidence (AOI) as well as the wavelength of the incident light. Response-versus-scan-angle (RVS) is defined as a dependence function of the scan mirror’s reflectance over AOI. An initial RVS for each RSB was measured prelaunch for both Terra and Aqua MODIS. Algorithms have been developed to track the on-orbit RVS variation using the measurements from the onboard calibrators, supplemented with the earth view (EV) trends from pseudoinvariant desert targets obtained at different AOI. Since the mission beginning, the MODIS characterization support team (MCST) has dedicated efforts in evaluating approaches of characterizing the on-orbit RVS. A majority of the approaches focused on fitting the data at each AOI over time and then deriving the relative change at different AOI. The current version of the on-orbit RVS algorithm, as implemented in the collection 6 (C6) level-1B (L1B), is also based on the above rationale. It utilizes the EV response trends from the pseudoinvariant Libyan desert targets to supplement the gain derived from the onboard calibrators. The primary limitation of this approach is the assumption of the temporal stability of these desert sites. Consequently, MCST developed an approach that derives the on-orbit RVS change using measurements from a single desert site, combined with the on-orbit lunar measurements. In addition, the EV and onboard responses are fit first as a function of AOI before fitting temporally in order to eliminate the dependence on the stability of the desert site. Comprehensive comparisons are performed with current C6 RVS results for both Terra and Aqua MODIS. Results demonstrate that this alternative method provides a supplemental means to monitor the on-orbit RVS for MODIS RSB.

Evaluation of VIIRS and MODIS Thermal Emissive Band Calibration Stability Using Ground Target

The S-NPP Visible Infrared Imaging Radiometer Suite (VIIRS) instrument, a polar orbiting Earth remote sensing instrument built using a strong MODIS background, employs a similarly designed on-board calibrating source—a V-grooved blackbody for the Thermal Emissive Bands (TEB). The central wavelengths of most VIIRS TEBs are very close to those of MODIS with the exception of the 10.7 µm channel. To ensure the long term continuity of climate data records derived using VIIRS and MODIS TEB, it is necessary to assess any systematic differences between the two instruments, including scenes with temperatures significantly lower than blackbody operating temperatures at approximately 290 K. Previous work performed by the MODIS Characterization Support Team (MCST) at NASA/GSFC used the frequent observations of the Dome Concordia site located in Antarctica to evaluate the calibration stability and consistency of Terra and Aqua MODIS over the mission lifetime. The near-surface temperature measurements from an automatic weather station (AWS) provide a direct reference useful for tracking the stability and determining the relative bias between the two MODIS instruments. In this study, the same technique is applied to the VIIRS TEB and the results are compared with those from the matched MODIS TEB. The results of this study show a small negative bias when comparing the matching VIIRS and Aqua MODIS TEB, implying a higher brightness temperature for S-VIIRS at the cold end. Statistically no significant drift is observed for VIIRS TEB performance over the first 3.5 years of the mission.

Impact of Terra MODIS Collection 6 on long-term trending comparisons with Landsat 7 ETM+ reflective solar bands

Recently, the Moderate Resolution Imaging Spectroradiometer (MODIS) Characterization Support Team (MCST) has worked closely with the science team members to make significant improvements in both Terra and Aqua MODIS sensors. These refinements are included in the newly released Collection 6 (C6) Level 1B (L1B) products, and they primarily mitigate the long-term drifts observed in the short-wavelength bands of the MODIS sensors. This letter focuses particularly on evaluating the improvement in the long-term on-orbit radiometric calibration stability of the Terra MODIS sensor by comparing the trends with the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) sensor. In this study, Terra MODIS L1B products from both Collection 5 (C5) and C6 were used to compare the long-term top-of-atmosphere (TOA) reflectance trending of the Terra MODIS reflective solar bands (RSB) with spectrally matching bands of the L7 ETM+ sensors over the Committee on Earth Observation Satellites (CEOS) reference pseudo-invariant calibration sites (PICS). In addition, intensive statistical tests were performed to support the assessment of the observed long-term drifts. The results from the newly processed Terra MODIS C6 L1B products clearly show the excellent calibration stability (long-term drift within 2%) of the MODIS sensor with the multi-year drifts within the specified calibration uncertainty.

AVHRR, MODIS, and VIIRS radiometric stability and consistency in SST bands

Monitoring of IR Clear‐Sky Radiances over Oceans for SST (MICROS; www.star.nesdis.noaa.gov/sod/sst/micros) is NESDIS near‐real time web‐based radiance monitoring system. It analyzes Model (Community Radiative Transfer Model, CRTM) minus Observation (M‐O) biases in brightness temperatures (BT) in three bands centered at 3.7 (IR37), 11 (IR11), and 12 µm (IR12), for several AVHRR (NOAA‐16, ‐17, ‐18, ‐19, Metop‐A, ‐B), VIIRS (Suomi National Polar Partnership, S‐NPP), and MODIS (Terra, Aqua) sensors. Double‐differences (DD) are employed to check BTs for radiometric stability and consistency. All sensors are stable, with the exception of two AVHRRs, onboard NOAA‐16 and to a lesser extent NOAA‐18, and generally consistent. VIIRS onboard S‐NPP, launched in October 2011, is well in‐family, especially after its calibration was fine‐tuned on 7 March 2012. MODIS M‐O biases were initially out‐of‐family by up to −0.6 K, due to incorrect CRTM transmittance coefficients. Following MICROS feedback, CRTM Team updated coefficients and brought MODIS back in‐family. Terra and Aqua BTs are very consistent in IR11 and IR12 but show cross‐platform bias of 0.3 K in IR37, likely attributed to MODIS characterization. Work with MODIS Characterization Support Team is underway to resolve this. Initial analyses of AVHRR onboard Metop‐B launched in September 2012 suggest that its BTs are offset from Metop‐A by up to ∼0.3 K. Overall, MICROS DDs are well suited to evaluate the sensors stability, but dedicated effort is needed to ensure consistent radiative transfer modeling (RTM) calculations for various sensors before DDs can be used in Global Space‐based Inter‐Calibration System (GSICS) quantitative applications.

Terra and Aqua moderate-resolution imaging spectroradiometer collection 6 level 1B algorithm

The moderate-resolution imaging spectroradiometer (MODIS) was launched on the Terra spacecraft on Dec.18, 1999 and on Aquaon May 4, 2002. The data acquired by these instruments have contributed to the long-term climate data record for more than a decade and represent a key component of NASA’s Earth observing system. Each MODIS instrument observes nearly the whole Earth each day, enabling the scientific characterization of the land, ocean, and atmosphere. The MODIS Level 1B (L1B) algorithms input uncalibrated geo-located observations and convert instrument response into calibrated reflectance and radiance, which are used to generate science data products. The instrument characterization needed to run the L1B code is currently implemented using time-dependent lookup tables. The MODIS characterization support team, working closely with the MODIS Science Team, has improved the product quality with each data reprocessing. We provide an overview of the new L1B algorithm release, designated collection 6. Recent improvements made as a consequence of on-orbit calibration, on-orbit analyses, and operational considerations are described. Instrument performance and the expected impact of L1B changes on the collection 6 L1B products are discussed.

Evaluation of detector-to-detector and mirror side differences for Terra MODIS reflective solar bands using simultaneous MISR observations

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the five Earth-observing instruments on-board the National Aeronautics and Space Administration (NASA) Earth-Observing System (EOS) Terra spacecraft, launched in December 1999. It has 36 spectral bands with wavelengths ranging from 0.41 to 14.4 μm and collects data at three nadir spatial resolutions: 0.25 km for 2 bands with 40 detectors each, 0.5 km for 5 bands with 20 detectors each and 1 km for the remaining 29 bands with 10 detectors each. MODIS bands are located on four separate focal plane assemblies (FPAs) according to their spectral wavelengths and aligned in the cross-track direction. Detectors of each spectral band are aligned in the along-track direction. MODIS makes observations using a two-sided paddle-wheel scan mirror. Its on-board calibrators (OBCs) for the reflective solar bands (RSBs) include a solar diffuser (SD), a solar diffuser stability monitor (SDSM) and a spectral-radiometric calibration assembly (SRCA). Calibration is performed for each band, detector, sub-sample (for sub-kilometre resolution bands) and mirror side. In this study, a ratio approach is applied to MODIS observed Earth scene reflectances to track the detector-to-detector and mirror side differences. Simultaneous observed reflectances from the Multi-angle Imaging Spectroradiometer (MISR), also onboard the Terra spacecraft, are used with MODIS observed reflectances in this ratio approach for four closely matched spectral bands. Results show that the detector-to-detector difference between two adjacent detectors within each spectral band is typically less than 0.2% and, depending on the wavelengths, the maximum difference among all detectors varies from 0.5% to 0.8%. The mirror side differences are found to be very small for all bands except for band 3 at 0.44 μm. This is the band with the shortest wavelength among the selected matching bands, showing a time-dependent increase for the mirror side difference. This study is part of the effort by the MODIS Characterization Support Team (MCST) in order to track the RSB on-orbit performance for MODIS collection 5 data products. To support MCST efforts for future data re-processing, this analysis will be extended to include more spectral bands and temporal coverage.

Cross calibration of SeaWiFS and MODIS using on-orbit observations of the Moon

Observations of the Moon provide a primary technique for the on-orbit cross calibration of Earth remote sensing instruments. Monthly lunar observations are major components of the on-orbit calibration strategies of SeaWiFS and MODIS. SeaWiFS has collected more than 132 low phase angle and 59 high phase angle lunar observations over 12 years, Terra MODIS has collected more than 82 scheduled and 297 unscheduled lunar observations over nine years, and Aqua MODIS has collected more than 61 scheduled and 171 unscheduled lunar observations over seven years. The NASA Ocean Biology Processing Group Calibration and Validation Team and the NASA MODIS Characterization Support Team use the USGS RObotic Lunar Observatory (ROLO) photometric model of the Moon to compare these time series of lunar observations over time and varying observing geometries. The cross-calibration results show that Terra MODIS and Aqua MODIS agree, band to band, at the 1%-3% level, while SeaWiFS and either MODIS instrument agree at the 3%-8% level. The combined uncertainties of these comparisons are 1.3% for Terra and Aqua MODIS, 1.4% for SeaWiFS and Terra MODIS, and 1.3% for SeaWiFS and Aqua MODIS. Any residual phase dependence in the ROLO model, based on these observations, is less than 1.7% over the phase angle range of -80° to -6° and +5° to +82°. The lunar cross calibration of SeaWiFS, Terra MODIS, and Aqua MODIS is consistent with the vicarious calibration of ocean color products for these instruments, with the vicarious gains mitigating the calibration biases for the ocean color bands.

Progress and lessons from MODIS calibration intercomparison using ground test sites

Calibration accuracy and stability are key performance parameters for Earth-observing satellites. They have direct impact on the quality of science data products derived from sensor observations. Meanwhile, calibration consistency between spectral bands or among sensors also plays a critical role when deriving data products using multiple spectral bands and constructing long-term climate data records (CDR) using observations made by different sensors. The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) Terra and Aqua missions. To maintain its on-orbit calibration accuracy and monitor its calibration stability, MODIS was designed and built with a set of onboard calibrators (OBCs), which can be operated continuously or on an as needed basis. In addition to regular onboard calibration activities, extensive efforts have been made by MODIS calibration scientists and members of the MODIS Characterization Support Team (MCST) to validate the calibration accuracy of each sensor and to quantify the calibration consistency between both Terra and Aqua MODIS. This paper provides a brief description of MODIS on-orbit calibration methodologies and an overview of recent progress and lessons learned from MODIS calibration intercomparison studies through the use of ground reference sites. Specifically, the simultaneous nadir overpass (SNO) and double difference approaches over invariant reference sites, such as the Dome C, Libya-4 desert, and Lake Tahoe buoy sites, are illustrated. Examples derived from different intercomparison approaches and their applications for different sensors and spectral bands are presented, focusing on examining their calibration differences. Results from the Libya-4 desert site show that Terra and Aqua MODIS 0.65 and 0.85 µm channels have been well calibrated, with excellent long-term stability of better than 1.0%. Using the SNO approach, the Terra and Aqua MODIS calibration consistency is found to be within 1.5% and 0.6% for the 0.65 and 0.85 µm channels, respectively, and 0.02 K and 0.04 K for the 11 and 12 µm channels, respectively.

Using Dome C for moderate resolution imaging spectroradiometer calibration stability and consistency

Currently, there are two nearly identical moderate resolution imaging spectroradiometer (MODIS) instruments operated in space: one on the Terra spacecraft launched in December 1999 and another on the Aqua spacecraft launched in May 2002. MODIS has 36 spectral bands with wavelengths covering from visible (VIS) to long-wave infrared (LWIR). Since launch, MODIS observations and data products have significantly enabled studies of changes in the Earth system of land, oceans, and atmosphere. In order to maintain its on-orbit calibration and data product quality, MODIS was built with a comprehensive set of on-board calibrators. MODIS reflective solar bands (RSB) are calibrated on-orbit by a system that consists of a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) on a regular basis. Its thermal emissive bands (TEB) calibration is executed on a scan-by-scan basis using an on-board blackbody (BB). The MODIS Characterization Support Team (MCST) at NASA/GSFC has been responsible for supporting sensor calibration and characterization tasks from pre-launch to post launch. In this paper, we describe current MCST efforts and progress made to examine sensor stability and inter-calibration consistency using observations over Dome Concordia, Antarctica. Results show that this site can provide useful calibration reference for Earth-observing sensors.