Moderate Resolution Imaging Spectroradiometer Reflective Solar Bands Research Articles

Cross-Calibration of Terra and Aqua MODIS Using RadCalNet

Moderate Resolution Imaging Spectroradiometer (MODIS) instruments onboard the Terra and Aqua spacecraft have been successfully operating for nearly two decades and providing complementary observations of the Earth's land, ocean, and atmosphere. Although the two MODIS instruments view the entire Earth's surface once every 2-3 days, simultaneous views between them are limited due to their varying orbits. Therefore, the intercomparison between these two instruments has been previously performed using a transfer instrument [such as Advanced Very-high-resolution Radiometer (AVHRR)] or using lunar measurements normalized using a common model, such as the USGS Robotic Lunar Observatory (ROLO). In recent years RadCalNet, a Committee on Earth Observation Satellites (CEOS) initiative, has provided SI-traceable Top-of-Atmosphere (TOA) reflectances from a coordinated network of instrumented land-based sites. RadCalNet facilitates a unique mechanism to perform cross-calibration of instruments by minimizing the uncertainties associated with overpass time differences. In this letter, the near-simultaneous TOA reflectance measurements from the Railroad Valley (RRV), US (RVUS) are used as a transfer to compare the on-orbit observations for the Terra and Aqua MODIS reflective solar bands (RSBs). Near-nadir overpasses from January 2013 to January 2019 are processed and matched up with near-simultaneous RadCalNet measurements. Results show that the visible (VIS)/near-infrared response (NIR) bands agree to within 2%, and the short-wave infrared (SWIR) bands agree to within 5%. Also discussed in this letter are the future efforts that will be undertaken to expand this comparison to include other instruments, other sites, and both nadir- and off-nadir views after compensation for bidirectional reflectance distribution function (BRDF) effects.

Results From the Deep Convective Clouds-Based Response Versus Scan-Angle Characterization for the MODIS Reflective Solar Bands

The Terra and Aqua Moderate-Resolution Imaging Spectroradiometer (MODIS) scan mirror reflectance is a function of the angle of incidence (AOI) and was characterized prior to launch by the instrument vendor. The relative change of the prelaunch response versus scan angle (RVS) is tracked and linearly scaled on-orbit using observations at two AOIs of 11.2° and 50.2° corresponding to the moon view and solar diffuser, respectively. As the missions continue to operate well beyond their design life of six years, the assumption of linear scaling between the two AOIs is known to be inadequate in accurately characterizing the RVS, particularly at short wavelengths. Consequently, an enhanced approach of supplementing the on-board measurements with response trends from desert pseudoinvariant calibration sites (PICS) was formulated in MODIS Collection 6 (C6). An underlying assumption for the continued effectiveness of this approach is the long-term (multiyear) and short-term (month to month) stability of the PICS. Previous work has shown that the deep convective clouds (DCC) can also be used to monitor the on-orbit RVS performance with less trend uncertainties compared with desert sites. In this paper, the raw sensor response to the DCC is used to characterize the on-orbit RVS on a band and mirror-side basis. These DCC-based RVS results are compared with those of C6 PICS-based RVS, showing an agreement within 2% observed in most cases. The pros and cons of using a DCC-based RVS approach are also discussed in this paper. Although this reaffirms the efficacy of the C6 PICS-based RVS, the DCC-based RVS approach presents itself as an effective alternative for future considerations. Potential applications of this approach to other instruments, such as Suomi National Polar-orbiting Partnership, Joint Polar Satellite Systems, and Visible Infrared Imaging Radiometer Suite, are also discussed.

Assessment of Terra MODIS On-Orbit Polarization Sensitivity Using Pseudoinvariant Desert Sites

The Moderate Resolution Imaging Spectroradiometer (MODIS) is currently flying on NASA’s Earth Observing System Terra and Aqua satellites, launched in 1999 and 2002, respectively. MODIS reflective solar bands in the visible wavelength range are known to be sensitive to polarized light based on prelaunch polarization sensitivity tests. After about five years of on-orbit operations, it was discovered that the polarization sensitivity at short wavelengths had shown a noticeable increase. In this paper, we examine the impact of polarization on measured top-of-atmosphere (TOA) reflectances based on MODIS Collection-6 L1B over pseudoinvariant desert sites. The standard polarization correction equation is used in combination with simulated at-sensor radiances using the second simulation of a satellite signal in the Solar Spectrum, Vector Radiative Transfer Code (6SV). We ignore the polarization contribution from the surface and a ratio approach is used for both 6SV-derived input parameters and observed TOA reflectances. Results indicate that significant gain corrections up to 25% are required near the end of scan for the 412 and 443 nm bands. The polarization correction reduces the seasonal fluctuations in reflectance trends and mirror side ratios from 30% and 12% to 10% and 5%, respectively, for the two bands. Comparison of the effectiveness of the polarization correction with the results from the NASA Ocean Biology Processing Group shows a good agreement in the corrected reflectance trending results and their seasonal fluctuations.

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.

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.

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.

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.

Intercomparison of On-Orbit Calibration Consistency Between Terra and Aqua MODIS Reflective Solar Bands Using the Moon

Two nearly identical Moderate Resolution Imaging Spectroradiometer (MODIS) sensors, one on the Terra and the other on the Aqua satellite, are currently operating in space, making continuous global observations in 36 spectral bands: 20 reflective solar bands (RSBs) and 16 thermal emissive bands. For MODIS RSB with wavelengths from 0.41 to 2.1 mum, the sensor-specified calibration-accuracy requirements are plusmn2% for reflectance and plusmn5% for radiance products. They are calibrated on-orbit by a solar diffuser (SD) and an SD stability monitor. In addition, lunar observations are scheduled regularly to monitor the RSB radiometric calibration stability. This letter describes an intercomparison method developed for evaluating the calibration consistency between Terra and Aqua MODIS RSBs and calibration differences among detectors in each spectral band. It presents intercomparison results derived from Terra and Aqua MODIS lunar observations made over their overlapped mission operation. This method uses predicted lunar irradiances derived from a lunar model to remove lunar-viewing-geometry differences among different observations made by each sensor. The results, excluding the bands which either have electronic crosstalk or saturate during lunar observations, show that the Terra and Aqua MODIS RSBs have been consistently calibrated to within plusmn1%. For the detectors within any one spectral band, the calibration differences are less than plusmn0.5%. The methodology developed here can be applied to other sensors for intercomparison studies.

Characterization of earthshine impacts on Moderate Resolution Imaging Spectroradiometer reflective solar bands calibration

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASA's Earth Observing System (EOS) missions. Its high quality land, ocean, and atmosphere data products heavily rely on accurate on-orbit calibration and characterization. MODIS reflective solar bands (RSB) on-orbit calibration is reflectance based and is provided by an on-board Solar Diffuser (SD). The Bi-directional Reflectance Factor (BRF) of the SD was characterized pre-launch. The earthshine (ES) contamination to the SD signal was estimated to be approximately 0.5%. On-orbit observations and geometric optical modeling results show that this contamination strongly depends on the wavelength, calibration viewing geometry, and the Earth surface type. In this paper, the contribution from the earthshine on MODIS RSB calibration is illustrated and characterized systematically. A simple approach is proposed to reduce its impact on the RSB calibration. This approach has been applied to both Terra and Aqua MODIS RSB calibration and proved to be effective in reducing the earthshine contaminations.

MODIS Polarization-Sensitivity Analysis

The moderate resolution imaging spectroradiometer (MODIS) is one of the primary instruments in the Earth Observing System (EOS). Currently, MODIS instruments are onboard the NASA EOS Terra and Aqua spacecraft launched in December 1999 and May 2002, respectively. The MODIS reflective solar bands (RSBs) are sensitive to the polarization of incident light, particularly for the visible bands. To derive accurate top-of-the-atmosphere radiances, it is essential to know the polarization sensitivity, characterized by a polarization factor and phase angle, of the instruments. From prelaunch polarization sensitivity measurements, the polarization factors and phase angles for all visible and near-infrared bands of both instruments are derived, analyzed, and compared. The polarization factors are wavelength, angle of incidence on the MODIS scan mirror, and detector-dependent. For Terra MODIS, they are also mirrorside-dependent. The 412-nm band has the largest polarization factor, which is about 0.04 for both instruments. The polarization factors of all other bands are either smaller than or close to 0.02, which is the polarization requirement for the MODIS RSB whose wavelengths are longer than 412 nm. The unexpected one-, three-, and four-cycle anomalies observed in the measurements are analyzed. These anomalies are shown to be likely due to the nonuniformity of the light source and the retro-reflected light from the MODIS optical system. Their impacts on the derived polarization parameters are estimated and discussed.

MODIS Reflective Solar Bands On-Orbit Lunar Calibration

The moderate resolution imaging spectroradiometer (MODIS) protoflight model on-board the Terra spacecraft and the MODIS flight model 1 on-board the Aqua spacecraft were launched on December 18, 1999 and May 4, 2002, respectively. They view the moon through the space view (SV) port approximately once a month to monitor the long-term radiometric stability of their reflective solar bands (RSBs). The lunar irradiance observed by MODIS depends on the viewing geometry. Algorithms were developed to select lunar views such that these geometric effects are minimized. In each MODIS lunar observation, the moon can be viewed in multiple scans. The lunar irradiance of a MODIS RSB can be derived from the response of all detectors of a spectral band in one scan which fully covers the moon, from that of one detector in multiple scans or from the response of all detectors in multiple scans. Based on lunar observations, a set of coefficients is defined and derived to trend MODIS system response degradation at the angle of incidence (AOI) of its SV port. It is shown that the degradation is both wavelength and mirror side dependent. Since launch, Terra and Aqua MODIS band 8 (412 nm) mirror side one have degraded 36% and 17%, respectively, at the AOI of the SV. A comparison between the lunar coefficients and those derived from the MODIS on-board solar diffuser (SD) calibrations shows that the response change of the MODIS RSB is both AOI and time dependent. Time-dependent response versus scan angle (RVS) lookup tables derived from lunar views, SD calibration, and Earth-view observations have been used to maintain the quality of the L1B data for both the Terra and Aqua MODIS RSB. The corrections provided by the RVS in the Terra and Aqua MODIS data from the 412-nm band are as large as 14% and 6.2%, respectively.