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Moderate Resolution Imaging Spectroradiometer (MODIS), whose openly public data have been used for over two decades to monitor and address global issues, has 16 thermal emissive bands (TEBs) with central wavelengths that range from 3.7 to 14.4 μm and are calibrated on-orbit using observations from its on-board blackbody. To maintain MODIS' rich, well-calibrated archive of multispectral imagery and data, Earth targets are regularly used to track its long-term stability as well as the consistency between the two sensors onboard the Terra and Aqua satellites. Moreover, these scenes can be used to compare MODIS Earth view data over the complete scan-angle range and evaluate the on-orbit performance of the TEBs response-versus-scan-angle (RVS) over the mission lifetime. This article focuses on evaluating the MODIS TEBs Collection (C6.1) radiometric calibration stability for both instruments using an in situ ocean target as reference [hereafter referred to as in situ sea surface temperature (SST)]. Furthermore, it will assess the calibration consistency between the MODIS sensors. Finally, it will analyze the on-orbit RVS stability for Terra and Aqua MODIS. Only cloud-free, nighttime MODIS TEB retrievals were used for the study. A normalization methodology is applied to standardize the MODIS data to the in situ SST. In addition, spectral corrections were derived between some of the Terra and Aqua MODIS TEBs by using a combination of the MODIS Atmospheric Profile product and MODerate resolution atmospheric TRANsmission (MODTRAN) simulations. Results indicate that most MODIS TEBs exhibit mission-long trends of ±0.50 K--with Terra band 30 presenting the largest downward drift due to residual electronic crosstalk effects. Moreover, the calibration consistency analysis over a warm ocean target demonstrated that the average Terra-to-Aqua MODIS bias for most bands is well within ±0.50 K (bands 27 and 30 show the largest--electronic crosstalk-related--biases). Finally, the MODIS TEBs RVS trends display changes of ±0.50 K (except for bands 25 and 27 at the end-of-scan angles) for both instruments. Overall, the MODIS TEBs remain well-calibrated and their RVSs aptly characterized.

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.

Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) are major instruments for Earth science observations. Nearly 40 MODIS scientific products and a wide range of VIIRS environmental data records are produced using their global observations. The consistency of the MODIS and VIIRS calibrated data is important for the study of Earth science. This article assesses the calibration consistency of the Aqua MODIS and VIIRS thermal emissive band (TEB) data. To remove the impact of the mismatched relative spectral response (RSR) on the comparisons, the simultaneous nadir observation data from the cross-track infrared sounder (CrIS) and the infrared atmospheric sounding interferometer (IASI) are used as references in two different methods to independently verify the consistency. The comparisons of the MODIS and VIIRS TEB calibrated data show that the brightness temperature (BT) differences for comparable bands between MODIS and VIIRS are in general within 0.2 K for BT larger than 230 K. The differences of all comparable MODIS and VIIRS TEBs are consistent over time. MODIS measurements agree better with N20 VIIRS than with Suomi National Polar-orbiting Partnership (S-NPP) VIIRS. The S-NPP VIIRS measurements are higher than N20 VIIRS, within 0.1 K for long-wave infrared (LWIR) bands and 0.3 K for band M13.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments onboard the Aqua and Terra satellites have been operated for nearly two decades, producing high-quality earth observation data sets suitable for a broad range of scientific studies regarding the earth’s land, ocean, and atmospheric processes. The high radiometric accuracy of MODIS reflective solar band (RSB) calibration has also served as benchmark measurements for on-orbit cross-calibration studies. As the two MODIS instruments have operated well beyond their design lifespan of six years, the measurements from the onboard calibrators alone become inadequate to characterize the sensor’s response at all scan angles, as evinced by long-term drifts observed at certain scan positions of the Aqua-MODIS 0.64- and 0.86- $\mu \text{m}$ bands in Collection 6 (C6) data set. The latest MODIS Level 1B C6.1 data set incorporates earth-view response trending from invariant desert sites as supplemental inputs to characterize the scan-angle calibration dependencies for all RSB. This article presents a deep convective cloud (DCC)-based calibration approach for an independent evaluation of the MODIS RSB response versus scan-angle (RVS) performance in C6.1. The long-term calibration stability and RVS differences in C6.1 have been significantly improved for Aqua-MODIS RSB. The observed RVS differences of more than 2% in Aqua-MODIS C6 bands 1 and 2 have been reduced to within 1% in C6.1. Some RSBs of Terra-MODIS have suffered temporal drifts up to ~2% and calibration shifts up to 3%, particularly around 2016 when the Terra satellite entered into safe mode. The DCC approach has been found very effective in tracking the on-orbit RVS changes over time.

The MODIS thermal emissive bands (TEB) radiometric calibration uses a quadratic function for the instrument response, and the calibration coefficients look-up tables (LUTs) are updated using the response of an on-board blackbody (BB). After more than 21 and 19 years on-orbit, the TEB performances for Terra and Aqua MODIS have been generally stable. However, contamination from electronic crosstalk, a known issue since prelaunch, has affected the L1B image quality and measurement accuracy. In addition to the photovoltaic (PV) longwave infrared (LWIR) bands crosstalk correction included in Terra MODIS Collection 6.1 (C6.1), a crosstalk correction for select detectors in the Terra and Aqua mid-wave infrared (MWIR) and Aqua PV LWIR bands are applied in C7. The mission-long crosstalk coefficients for the selected detectors are derived and populated in the form of LUTs. The crosstalk correction is applied to both on-orbit calibration and the algorithm used to generate Earth-view L1B products. Among these detectors, the Aqua MODIS band 24 detector 1 crosstalk has the largest impact on image quality, with striping observed over cold scenes for both Terra and Aqua MODIS. The images of C7 L1B and C6.1 are compared to assess the impact of the correction. Additional assessments using Earthview measurements and inter-comparison also revealed the need for improvement of calibration stability and consistence for select bands. Additional improvements for long-term stability and mirror side consistence were developed using quasideep convective clouds (qDCC), Dome-C, ocean, desert, and inter-comparison with other instruments.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua polar-orbiting satellite has long been known to be subjected to electronic crosstalk. Its signatures are clearly visible in images of the lunar disk by various bands, which are routinely obtained during scheduled roll maneuvers. Electronic crosstalk can potentially impact Level 1B products, causing image artifacts such as striping and radiometric bias. However, a comprehensive effort in mapping the sending/receiving bands and detectors, deriving the crosstalk coefficients, and assessing the impact of the crosstalk contamination on the Level 1B product for Aqua’s mid-wave IR bands (3.75– 4.52 $\mu \text{m}$ ) had been lacking. In this work, we surveyed lunar images by the Aqua MODIS bands that are connected to electronic output 2 of the SWIR/MWIR FPA, identifying all detectors affected by electronic crosstalk contamination and determined which bands and detectors are sending the contaminating signal. By assuming a linear model to describe the contamination, we derived linear crosstalk coefficients for the bands/detectors concerned from lunar images and used these to generate corrected Earth Level 1B images for the mid-wave IR bands and assessed the impact of the electronic crosstalk on the Level 1B imagery.

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the NOAA-20 (N20) satellite was launched on November 18, 2017. The N20 VIIRS reflective solar bands (RSBs) are calibrated on-orbit using a solar diffuser. An accurate on-orbit calibration is crucial to the high-quality downstream products facilitating atmosphere, ocean and land applications. In this study, the stability of the Level 1B (L1B) reflectance product is investigated using measurements over deep convective clouds (DCCs) for M-bands M1-M5, M7-M11, and I-bands I1-I3. The methodologies developed previously for Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) sensors and Suomi National Polar-orbiting Partnership (SNPP) VIIRS are extended and applied to the N20 RSB to derive DCC-based trends. The Collection 2 L1B data produced by NASA Land Science Investigator-led Processing Systems (SIPS) is used to evaluate the performance of the N20 VIIRS RSB calibration. At nadir, the reflectance trends for M1, M5, M8-M11, and I3 are insignificant compared to their corresponding variations (STDs) except for bands M2-M4, M7, and I1-I2, whose trends are larger than or equivalent to their STDs. The reflectance is relatively stable compared to their STDs for all the study RSBs at six aggregation zones across the entire scan angle range. Also discussed in this paper are the detector-to-detector differences and half-angle mirror side differences using the DCCs. Future applications using DCCs, which include an intercomparison with SNPP VIIRS, are also discussed.

MODIS instruments have been operating over 21 and 19 years on the Terra and Aqua spacecraft, respectively. As both instruments continue to operate beyond their design lifetime of 6 years, relying on the on-board calibrators alone is proving a challenge to accurately characterize their response at all scan angles, especially for the reflective solar bands. This study presents an alternative approach that relies on Earth view responses at multiple scan angles from a single site over Dome Concordia (Dome C), Antarctica, one of the most homogeneous land surfaces uniformly covered by snow. This approach has advantages of more frequent overpasses over the entire scan angle range than the desert sites.

Terra and Aqua MODIS have successfully operated for more than 20 and 18 years, respectively, and far exceeded their designed lifetimes of 6 years. MODIS has 36 spectral bands, among which 16 are the thermal emissive bands (TEB) covering a wavelength range from 3.75 to 14.24 μm. Observations from the MODIS TEB have been used to generate a number of data products, such as surface/cloud/atmospheric temperatures, cloud top altitude, and water vapor properties. To a large extent, the quality of the MODIS L1B radiance product has been maintained through the entirety of each mission via extensive on-orbit calibration and validation efforts. MODIS TEB calibration uses a quadratic algorithm and is referenced to an on-board blackbody with its temperature measurements traceable to the NIST standard. We provide an overview of the MODIS instrument operations, key TEB calibration activities, and algorithms used in the latest L1B data Collection 6.1 (C6.1) and describe the TEB on-orbit performance for both Terra and Aqua MODIS. The TEB gain trends have been stable to within 5% over the Terra mission after 2003 under the same electronic configuration, except for bands 27 to 30, and within 3% over the Aqua mission. The Terra MODIS optical crosstalk correction implemented right after launch has been very effective in removing the ghost images in its photoconductive detector bands 32 to 36. Starting from C6.1, an electronic crosstalk correction algorithm has also been applied to the Terra MODIS photovoltaic detector long-wave infrared bands 27 to 30. The noise characterization performance remains stable with most TEB detectors continuously meeting their design requirements. A gradual loss of the Aqua MODIS radiative cooler margin reached its maximum in 2013 and has been slowly recovering since then. On-orbit changes in TEB response versus scan angle are extremely small based on pitch maneuver observations and assessments using vicarious approaches. The remaining challenges in TEB calibration and proposed improvements for the upcoming data collection with L1B data production and reprocessing are discussed.

Aqua MODIS has successfully operated for more than 18 years since its launch in May 2002 and has generated numerous science products in support of studies of the Earth’s system and its changes via a set of geophysical and environmental parameters. On-orbit calibration and characterization activities have played a vital role in maintaining the quality of MODIS data products. In addition to data collected from sensor on-board calibrators (OBC), near-monthly lunar observations and select earth view targets have been used to monitor and characterize on-orbit changes in sensor responses and to derive and update the calibration look-up tables. In this paper, we provide an overview of Aqua MODIS on-orbit operations, calibration activities and approaches, and algorithm improvements and also illustrate the sensor on-orbit performance using examples derived from various calibration sources and targets. We will focus on issues identified from instrument operations and calibrations, such as solar diffuser degradation, electronic crosstalk, variations in the cold focal plane temperatures, and changes in response versus scan-angle. Also discussed in this paper are remaining challenges and future improvements.

Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) have successfully operated since their launches in 1999 and 2002, respectively, and generated various data products to support the Earth remote sensing disciplines and users worldwide for their research activities and applications, including studies of the Earth system, and its changes over time and geographic regions. The MODIS data have also significantly contributed to the continuity of multi-decadal satellite data records and led to major advances in the Earth remote sensing field. The long-term data records from MODIS observations have been and will continue to be extended by the Visible Infrared Imaging Radiometer Suite (VIIRS) instruments, currently operated aboard the Suomi-National Polar-Orbiting Partnership (NPP) and NOAA-20 satellites. The data quality of satellite instruments strongly depends on their calibration accuracy and stability. In order to help scientists and users gain a better understanding of MODIS and VIIRS data quality, this paper provides an overview of their on-orbit calibration methodologies, approaches, and results derived from instrument on-board calibrators and lunar observations, as well as select Earth view targets. What is also discussed is the calibration consistency between MODIS and VIIRS and its potential impact on producing multi-sensor long-term data records. As illustrated, the overall performance of both MODIS and VIIRS continues to meet their design requirements.

An accurate on-orbit characterization of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors onboard the Terra and Aqua satellites is essential to satisfy the requirements from the scientific community for high-quality operational and research products. In this study, NASA’s C6.1 L1B data are utilized to assess the stability of the calibrated top of atmosphere reflectance retrieved from individual detectors within a spectral band over deep convective clouds (DCCs) and desert sites. The results from these invariant Earth targets show detector-to-detector (D2D) differences in the L1B reflectance products for select MODIS reflective solar bands. For Aqua MODIS, the D2D differences have no significant change over time for the bands studied. However, the D2D differences exhibit an increase in magnitude over time for Terra MODIS bands 1, 3-7, and 26, which can induce noticeable striping and higher uncertainties in the downstream data products. The D2D stability assessment results over DCCs are consistent with those observed over desert sites. In addition, the view-angle dependence of the D2D differences is investigated by separating the frames along scan into five zones. The D2D assessment is performed for each of the five zones, and the results will further benefit any improvements in the calibration of a detector-dependent response versus scan angle that can be considered for future MODIS Level 1B Collections.

The Moderate Resolution Imaging Spectroradiometer (MODIS), a key instrument of the NASA Earth Observing System missions, has operated for over 20 years and 18 years onboard the Terra and Aqua satellites, respectively. Sixteen thermal emissive bands (TEBs) are distributed on two cold focal place assemblies with photovoltaic (PV) HgCdTe detectors for bands 20-25 and 27-30 and photoconductive (PC) HgCdTe detectors for bands 31-36. This paper evaluates the performance of the latest MODIS Collection 6.1 (C6.1) Level-1B products with a focus on the relative differences between the Terra and Aqua MODIS TEBs. The Infrared Atmospheric Sounding Interferometer (IASI) onboard the Metop-A satellite serves as a reference. Observations obtained from the simultaneous nadir overpass between the Terra (or Aqua) and Metop-A satellites are used to track the brightness temperature differences between MODIS and IASI. The assessment is based on the temperature differences as a function of scene temperature and their trends in comparison with IASI from years 2007 to 2020. The results confirm that the calibration of Terra MODIS bands 27-30 significantly improved in C6.1 after implementing the electronic crosstalk correction. In spite of these corrections, Terra MODIS shows a larger drift in PV LWIR bands 27-30 than Aqua MODIS when compared to IASI.A sensitivity study is performed, where the center wavelength of the MODIS relative spectral response is shifted, and as a result some of these bands show a significant improvement in scene temperature dependency and their consistency with IASI measurements.

The Moderate-Resolution Imaging Spectroradiometer (MODIS) has been maintained its nominal operations since it was launched in December 1999 on Terra and May 2002 on Aqua spacecraft, respectively. MODIS is a scanning radiometer with a two-sided mirror that scans the Earth over an angular range from -55° to +55° off nadir and covers a wavelength range from 0.4 to 14.4 μm in 36 spectral bands. The sensitivity to polarization was fully characterized during prelaunch characterization for each reflective solar band (RSB), detector and mirror side as a function of scan angle. It was observed that while on-orbit polarization sensitivity for Aqua MODIS has been stable over the course of mission, the Terra MODIS polarization sensitivity at a few short wavelengths showed a significant increase near the end of scan angle after a few years in the mission. Our previous study examined the impact of polarization on the measured top-of-atmosphere reflectances obtained over the pseudo-invariant desert sites. There are up to 25% change in gain and 30% seasonal variation in select Terra RSBs due to the increase in polarization sensitivity. In this study, we examine relative changes in the polarization sensitivity of detectors within a band. Results from prelaunch tests indicate that there is a noticeable detector dependent polarization sensitivity and it is expected that such a dependency could change on orbit, particularly for Terra MODIS. Results of this study provide useful information on polarization impact and implementation of the correction algorithm.

The Moderate-Resolution Imaging Spectroradiometer (MODIS) is the key instrument of the NASA’s Earth Observing System (EOS) Terra and Aqua missions, launched in December 1999 and May 2002, respectively. The Visible Infrared Imaging Radiometer Suite (VIIRS) expands the MODIS legacy, launched onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite in October 2011 and NOAA20 satellite in November 2017, respectively. The MODIS and VIIRS sensors have a similar design with spectrally matched reflective solar spectral bands (RSB). Their on-board calibration components include a solar diffuser and a solar diffuser stability monitor for RSB, a V-grooved blackbody for the thermal emissive bands (TEB), and a space view as a background reference. This study evaluates the calibration consistency of the Terra and Aqua MODIS RSB based on the current Collection 6.1 L1B data products generated by the NASA MODAPS, which have used consistent calibration coefficient look up tables (LUT) for the entire data-record. In the case of SNPP and NOAA20 VIIRS, the latest L1B data produced by NASA Land SIPS are used. Several independent vicarious approaches are used to examine the stability and consistency of the at-sensor reflectance among MODIS and VIIRS instruments. Vicarious approaches include observations from simultaneous nadir overpasses (SNO), the Libya-4 desert and Dome C snow sites, as well as deep convective clouds (DCC). Impact of existing band spectral difference on the reflectance is corrected using hyperspectral observations provided by Europe Space Agency’s SCIAMACHY sensor. Results of this study provide comprehensive assessments of calibration performance, radiometric agreement and associated uncertainties.

The MODerate-resolution Imaging Spectroradiometer (MODIS) is a premier Earth-observing sensor of the early 21st century, flying onboard the Terra (T) and Aqua (A) spacecraft. Both instruments far exceeded their six-year design life and continue to operate satisfactorily for more than 15 and 13 years, respectively. The MODIS instrument is designed to make observations at nearly a 100% duty cycle covering the entire Earth in less than two days. The MODIS sensor characteristics include a spectral coverage from 0.41 to 14.4 $\mu\mbox{m}$ , of which those wavelengths ranging from 3.7 to 14. 4 $\mu\mbox{m}$ cover the thermal infrared region which is interspaced in 16 thermal emissive bands (TEBs). Each of the TEB contains ten detectors which record samples at a spatial resolution of 1 km. In order to ensure a high level of accuracy for the TEB-measured top-of-atmosphere radiances, an onboard blackbody (BB) is used as the calibration source. This paper reports the noise characterization and performance of the TEB on various counts. First, the stability of the onboard BB is evaluated to understand the effectiveness of the calibration source. Next, key noise metrics such as the noise equivalent temperature difference and the noise equivalent dn difference (NEdN) for the various TEBs are determined from multiple temperature sources. These sources include the nominally controlled BB temperature of 290 K for T-MODIS and 285 K for A-MODIS, as well as a BB warm up–cool down cycle that is performed over a temperature range from roughly 270 to 315 K. The space-view port that measures the background signal serves as a viable cold temperature source for measuring noise. In addition, a well characterized Earth-view target, the Dome Concordia site located in the Antarctic plateau, is used for characterizing the stability of the sensor, indirectly providing a measure of the NEdN. Based on this rigorous characterization, a list of the noisy and inoperable detectors for the TEB for both instruments is reported to provide the science user communities quality control of the MODIS Level 1B calibrated product.

Terra and Aqua MODIS have provided continuous global observations for scientific applications for more than 20 and 18 years, respectively. The radiometric calibration of the MODIS thermal emissive bands (TEB) is based on a quadratic approximation of the instrument response. The calibration coefficients look-up tables (LUTs) are updated using the response of the MODIS on-board blackbody (BB) with the response background subtracted by space view. The quarterly on-board BB warm-up and cool-down activity temperature ranges from 270 K to 315 K, and the derived offset has a relatively large uncertainty. Electronic cross-talk, an artifact that affects both instrument calibration and Earth view (EV) radiance retrievals, is corrected based on lunar observations. Calibration assessments using EV observations (e.g. ocean, desert, Antarctic Dome Concordia, and deep convective clouds) provide useful information to evaluate the impact of the Terra safe mode (February 2016) and Aqua MODIS formatter reset (January 2018) events on both MODIS instruments. This study focuses on the TEB radiometric calibration algorithm improvements for future collections based on calibration assessments using EV measurements and analytical modeling. Measurement stability and consistency over specific Earth scenes with a wide temperature range, as well as their brightness temperature (BT) dependency, are used for bias estimations in the calibration coefficients. Calibration coefficients are derived and updated after adjusting the current fitting algorithm. Thereafter, using the test LUTs, their impact on the Level 1B (L1B) data for different EV scenes is analyzed.

Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua has been in operation providing continuous global observations for science research and applications since 2002. The long-term stability of thermal emissive bands (TEBs) of Aqua MODIS was monitored through inter-comparisons with measurements by hyperspectral infrared sensors, such as AIRS on Aqua and IASI on MetOp satellite, or through long term vicarious monitoring over cold targets, such as Dome- C and deep convective clouds (DCC). In this paper, a radiative transfer modeling-based simulation model using Community Radiative Transfer Model (CRTM) is developed to perform the long-term monitoring of the stability of TEBs of Aqua MODIS. CRTM is a fast-radiative transfer model for calculations of radiances for satellite infrared radiometers and is able to output infrared radiance and brightness temperature at spectral bands of MODIS. Long term European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis data, such as temperature and humidity profiles, are used as inputs to the CRTM simulation. By confining the area of interest to be over low to middle latitude ocean, the long-term stabilities of selected Aqua MODIS TEBs are monitored through Observation–Background (O-B) brightness temperature (BT) bias between MODIS measurements and BT retrieval from CRTM simulation. The consistency and relative stability between Aqua MODIS and ECMWF reanalysis data for surface channels of MODIS are evaluated. In addition, the radiative transfer modeling with CRTM enables us to evaluate the impacts of long term variation of global CO2 distribution on the O-B BT biases for CO2 channels of Aqua MODIS through comparison of simulations with constant or long term variable CO2 as inputs. The O-B analysis with RTM show that Aqua MODIS surface channels are all radiometrically stable with yearly BT bias drift less than 0.004K/year for B20, B22, B23 and ~0.01K/year for B31 and B32. The CO2 absorption channels of Aqua MODIS, e.g. B33-B36, are stable with BT bias drift < 0.005K/year.

The intercalibration between a broadband and a hyperspectral satellite Earth observation system requires the convolution of the hyperspectral data with the spectral response functions (SRFs) of the corresponding broadband channels. There are two potential issues associated with the convolution procedure. First, the finite resolution of a hyperspectral spectrum, that is, the deviation from the highly accurate line-by-line monochromatic radiances, will contribute to convolution errors. The magnitude of the errors depends on the spectral resolution and the SRF shape of the hyperspectral instrument. This type of the convolution error has not been well recognized, and there is a lack of corresponding discussion in most published papers. Although it is small as compared with the instrument accuracy of existing hyperspectral sounders, the error is deemed to be signicant when it is compared with the stringent calibration requirement imposed by future climate missions like the Climate Absolute Radiance and Refractivity Observatory (CLARREO). Second, some broadband channels are insufficiently covered by the hyperspectral data, causing spectral gaps that lead to convolution errors. Although several methods have been developed to fill the spectral gaps and hence compensate for the second type of convolution error, the correction accuracy may still need improvement especially when a large spectral gap needs to be lled. This paper presents a methodology to accurately quantify and compensate for both types of convolution errors. This methodology utilizes the available hyperspectral information to correct the scene-dependent convolution errors due to either the limited spectral resolution or spectral gaps. We use simulations to characterize the intercalibration errors between the Moderate resolution Imaging Spectroradiometer (MODIS) and current operational infrared sounders. We demonstrate that convolution errors can be effectively removed to meet the highly accurate intersatellite calibration requirement proposed by the Climate Absolute Radiance and Refractivity Observatory. Our methodology is also validated using real satellite data for the intercalibration between Aqua MODIS and Aqua Atmospheric Infrared Sounders (AIRS). Our study demonstrates that the accurate characterization and correction for the convolution errors greatly reduces the scene-dependent and spectrally dependent errors, being critical to the consistency check between Infrared Atmospheric Sounding Interferometer (IASI) and AIRS using the double-difference method. The convolution correction also facilitates the evaluation for other intercalibration errors (e.g., the drift of MODIS SRFs). Our derived SRF shift values from MODIS-AIRS (after convolution error corrections) and from MODIS-IASI intercalibration are consistent with each other. We further extend the methodology to study the calibration of a broadband channel which is either completely or largely uncovered bya hyperspectral measurement.The large spectral gap-filling methodology is validated by demonstrating the accurate prediction of the MODIS radiance of band 29 using the Cross-track Infrared Sounder spectra, with the real IASI spectral data being used as the reference.

This paper evaluates the calibration quality during the blackbody (BB) warm-up cool-down cycle for thermal emissive bands onboard Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) and Suomi National Polar-orbiting Partnership (S-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS). This evaluation utilizes data from Aqua MODIS Collection 6 Level-1B products and VIIRS Sensor Data Records in 6-min granule format provided by the NASA Land Science Investigator-led Processing System. Nearly simultaneous hyperspectral measurements from the Aqua Atmospheric Infrared Sounder (AIRS) and the S-NPP Cross-track Infrared Sounder (CrIS) are used as references for MODIS and VIIRS, respectively. Each AIRS footprint of 13.5 km is co-located with multiple MODIS pixels while each CrIS field of view of 14 km is co-located with multiple VIIRS pixels. The corresponding AIRS-simulated MODIS and CrIS-simulated VIIRS radiances are derived by convolutions based on sensor-dependent relative spectral response functions. In this paper, the analysis mainly focuses on the bands that are used in sea surface temperature products. The results show that there is virtually no impact for MODIS bands 22 and 23 and bands 31 and 32 for a BB temperature below 290 K; however, when the BB temperature increases above 290 K, the impact is up to 0.3 K for bands 22 and 23 and 0.05 K for bands 31 and 32, respectively. For VIIRS, BB temperature-dependent drifts are observed in M15 and M16, which can reach 0.15 and 0.1 K, respectively, over the operational BB temperature range and the VIIRS brightness temperature range.

The Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) has been chosen by the Global Space-based Inter-Calibration System operational community as the reference sensor in cross-sensor calibration. A number of geostationary orbit and low-earth orbit sensors use the 0.64- $\mu \text{m}$ band from Aqua MODIS as a calibration reference. After over 15 years on-orbit, the performance characteristics of the MODIS instrument have changed, with effects evident at short wavelengths. MODIS employs a reflectance-based calibration using the solar diffuser measurements with the monthly lunar observations facilitating a response versus scan angle characterization on-orbit. As the instrument continues to operate beyond its design lifetime of 6 years, the on-board calibrators alone are insufficient to accurately characterize the instrument’s response at all scan angles. This results in a long-term reflectance drift, particularly observed in the 0.64- and 0.85- $\mu \text{m}$ bands, while observing the temporally invariant desert sites. Long-term reflectance drifts of up to 2% and 3% are observed at the beginning of scan for the 0.64- and 0.85- $\mu \text{m}$ bands, respectively. An approach using earth-view response to supplement the on-board calibrator measurements has been shown to overcome these inadequacies and is now implemented for the 0.64-, 0.85-, 0.46-, and 0.55- $\mu \text{m}$ land bands of Aqua MODIS. This paper presents the details related to the algorithm implementation and an independent evaluation using the Dome Concordia site and deep-convective clouds. This approach has been reviewed, tested, and approved by the MODIS science team and has been implemented in the forward production of the MODIS L1B Collection 6 starting July 9, 2016. This enhanced approach has also been adopted in the MODIS L1B Collection 6.1 reprocess for the entire mission to facilitate an improved quality of downstream science products. The results with the enhanced approach reduce the reflectance drifts to within 0.5% for most cases.

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.

On December 18, 2015, the Terra spacecraft completed 16 years of successful operation in space. Terra has five instruments designed to facilitate scientific measurements of the earth’s land, ocean, and atmosphere. The Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multiangle Imaging Spectroradiometer (MISR) instruments provide information for the temporal studies of the globe. After providing over 16 years of complementary measurements, a synergistic use of the measurements obtained from these sensors is beneficial for various science products. The 20 reflective solar bands (RSBs) of MODIS are calibrated using a combination of solar diffuser and lunar measurements, supplemented by measurements from pseudoinvariant desert sites. MODIS views the on-board calibrators and the earth via a two-sided scan mirror at three spatial resolutions: 250 m using 40 detectors in bands 1 and 2, 500 m using 20 detectors in bands 3 and 4, and 1000 m using 10 detectors in bands 8–19 and 26. Simultaneous measurements of the earth’s surface are acquired in a push-broom fashion by MISR at nine view angles spreading out in the forward and backward directions along the flight path. While the swath width for MISR acquisitions is 360 km, MODIS scans a wider swath of 2330 km via its two-sided scan mirror. The reflectance of the MODIS scan mirror has an angle dependence characterized by the response versus scan angle (RVS). Its on-orbit change is derived using the gain from a combination of on-board and earth-view measurements. The on-orbit RVS for MODIS has experienced a significant change, especially for the short-wavelength bands. The on-orbit RVS change for the short-wavelength bands (bands 3, 8, and 9) at nadir is observed to be greater than 10% over the mission lifetime. Due to absence of a scanning mechanism, MISR can serve as an effective tool to evaluate and monitor the on-orbit performance of the MODIS RVS. Furthermore, it can also monitor the detector and scan-mirror differences for the MODIS bands using simultaneous measurements from earth-scene targets, e.g., North Atlantic Ocean and North African desert. Simultaneous measurements provide the benefit of minimizing the impact of earth-scene features while comparing the radiometric performance using vicarious techniques. Long-term observations of both instruments using select ground targets also provide an evaluation of the long-term calibration stability. The goal of this paper is to demonstrate the use of MISR to monitor and enhance the on-orbit calibration of the MODIS RSB. The radiometric calibration requirements for the MODIS RSB are ±2% in reflectance and ±5% in radiance at typical radiance levels within ±45° of nadir. The results show that the long-term changes in the MODIS reflectance at nadir frames are generally within 1%. The MODIS level 1B calibrated products, generated after correcting for the on-orbit changes in the gain and RVS, do not have any correction for changes in the instrument’s polarization sensitivity. The mirror-side-dependent polarization sensitivity exhibits an on-orbit change, primarily in the blue bands, that manifests in noticeable mirror side differences in the MODIS calibrated products. The mirror side differences for other RSB are observed to be less than 1%, therefore demonstrating an excellent on-orbit performance. The detector differences in the blue bands of MODIS exhibit divergence in recent years beyond 1%, and a calibration algorithm improvement has been identified to mitigate this effect. Short-term variations in the recent year caused by the forward updates were identified in bands 1 and 2 and are planned to be corrected in the next reprocess.

The primary objective of the CLARREO Pathfinder (CPF) mission is to demonstrate essential measurement technologies to enable highly accurate decadal change observations traceable on-orbit to SI standards. Another important objective of the CPF is to demonstrate inter-calibration with the VIIRS sensor and to show that such high-accuracy reference inter-calibration is achievable. To satisfy this requirement, it is important to understand and quantify uncertainties in VIIRS sensor measurements. Based on prelaunch test results, the reflective solar bands of both SNPP and NOAA-20 VIIRS have exhibited polarization sensitivities in the shortest wavelength bands, with an unexpectedly larger sensitivity observed for NOAA-20 VIIRS, particularly in bands M1 to M4. In this study, we use VIIRS Level-1B reflectance data collected over the Pseudo-Invariant Calibration Sites over North African desert region to examine the polarization associated uncertainty for these four bands. Impact due to BRDF on the reflectance data is considered by comparing baseline results obtained from the same viewing and solar zenith angles under relatively low polarization sensitivity. Impact due to detector relative difference in polarization sensitivity is examined by normalizing reflectances by the value of the middle detector. Results of this study provide useful information on VIIRS uncertainty contribution due to polarization when conducting CPF and VIIRS inter-calibration.

Since its launch in December 1999, Terra Moderate Resolution Imaging Spectroradiometer (MODIS) has successfully operated for more than 20 years, with its observations generating a broad range of science data products that have greatly enabled the remote sensing community and users worldwide in their studies of many key geophysical parameters of the Earth’s systems. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.4 μm, which are calibrated by a set of onboard calibrators (OBCs). Also contributing to the sensor’s mission-long on-orbit calibration and characterization are near-monthly scheduled lunar observations and multiple time series of the sensor’s responses over select ground targets at a variety of scan angles. To a large extent, the quality of MODIS data products relies strongly on the dedicated efforts to operate and calibrate the instrument, to derive and update calibration parameters, and to develop and implement new calibration strategies and algorithms in response to on-orbit changes of the sensor’s characteristics and its OBC functions. We provide an overview of the Terra MODIS on-orbit operation and calibration activities over the last 20 years, including changes made to extend and preserve the instrument and OBC functions and their operation strategies. It also illustrates the sensor’s on-orbit performance with results derived from its OBC, lunar observations, and select ground targets and discusses major changes in sensor characteristics and corrections applied to the L1B algorithms as well as calibration lookup table updates. To date, the Terra MODIS instrument and its OBCs continue to operate and function normally. Except for those identified prelaunch, most spectral bands and detectors continue to meet their specified calibration requirements. Also discussed in our paper are lessons learned from Terra MODIS operation and calibration, as well as future efforts to further extend and maintain the quality of its long-term data records.

MODIS has 16 Thermal Emissive Bands (TEBs) whose wavelengths range from 3.7μm to 14.4μm and are calibrated using scan-by-scan observations of its on-board blackbody (BB). Specific Earth surface targets are used to track the long-term consistency, stability, and relative bias between the two MODIS instruments onboard the Terra and Aqua satellites. Previous studies displayed that cold targets have exhibited a relative bias (between Terra and Aqua MODIS Band 31 (11μm)) of less than 0.10K for more than a decade. However, these brightness temperatures are typically outside the MODIS BB calibration range. Hence, in-situ and remotely-sensed (RS) sea surface temperature (SST) measurements provide useful references for the more characteristic higher scene temperatures. Prior literature indicates that the MODIS TEBs have not been extensively evaluated for calibration stability using warmer targets for Collection 6.1 (C6.1), as it was recently implemented. This manuscript expands on the techniques previously applied to all TEBs using SST ocean buoys as reference for C6 by extending the long-term calibration stability and relative bias (between Terra and Aqua MODIS) discussion to present day and incorporating RS data provided by The Group for High Resolution Sea Surface Temperature (GHRSST) as proxy.

The first Visible Infrared Imaging Radiometer Suite (VIIRS) is onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. As a primary sensor, it collects imagery and radiometric measurements of the land, atmosphere, cryosphere, and oceans in the spectral regions from visible (VIS) to long-wave infrared. NASA's National Polar-orbiting Partnership (NPP) VIIRS Characterization Support Team has been actively involved in the VIIRS radiometric and geometric calibration to support its Science Team Principal Investigators for their independent quality assessment of VIIRS Environmental Data Records. This paper presents the performance assessment of the radiometric calibration stability of the VIIRS VIS and NIR spectral bands using measurements from SNPP VIIRS and Aqua MODIS simultaneous nadir overpasses and over the invariant surface targets at the Libya-4 desert and Antarctic Dome Concordia snow sites. The VIIRS sensor data records (SDRs) used in this paper are reprocessed by the NASA SNPP Land Product Evaluation and Analysis Tool Element. This paper shows that the reprocessed VIIRS SDRs have been consistently calibrated from the beginning of the mission, and the calibration stability is similar to or better than MODIS. Results from different approaches indicate that the calibrations of the VIIRS VIS and NIR spectral bands are maintained to be stable to within 1% over the first three-year mission. The absolute calibration differences between VIIRS and MODIS are within 2%, with an exception for the 0.865- μm band, after correction of their spectral response differences.

The newly launched (November 18, 2017) polar-orbiting satellite of the Joint Polar Satellite System (JPSS-1), now transitioned to NOAA-20, is the follow-on mission to the SNPP (Suomi National Polar-orbiting Partnership) satellite, launched six years ago. NOAA-20 leads SNPP by a half orbit or about 50 minutes. The Visible Infrared Imaging Radiometer Suite (VIIRS) is a key sensor onboard both NOAA-20 and SNPP spacecraft with nearly identical band spectral responses. Similar to the heritage sensor MODIS, VIIRS has on-board calibration components including a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB), a V-groove blackbody for the thermal emissive bands (TEB), and a space view (SV) as background reference for calibration. This study provides an initial assessment of calibration of the NOAA-20 VIIRS reflective solar bands (RSB) by inter-comparison with measurements from SNPP VIIRS using various vicarious approaches. The first approach is based on a double difference method using observations from simultaneous nadir overpasses (SNO) with Aqua MODIS. The second is from the collected reflectances over the widely used Liby-4 desert site from 16-day repeatable orbits so each data point has the same viewing geometry relative to the site. The third approach is to use the frequent overpasses over the Dome C snow site. Results of this study provide useful information on NOAA-20 VIIRS post-launch calibration assessment and preliminary analysis of its calibration stability and consistency for the first 6 months

Aqua MODIS is the second MODIS instrument of NASA’s Earth Observation System and has operated for over sixteen years since its launch in 2002. MODIS has sixteen thermal emissive bands (TEBs) located on two separate cold focal plane assemblies (CFPA). The TEBs are calibrated every scan using observations of an onboard blackbody (BB) and a space view port. Low saturation temperatures (Tsat) of Aqua MODIS bands 33, 35, and 36 cause these bands to saturate when the BB temperature is higher than their Tsat values during a BB warm-up cool-down (WUCD) cycle, therefore impacting the ability to perform nominal calibration. In addition, starting from around 2006, the CFPA temperature showed gradual variation from its nominally-controlled operating temperature due to a loss of its radiative cooler margin and the magnitude of its fluctuation reaching a maximum in 2013. The MODIS Characterization Support Team currently uses a correction that is dependent on the CFPA temperature to provide a gain estimate for the saturated scans during the BB WUCD. This gain estimation has been implemented in the Aqua MODIS Collection 6 (C6) and C6.1 L1B products. This paper evaluates the quality of the calibrated radiance of Aqua MODIS bands 33, 35, and 36 using simultaneous nadir observations from the Atmospheric Infrared Sounder (AIRS), which is also onboard the Aqua satellite. Our analysis results show that the differences between AIRS and Aqua MODIS can be controlled well within the fluctuation range compared to the periods when the BB signals for these bands are not saturated.

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a key sensor carried on the newly launched (November 18, 2017) Joint Polar Satellite System-1 (JPSS-1), now transitioned to NOAA-20, and the Suomi National Polar-orbiting Partnership (SNPP) satellite. The two VIIRS sensors are nearly identical in design. Its on-board calibration components include a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB), a V-groove blackbody for the thermal emissive bands (TEB), and a space view (SV) port for background subtraction. These on-board calibrators are located at fixed scan angles. The VIIRS response versus scan angle (RVS) was characterized prelaunch in lab ambient conditions and is currently used to calibrate the on-orbit response for all scan angles relative to the calibrator’s scan angle. A spacecraft level pitch maneuver was scheduled during the initial intensive Cal/Val testing for both the NOAA-20 and SNPP. The pitch maneuver provided a rare opportunity for VIIRS to make observations of deep space over the entire scan angle range, which can be used to characterize the TEB RVS. This study provides our analysis of the NOAA-20 pitch maneuver data and assessment of the derived TEB RVS. A comparison between the RVS determined by the pitch maneuver observations and prelaunch lab measurements is conducted for each band, detector, and mirror side of the half angle mirror.

Terra MODIS has provided continuous global observations for science research and applications for more than 18 years. The MODIS Thermal emissive bands (TEB) radiometric calibration uses a quadratic function for instrument response. The calibration coefficients are updated using the response of an on-board blackbody (BB) in quarterly warm-up and cool-down (WUCD) events. As instrument degradation and electronic crosstalk of long-wave infrared (LWIR) bands 27 to 30 developed substantial issues, accurate calibration is crucial for a high-quality L1B product. The on-board BB WUCD temperature ranges from 270 K to 315 K and the derived nonlinear response has a relatively large uncertainty for the offset, especially for these LWIR bands, which affects the measurements of low brightness temperature (BT) scenes. In this study, the TEB radiometric calibration impact on the L1B product is assessed using selected cold targets and the measurements during regular lunar rolls. The cold targets include Antarctic Dome Concordia (Dome-C) and deep convective clouds (DCC) for the calibration assessment, focusing on bands 27 to 30. Dome-C area is covered with uniformly-distributed permanent snow, and the atmospheric effect is small and relatively constant. Usually the DCC is treated as an invariant earth target to evaluate the reflective solar band calibration. The DCC can also be treated as a stable target to assess the performance of TEB calibration. During a scheduled lunar observation event with a spacecraft roll maneuver to view the moon through the space view port, the instrument cavity provides a stable reference for calibration assessment. The long-term trending of BT measurements and the relative difference between scan mirror sides and detectors are used for the assessment of the calibration consistency and stability. The comparison of L1B products over the selected targets before and after the calibration coefficients update can be used to assess the impact of a calibration look-up table (LUT) update. This assessment is beneficial for future calibration algorithm and LUT update procedure improvements for enhancing the L1B product quality.

The MODIS instruments on the Terra and Aqua spacecraft are cross-track scanning radiometers that view the Earth scene, a space view port, and the on-board calibrators using a two-sided scan mirror. The reflectivity of the scan mirror varies with the angle of the incident light, and changes in this response versus scan angle (RVS) need to be tracked on orbit in order to maintain accurate calibration. In this paper, we review various methods of RVS calibration of the reflective solar bands using pseudo-invariant desert targets and discuss potential advantages and disadvantages for use in future calibration.

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite has operated successfully since its launch in October, 2011. The calibration using regular solar diffuser (SD) observations for the 14 reflective solar bands facilitates the generation of high-quality calibrated products, known as sensor data records (SDR). As SNPP VIIRS nears six years on-orbit, monitoring the on-orbit calibration performance using multiple techniques is vital. One such technique, using the deep convective clouds (DCC), has been widely used to monitor the on-orbit calibration performance of earth observing sensors, such as the MODIS instrument on Terra and Aqua spacecrafts. In this study, the DCC technique is utilized to evaluate the stability of SNPP VIIRS SDR reflectance product for 10 moderate resolution bands (M-bands, M1–M5 and M7– M11) and three imaging bands (I-bands, I1–I3). An empirical Bidirectional Reflectance Distribution Function (BRDF) correction for the long-term reflectance measurements over DCC is formulated and implemented. The fluctuation in the long-term trending is analyzed to evaluate the uncertainties. The BRDF-corrected reflectances over DCC are used to evaluate the stability of the response versus scan-angle (RVS) and its effects in the SDR products. The RVS effects are analyzed based on the difference in mode reflectances over DCC among six different frame zones. Results indicate that the RVS effects should be monitored for an improved VIIRS RSB calibration in the future.

Over the years, data from Terra and Aqua MODIS (The Moderate Resolution Imaging Spectroradiometer) has provided invaluable information about Earth's atmosphere, land and oceans. Both MODIS sensors have exceeded their designed lifetime of 6 years and are still in operation. Therefore, it is important to understand the on- orbit performance of both sensors. The thermal emissive bands (TEB) on MODIS are comprised of 16 spectral bands with wavelengths from 3.7 to 14.4 μm. The TEB are calibrated on orbit on a scan-by-scan basis using a blackbody calibration source. Since the mission beginning, a steady increase in electronic crosstalk has been observed in Terra MODIS bands 27-30. The MODIS Characterization Support Team (MCST) at NASA/GSFC has recently derived a set of correction factors that correct these bands for the entire mission. These corrections are being applied in Terra Collection 6.1. In this study, the effectiveness of this crosstalk correction is assessed. First, the observed brightness temperatures over Dome C for Terra with and without the crosstalk correction are compared. Then, the calibration consistency and stability between the TEB of Terra, with the crosstalk correction, and Aqua MODIS are also assessed. Finally, the relative bias between the two MODIS instruments is evaluated using the near-surface temperature measurements from an Automatic Weather Station (AWS).

Since launch, the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on the Terra and Aqua spacecraft have operated successfully for more than 14 and 12 years, respectively. A key instrument for National Aeronautics and Space Administration Earth Observing System missions, MODIS was designed to make continuous observations for studies of Earth's land, ocean, and atmospheric properties and to extend existing data records from heritage Earth observing sensors. The 16 thermal emissive bands (TEBs) (3.75–14.24 $\mu\mbox{m} $ ) are calibrated on orbit using a temperature controlled blackbody (BB) . Both Terra and Aqua MODIS BBs have displayed minimal drift over the mission lifetime, and the seasonal variations of the BB temperature are extremely small in Aqua MODIS. The long-term gain and noise equivalent difference in temperature performance of the 160 TEB detectors on both MODIS instruments have been well behaved and generally very stable. Small but noticeable variations of Aqua MODIS bands 33–36 (13.34–14.24 $\mu\mbox{m} $ ) response in recent years are primarily due to loss of temperature control margin of its passive cryoradiative cooler. As a result, fixed calibration coefficients, previously used by bands when the BB temperature is above their saturation temperatures, are replaced by the focal-plane-temperature-dependent calibration coefficients. This paper presents an overview of the MODIS TEB calibration, the on-orbit performance, and the challenging issues likely to impact the instruments as they continue operating well past their designed lifetime of six years.

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission was recommended by the National Research Council in 2007 to conduct highly accurate and International System of Unit-traceable decadal change observations and provide an on-orbit intercalibration standard with high accuracy for relevant Earth observing sensors. The goal of reference intercalibration is to enable rigorous observations of critical climate change variables, including reflected broadband radiation, cloud properties, and changes in surface albedo, including snow and ice albedo feedback, to be made consistently among different sensors. This requires the CLARREO Reflected Solar Spectrometer (RSS) to provide highly accurate spectral reflectance measurements to establish an on-orbit reference with a radiometric accuracy requirement better than 0.3% $(\mathrm{k} =2) $ for existing sensors. In this paper, MODTRAN-simulated top-of-atmosphere spectral data and spectral measurements from the SCIAMACHY instrument on Envisat are used to determine sensitivity of intercalibration uncertainty on key design parameters of the CLARREO spectrometer: spectral range, sampling and resolution. Their impact on intercalibration uncertainty for MODIS and VIIRS imagers is estimated for various surface types (ocean, vegetation, desert, snow, deep convective clouds, clouds and all-sky) . Results indicate that for the visible to near-infrared spectral region (465–856 nm) , the RSS instrument under current design concept produces uncertainties of 0.16% for the spectral range and 0.3% for the sampling and resolution. However, for the water vapor absorption bands in the short wavelength infrared region (1242–1629 nm) , the same requirement is not met for sampling and resolution due to their high sensitivity to the influence of atmospheric water vapor.

The Moderate Resolution Imaging Spectroradiometer (MODIS) Calibration Team has recently released the Collection 6 (C6) radiances, which offer broad improvements over Collection 5 (C5). The recharacterization of the solar diffuser, lunar measurements, and scan mirror angle corrections removed most of the visible channel calibration drifts. The visible band calibration stability was validated over the Libyan Desert, Dome-C, and deep convective cloud (DCC) invariant Earth targets, for wavelengths less than 1 μm. The lifetime stability of Terra and Aqua C6 is both within 1%, whereas the Terra C5 degradation exceeded 2% for most visible bands. The MODIS lifetime radiance trends over the invariant targets are mostly within 1%; however, the band-specific target fluctuations are inconsistent, which suggests that the stability limits of the invariant targets have been reached. Based on Terra- and Aqua-MODIS nearly simultaneous nadir overpass (NSNO) radiance comparisons, the Terra and Aqua C6 calibration shows agreement within 1.2%, whereas the C5 calibration exceeds 2%. Because the MODIS instruments are alike, the same NSNOs are used to radiometrically scale the Terra radiances to Aqua. For most visible bands, the Terra-scaled and Aqua C6 radiances are consistent to within 0.5% over Dome-C, DCC, and for geostationary visible imagers having similar spectral response functions, which are used as transfer radiometers. For bands greater than 1 μm, only minor calibration adjustments were made, and the C6 calibration is stable within 1% based on Libya-4.

Terra and Aqua MODIS instruments have continued to operate normally since their launch in December 1999 and May 2002. MODIS reflective solar bands (RSB) with wavelengths ranging from 0.41 to 2.3 μm are calibrated on-orbit by an on-board solar diffuser (SD) and a solar diffuser stability monitor (SDSM). In addition, a spectroradiometric calibration assembly (SRCA) is used regularly to characterize and track on-orbit changes in RSB spectral, spatial, and radiometric performance. On a near-monthly basis, lunar observations are scheduled and performed to support sensor on-orbit calibration and characterization, such as radiometric calibration stability monitoring for the RSB. This paper provides a brief review of MODIS design requirements related to its RSB calibration performance and an assessment of MODIS RSB on-orbit calibration stability on both the short- and long-term timescales. Examples from different approaches are presented to demonstrate on-orbit performance of MODIS RSB calibration stability.

Signatures of electronic crosstalk contamination are clearly seen in Aqua MODIS Moon images from bands 20 to 30 (1.375 and 3.75 - 9.73 μm). Electronic crosstalk can potentially impact L1B products, causing image artifacts such as striping and radiometric bias, the severity of which will depend on the amount of signal leaked, how it compares to the flux levels in the receiving image, and on the ability of the calibration in absorbing the contamination. In this paper, we address two distinct manifestations of electronic crosstalk contamination seen in Aqua MODIS lunar images that we call the simple and complex ghosts. The simple ghosts appear in lunar images from bands 20 to 30, affect detector 1 only, and each ghost is caused by signal leak from one sending detector only. The complex ghosts form negative regions in Aqua MODIS lunar images from bands 27 to 30 and are caused by multiple sending detectors and bands. We map the crosstalk signatures back to their respective sending bands/detectors, by determining the displacement between the main lunar image and the crosstalk ghosts. We assume the contaminating signal is proportional to the signal of the sending band/detector and derive linear crosstalk coefficients for the simple ghosts in bands 20 to 26 and 28 to 30 and for the complex ghosts in band 29 for the entire mission. The linear crosstalk coefficients can then be applied in the correction of L1A Earth and calibration data that will generate corrected L1B images from which we can assess the impact the contamination has on the L1B product.

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP (National Polar-orbiting Partnership) satellite has been in operation for over five years. VIIRS has 22 bands with a spectral range from 0.4 μm to 2.2 μm for the reflective solar bands (RSB). The Earth view swath covers a distance of ~3000 km over scan angles of ± 56.0° off nadir. The on-board calibration of the RSB relies on a solar diffuser (SD) located at a fixed scan angle and a solar diffuser stability monitor (SDSM). The response versus scan angle (RVS) was characterized prelaunch in ambient conditions and is currently used to determine the on-orbit response for all scan angles relative to the SD scan angle. Since the RVS is vitally important to the quality of calibrated level 1B products, it is important to monitor its on-orbit stability, particularly at the short wavelengths (blue) where the most degradation occurs. In this study, the RVS stability is examined based on reflectance trends collected at various scan angles over the selected pseudo-invariant desert sites in Northern Africa and the Dome C snow site in Antarctica. These trends are corrected by the site dependent BRDF (bi-directional reflectance function) model to reduce seasonally related fluctuations. The BRDF corrected trends are examined so any systematic drifts in the scan angle direction would indicate a potential change in RVS. The results of this study provide useful information on VIIRS RVS on-orbit stability performance.

The Moderate-Resolution Imaging Spectroradiometer (MODIS), launched in 1999 on Terra and 2002 on Aqua spacecraft respectively, is a scanning radiometer that covers a wavelength range from 0.4 μm to 14.4 μm and scans the Earth over an angular range from -55° to +55°. After a few years in the Terra mission, it became extremely challenging to characterize the changes in the sensor gain and response versus scan angle (RVS) at short wavelengths due to significant degradation and increased polarization sensitivity. To better characterize the system-level degradation, the MODIS Characterization Support Team (MCST) developed an enhanced approach in Collection-6 (C6) L1B algorithm by supplementing the on-board calibration data with the Earth-scene response trends at various scan angles obtained from the pseudo-invariant desert sites. However, the trends at short wavelengths experienced significant impact due to the increased polarization sensitivity, especially at the end of scan. In this study, a polarization correction algorithm developed by MCST is applied to the Terra MODIS RSB response trends obtained from the desert sites. The trends after polarization correction are used to derive the gain and RVS based on the existing MODIS C6 calibration algorithm. Impact of the polarization correction is examined for gain, RVS and their fitting uncertainties over the entire mission. The results of this study provide useful information on how to further improve accuracy and stability of the calibrated L1B product.

The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key scientific instrument that was launched into Earth orbit by NASA in 1999 on board the Terra (EOS AM) satellite and in 2002 on board the Aqua (EOS PM) satellite. Terra and Aqua MODIS collect the entire Earth’s images every 1 to 2 days in 36 spectral bands. MODIS band 1 (0.620- 0.670 μm) and band 2 (0.841-0.876 μm) have nadir spatial resolution of 250 m and their measurements are crucial to derive key land surface products. This study evaluates the performance of the Collection 6 (C6, and C6.1) L1B of both Terra and Aqua MODIS bands 1 and 2 using Simultaneous Nadir Overpass (SNO) data to compare with AVHRR/3 sensors. We examine the relative stability between Terra and Aqua MODIS in reference to NOAA N15 and N19 the Advanced Very High Resolution Radiometer (AVHRR/3). The comparisons for MODIS to AVHRR/3 are over a fifteenyear period from 2002 to 2017. Results from this study provide a quantitative assessment of Terra and Aqua MODIS band 1 and band 2 calibration stability and the relative differences through the NOAA N15 and N19 AVHRR/3 sensors.

The 16 thermal emissive bands (TEB) of MODIS instruments on-board Terra and Aqua satellites are located at two cold focal plane assemblies (CFPA), which are cryogenically cooled by a passive radiative cooler. The CFPA temperatures are controlled at a nominal value of 83 K. For Aqua MODIS, the cooler margin has gradually decreased since launch, which deteriorates the CFPA temperature control. Since 2006, Aqua CFPA temperature has deviated from its 83 K baseline and been fluctuating with the instrument temperature seasonally, daily and per orbit. The magnitude of the fluctuation steadily increased until reached its peak at 0.7 K in 2013 and has been decreasing. The gains of TEB detectors, especially those of the photoconductive bands 31-36, change with the CFPA temperature by up to 8%, as is demonstrated by both pre-launch calibration and on-orbit monitoring. In this paper, the CFPA temperature and its related telemetries are provided. The impact of the fluctuation to TEB radiometric calibration is also assessed. Overall, because the calibration is normally performed on a scan-by-scan basis based on the observation of an onboard blackbody (BB), the detector gain change can be retrieved nearly real time. Therefore, the impact is not significant overall. However, for bands 33, 35 and 36, their detectors saturate when observing BB at BB temperature above certain saturation limits during quarterly held BB warmup-cooldown activities. Since there is no valid scan-by-scan calibration during these periods, a special treatment has to be applied to calibrate these bands to reflect the detector gain fluctuation.

Observations in the Terra MODIS PVLWIR bands 27 – 30 are known to be influenced by electronic crosstalk from those bands as senders and into those same bands as receivers. The magnitude of this crosstalk affecting L1B radiances has been steadily increasing throughout the mission lifetime, and has resulted in several detectors within these bands to be unusable for making L2 and L3 science products. In recent years, the crosstalk contamination has been recognized as compromising the climate quality status of several MODIS L2 and L3 science products that depend on the PVLWIR bands. In response, the MODIS Characterization Support Team (MCST) has undertaken an effort to generate a crosstalk correction algorithm in the operational L1B radiance algorithm. The correction algorithm has been tested and established and crosstalk corrected L1B radiances have been tested in several Terra MODIS L2 science product algorithms, including MOD35 (Cloud Mask), MOD06 (Cloud Fraction, Cloud Particle Phase, Cloud Top Properties), and MOD07 (Water Vapor Profiles). Comparisons of Terra MODIS to Aqua MODIS and Terra MODIS to MetOp-A IASI show that long-term trends in Collection 6 L1B radiances and the associated L2 and L3 science products are greatly improved by the crosstalk correction. The crosstalk correction is slated for implementation into Collect 6.1 of MODIS processing.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments have successfully operated for more than 17 and 15 years, respectively, on-board the NASA’s Earth Observing System (EOS) Terra and Aqua spacecraft. MODIS level 1B (L1B) data products include top of the atmosphere (TOA) reflectance factors for the reflective solar bands (RSB) and radiances for both the RSB and the thermal emissive bands (TEB), and their associated uncertainty indices (UI) at a pixel-by-pixel level. This paper provides a brief review of MODIS L1B calibration algorithms, including improvements made in recent years. It presents an update of sensor calibration uncertainty assessments with a focus on several new contributors resulting from changes in sensor characteristics and on-orbit calibration approaches and the impact due to these changes on the L1B data quality. Also discussed in this paper are potential changes that could be made to continue improving the quality of MODIS L1B uncertainty product.

Since launch on May 04, 2002, Aqua MODIS has successfully operated for 15 years and continuously produced from its observations many data products in support of a broad range of scientific research activities and applications. Its overall mission success has relied heavily on the dedicated efforts to operate and calibrate the instrument and to track and correct on-orbit changes in sensor responses. This paper provides an overview of Aqua MODIS instrument operation and calibration activities, algorithm improvements, and look-up-table (LUT) updates. Results derived from various on-orbit calibration targets are presented to demonstrate sensor long-term performance. Also discussed in this paper are challenging issues identified and future efforts to maintain instrument calibration and data quality.

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.

The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, unlike its predecessor Terra MODIS, had always been thought to have been spared from significant deleterious impacts of electronic crosstalk on its imagery. However, recent efforts brought to our attention the presence of striping artifacts in Aqua MODIS images from band 24 ( $4.47~\mu \text{m}$ ), which upon further inspection proved to have a noticeable impact on the quality of the L1B product and to have been present since the beginning of the mission, in 2002. Using images of the moon from scheduled lunar observations, we linked the artifacts with electronic crosstalk contamination of the response of detector 1 of band 24 by signal sent from the detector 10 of band 26 ( $1.375~\mu \text{m}$ ), a neighboring band in the same focal plane assembly. In this letter, we report on these findings, the artifact mitigation strategy adopted by us, and on our success in restoring band 24 detector 1 behavior and image quality.

An accurate on-orbit characterization of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership satellite is essential to satisfy the requirements from NOAA, NASA, and the general research community for high-quality operational and research products. NASA’s Land Science Investigator-led Processing System (LSIPS) sensor data records (SDRs) are utilized to assess the stability of the calibrated top of atmosphere reflectance over the deep convective clouds (DCCs) and over the Libya-4 desert site. The results from DCCs and desert show detector-to-detector (D2D) reflectance differences in the VIIRS reflective solar bands, 10 moderate-resolution bands (M-bands, M1 to M5, M7 to M11), and three imagery resolution bands (I-bands, I1-I3). More importantly, the D2D differences exhibit an increase in magnitude over time for bands M1 to M5 and I1 to I2, which induces noticeable striping and higher uncertainties in the downstream data products. The largest D2D difference is observed in bands M1 and M2, with magnitudes greater than 1.8% in trends among the 16 detectors and standard deviations less than 0.12% over time. The D2D stability assessment results over DCCs are consistent with those observed over the Libya-4 desert site. To improve the quality of the calibrated SDR reflectance data, NASA’s VIIRS characterization support team has improved the calibration algorithm to incorporate corrections based on these DCC measurements to mitigate the striping (detector differences) observed in the current version of the SDR. These improvements are planned to be included in the next mission reprocessing of the LSIPS land products.