The MODIS instruments on the Terra and Aqua spacecrafts have successfully operated for more than 23 and 21 years, respectively, and have far exceeded their designed lifetimes of 6 years. The visible, near infrared, and short-wave infrared spectral bands, with wavelengths from 0.41 to 2.2 μm, are calibrated using the on-board solar diffuser. The mid-wave infrared and long-wave infrared spectral bands are calibrated using a blackbody. The sustained calibration and characterization efforts undertaken by the MODIS Characterization Support Team (MCST) have resulted in several upgrades to the Level-1B (L1B) algorithms over the mission lifetime. The latest version of the L1B algorithm, designated as Collection 7 (C7), was developed based upon observed performance of the current operational L1B product (C6.1), changing instrument behavior, and feedback from the science community. This paper provides an overview of the C7 algorithm and its improvements over the previous data collection, as well as some of the updates that have been incorporated since the first delivery in early 2021.

The MODIS instrument on NASA’s Terra and Aqua spacecrafts is a multispectral radiometer with a whiskbroom scanning design, using a two-sided scan mirror as the primary Earth-facing optical element. Accurate characterization of the on-orbit changes of both sides of the scan mirror is crucial to the calibration of the Level 1B data products. This paper reviews the performance of the Terra and Aqua MODIS scan mirrors over the missions for both the reflective solar and thermal emissive bands, including degradation in gain and response versus scan angle, and differences between the two mirror sides. Particular attention is given to a recent electronic reset event experienced by Terra MODIS in 2022 that led to changes in the zero-signal bias and gain characterization between the two mirror sides. Calibration algorithm changes were made to mitigate the impact of this event, leading to reduced mirror side striping in the Earth scene imagery.

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$14.4~\mu \text{m}$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 anin situocean target as reference [hereafter referred to asin situsea 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 thein situSST. 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 NOAA-20 (N20) satellite was launched on November 18, 2017 carrying the second Visible Infrared Imaging Radiometer Suite (VIIRS) instrument. Immediately following the launch, the VIIRS passed a series of intensive calibration and validation tests, after which regular calibration and operation activities have continued successfully for more than three years. The production of NASA Collection 2 Level 1B (C2 L1B) for N20 VIIRS began in summer 2019. In this article, we evaluate the early mission performance of the N20 VIIRS reflective solar bands (RSB) covering the first three full years of operation. The calibrated RSB gains are calculated primarily from the onboard solar diffuser (SD) and used in generating the C2 L1B reflectance and radiance products. We also show the on-orbit performance of the instrument noise, signal-to-noise ratio (SNR), and a reflectance uncertainty assessment. Comparisons are made to the first three years of operation of the first VIIRS instrument, aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite. We evaluate the long-term stability of the calibrated N20 RSB reflectance product by looking at the long-term trends of lunar observations and data from the pseudo-invariant Libya 4 desert site. The N20 RSB have had excellent early mission performance, with changes in the gain of less than 0.5% in the first three years across all detectors, stable L1B reflectance, and very stable values of detector SNR and reflectance uncertainty.

The Earth-observing Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership (SNPP) satellite regularly performs on-orbit radiometric calibration of its reflective solar bands (RSBs), primarily through observations of an onboard sunlit solar diffuser (SD). The on-orbit change of the SD bidirectional reflectance distribution function (BRDF) value, quantified by a numerical factor called the H-factor, is determined by the onboard SD stability monitor. Our previous study showed that the H-factor is solar angle- and view direction-dependent. In this study, we determine the dependence of the H-factor on the detector SD view footprint location. We fit an empirical model to the NASA Collection 1 SNPP VIIRS Level 1B (L1B) spectral reflectance difference across the detectors in an RSB over uniform Earth scenes of the Libya 4 desert and deep convective clouds (DCCs). We apply the model predicted SD-positional-dependent H-factor to calibrate the RSBs. Under this new calibration scheme, the original unreal striping is removed from the homogeneous Libya 4 desert and the DCC images, as well as the original unreal striping from the Dunhuang desert image. The SD-positional-dependent H-factor has been used to calculate the SNPP VIIRS RSB radiometric correction factor for the NASA Collection 2.0 SNPP VIIRS L1B products.

The inter-comparison of MODIS reflective solar bands onboard Aqua and Terra is very important for assessment of each instrument's calibration. One of the limitations is the lack of simultaneous nadir overpasses. Their measurements over a selected Earth view target have significant differences in solar and view angles, which magnify the effects of atmospheric scattering and Bidirectional Reflectance Distribution Function (BRDF). In this work, an inter-comparison technique is formulated after correction for site's BRDF and atmospheric effects. The reflectance measurements over Libya desert sites 1, 2, and 4 from both the Aqua and Terra MODIS are regressed to a BRDF model with an adjustable coefficient accounting for calibration difference. The ratio between Aqua and Terra reflectance measurements are derived for bands 1 to 9 and the results from different sites show good agreement. For year 2003, the ratios are in the range of 0.985 to 1.010 for band 1 to 9. Band 3 shows the lowest ratio 0.985 and band 1shows the highest ratio 1.010. For the year 2014, the ratio ranges from approximately 0.983 for bands 2 and 1.012 for band 8. The BRDF corrected reflectance for the two instruments are also derived for every year from 2003 to 2014 for stability assessment. Bands 1 and 2 show greater than 1% differences between the two instruments. Aqua bands 1 and 2 show downward trends while Terra bands 1 and 2 show upward trends. Bands 8 and 9 of both Aqua and Terra show large variations of reflectance measurement over time.

This work takes advantage of the stable distribution of deep convective cloud (DCC) reflectance measurements to assess the calibration stability and detector difference in Visible Infrared Imaging Radiometer Suite (VIIRS) reflective bands. VIIRS Sensor Data Records (SDRs) from February 2012 to June 2015 are utilized to analyze the long-term trending, detector difference, and half angle mirror (HAM) side difference. VIIRS has two thermal emissive bands with coverage crossing 11 μm for DCC pixel identification. The comparison of the results of these two processing bands is one of the indicators of analysis reliability. The long-term stability analysis shows downward trends (up to approximately 0.4% per year) for the visible and near-infrared bands and upward trends (up to 0.5% per year) for the shortand midwave infrared bands. The detector difference for each band is calculated as the difference relative to the average reflectance over all detectors. Except for the slightly greater than 1% difference in the two bands at 1610 nm, the detector difference is less than 1% for other solar reflective bands. The detector differences show increasing trends for some short-wave bands with center wavelengths from 400 to 600 nm and remain unchanged for the bands with longer center wavelengths. The HAM side difference is insignificant and stable. Those short-wave bands from 400 to 600 nm also have relatively larger HAM side difference, up to 0.25%. Comparing the striped images from SDR and the smooth images after the correction validates the analyses of detector difference and HAM side difference. These analyses are very helpful for VIIRS calibration improvement and thus enhance product quality.

The intercomparison of MetOp-A and MetOp-B Advanced Very High Resolution Radiometer visible and near-infrared (NIR) channels over the Libyan Desert with consideration of the effect from the bidirectional reflectance distribution function (BRDF) is used for MetOp-B postlaunch calibration update. In order to remove this effect from the comparison, two methods are investigated: BRDF effect modeling in direct comparison and seasonal oscillation regression. The BRDF modeling method employs both a simple linear model and an empirical model. The seasonal oscillation method has improved by adding a curve regression, where the seasonal oscillation is derived from MetOp-A measurement data since its launch and used for the regression of MetOp-B measurements. Using these methods, the reflectance ratios of MetOp-B over MetOp-A in three reflective solar channels have been derived, which are 1.034 for channel 1, 0.912 for channel 2, and 0.805 for channel 3A. The uncertainties of the derived ratios are estimated in the range of 4.7%-6.4%. The model accuracy and uncertainty have been discussed. Initial calibration updates based on these results have been delivered for the MetOp-B operational L1B product, and a routine update is performed monthly. The methods used in this work are also applicable to the intercomparison of other visible and NIR instruments.

This paper presents a generic self-emission model for infrared (IR) radiometers where the relationship between the instrument self-emission and calibration intercept is analyzed. This model is applied to both the prelaunch and on-orbit MetOp/High-Resolution IR Radiation Sounder (HIRS) calibration data. For MetOp/HIRS, the changes in intercept due to self-emission variations between calibration events are up to 1% of the onboard blackbody calibration radiance. The impact is dependent on Earth scene brightness temperature and channel wavelength. The investigation shows that the intercept variation due to self-emission is significant and, if not corrected, can induce errors of up to 1 K for Earth scenes with 285 K brightness temperature and higher for Earth scenes with lower brightness temperatures. Based on this self-emission model, an improved in-flight calibration algorithm is proposed to reduce systematic calibration errors. We believe that this model can improve accuracy and consistency of level 1b radiance for climate studies and can be applied to other IR radiometers.

Sixteen Moderate Resolution Imaging Spectroradiometer thermal emissive bands (TEBs) cover the wavelength from 3.75 to 14.24 μm. TEB calibration uses data collected from the detector responses to the onboard blackbody (BB) and space view. The BB was designed to operate either at a constant temperature for detector linear gain calibration or at temperatures varying from ambient (~270 K) to 315 K for on-orbit characterization of nonlinear coefficients. In this paper, we assess TEB on-orbit calibration performance in two aspects: One is to review the calibration trending on the orbital, daily, and multiyear timescales, and the other is to analyze the on-orbit calibration radiance uncertainty and its impact on the calibration. The calibration trending confirms the detector response dependence on the instrument temperature. The temperature trending and prelaunch characterization provide the basis for determining the calibration radiance source temperature range and uncertainties. An analytical approach was used to assess the impacts of onboard radiance uncertainties. The BB emission uncertainty, resulting from the temperature measurement error and emissivity uncertainty, causes a calibration uncertainty up to 0.3%, a value decreasing with the band wavelength. The BB nonblackness effect is analyzed and found to be insignificant. For the band with the lowest BB emissivity, the nonblackness affects the calibration radiance by less than 0.08%. The cavity emission uncertainty and the scan-mirror emission uncertainty both cause a less than 0.1% calibration uncertainty. The analysis of the nonlinear calibration coefficient uncertainty shows that its effect on the low Earth-view brightness-temperature range varies by band and is generally insignificant.

Since launch, Terra and Aqua MODIS have successfully operated for more than 10 years and 8 years, respectively. Data products derived from MODIS observations have been widely distributed to the science and user community, enabling a broad range of applications. MODIS collects data in 36 spectral bands, covering wavelengths from visible (VIS) to long-wave infrared (LWIR). They are calibrated on-orbit by a set of on-board calibrators (OBC). This paper provides an overview of instrument operation, calibration, and performance, including lessons learned. Though having exceeded their design lifetime of 6 years, both Terra and Aqua MODIS continue to perform well, collect useful data, and support Earth remote sensing applications.

MODIS has 16 thermal emissive bands (TEB) covering wavelengths from 3.75 to 14.24 ¿m. They are calibrated on a scan-by-scan basis using a quadratic calibration algorithm and data collected from detector responses to the instrument on-board blackbody (BB) and space view (SV). The MODIS on-board BB was designed to be capable of operating at temperatures varying from instrument ambient (about 270 K) to 315 K. This function has allowed the TEB nonlinear calibration coefficients to be characterized on-orbit and updated as needed. Following a brief description of MODIS TEB calibration methodologies and BB on-orbit operations, this paper provides an assessment of algorithm performance. Variations of detector short-term and long-term responses and their calibration impact are examined and quantified. Individual contributions from the BB, instrument scan cavity, and scan mirror thermal emissions are analyzed. A comparison is also made of Terra and Aqua TEB on-orbit performance.