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.

The reflective solar bands (RSB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (SNPP) satellite is calibrated by a solar diffuser (SD) whose performance is itself monitored by a solar diffuser stability monitor (SDSM). In this study, we describe the calibration algorithm of the SDSM, analyze the current two and a half years of calibration data, and derive the performance result for the SD, commonly called SD degradation or H-factors. The application of the newly derived vignetting functions for both the SD screen and the SDSM sun-view screen effectively removes the seasonal oscillations in the derived SD degradation and significantly improves the quality of the H-factors. The full illumination region, the so-called “sweet spot,” for both SD view and SDSM sun view is carefully examined and selected to ensure a consistent and an optimal number of valid data samples to reduce the sample noise owing to inconsistent or lack of samples. The result shows that SD degrades much faster at short wavelength as expected, about 28.5% at 412 nm but only 1.2% at 935 nm up to date. The performance of the SD degrades exponentially with time until 7 November 2013 but has since become flat. This sudden flattening of the SD degradation is a new phenomenon never previously observed for the degradations of the SD on VIIRS or other satellite sensors. The overall result shows that SDSM is essentially functioning without flaws in catching the on-orbit degradation of the SD. The most significant and direct impact of this work would be on the quality of the ocean color products that depend sensitively on moderate RSB (RSB) (M1–M8, M10, and M11). Two very important and key questions on the performance of the SD are also raised. One pertains to the directional dependence of the SD degradation result, and it is shown that the SD does not degrade uniformly in all directions as has been assumed by all SD calibration analyses. This has a definitive impact on the RSB calibration. Another is on the degradation of the SD at the shortwave infrared (SWIR) wavelengths, and it is shown that the zero degradation input for the RSB calibration would be erroneous. Last, the impact of the stray light on the SD since “first light” is cleanly exhibited in the improved SD degradation result.

The Visible Infrared Imaging Radiometer Suite (VIIRS) is now entering its fourth year of in-orbit global environmental observation and is producing a wide range of scientific output. The ocean color products in particular require a level of accuracy from the reflective solar bands (RSBs) that is a magnitude higher than the specification. In this work, we present an updated and completed core calibration pipeline that achieves the best sensor data records (SDR) to date and helps the ocean color products to reach maturity. We review the core calibration methodology of the RSBs and describe each essential input, including the solar diffuser stability monitor, the solar diffuser (SD), and lunar calibrations. Their associated issues, along with the successful mitigation and improved results, are described and presented. In particular, we illuminate the inaccuracy suffered due to the evolving angular dependence in the degradation of the on-board SD that impacts the heart of the RSB calibration, but also show that lunar-based calibration instead provides the correct long-term baseline for the successful restoration of the core methodology. The new look-up tables, which combine the coefficients from the SD-based and lunar-based calibrations, produce the optimal result, with an estimated accuracy of $\sim 0.2\%$. This hybrid approach highlights significant progress in the VIIRS RSB calibration and marks a completion of the core calibration result upon which other physical impacts or scientific issues can then be more accurately examined. We demonstrate the significant improvement and its impact on the ocean color products by comparing the current official output to the newly generated result. Lastly, we point out that this hybrid calibration coefficients scheme is made possible by a VIIRS design and layout change over its predecessor, the Moderate Resolution Imaging Spectroradiometer, that allows both the SD and the moon to be viewed by the RSB at the same angle of incidence. Thus, this design element warrants serious consideration for other satellite sensors utilizing a similar calibration methodology.

The reflective solar bands (RSBs) of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-Orbiting Partnership satellite are calibrated by a solar diffuser (SD) panel whose performance is itself monitored by an accompanying solar diffuser stability monitor (SDSM). In this comprehensive work we describe the SD-based calibration algorithm of the RSBs, analyze the calibration data, and derive the performance results—the RSB calibration coefficients or F-factors—for the current three and a half years of mission. The application of the newly derived product of the SD bidirectional reflectance factor and the vignetting function for the SD screen and the newly derived SD degradation, so-called H-factors, effectively minimizes the artificial seasonal patterns in the RSB calibration coefficients due to the errors of these ingredient inputs. The full illumination region, the “sweet spot,” during calibration events for SD view is carefully examined and selected to ensure high data quality and to reduce noise owing to non-fully illuminated samples. A time-dependent relative spectral response (RSR), coming from the large out-of-band contribution and the VIIRS optical system wavelength-dependent degradation, is derived from an iterative approach and applied in the SD calibration for each RSB. The result shows that VIIRS RSBs degrade much faster at near-infrared (NIR) and shortwave-infrared (SWIR) wavelength ranges due to the faster degradation of the rotating telescope assembly against the remaining part of the system. The gains of the VIIRS RSBs have degraded 2.0% (410 nm, Band M1), 0.2% (443 nm, Band M2), $-0.3\%$ (486 nm, Band M3), 0.2% (551 nm, Band M4), 6.2% (640 nm, Band I1), 11.0% (671 nm, Band M5), 21.3% (745 nm, Band M6), 35.8% (862 nm, Band I2), and 35.8% (862 nm, Band M7), respectively, since launch and 24.8% (1238 nm, Band M8), 18.5% (1378 nm, Band M9), 11.5% (1610 nm, Band I3), 11.5% (1610, Band M10), and 4.0% (2250 nm, Band M11), respectively, since 20 January 2012. It is established that the SD calibration accurately catches the on-orbit RSB degradation according to the instrument design and the calibration algorithm. However, due to the inherent nonuniform degradation of the SD affecting especially the short wavelength bands and the lack of capability of the SDSM calibration to catch degradation beyond 935 nm, the direct and the unmitigated application of the SD calibration result will introduce nonnegligible error into the calibration coefficients resulting in long-term drifts in the sensor data records and consequently the high-level products. We explicitly unveil the effect of the nonuniformity in SD degradation in the RSB calibration coefficients but also briefly discuss a critical yet simple mitigation to restore the accuracy of the calibration coefficients based on lunar observations. The methodology presented here thus remains intact as the cornerstone of the RSB calibration, and our derived RSB calibration coefficients represent the optimal result. This work has the most impact on the quality of the ocean color products that sensitively depend on the moderate visible and NIR bands (M1–M7), as well as the SWIR bands (M8, M10, and M11).

The radiometric calibration for the reflective solar bands (RSB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (SNPP) platform has reached a mature stage after four years since its launch. The characterization of the vignetting effect of the attenuation screens, the bidirectional reflectance factor of the solar diffuser, the degradation performance of the solar diffuser, and the calibration coefficient of the RSB have all been made robust. Additional investigations into the time-dependent out-of-band relative spectral response and the solar diffuser degradation non-uniformity effect have led to newer insights. In particular, it has been demonstrated that the solar diffuser (SD) degradation non-uniformity effect induces long-term bias in the SD-calibration result. A mitigation approach, the so-called Hybrid Method, incorporating lunar-based calibration results, successfully restores the calibration to achieve ~0.2% level accuracy. The successfully calibrated RSB data record significantly impacts the ocean color products, whose stringent requirements are especially sensitive to calibration accuracy, and helps the ocean color products to reach maturity.

The assumption of angular dependence stability of the solar diffuser (SD) throughout degradation is critical to the on-orbit calibration of the reflective solar bands (RSBs) in many satellite sensors. Recent evidence has pointed to the contrary, and in this work, we present a thorough investigative effort into the angular dependence of the SD degradation for the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite and for the twin Moderate-resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua spacecrafts. One common key step in the RSB calibration is the use of the SD degradation performance measured by an accompanying solar diffuser stability monitor (SDSM) as a valid substitute for the SD degradation factor in the direction of the RSB view. If SD degradations between these two respective directions do not maintain the same relative relationship over time, then the unmitigated use of the SDSM-measured SD degradation factor in the RSB calibration calculation will generate bias, and consequently, long-term drift in derived science products. We exploit the available history of the on-orbit calibration events to examine the response of the SDSM and the RSB detectors to the incident illumination reflecting off SD versus solar declination angle and show that the angular dependency, particularly at short wavelengths, evolves with respect to time. The generalized and the decisive conclusion is that the bidirectional reflectance distribution function (BRDF) of the SD degrades nonuniformly with respect to both incident and outgoing directions. Thus, the SDSM-based measurements provide SD degradation factors that are biased relative to the RSB view direction with respect to the SD. The analysis also reveals additional interesting phenomena, for example, the sharp behavioral change in the evolving angular dependence observed in Terra MODIS and SNPP VIIRS. For SNPP VIIRS the mitigation for this “SD degradation nonuniformity effect” with respect to angles relies on a “hybrid methodology” using lunar-based calibration to set the reliable long-term baseline. For MODIS, the use of earth targets in the major release Collection 6 to improve calibration coefficients and time-dependent response-versus-scan-angle characterization inherently averts the use of SD and its associated issues. The work further supports that having an open-close operational capability for the space view door can minimize SD degradation and its associated effects due to solar exposure, and thus provide long-term benefits for maintaining calibration and science data accuracy.