Kevin A. Twedt

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a key Earth-observing satellite sensor utilized to generate nearly 30 products for environmental studies and weather forecasting. Fourteen of the 22 VIIRS spectral bands are reflective solar bands (RSBs), which are calibrated on orbit through a sunlit solar diffuser (SD). Upon exposure to the Sun, the SD’s bidirectional reflectance distribution function (BRDF) changes its value, quantified by the H-factor. We observe that the H-factor depends on the location of the SD surface. Here, we compare the H-factor SD positional dependence across the three VIIRS instruments currently in operation, aboard the SNPP, the N20, and the N21 satellites. We determine the relative amount of solar exposure using the temporal trend difference in the H-factor before and after the nadir door is opened. Using the relative amount of solar exposure, we derive the striping model coefficients which generally agree well with the published values. Hence, our methodology can be applied early to destripe future VIIRS RSBs.

The MODIS reflective solar bands (RSB) are calibrated on orbit using data from several different stable sources. An on-board solar diffuser (SD) provides baseline reflectance calibration, traceable to pre-launch measurements, with a solar diffuser stability monitor (SDSM) to track on-orbit changes in the SD reflectance. Monthly observations of the Moon and regular data from select Earth targets, such as Libya desert sites and deep convective clouds, are used to track RSB gain changes at different MODIS scan angles. Data from all these sources are combined to produce the calibrated Collection 6.1 and Collection 7 Level 1B imagery. Since about 2022, both the Terra and Aqua satellites have been gradually drifting away from their historically stable orbits due to fuel limitations. We review the impact of orbit drift on the calibration data and the corresponding significant changes made to the MODIS RSB calibration algorithms. One challenge has been changes in the solar illumination angle to the SD, leading to large drops in the SD and SDSM signals, part of which can be characterized using data from 2024 yaw maneuvers. Orbit drift also affects the phase angle at which MODIS views the Moon, and a phase-angle correction has been investigated. For the Earth scenes, the drift impacts solar illumination and satellite viewing angles and an improved understanding of the scene bi-directional reflectance function is needed to continue to rely on these data. With these algorithm adjustments, we expect that high quality MODIS RSB calibration can be maintained through the end of the Terra and Aqua missions.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on-board NASA’s Earth Observing System Aqua satellite has continued providing Earth-science measurements for over two decades. Understanding the changes in instrument response throughout this extended mission is critical to providing reliable, quality data to the science community. To assist in this understanding, MODIS is equipped with on-board calibrators including the unique spectro-radiometric calibration assembly (SRCA). The SRCA can be commanded to provide calibrated signals to measure the radiometric response, spatial alignment, and spectral behavior of MODIS detectors. Using the SRCA in its spatial configuration provides the ability to characterize the relative registration of all 36 MODIS spectral bands, in both along-scan and along-track directions. The spectral configuration leverages an internal monochromator to measure the relative spectral response functions (RSRs) of MODIS detectors from 400 nm to 1 μm. This paper provides a comprehensive review of the SRCA spatial and spectral calibrations, their traceability to pre-launch measurements, performance against specifications, and investigation of measurement uncertainties. Recently, the SRCA was commanded into a special configuration to investigate signal cross-contamination (crosstalk) among MODIS bands. Measurements and lessons learned from this testing are also included.

The MODIS instrument on the Aqua satellite has provided more than 24 years of continuous, global Earth imagery since its launch in 2002, covering a wide spectral range from 0.4 to 14.4 μm with high radiometric precision. Aqua MODIS, in a sun-synchronous polar orbit with an early afternoon crossing time, provides complementary observations to its predecessor MODIS instrument on the Terra platform, launched in 1999 in a morning orbit. Together, the two MODIS represented a significant advance over similar legacy Earth remote sensors, with an improved combination of spatial and spectral resolution, radiometric precision, and a complex suite of on-board calibrators for characterizing the performance and maintaining calibration over the missions. Although the two MODIS have identical designs, their prelaunch and on-orbit performances have distinct differences. This paper gives a comprehensive review of Aqua MODIS on-orbit operation, calibration, and performance. The current calibration algorithms and associated uncertainties are discussed, including how they have evolved over time to accommodate changes in instrument behavior. Trends of key performance metrics are shown across the spectral bands, as well as evaluations of the on-board calibrator performance and comparisons to the performance of Terra MODIS. Aqua MODIS has maintained absolute reflectance accuracy within 2% for the reflected solar bands and radiance accuracy within 1% for most thermal bands and has demonstrated even more stringent long-term stability as verified by comparisons with other sensors and ground targets. The success of both MODIS sensors for more than two decades demonstrates the value of detailed prelaunch and on-orbit characterization and continued calibration adjustments in building long-term stable data records.

For over 26 years, the MODIS instrument on the Terra platform has generated a broad range of scientific products, enabling the remote sensing community worldwide for the study of many geophysical parameters of the Earth’s system. Together with the complementary observations from the MODIS instrument on the Aqua platform, a continuous and consistent data record dating back to the year 2000 has been generated. MODIS, a major advance over its heritage sensors in terms of its spectral, spatial, and temporal resolutions, provides Earth scene imagery via 36 spectral bands ranging from 0.4 to 14.4 μm at three different spatial resolutions with a wide swath of 2330 km. In addition, significantly enhanced are dedicated calibration efforts to help produce and maintain MODIS data quality through its entire mission. We present a comprehensive summary of Terra MODIS operations, the calibration strategies developed and implemented, and its on-orbit performance over the 26 years of successful operation. Terra MODIS has maintained radiometric uncertainties of less than 2% for reflective solar bands and better than 0.1 K for thermal emissive bands over its 26-year mission, far exceeding its 6-year design life. The mission’s success in maintaining long-term data quality demonstrates the critical importance of comprehensive on-board calibration systems and continuous performance monitoring for multiyear Earth observations. This calibration heritage provides essential guidance for designing future multidecadal Earth observing missions.

The Visible Infrared Imaging Radiometer Suite (VIIRS) is an Earth-observing satellite sensor. Out of the 22 VIIRS spectral bands, five are the short-wave infrared (SWIR) bands. Earth views of these bands for the VIIRS instrument aboard the NOAA-20 (N20) satellite have unexpected striping. The scene spectral radiance is retrieved by using a cubic polynomial of the background removed detector digital number with the offset set to zero. To minimize the striping, we develop an algorithm to find the offsets from the striping assessments of the satellite images of the Sahara Desert, deep convective clouds (DCCs) around the equator, and the Moon. We use the average of the offsets obtained from these three scenes and the other coefficients in the original cubic polynomial to update the polynomial coefficient look-up-table that is used for the NASA N20 VIIRS top-of-the-atmosphere spectral radiance and reflectance retrievals in Collection 2.1. After the application of the derived offsets, the striping from the scenes and the Libya 4 Desert is significantly reduced, except bands I3 and M10 over the DCC scene. For the DCC scene, the original striping for bands I3 and M10 is reduced but not significantly. In the case that the quadratic term in the polynomial also contributes to the observed striping, we further develop an algorithm to find both the offset and the quadratic term coefficient from striping assessments of two scenes whose spectral radiance levels differ significantly.

The MODIS instrument has been operating on both the Terra and Aqua spacecraft for more than 20 years. After the last orbital maintenance maneuvers performed for Terra and Aqua in early 2020 and 2021, respectively, their orbits have been drifting. The orbit drift brings a challenge of maintaining a consistent radiometric calibration for the MODIS reflective solar bands (RSB). In the current Collection 6.1 and upcoming Collection 7 Level 1B (L1B) data products, the calibration of the RSB relies on measurements from an on-board solar diffuser (SD) combined with ancillary data from scheduled lunar observations and pseudo-invariant Earth targets. The lunar and Earth view data is collected at various scan angles to track changes in the response versus scan angle (RVS). The orbit drift causes changes in the solar illumination and satellite view angles to the Earth targets. The simultaneous nadir overpass (SNO) approach provides a direct comparison of spectrally matching bands between MODIS and any one of the VIIRS sensors on the SNPP, NOAA-20 and NOAA-21 satellites, in which impacts of scene variability, solar illumination and sensor view angles are largely reduced. This study shows how the orbit drift affects frequency, location and angular match between MODIS and VIIRS crossovers. We extended the SNO approach from MODIS near-nadir views to off-nadir scan angles to examine the stability of MODIS to VIIRS reflectance ratios at various scan angles. Trending results starting from 2018 based on SNO with NOAA-20 VIIRS are presented for Terra and Aqua MODIS bands 1-4, 8 and 9.

The NOAA-21 VIIRS has successfully operated since its launch in November 2022 in an orbital constellation that includes the S-NPP and NOAA-20 launched in 2011 and 2017, respectively. Each VIIRS instrument makes daily global observations with 22 spectral bands, covering wavelengths from 0.41 to 12μm. It includes a day-night band (DNB) with a nominal spectral bandwidth of 0.5-0.9μm. The VIIRS instrument is regularly calibrated on orbit by a set of on-board calibrators (OBC), which include a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), and a space view (SV) port. On-orbit calibration activities and strategies also include near-monthly lunar observations performed via spacecraft roll maneuvers. The lunar observations are often used along with SD measurements to help improve on-orbit calibration of the reflective solar bands. In this paper, we provide an update of NOAA-21 VIIRS on-orbit calibration and performance since launch, and a comprehensive assessment of on-orbit changes of its spectral band responses before and after the second mid-mission outgas (MMOG) performed in February 2024. In addition to detector noise characterization, the OBC performance of NOAA-21 VIIRS is illustrated and compared with that of S-NPP and NOAA-20 over the same operating period. Results show that the NOAA-21 VIIRS overall performance is comparable to or better than its predecessors.

Dome Concordia (Dome C) in Antarctica is an excellent calibration site for polar-orbiting Earth observation instruments due to its spectral, spatial, and temporal uniformity. These instruments also observe Dome C multiple times a day and at a variety of geometries. The MODIS Characterization Support Team uses regular observations of Dome C by Aqua and Terra MODIS to help validate and improve the calibration of the detector gain and response versus scan angle of the reflective solar bands used to generate NASA’s Level 1B reflectance products. The reflectance trends at Dome C are typically assessed on a yearly basis, due to a six-month sunlit observation period. In this work, we increase the temporal resolution of the trends from yearly to bi-monthly and reduce measurement noise using a reflectance-based snow BRDF model. We show results for Terra and Aqua MODIS BRDF-normalized reflectance using the Collection 7 calibration for bands 1-4, 8-9, and 17. The BRDF model significantly reduces the variations in the bi-monthly reflectance trends with the best results observed near nadir and for the blue bands 3, 8, and 9. The higher temporal sampling allows for better real-time identification of any calibration errors during the sunlit season. In addition, due to its polar location, Dome C is largely insensitive to the recent orbit drift of the Terra and Aqua satellites which has created challenges for MODIS calibration based on other on-board and Earth targets. Combined, these advantages will make Dome C a particularly important calibration reference target during the final years of the Terra and Aqua missions.

The MODIS instruments on NASA’s Terra and Aqua satellites each use an on-board solar diffuser (SD) as the primary calibration source for the reflective solar bands (RSB). The reflectance properties of the MODIS solar diffusers were extensively characterized prior to launch and those measurements form the basis for the absolute reflectance and radiance calibration of the MODIS Level-1B data products. On orbit, additional characterization was done during early mission yaw maneuvers to verify the angular dependence of the SD reflectance and measure the transmission of an optional attenuation screen that can be placed in front of the SD. To account for degradation of the SD reflectance from cumulative solar radiation exposure, MODIS is also equipped with an on-board solar diffuser stability monitor (SDSM). Regular SD and SDSM calibrations have been made throughout the duration of the Terra and Aqua missions, for more than 20 years now. In the past few years, both Terra and Aqua have been drifting away from their historically maintained orbital planes. Observations of the SD continue to be made, but with incident solar angles that are outside the limits of the pre-launch and early mission characterizations, which presents a problem for accurate calibration. Recently, in August 2024, an additional set of yaw maneuvers was performed for Aqua to help characterize the upcoming drift-induced changes in SD measurements. We discuss the current state of Terra and Aqua MODIS SD calibration, the results of the Aqua yaw maneuvers, and the various options available to extend the SD measurements and RSB calibration through the end of the Terra and Aqua missions.