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.
The VIIRS instrument onboard the NOAA-21 spacecraft has successfully operated since its launch on November 18, 2022. A panchromatic channel in VIIRS, referred to as the day-night band (DNB), was designed with multiple gain stages resulting in a large dynamic range and high sensitivity such that its detectors can make observations during both spacecraft day and spacecraft night. The on-orbit calibration performance is monitored via the gain trending of low-gainstage (LGS), gain ratios of mid-gain-stage (MGS) to LGS, and high-gain-stage (HGS) to MGS, as well as dark offsets in all modes, detectors and HAM sides. Contamination of DNB images due to straylight has been observed in previous VIIRS builds. Data from monthly new moon observations have been used to estimate the straylight impact such that a look-up-table (LUT) has been built each month to correct the contaminated images. In this paper, the calibration algorithm and performance of NOAA-21 VIIRS DNB have been presented, together with the comparisons to previous VIIRS instruments onboard the S-NPP and NOAA-20 spacecraft.
The MODIS instrument onboard the Terra and Aqua satellites provides key measurements of various environmental parameters such as the land, ocean, and atmosphere. After over two decades of successful operations, both sensors experienced anomalies in the year 2022. In March, the Aqua spacecraft and, subsequently, the MODIS instrument entered a safe mode, and Terra MODIS experienced a Command Processor and Format Processor (CP/FP) reset. Separately, the Terra constellation exit maneuver (CEM) was performed in October, which included the transition of the MODIS instrument into a safe configuration as well. While MODIS has 16 infrared channels referred to as the thermal emissive bands (TEB), only the longwave infrared bands (27-30) were significantly impacted due to an increase in electronic crosstalk contamination after the Aqua MODIS sensor entered into safe mode. Crosstalk corrections have been applied to these bands to maintain the Level 1B product quality. Although to a lesser extent, the same MODIS bands were affected due to a slight increase in electronic crosstalk contamination after the Terra CEM was completed. Lastly, the Terra MODIS CP/FP reset had an effect on the digital output that transferred onto its photovoltaic bands due to their calibration algorithm. This paper presents the impacts of these events on the instruments’ TEB performance, and the subsequent changes made to their respective calibration algorithms.
VIIRS day-night band (DNB) covers a wavelength range from 500 nm to 900 nm, has three gain stages enabling a dynamic range of 7 orders of magnitude, and is calibrated by a solar diffuser. In this paper, the calibration uncertainty of the DNB is analyzed for both SNPP and NOAA-20 VIIRS instruments. It is shown that the uncertainties of the DNB for all gain stages, detectors, half angle mirror sides, and aggregation modes are much smaller than the uncertainty specifications of the band, which is 5%, 10%, and 100% for low, middle, and high gain stage, respectively.
MODIS is an imaging spectroradiometer onboard NASA’s Terra and Aqua satellites with visible to short-wave infrared capability enabled through 20 reflective solar bands (RSB) in the wavelength range of 400 to 2200 nm. RSB gain changes are monitored by using observations of a fully sunlit solar diffuser (SD) for the on-orbit radiometric calibration. SD signal changes, due to degradation of the SD surface, are monitored by using the SD Stability Monitor (SDSM), which has 9 detectors in the wavelength range of 400 to 936 nm. These calibration approaches depend on the geometry of the spacecraft’s orbit. Both Terra and Aqua have recently started to exit their respective constellations and are (or soon will be) in the process of drifting from their nominal orbits. These orbital changes will cause changes in the solar diffuser (SD) and solar diffuser stability monitor (SDSM) viewing geometry and calibration conditions. This, in turn, will drive a variation in the calibration parameters used to calculate reference adjustments for the MODIS reflective solar bands (RSB). We examine the expected effect of orbital drift on the reflectance and transmittance functions, the evaluation of on-going SDSM detector response, the variation of SD surface incidence angle ranges, and the estimation of fully-illuminated observing condition for SD signal monitoring.
The S-NPP and NOAA-20 VIIRS instruments have successfully operated since their launches on October 28, 2011 and November 18, 2017, respectively. A panchromatic channel onboard VIIRS is referred to as the day-night band (DNB), was designed with a large dynamic range and high sensitivity, such that its detectors can make observations during both daytime and nighttime. The DNB uses an onboard solar diffuser (SD) panel for low gain stage calibration, and the SD observations are also carefully selected to compute gain ratios between low-to-mid and mid-to-high gain stages. In this paper, we present the S-NPP and NOAA-20 VIIRS DNB calibration performed by the NASA VIIRS Characterization Support Team (VCST) to generate the calibration coefficient look up tables (LUTs) for the latest NASA Level 1B Collection 2 products. This activity supports the NASA Earth science community by delivering consistent VIIRS sensor data products via the Land Science Investigator-led Processing Systems. The DNB stray light contamination and its different behavior have been highlighted between two instruments. Its estimate and correction methods as well as performance are illustrated.
The MODIS imaging spectroradiometer instruments on-board NASA's Terra and Aqua satellites have 20 reflective solar bands (RSB) covering a wavelength range from 400 to 2200 nm. Radiance is calculated from processing raw signals with background, temperature, and electronic contamination corrected. Measured gain is calibrated with a fully Sunlit solar diffuser (SD) at a stable radiance level, considering a slowly changing SD reflectance degradation. These measurements provide time-dependent gain adjustment factors, and the calibration assumes a linear response for each band and detector. Hence, an analysis of the dependence on different radiance levels is warranted. The MODIS design has no mechanism for varying radiance levels, except for an attenuator screen. However, it has been in static configuration for Terra since mid-2003. An external source of radiance attenuation can be utilized during solar eclipse events, while maintaining high stability and accuracy of solar calibration standards. Due to its long mission lifetime, Terra has seen several Sun-Moon near-conjunction events when it coincides with the orbit path where the SD is directly illuminated. As of August 2020, we have identified 7 viable partial solar eclipses in the Terra mission data. We will discuss several results of our study, including comparison of measured SD signal to predicted radiance reduction based on a solar disk radiance model; nominal and outlier behavior as a function of bands, detectors, and mirror-sides; and comparison with other data sets. Our main conclusion from this study is that there is no notable correlation of detector-dependent trend with radiance level for most RSB bands.
Bryan J. Holler, Stefanie N. Milam, James M. Bauer, Charles Alcock, Michele T. Bannister, Gordon L. Bjoraker, Dennis Bodewits, Amanda S. Bosh, Marc W. Buie, Tony L. Farnham, Nader Haghighipour, Paul S. Hardersen, Alan W. Harris, Christopher M. Hirata, Henry H. Hsieh, Michael Kelley, Matthew M. Knight, Emily A. Kramer, Andrea Longobardo, Conor A. Nixon, Ernesto Palomba, Silvia Protopapa, Lynnae C. Quick, Darin Ragozzine, Vishnu Reddy, Jason D. Rhodes, Andy S. Rivkin, Gal Sarid, Amanda A. Sickafoose, Amy A. Simon, Cristina A. Thomas, David E. Trilling, Robert A. West
We present a community-led assessment of the solar system investigations achievable with NASA’s next-generation space telescope, the Wide Field Infrared Survey Telescope (WFIRST). WFIRST will provide imaging, spectroscopic, and coronagraphic capabilities from 0.43 to 2.0 μm and will be a potential contemporary and eventual successor to the James Webb Space Telescope (JWST). Surveys of irregular satellites and minor bodies are where WFIRST will excel with its 0.28 deg2 field-of-view Wide Field Instrument. Potential ground-breaking discoveries from WFIRST could include detection of the first minor bodies orbiting in the inner Oort Cloud, identification of additional Earth Trojan asteroids, and the discovery and characterization of asteroid binary systems similar to Ida/Dactyl. Additional investigations into asteroids, giant planet satellites, Trojan asteroids, Centaurs, Kuiper belt objects, and comets are presented. Previous use of astrophysics assets for solar system science and synergies between WFIRST, Large Synoptic Survey Telescope, JWST, and the proposed Near-Earth Object Camera mission is discussed. We also present the case for implementation of moving target tracking, a feature that will benefit from the heritage of JWST and enable a broader range of solar system observations.
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