Validation of CERES Flight Model 5 in-orbit calibrations using lunar observations Janet L. Daniels*a, George L. Smitha, Susan Thomasa, Kory J. Priestleyb aScience Systems and Applications, Inc., Hampton, Virginia, U. S.; bNASA Langley Research Center, Hampton, Virginia, U. S. ABSTRACT Scientific studies require radiation fluxes over the Earth to be accurate within 1% for shortwave fluxes and 0.5% for outgoing longwave fluxes. The validation of in-orbit instrument performance requires both stability in calibration source and also calibration corrections to compensate for instrument changes. The Moon offers an external source whose signal variance is predictable and non-degrading. CERES detectors register the signal output from the entire face of the Moon. Lunar observations performed by CERES Flight Models (FM) 1 through 4 have been successful in assisting validation of radiances to the required accuracy. CERES Flight Model 5 (FM-5) is on Suomi/NPP spacecraft, orbiting since October 2011. This paper uses lunar measurements to validate detector output of FM-5. These measurements are adjusted to remove orbital effects due to variations in distance between Moon and Sun, distance between the satellite and the Moon and lunar phase angle. These effects create a total variation in lunar irradiance of 20% in the total channel and 8% in the shortwave channel. The change in orientation of the Moon as seen by the detector is called libration and causes variations of about 1% of the irradiance. A consistent dataset spanning at least 2 years in length is required to remove variations due to libration. The major uncertainties remaining in the measurements are assumed to be due to changes of the spectral responses of the channels due to degradation of optical surfaces in orbit. The results demonstrate that lunar observations can be used to validate FM-5 measurements. Keywords: Clouds and Earth Radiant Energy System, Earth radiation budget, lunar measurements, radiometry, remote measurements, calibration, validation. 1. INTRODUCTION The understanding of inter-annual climate variations requires measurements of the Earth’s radiation budget over a period of years. The Clouds and Earth Radiant Energy System (CERES) Project was created for this need1. CERES Flight Models FM-1 and -2 have been operating on the Terra spacecraft starting in February 2000 and on Aqua FM-3 and -4 since June 2002. To assure continuity of the Earth Radiation Budget Climate Data Record, CERES FM-5 was placed into orbit on the Suomi/NPP in October 20112. The radiation fluxes over the Earth should be accurate within 1% for shortwave fluxes and 0.5% for outgoing longwave fluxes. Maintaining this accuracy demands frequent in-flight calibration3,4. The basis of in-flight calibration is the Internal Black Body System and the Shortwave Internal Calibration System5. These are augmented by using Deep Convective clouds as a vicarious calibration target6. A thorough validation protocol7 is implemented to assure the resulting accuracy. The first set of validation methods is comparing radiances from the different CERES instruments. The FM-1 and -2 instruments can be compared directly as they are both on the Terra spacecraft. Likewise the FM-3 and -4 on Aqua can be compared. FM-1 and FM-3 can be compared near 70oN where the orbits cross8. Similarly FM-5 measurements are compared with FM-1 radiances9. Because the Aqua and Suomi/NPP orbits are so close, there are ways to compare FM-5 and FM-3 frequently10. The Tropical Mean Method11 provides a check between the channels. Daniels et al.12 developed a method of validation using lunar observations near full moon by the Clouds and the Earth’s Radiant Energy System (CERES) and applied the method to the four CERES instruments on the Terra and Aqua spacecraft. The present paper uses lunar observations to validate the FM-5 measurements from 2011 to 2019. First the CERES lunar observation method and its application to each of the three channels are described, then the lunar observation used to compute the changes of each of the FM-5 channels. The results are compared to those obtained from FM-1 through -4. *[email protected]; phone 1 757 864-2778; fax 1 757 864-6319; nasa.gov 2. METHODOLOGY The validation of in-orbit instrument performance requires both stability in calibration source and also a method to correct calibrations to compensate for instrument changes. Unlike internal calibration systems, the Moon is an external source whose signal variance is predictable and non-degrading. Figure 1 shows the viewing geometry during lunar observations. The observations are made near full Moon so as to have near the brightest and hottest condition to get a good dynamic range. At this time the Moon is near the Sun-Earth line. As the Suomi/NPP spacecraft comes over Antarctica, the instrument elevation gimbal is fixed and FM-5 is rotated slowly in azimuth to bring the Moon through the field of view several times during the viewing opportunity. Thus, there are several measurements for each full Moon. Because the instrument is not scanning in elevation angle and the rotation in azimuth is slow compared to the detector response time, the dynamic point response function does not apply. The static point response function is constant over the field of view except near the edges, where the optical blur (0.16o) causes it to decrease. Figure 1: Suomi/NPP orbital location during lunar observations. Figure 2 shows the image of the Moon at mean Earth-Moon distance within the CERES field of view. Because of the variation of albedo over its surface, as the Moon rotates relative to the instrument the net albedo changes. These librations create changes in the measurements. The radius of the Moon’s image varies inversely with the Satellite-to-Moon distance due to the ellipticity of the Moon’s orbit, so the area of the image and the measurement decrease as the inverse square of the distance. Figure 2: Image of Moon in CERES field of view. The insolation on the Moon varies because of the variation of the distance between Sun and Moon. The measurement also varies due to the change of distance between the instrument and the Moon. The lunar phase angle, i.e. the lunar central angle between the lunar sub-solar point and the sub-satellite point, determines how well the part of the Moon which is viewed is sun-lit. Daniels et al.12 demonstrated that when these factors are taken into account, there is about a 1% variation remaining, due to lunar libration. They showed that these changes of irradiance for the three channels averaged out well below the accuracy requirements over a two year period. This procedure is be used for validating the in-flight calibration of FM-5. 3. ANALYSIS Edition 1-CV data are used in this paper. For each channel, lunar measurements were normalized. Figure 3a shows the normalized measurements by the total channel for lunar observations from February 2012 through June 2016. The satellite- to-Moon distances for the measurements are also shown, as a solid line. The total channel measurements are adjusted to the mean satellite-to-Moon distance using the inverse square and is shown in figure 3b. There is still a strong cycle present. The distance from Moon-to-Sun is also plotted and the two curves are quite similar and are out of phase 180o. The inverse square relation is used to adjust the measurements to a distance of one astronautical unit and the result is plotted in figure 3c. The time variation is nearly periodic with a range of ±0.02. Much of this variation is due to the lunar libration. Figure 4 shows the shortwave channel measurements analyzed in the same manner. The scatter of the measurements for each month are greater than for the total channel. The albedo of the Moon is about 0.1, so the variations of albedo over the surface of the Moon cause a large relative change in the solar radiation. Because the total channel is measuring both the reflected and the emitted longwave fluxes, these variations of albedo make very little difference to that channel. When the satellite to Moon distance and the Moon to Sun distance are taken into account, the variations due to lunar librations remain in figure 4c. These are at the ±0.02 level, as for the total channel. The longwave channel measurements are shown in figure 5, which looks very much like those of the total channel in figure 3. As noted earlier, 0.9 of the lunar radiation is emitted radiation. a) b) c) Figure 3: Total channel radiances: (a) Variation of normalized radiances and satellite to Moon distance (solid line), (b) Variation of normalized radiances adjusted for satellite to distance and Moon to Sun distance (solid line), (c) Radiance after adjustments for satellite-to-Moon and Moon-to-Sun distances. a) b) c) Figure 4: Shortwave channel radiances: (a) Variation of normalized radiances and satellite to Moon distance (solid line), (b) Variation of normalized radiances adjusted for satellite to distance and Moon to Sun distance (solid line), (c) Radiance after adjustments for satellite-to-Moon and Moon-to-Sun distances. a) b) c) Figure 5: Window channel radiances: (a) Variation of normalized radiances and satellite to Moon distance (solid line), (b) Variation of normalized radiances adjusted for satellite to distance and Moon to Sun distance (solid line), (c) Radiance after adjustments for satellite-to-Moon and Moon-to-Sun distances. The effect of lunar phase angle (LPA) on the measurements is determined empirically. Figure 6 shows the measurements after they have been adjusted to a mean satellite-to-Moon distance and Moon-to-Sun distance of 1 A.U.