Measurement method of the solar diffuser bidirectional reflectance distribution function in the short-wave infrared band

In the solar reflective band, an on-board calibration method based on a solar diffuser (SD) can realize full aperture, full field of view, and end-to-end absolute radiometric calibration of optical remote sensors. The SD’s bidirectional reflectance distribution function (BRDF) is a key parameter that affects the accuracy of the on-board calibration. High-accuracy measurement of the SD BRDF is required in the laboratory before launch. Due to the uncertainty of the goniometer system, polarization effects, and other factors, the measurement uncertainty of the SD BRDF at large incident angles is much higher than that at a 0° incident zenith angle and 45° reflection zenith angle. In this paper, an absolute BRDF measurement facility is reported. The goniometric system consists of a high-brightness integrating sphere as a radiation source, a six-axis robot arm, and a large rotation stage. The measurement wavelength range was from 350 nm to 2400 nm. An improved data processing method based on the reciprocity theorem was proposed to reduce the measurement uncertainty of the SD BRDF at large incident angles. At an incident zenith angle of 75°, the improved data processing method reduced the measurement uncertainty of the SD BRDF by 52% at 410 nm to 480 nm, by 70% at 480 nm to 1000 nm, and by 20% at other bands compared to the absolute measurement method. The influence of the radiation source, goniometer system, detection system, and other factors on the measurement uncertainty are analyzed in this paper. The results show that the measurement uncertainty (coverage factor k = 2) of the SD BRDF was better than 1.04% at 350 nm to 410 nm, 0.60% at 410 nm to 480 nm, 0.43% at 480 nm to 1000 nm, and 0.86% at 1000 nm to 2400 nm.

On-orbit radiometric calibration of the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor onboard the National Oceanic and Atmospheric Administration’s (NOAA) NOAA-20 satellite is dependent on Solar Diffuser (SD) and Solar Diffuser Stability Monitor (SDSM) observations for the Reflective Solar Bands (RSBs). The time-dependent degradation of the SD Bidirectional Reflectance Distribution Function (BRDF) (called H-factor) is measured by the SDSM. To reduce direct illumination from the Sun, SD and SDSM have two independent screens called SD Screen (SDS) and SDSM Sun view screen. Once the instrument is on-orbit, the product of the SD BRDF and SDS transmittance can only be measured by VIIRS and SDSM detectors through the Rotating Telescope Assembly (RTA) and SDSM SD viewport, respectively. Using pre-launch parameter tables, the initial H-factor trends showed abnormal oscillations. To validate the functionality of the pre-launch tables, the NOAA-20 VIIRS spacecraft performed 15 yaw manoeuvres on 25 January and 26 January 2018 as a part of post-launch tests. During the yaw manoeuvres, on-orbit SD and SDSM data were collected and used to characterize the SD screen transmittance functions with BRDFs and the SDSM Sun view screen transmittance functions. On-orbit estimated SD/SDSM BRDFs and SDSM Sun screen transmittance functions showed some differences from the pre-launch version, especially with the SDSM Sun view screen transmittance function. This problem was not resolved by applying the initial yaw manoeuvre-derived table. As an alternative approach, NOAA VIIRS team applied a methodology to add fine features in the SDSM Sun transmittance function from the one-year regular on-orbit SDSM collections to cover a full operational range of the solar azimuth angle in the SDSM Sun viewport. After applying additional SDSM detector gain corrections, the yaw manoeuvre and on-orbit regular SDSM data points are aligned that reduced abnormally large degradations in SDSM detector 6, 7, and 8. Finally, 1% level H-factor oscillations are reduced to 0.2% level. Corresponding SD F-factors, scaling calibration coefficients, are calculated and deliver to the NOAA’s Interface Data Processing Segment (IDPS) to generate the official VIIRS Science Data Record (SDR) products.

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a passive scanning Earth observing satellite radiometer. The VIIRS has 22 spectral bands with design center wavelengths from 0.41 to 12.01 m, providing data to generate more than 20 Earth’s biogeophysical parameters. Fourteen of the 22 VIIRS bands are the reflective solar bands (RSBs), detecting Earth reflected sunlight. To ensure data quality, regular on-orbit radiometric calibrations of the RSBs are performed, mainly through observations of an onboard solar diffuser (SD). The spectral radiance provided by the sunlit SD depends on the SD screen transmittance which is a function of the solar vector orientation. Additionally, on orbit the SD’s bidirectional reflectance distribution function (BRDF) changes its value due to solar bombardment. The BRDF change is derived from the SD stability monitor (SDSM) measurements. The SDSM views the Sun through a screen with through holes (the SDSM screen) and the SD at almost the same time. The time series of the ratio of the signal strengths is a measure of the SD BRDF on-orbit change. Hence the measurements of the on-orbit SD BRDF change depends on the SDSM screen relative transmittance which is also solar vector orientation dependent. In this paper for both the SNPP and the NOAA-20 VIIRS instruments we examine the solar vector orientation knowledge error through matching the SDSM screen relative effective transmittances derived from the calibration data collected on the yaw maneuver and the regular orbits.

To ensure data quality, the Earth-observing Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polar-orbiting Partnership satellite regularly performs on-orbit radiometric calibration of its 22 spectral bands. The primary radiance source for the calibration of the VIIRS reflective solar bands (RSBs) is a sunlit onboard solar diffuser (SD). During the calibration process, sunlight goes through a perforated plate (the SD screen) and then strikes the SD. The sunlight, scattered off the SD of near-Lambertian property, is used for the calibration. Consequently, the spectral radiance of the scattered sunlight is proportional to the product of the SD screen transmittance and the SD bidirectional reflectance distribution function (BRDF) value at the observation direction. The BRDF value is decomposed to the product of its initial value at launch and a numerical degradation factor that quantifies the decrease from the initial value. The degradation factor is determined by an onboard SD stability monitor (SDSM). During the BRDF degradation factor determination process, the SDSM receives the SD scattered sunlight and the sunlight that goes through another perforated plate at almost the same time. The ratio of the signal strengths from the two observations is used to determine the BRDF degradation factor. Consequently, the RSB radiometric calibration requires the accurate knowledge of the product of the SD screen transmittance and the initial BRDF value as sensed by the RSB and the SDSM detectors. We use both yaw maneuver and a small portion of regular on-orbit data to determine the products.

The advanced geosynchronous radiation imager (AGRI) is a geostationary sensor whose reflective solar band is calibrated by a solar diffuser (SD). The SD bidirectional reflectance distribution function (BRDF) degrades over time in the space environment. This degradation can be measured through the SD reflectance degradation monitor (SDRDM). The SDRDM calibration data are normally collected by three filtered detectors, covering wavelengths from 0.45μm to 0.90μm. The SD reflectance degradation can be derived by trending the ratio of the background-subtracted solar-angle corrected SDRDM sun and SD view responses. The conventional monitoring methods rely on geometry factors of the sun view port and the relative BRDF of the SD, and these parameters can cause uncertainties. Aiming at these uncertain factors, the present study collects calibration data at the same solar angle. This method compares the detector digital counts obtained at different times but with the same solar angle. Consequently, it cancels out the angle-dependent parameter to obtain the ratio of the BRDF degradation factors. The obtained results show that monitoring uncertainty of the proposed method is less than 0.18%, while the corresponding monitoring error is less than 0.66%. This method can be applicated in SD's BRDF monitoring.

To radiometrically calibrate its reflective solar bands, the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership satellite observes a sunlit onboard solar diffuser (SD). The degradation factor of the SD bidirectional reflectance distribution function (BRDF) is determined by an onboard solar diffuser stability monitor (SDSM). We improve the accuracy of the measured SD BRDF degradation factor in four ways. First, we remove the bias in the previously computed relative product of the SD screen transmittance and the BRDF at t 0 (when the BRDF degradation just started) τ R/SD, eff BRDF( t 0 ). The bias is introduced by the angular dependence of the SD BRDF degradation factor. Second, to computeτ R/SD, eff BRDF( t 0 ), we use both the yaw maneuver and a small portion (~ 3 months) of regular on-orbit data. Third, we average the computed degradation factors over a large angular range of an orbit, reducing the impact of the solar power and detector gain noise. And fourth, we center the average of computed degradation factor at a fixed angle relative to the SD surface normal vector to remove the variation due to the dependence of the degradation factor on solar radiation energy incident angle. We fit the degradation factor to a smooth function of time and use the fitting residuals to estimate the accuracy of the degradation factors measured by the SDSM on a per orbit basis.

The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (SNPP) satellite carries out radiometric calibration of its reflective solar bands primarily through observing a sunlit onboard solar diffuser (SD). The SD bidirectional reflectance distribution function (BRDF) degrades over time. The degradation factor is determined by an onboard solar diffuser stability monitor (SDSM) which observes the Sun through a pinhole screen and the sunlit SD. The transmittance of the SDSM pinhole screen over a range of solar angles was determined prelaunch and used initially to determine the BRDF degradation factor. The degradation factor versus time curves were found to have a number of very large unphysical undulations likely due to the inaccuracies in the prelaunch determined SDSM screen transmittance. To validate and if necessary to refine both the SD and the SDSM screen transmittances, satellite yaw maneuvers were carried out. With the yaw maneuver data determined SDSM screen transmittance, the computed BRDF degradation factor curves still have large unphysical ripples, indicating that the projected solar horizontal angular step size in the yaw maneuver data is too large to resolve the transmittance at a fine angular scale. We develop a methodology to use both the yaw maneuver and regular on-orbit data to determine the SDSM screen transmittance at a fine angular scale with a relative error standard deviation from 0.00029 (672 nm; detector 5) to 0.00074 (926 nm; detector 8). With the newly determined SDSM screen transmittance, the computed BRDF degradation factor behaves much more smoothly over time.

The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership satellite performs radiometric calibration of its reflective solar bands (RSBs) primarily by observing an onboard solar diffuser (SD). The SD optical scattering property is measured by a bidirectional reflectance distribution function (BRDF). Once on orbit, the BRDF degrades over time and the degradation factor is determined by an onboard solar diffuser stability monitor (SDSM) which observes the Sun and the sunlit SD at almost the same time. We showed in a previous SPIE paper that the BRDF degradation factor is angle dependent. Consequently, due to that the SDSM and the VIIRS telescope SD views have very different angles, applying the BRDF degradation factor determined from the SDSM without any adjustments to the VIIRS RSB calibration can result in large systematic errors. In addition, the BRDF angular dependence impacts the determination of the SD screen transmittance viewed by both the SDSM detectors and the VIIRS telescope. We first use yaw maneuver data to determine the product of the SD attenuation screen transmittance and the BRDF at the initial time (when the BRDF just started to degrade) viewed by the VIIRS telescope, removing the impact of the SD BRDF degradation factor angular dependence over satellite orbits. By attributing the large bumps observed in the initially computed VIIRS detector gains for the M1-M4 bands to the angular dependence of the BRDF degradation factor and matching the computed VIIRS detector gains from the SD and the lunar observations, we find the relation between the BRDF degradation factors in the VIIRS telescope and SDSM SD view directions.

The first VIIRS instrument is aboard the Suomi National Polar-orbiting Partnership satellite. The instrument has 14 reflective solar bands (RSBs) to passively collect photons reflected from the Earth surface in the design wavelengths from 412 to 2250 nm. The instrument uses a solar diffuser (SD) to radiometrically calibrate its RSBs. When lit by the Sun through an attenuation screen (the SD screen), the SD diffusely reflects off the incident sunlight to act as a radiance source for the calibration. An onboard solar diffuser stability monitor (SDSM) yields the on-orbit change of the SD bidirectional reflectance distribution function (BRDF) by comparing the signal strength from the SD with that from the Sun attenuated by another attenuation screen (the SDSM screen). Complications arise due to the discovery that the on-orbit change of the BRDF is angle dependent. Additionally, the SDSM does not cover the wavelengths for the short-wave infrared bands in the RSBs. Furthermore, satellite yaw maneuvers were performed in the early mission to yield data for refining the prelaunch SDSM screen relative effective transmittance and the relative product of the SD screen transmittance and the BRDF at the mission start. But the yaw maneuver data are coarse in the solar azimuth angles and thus are unable to yield accurate values between the measurement angles. Over the years of performing on-orbit radiometric calibration through the SD for the VIIRS RSBs, we have developed several highly effective calibration algorithms to address the issues mentioned above. We review these algorithms.

The Visible Infrared Imaging Radiometer Suite (VIIRS) on the National Oceanic and Atmospheric Administration-20 (NOAA-20) satellite performs on-orbit radiometric calibration based on regular solar diffuser (SD) observations illuminated by the Sun at the termination point near the South Pole. Due to exposure to the ultraviolet portion of the solar irradiance spectrum, the SD bidirectional reflectance distribution function (BRDF) has been degrading over time. The SD degradation (called H-factor) was measured by the on-board calibrator called the SD stability monitor (SDSM). Nevertheless, over two years of operation, there have been systematic on-orbit calibration differences between the SD-based and independent moon-based calibration results. In this study, the NOAA VIIRS team used a surface roughness Rayleigh scattering (SRRS) model as a baseline SD degradation, simulated on-orbit center wavelength-based approach of SD degradation, and a new SDSM relative spectral response (RSR)-dependent SD degradation estimation method to evaluate the degradation. There were time-dependent growing differences between the SDSM RSR-applied H-factors and center wavelength interpolated H-factors especially in the short wavelength detectors (SDSM detector 1-4). The NOAA-20 SD-based calibration coefficients (SD F-factors) were reprocessed using the RSR-applied H-factors, and the new SD F-factors show similar long-term trends compared with the independent monthly lunar F-factors. The newly processed SD F-factor suggested that the NOAA-20 VIIRS detectors in the reflective solar bands (M1-M11 and I1-I3) showed very stable responses within 0.5% level over the two years of on-orbit operation.

The measurement and long-term monitoring of global total ozone by ultraviolet albedo measuring satellite instruments require accurate and precise determination of the Bi-directional Reflectance Distribution Function (BRDF) of laboratory-based diffusers used in the pre-launch calibration of those instruments. To assess the ability of laboratories to provide accurate UltraViolet (UV) diffuse BRDF measurements, a BRDF measurement comparison was initiated by the NASA Total Ozone Mapping Spectrometer (TOMS) Project. From December 1998 to September 1999, NASA's Goddard Space Flight Center (GSFC), TPD TNO (formerly the TNO Institute of Applied Physics), and the National Institute of Standards and Technology (NIST) made BRDF measurements on four Spectralon diffusers used in the pre-launch calibration of three TOMS instruments. The diffusers were measured at the six TOMS wavelengths and at the incident and scatter angles used in the TOMS pre-launch calibration. The participation of GSFC, TPD TNO, and NIST in the comparison establishes a link between the diffuser calibrations of the on-orbit TOMS instruments, the Ozone Monitoring Instrument (OMI), and a national standards laboratory. The results of the comparison show that all of the BRDF measurements on the four diffusers agreed within +0.85 % to -1.10 % of the average BRDF and were well within the combined measurement uncertainties of the participating laboratories.

The Bidirectional Reflectance Distribution Function (BRDF) has a well defined and studied diffuse measurement standard in the ultraviolet, visible and near infrared (NIR), Spectralon®. It is predictable, stable, repeatable, and has low surface variation because it is a bulk scatterer. In the mid-wave IR (MWIR) and long-wave IR (LWIR), there is not such a well-defined standard. There are well-defined directional hemispherical reflectance (DHR) standards, but the process of integrating BRDF measurements into DHR for the purpose of calibration is problematic at best. Direct BRDF measurement standards are needed. This study systematically investigates the BRDF and its variation for six potential MWIR diffuse BRDF standards. The currently accepted reflectance standard in the MWIR, Infragold®, is compared against two alternative gold-electroplated arc-sprayed aluminum samples, a silver-painted arc-sprayed aluminum sample, a black-paint sample, Spectralon®, and a novel a laser beam diffuser that has been gold coated. Diffuseness is compared by fitting the data to BRDF models, and repeatability is measured by using the standard deviation and percent difference from the mean calculated from multiple BRDF measurements across the surface of the samples.

Radiometric calibration of the Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (VIIRS) reflective solar bands relies mainly on the onboard solar diffuser (SD) observations. The SD reflectance degrades over time due to the exposure to solar ultraviolet radiation. The uncertainties embedded in characterizing the SD bidirectional reflectance distribution function (BRDF) directly affect the accuracy of sensor radiometric calibration coefficients, such as F-factors, which are proxies of detector gain. The Moon-based radiometric calibration provides an independent way of validating and correcting the SD-based calibration. This study focuses on the comparison of the long-term SD F-factors with lunar F-factors by using two independent lunar irradiance models, i.e., Miller and Turner (MT) model and the Global Space-based Inter-Calibration System Implementation of ROLO (GIRO) model. To monitor the long-term detector response changes, the lunar F-factor differences are matched to the SD F-factors by applying the best fit scaling factors. Overall, the two lunar F-factors agree well, within 2% of one sigma standard deviation in the reflective solar bands compared to the SD F-factors. The lifetime standard deviations of difference between the GIRO-based lunar and SD F-factors show better long-term match than that of MT-based lunar F-factors. The GIRO-based lunar F-factors show increasing differences over time in comparison with the SD F-factors especially for bands M1 to M4, which indicates the underestimation of the VIIRS detector degradation by SD F-factors for these bands. Using standard SD calibration method and the GIRO-based lunar model, long-term difference between the lunar and SD F-factors shows there are 1.6%, 1.3%, 1.0%, and 0.9% increases in lunar F-factor trend for bands M1 to M4 at the end of year 2015. To mitigate these time-dependent biases, NOAA Ocean Color (OC) group and NASA VIIRS characterization support team (VCST) developed lunar correction methods and applied them to their specific products. However, the amounts of band-dependent lunar corrections are not consistent between these two teams, especially in the short-wavelength bands from M1 to M4, depending on the versions of lunar models and SD F-factor calculation algorithms. Using the standard SD F-factor algorithm and the multi-agency endorsed GIRO model, we derived lunar correction factors based on the quadratic fits between the SD and lunar F-factors. The differences with the NOAA OC group and NASA VCST team are compared and described in this study.

Ball Aerospace uses several techniques in radiance calibrations of the SBUV/2 instruments. The instrument Primary Test Fixture (PTF) and Normal Incidence Test Fixture (NITF) both use Spectralon diffusers as radiance targets. Diffuser BRDF (Bidirectional Reflectance Distribution Function) is measured for a central spot at several scatter angles and at several wavelengths. Weighted BRDF is then calculated across the instrument FOV, based on diffuser BRDF measurements, spatial uniformity test data, instrument vignetting, and test geometry. This weighted BRDF curve is then fitted spectrally to determine BRDF at each wavelength of the SBUV/2 instrument. The PTF and NITF have their own BRDF curves, since each fixture has a unique diffuser plate and test geometry. A third test fixture is used for the last SBUV/2 instrument radiance calibration, using a Labsphere Uniform Source System (USS) and an external source for reference. The large aperture of the sphere provides a uniform radiance target with no need for BRDF knowledge. Comparison of instrument calibrations from all three radiance targets shows a small discrepancy of about ±1% among these calibration methods, which indicates that BRDF calculations for both PTF and NITF test diffusers are acceptable.

Many applications require quantitative measurements of surface light scattering, including quality control on production lines, inspection of painted surfaces, inspection of field repairs, etc. Instruments for measuring surface scattering typically fall into two main categories, namely bidirectional reflectometers, which measure the angular distribution of scattering, and hemispherical directional reflectometers, which measure the total scattering into the hemisphere above the surface. Measurement of the bi-directional reflectance distribution function (BRDF) gives the greatest insight into how light is scattered from a surface. Measurements of BRDF, however, are typically very lengthy measurements taken by moving a source and detector to map the scattering. Since BRDF has four angular degrees of freedom, such measurements can require hours to days to complete. Instruments for measuring BRDF are also typically laboratory devices, although a field- portable bi-directional reflectometer does exist. Hemispherical directional reflectance (HDR) is a much easier measurement to make, although care must be taken to use the proper methodology when measuring at wavelengths beyond 10 micrometer, since integrating spheres (typically used to make such measurements) are very energy inefficient and lose their integrating properties at very long wavelengths. A few field- portable hemispherical directional reflectometers do exist, but typically measure HDR only at near-normal angles. Boeing Defense and Space Group and Surface Optics Corporation, under a contract from the Air Force Research Laboratory, have developed a new hand-held instrument capable of measuring both BRDF and HDR using a unique, patented angular imaging technique. A combination of an hemi-ellipsoidal mirror and an additional lens translate the angular scatter from a surface into a two-dimensional spatial distribution, which is recorded by an imaging array. This configuration fully maps the scattering from a half-hemisphere above the surface with more than 30,000 angularly-resolved points and update rates to 60 measurements per second. The instrument then computes HDR from the measured BDR. For ease of use, the instrument can also compare both the BRDF and HDR to preset limits, generating a Pass/Fail indicator for HDR and a high-acceptable-low image display of BRDF. Beam incidence elevation is variable from normal incidence ((theta) equals 0 degrees) to 5 degrees off grazing ((theta) equals 85 degrees), while scattering is measured to nearly 90 degrees off normal. Such capability is extremely important for any application requiring knowledge of surface appearance at oblique viewing angles. The current instrument operates over the range of 3 micrometer to 12 micrometer, with extension into the visible band possible.