Spectralon® is a high reflectance excellent diffuser used to reflect sunlight for use as a calibrator for on-orbit and ground instruments. Radiometric calibration of the reflective bands in the 0.4 to 2.5μm wavelength range is performed by measuring the sunlight reflected from Spectralon®. Reflected sunlight is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon®. On-orbit exposure to sunlight results in solarization due to solar UV and the presence of residual contamination. Spectralon® quality is checked at start of build by measuring the change in reflectance on exposing a witness sample to 100 hours Solar UV as an indication of on orbit performance. For JPSS J2, the witness samples accompanied the sensor till 30 days before launch. Measuring the reflectance change on exposure to Solar UV of the witness samples accompanying the sensor through build and test is a better indication of on orbit performance as this includes any additional contamination during the build and test phase.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite as well as Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometric and imagery data in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective solar bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from Solar Diffuser Assembly (SDA). The Solar Attenuation Screen (SAS) is designed to adjust the amount of sunlight reaching the SDA so that the albedo levels seen by VIIRS are comparable to VIIRS earth view while rejecting light reflected from the earth. As the throughput varies with sun angle of incidence, the SAS transmittance was characterized over the as use angular range (13-32 degrees in azimuth and 15-18.5 degrees in declination) with an uncertainty better than 0.2%. The setup of the test station allows for the SAS’s transmission and modulation to be measured in the as used configuration. The results of the SAS transmittance was then combined with the Bidirectional Reflectance Distribution Function (BRDF) of the SDA to calculate the albedo levels over the sun angular range used for calibration. The SAS transmittance angular variation was fit to a model to calculate the spatial signal modulation. This paper presents the SAS transmission and its spatial signal modulation for JPSS-J3 and JPSS-J4.
Spectralon® is a high reflectance excellent Lambertian diffuser used to reflect sunlight for use as a calibrator for on-orbit and ground instruments. Radiometric calibration of the reflective bands in the 0.4 to 2.5μm wavelength range is performed by measuring the sunlight reflected from Spectralon® . Reflected sunlight is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon® . On-orbit exposure to sunlight results in solarization due to solar UV. Previously, the rate / amount of solarization has varied as observed from on orbit measurements as well as laboratory UV exposure testing of samples. A method for determining whether a particular batch of Spectralon® has low solarization has been developed. This method relies on hemispherical reflectance measurements in the 0.25- 0.5 μm wavelength range before and after Spectralon® bake out. This method is reliable for as-made Spectralon® , not for contamination verification after shipment. We have also determined that additional Spectralon® bake outs do not change the as-made Spectralon® solarization rate. Knowledge of possible Spectralon solarization is important prior to its shipment to customers and eventual deployment in satellite and ground-based instrument calibration.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) and the Joint Polar Satellite System JPSS-1 as well as the upcoming JPSS-2, JPSS-3, and JPSS-4. VIIRS collects Earth radiometry and imagery in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from the Solar Diffuser Assembly (diffuser is Spectralon®). Spectralon® is known to solarize due to sun UV exposure at the blue end of the spectrum (~0.4 – 0.6+ μm) as seen by laboratory tests as well as on orbit data from Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua and VIIRS on NPP and NOAA-20. Similar to MODIS, the VIIRS uses a Solar Diffuser Stability Monitor (SDSM) to monitor the change in the Solar Diffuser reflectance in the 0.4 – 0.94 μm wavelength range to correct the calibration coefficients. The SDSM measures the ratio of sun light reflecting from the Solar Diffuser to a direct view of the sun at 8 different spectral bands close to the VIIRS bands in spectral regions where solarization effects are present and absent. The spectral response of these bands is critical to calculating the corrections to the radiometric coefficients. The Relative Spectral Response of the 8 SDSM bands for JPSS-3 and JPSS-4 is presented. The out of band response for spectral regions where most of the solarization occurs (< 600 nm) is <0.15% showing minimal mixing with somewhat larger mixing in the low solarization region.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite and the Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometric and imagery data in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from Spectralon. Reflected sun light is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon. Previously BRDF measurements of the Spectralon for JPSS J1 and J2 in the 0.4 – 1.63 μm wavelength using PASCAL (Polarization And Scatter Characterization Analysis of Lambertian materials) have been presented. However calibration is needed outside this measurement range at 2.25 μm. This paper presents an extrapolation of the measurements to 2.25 μm based on BRDF measurements in the as use geometry at lower wavelengths and hemispherical reflectance measurements of witness samples from 0.4 to 2.5 μm. The extrapolation relies on the spectral variation of BRDF for J1, J2, and J3 Spectralon panels as well as the hemispherical reflectance of witness samples with extrapolation provided for the entire range of sun angles of incidence. The results are compared with J1 witness sample extrapolation done by NASA. J3 BRDF results are also presented.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite as well as Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometric and imagery data in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective solar bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from Solar Diffuser Assembly (SDA). The Solar Attenuation Screen (SAS) is designed to adjust the amount of sunlight reaching the SDA so that the albedo levels seen by VIIRS are comparable to VIIRS earth view while rejecting light reflected from the earth. As the throughput varies with sun angle of incidence, the J2 SAS transmittance was characterized over the as use angular range (13-32 degrees in azimuth and 15- 18.5 degrees in declination) with an uncertainty better than 0.2%. The results of the SAS transmittance was then combined with the Bidirectional Reflectance Distribution Function (BRDF) of the SDA to calculate the albedo levels over the sun angular range used for calibration. This paper will present the spatial signal modulation of the SAS. The setup of the test station allows for the SAS’s transmission and modulation to be measured in the as used configuration.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership satellite as well as the Joint Polar Satellite System (JPSS). Emissive band calibration in the 3.7 to 12.5 μm wavelength is performed based on spectral emittance of a blackbody. This paper presents the results of the optical characterization for two builds for JPSS J3 and J4, of the VIIRS blackbody. This characterization is a combination of measurement and analysis.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite as well as the upcoming Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometric and imagery data in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from Spectralon®. Reflected sun light is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon. This paper presents the BRDF measurements of the Spectralon for JPSS J2 in the 0.4 – 1.63 μm wavelength using PASCAL (Polarization And Scatter Characterization Analysis of Lambertian materials) with an uncertainty better than 1.2%. PASCAL makes absolute measurements of the BRDF in an analogous fashion to the National Institute of Standards and Technology (NIST) Spectral Tri-function Automated Reflectance Reflectometer (STARR) facility. Unique additional features of this instrument include the ability to vary the sample elevation and roll / clock the sample about its normal, allowing measurement of BRDF in the as used geometry. Comparison of BRDF in the as used configuration for NPP, J1, and J2 shows variation of up to 3%. The sign of the change from panel to panel depends on the angle of incidence and view angle. The results demonstrate lot to lot variability in Spectralon and emphasize the necessity of characterizing each panel. A pattern in the BRDF variation is also presented.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on Suomi National Polar-orbiting Partnership (S-NPP) satellite launched on October 28, 2011 into a polar orbit of 824 km nominal altitude and the JPSS sensors currently being built and integrated. VIIRS collects radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the visible and infrared spectrum from 0.4 to 12.5 μm. Interference filters assembled in ‘butcher-block’ arrays mounted adjacent to focal plane arrays provide spectral definition. Out-of-band signal and out-of-band optical cross-talk was observed for bands in the 0.4 to 1 μm range in testing of VIIRS for S-NPP. Optical cross-talk is in-band or out-of-band light incident on an adjacent filter or adjacent region of the same filter reaching the detector. Out-of-band optical cross-talk results in spectral and spatial ‘impurities’ in the signal and consequent errors in the calculated environmental parameters such as ocean color that rely on combinations of signals from more than one band. This paper presents results of characterization, specification, and coating process improvements that enabled production of filters with significantly reduced out of band light for Joint Polar Satellite System (JPSS) J1 and subsequent sensors. Total transmission and scatter measurements at a wavelength within the pass band can successfully characterize filter performance prior to dicing and assembling filters into butcher block assemblies. Coating and process development demonstrated performance on test samples followed by production of filters for J1 and J2. Results for J1 and J2 filters are presented.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite in orbit as well as for the upcoming Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometry and imagery in 22 spectral from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength range is performed by measuring the sunlight reflectance from Solar Diffuser Assembly (diffuser is Spectralon®). Spectralon® is known to solarize due to sun UV exposure at the blue end of the spectrum (~0.4 – 0.6+ μm) as seen by laboratory tests as well as on orbit data from MODIS and NPP. VIIRS uses a Solar Diffuser Stability Monitor (SDSM) to monitor the change in the Solar Diffuser reflectance in the 0.4 – 0.94 μm wavelength range to correct the calibration constants. The SDSM measures the ratio of sun light reflecting from the Solar Diffuser to a direct view of the sun. As the intensity of the light reaching the SDSM in both Solar Diffuser view and sun view is a function of the sun’s angle of incidence (AOI), the SDSM response to sun AOI has to be characterized. This paper presents details of the test setup including an extended collimated source simulating the sun across all SDSM bands. The prelaunch characterization results for the JPSS-1 (J1) VIIRS SDSM are presented. Comparison with NPP on orbit yaw maneuver SDSM results shows similar behavior demonstrating that the J1 test successfully characterized the SDSM response to sun AOI.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting Partnership (NPP) satellite as well as the upcoming Joint Polar Satellite System (JPSS). VIIRS collects Earth radiometric and imagery data in 22 spectral bands from 0.4 to 12.5 μm. Radiometric calibration of the reflective bands in the 0.4 to 2.5 μm wavelength range is performed by measuring sunlight reflectance from Spectralon®. Reflected sun light is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon®. This paper presents the BRDF measurements of the Spectralon® for JPSS-1 VIIIRS in the 0.4 – 1,63 μm wavelength using PASCAL (Polarization And Scatter Characterization Analysis of Lambertian materials) with an uncertainty better than 0.8%. PASCAL makes absolute measurements of BRDF in an analogous fashion to the National Institute of Standards and Technology (NIST) Spectral Tri-function Automated Reflectance Reflectometer (STARR) facility. BRDF is calculated on the basis of the measured incident power, reflected power and geometric factors thus eliminating the need for a standard. The detector rotates about the sample in the plane defined by the source beam and the azimuth rotation of the sample. Unique additional features of this instrument include the ability to vary the sample elevation and roll / clock the sample about its normal thus allowing BRDF measurements in the as use geometry. A description of PASCAL, validation and the BRDF results of the Solar Diffuser are presented. Part dependent non-uniformity of Spectralon® was also observed.
The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor on the Joint Polar-orbiting Satellite System (JPSS)
mission has a solar diffuser as a reflective band calibrator. Due to UV solarization of the solar diffuser, the Solar
Diffuser Stability Monitor (SDSM) is on-board to track the reflectance change of the solar diffuser in visible to near IR
wavelengths. A 100 cm Sphere Integrating Source (SIS) has been used to configure and test the SDSM on the ground
since MODerate resolution Imaging Spectroradiometer (MODIS) programs. Recent upgrades of the radiance transfer
and BRDF measurement instruments in Raytheon have enabled more spectral data and faster measurement time with
comparable uncertainty to the previous methods. The SIS has a Radiance Monitor, which has been mainly used as a SIS
real-time health checker. It has been observed that the Radiance Monitor response is sufficiently linear and stable thus
the Radiance Monitor can be used as a calibrator for ground tests. This paper describes the upgraded SIS calibration
instruments, and the changes in the calibration philosophy of the SIS for the SDSM bands.
The Visible/Infrared Imaging Radiometer Suite (VIIRS) is a key sensor on the Suomi National Polar-orbiting
Partnership (NPP) satellite launched on October 28, 2011 into a polar orbit of 824 km nominal altitude. VIIRS collects
radiometric and imagery data of the Earth’s atmosphere, oceans, and land surfaces in 22 spectral bands spanning the
visible and infrared spectrum from 0.4 to 12.5 μm. The radiometric response for VIIRS spectral bands in the 600 – 2300
nm wavelength range (I1, M5, M6, M7 / I2, M8, M9, M10 / I3, M11), which started with significant signal to noise ratio
margin at beginning of life, has shown some degradation on orbit. This degradation has been correlated with UV
exposure of the VIIRS optics. UV exposure of witness samples from the Rotating Telescope Assembly (RTA) mirrors
by Aerospace Corporation showed reflectance loss with the same spectral signature as the response degradation observed
for VIIRS. The investigation and root cause determination for the VIIRS response degradation are discussed in separate
papers.
A model of VIIRS throughput degradation has been developed from witness sample UV exposure test results made by
Aerospace. A direct relationship is assumed between UV dose (fluence) and the reflectance degradation of the RTA
mirrors. The UV dose on orbit for the primary mirror is proportional to the incident earthshine and its solid angle of
view. For subsequent mirrors the UV dose is weighted by solid angle and preceding mirror UV reflectance. UV dose is
converted to reflectance change based on witness sample exposure measurements. The change in VIIRS throughput is
calculated by multiplying the reflectance of the four RTA mirrors and agrees with the on-orbit measured response
changes as a function of UV exposure time. Model predictions of the radiometric sensitivity for the affected VIIRS
bands show positive margin at end of life for all affected bands.
Accurate characterization of the reflection and scatter properties of materials is critical for their use in optical systems.
PASCAL (Polarization And Scatter Characterization and Analysis of Lambertian materials) makes BRDF (Bidirectional
Reflection Distribution Function) measurements with a polarized light source and can measure with / without an analyzer
in series with the detector in the 400 - 1700 nm wavelength range. The entire incident light beam is collected by the
detector assembly. With the sample in place, a precision circular aperture is used to collect the light. BRDF is
calculated on the basis of the incident power and geometric factors eliminating the need for a standard characterized at
another laboratory. The measured uncertainties of the geometric factors are comparable to those of the National Institute
of Standards and Technology (NIST) Spectral Tri-function Automated Reflectance Reflectometer (STARR) facility.
Spectral definition is achieved with band pass filters. As in typical BRDF instruments, the detector rotates about the
sample in the plane defined by the source beam and the azimuth rotation of the sample. Unique additional features of
this instrument include the ability to vary the sample elevation and sample roll. Comparisons with measurements made
at NIST are presented. Measurements with this instrument demonstrate the importance of sample orientation, roll, with
variations of 2.5% observed. The roll dependence can vary with polarization. The minimum sample size measurable is
5 cm diameter with the maximum sample size of 22 X 27 cm.
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