Environmental Data Records (EDR) from the Visible Infrared Imaging Radiometer Suite (VIIRS) have a need for Reflective Solar Band (RSB) calibration errors of less than 0.1%. Throughout the mission history of VIIRS, the overall instrument calibrated response scale factor (F factor) has been calculated with a manual process that uses data at least one week old and up to two weeks old until a new calibration Look Up Table (LUT) is put into operation. This one to two week lag routinely adds more than 0.1% calibration error. In this paper, we discuss trending the solar diffuser degradation (H factor), a key component of the F factor, improving H factor accuracy with improved bidirectional reflectance distribution function (BRDF) and attenuation screen LUTs , trending F factor, and how using RSB Automated Calibration (RSBAutoCal) will eliminate the lag and look-ahead extrapolation error.
A primary sensor on-board the Suomi-National Polar-orbiting Partnership (SNPP) spacecraft, the Visible Infrared Imaging Radiometer Suite (VIIRS) has 22 bands: 7 thermal emissive bands (TEBs), 14 reflective solar bands (RSBs) and a Day Night Band (DNB). The RSBs cover the spectral wavelengths between 0.412 to 2.25 μm and have three (I1-I3) 371m and eleven (M1-M11) 742m spatial resolution bands. A VIIRS Key Performance Parameter (KPP) is the Ocean Color/Chlorophyll (OCC) which uses moderate bands M1 (0.412μm) through M7’s (0.865 μm) calibrated Science Data Records (SDRs). The RSB SDRs rely on prelaunch calibration coefficients which use a quadratic algorithm to convert the detector’s response to calibrated radiance. This paper will evaluate the performance of these prelaunch calibration coefficients using SDR comparisons between bands with the same spectral characteristics: I2 with M7 (0.865 μm) and I3 with M10 (1.610 μm). Changes to the prelaunch calibration coefficient’s offset term c0 to improve the SDR’s performance at low radiance levels will also be discussed.
On October 28th, 2011, the Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched on-board the Suomi
National Polar-orbiting Partnership (NPP) spacecraft. The instrument has 22 spectral bands: 14 reflective solar
bands (RSB), 7 thermal emissive bands (TEB), and a Day Night Band (DNB). The DNB is a panchromatic, solar
reflective band that provides visible through near infrared (IR) imagery of earth scenes with radiances spanning 7
orders of magnitude. In order to function over this large dynamic range, the DNB employs a focal plane array (FPA)
consisting of three gain stages: the low gain stage (LGS), the medium gain stage (MGS), and the high gain stage
(HGS). The final product generated from a DNB raw data record (RDR) is a radiance sensor data record (SDR).
Generation of the SDR requires accurate knowledge of the dark offsets and gain coefficients for each DNB stage.
These are measured on-orbit and stored in lookup tables (LUT) that are used during ground processing. This paper
will discuss the details of the offset and gain measurement, data analysis methodologies, the operational LUT update
process, and results to date including a first look at trending of these parameters over the early life of the instrument.
The Suomi National Polar-orbiting Partnership (NPP) satellite was launched on Oct. 28, 2011, and began the
commissioning phase of several of its instruments shortly thereafter. One of these instruments, VIIRS, was found to
exhibit a gradual but persistent decrease in the optical throughput of several bands, with the near-infrared bands being
more affected than those in the visible. The rate of degradation quickly increased upon opening of the nadir door that
permits the VIIRS telescope to view the earth. Simultaneously, a second instrument on NPP, the Solar Diffuser Stability
Monitor (SDSM), was experiencing a similar decrease in response, leading the investigation team to suspect that the
cause must be the result of some common contamination process. This paper will discuss a series of experiments that
were performed to demonstrate that the VIIRS and SDSM response changes were due to separate causes, and which
enabled the team to conclude that the VIIRS sensor degradation was the result of ultraviolet light exposure of the
rotating telescope assembly. The root cause investigation of the telescope degradation will be addressed in a separate
paper.
The Visible-Infrared Imaging Radiometer Suite (VIIRS) is an instrument on-board the Suomi National Polar-orbiting
Partnership (NPP) spacecraft, which launched on October 28, 2011. VIIRS performs measurements in 14 reflective
solar bands (RSBs) spanning wavelengths from 412 nm to 2.25 um, which are calibrated by using solar radiance
reflected from a Solar Diffuser (SD). The SD reflectance degrades over time, and a Solar Diffuser Stability Monitor
(SDSM) is used to track the changes. The ratio between the calculated solar radiance reflected from the SD and the
VIIRS measurement of this radiance using the pre-launch calibration coefficients is known as the “F factor.” The F
factor is applied in the ground processing as a scale correction to the pre-launch calibration coefficients used to generate
the calibrated radiances and reflectances comprising the Sensor Data Records (SDRs). The F factor is trended over time
to track instrument response degradation. The equation for calculating expected solar radiance, and the coefficients used
to convert the raw digital numbers measured by the detectors into radiance and reflectance values, are based on
parameters stored in various Look-Up Tables (LUTs). This paper will discuss on-orbit RSB calibration for VIIRS, along
with a description of the processing methodology, which includes operational LUT updates based on off-line
calculations of F factor trending behavior.
The Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched onboard the Suomi National Polar-orbiting
Partnership (NPP) spacecraft on October 28, 2011. Among the bands on VIIRS are 14 reflective solar bands (RSBs).
The RSBs are calibrated using the sun as a source, after attenuation and reflection of sunlight from a Solar Diffuser (SD).
The reflectance of the SD is known to degrade over time, particularly at the blue end of the visible spectrum. VIIRS
incorporates a separate instrument, a Solar Diffuser Stability Monitor (SDSM), in order to measure and trend the SD
Bidirectional Reflectance Distribution Function BRDF changes over time. Inadequate knowledge of the SDSM screen
transmission as a function of solar geometry and SDSM detector dependent modulation effects require a unique
processing methodology to eliminate unphysical artifacts from the SD BRDF trending. The unique methodology is used
to generate periodic updates to operational Look-up Tables (LUTs) used by the Sensor Data Record (SDR) operational
code to maintain the calibration of the RSBs. This paper will discuss on-orbit SD BRDF behavior along with the
processing methodology used to generate RSB LUT updates incorporating the trended SD BRDF behavior.
A magnetically actuated optical phase modulator is described. The phase of a reflected optical beam is modulated by deflecting a Mylar® membrane, coated with a magnetic iron/nickel film, with the field of an electromagnet. A phase change of for light of wavelength 0.633 µm was achieved with a driving voltage of only 4 V, much smaller than the voltage required for comparable, electrostatically-actuated devices. The modulator can be scaled to micron dimensions for fabrication in arrays, scaled for even lower drive voltage, and operated at megahertz frequencies.
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