Kameron Rausch

Operational calibration of VIIRS reflective solar band sensor data records

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.

VIIRS solar diffuser bidirectional reflectance distribution function (BRDF) degradation factor operational trending and update

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.

Discovery and characterization of on-orbit degradation of the VisibleInfrared Imaging Radiometer Suite (VIIRS) Rotating TelescopeAssembly (RTA)

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.

VIIRS day-night band gain and offset determination andperformance

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.

VIIRS reflective solar bands on-orbit calibration coefficient performance using imagery and moderate band intercomparisons

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.

Mission history of reflective solar band calibration performance of VIIRS

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.

SUOMI NPP VIIRS reflective solar band radiometric calibration

The Visible-Infrared Imaging Radiometer Suite (VIIRS) is a primary sensor onboard the National Polar-orbiting Partnership (NPP) satellite launched on October 28, 2011. It collects data over the spectral range of 0.4-12.5 um and consists of 15 reflective solar bands (RSB) and 7 thermal emissive bands (TEB). Like the heritage sensor MODIS (MOderate Resolution Imaging Spectroradiometer), VIIRS has a set of On-Board Calibrators (OBC) that provide data for radiometric calibration of the instrument in offline ground processing. The reflective bands use sunlight reflected from a Solar Diffuser (SD) after passing through an attenuating Solar Diffuser Screen (SDS) as a reference illumination source. The emissive bands use an On-Board Calibrator Blackbody (OBC BB) maintained at a constant elevated temperature as a reference illumination source. The Space View (SV) port provides a dark reference scene for background subtraction to remove any detector biases for both reflective and emissive bands.

NPP VIIRS early on-orbit solar diffuser degradation results

The Visible-Infrared Imaging Radiometer Suite (VIIRS) was launched October 28, 2011 on-board the Suomi National Polar-orbiting Partnership (NPP) spacecraft as a primary sensor. It has 22 bands: 14 reflective solar bands (RSBs), 7 thermal emissive bands (TEBs) and a Day Night Band (DNB). The RSBs are calibrated using the sun as a source, after attenuation and reflection of sunlight from a Solar Diffuser (SD). To track SD degradation over time, VIIRS incorporates a separate instrument called the Solar Diffuser Stability Monitor (SDSM). The SDSM is a ratio radiometer using views of attenuated direct solar illumination and solar illumination reflected off the SD to track relative change in the SD reflectance over time. This paper will describe the SDSM design and analysis methodology as well as compare the SD degradation trends with its heritage sensor MODIS.