David Moyer

VIIRS thermal emissive bands calibration algorithm and on-orbit performance

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). There are 2 TEBs with a resolution of 371 m and 5 with 742 m which cover the spectral wavelengths between 3.7 to 12 μm. In addition to sea surface temperature (SST), a VIIRS Key Performance Parameter (KPP), the TEB detector dynamic range and spectral placement allow cloud, atmospheric and surface temperatures as well as water vapor to be measured. VIIRS TEB on-orbit calibration uses a quadratic algorithm with its calibration coefficients derived from pre-launch measurements and an on-board calibration blackbody (OBC BB) to provide scan-to-scan gain drift compensation. This paper will discuss the calibration methodology, OBC BB performance and stability, detector noise equivalent delta temperature and radiometric performance.

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.

On-Orbit Characterization of S-NPP VIIRS Transmission Functions

The Suomi National Polar-orbiting Partnership (S-NPP) spacecraft executed a series of yaw maneuvers on February 15 and 16, 2012. Data collected during these maneuvers were used to characterize the transmission functions of the Visible Infrared Imager Radiometer Suite (VIIRS) instrument solar diffuser (SD) and solar diffuser stability monitor (SDSM) views. On orbit, only the product of the attenuation screen transmittance and SD bidirectional reflectance distribution function (BRDF) can be measured for the VIIRS detector and SDSM SD views. For the SDSM solar view, the attenuation screen transmittance was also measured. The angular sampling provided by the yaw maneuver data of this solar view was too coarse to capture the fine structure of the transmission function; a model was developed to include this structure in the vignetting function by combining solar observation data from the first nine months of the mission with the yaw maneuver-derived vignetting function. The derived transmission functions were delivered for implementation in the operational processing stream (the derived VIIRS detector view transmittance produced up to 0.4% difference in the instrument responsivity, and SDSM transmission functions impacted the BRDF tracking by up to 3.0%). An uncertainty analysis was also conducted on all transmission functions delivered.

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.

S-NPP VIIRS Thermal Emissive Bands On-Orbit Calibration and Performance

Presented is an assessment of the on-orbit radiometric performance of the thermal emissive bands (TEB) of the Suomi National Polar-orbiting Partnership (S-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) instrument based on data from the first 2 years of operations—from 20 January 2012 to 20 January 2014. The VIIRS TEB are calibrated on orbit using a V-grooved blackbody (BB) as a radiance source. Performance characteristics trended over the life of the mission include the F factor—a measure of the gain change of the TEB detectors; the Noise Equivalent differential Temperature (NEdT)—a measure of the detector noise; and the detector offset and nonlinear terms trended at the quarterly performed BB warm-up cool-down cycles. We find that the BB temperature is well controlled and stable within the 30mK requirement. The F factor trends are very stable and showing little degradation (within 0.8%). The offsets and nonlinearity terms are also without noticeable drifts. NEdT is stable and does not show any trend. Other TEB radiometric calibration-related activities discussed include the on-orbit assessment of the response versus scan-angle functions and an approach to improve the M13 low-gain calibration using onboard lunar measurements. We conclude that all the assessed parameters comply with the requirements, and the TEB provide radiometric measurements with the required accuracy.

Analysis of Suomi–NPP VIIRS vignetting functions basedon yaw maneuver data

The Suomi – NPP Visible Infrared Imager Radiometer Suite (VIIRS) reflective bands are calibrated on-orbit via reference to regular solar observations through a solar attenuation screen (SAS) and diffusely reflected off a Spectralon ® panel. The degradation of the Spectralon panel BRDF due to UV exposure is tracked via a ratioing radiometer (SDSM) which compares near simultaneous observations of the panel with direct observations of the sun (through a separate attenuation screen). On-orbit, the vignetting functions of both attenuation screens are most easily measured when the satellite performs a series of yaw maneuvers over a short period of time (thereby covering the yearly angular variation of solar observations in a couple of days). Because the SAS is fixed, only the product of the screen transmission and the panel BRDF was measured. Moreover, this product was measured by both VIIRS detectors as well as the SDSM detectors (albeit at different reflectance angles off the Spectralon panel). The SDSM screen is also fixed; in this case, the screen transmission was measured directly. Corrections for instrument drift and degradation, solar geometry, and spectral effects were taken into consideration. The resulting vignetting functions were then compared to the pre-launch measurements as well as models based on screen geometry.

JPSS-1 VIIRS Pre-Launch Response Versus Scan Angle Testing and Performance

The Visible Infrared Imaging Radiometer Suite (VIIRS) instruments on-board both the Suomi National Polar-orbiting Partnership (S-NPP) and the first Joint Polar Satellite System (JPSS-1) spacecraft, with launch dates of October 2011 and December 2016 respectively, are cross-track scanners with an angular swath of ±56.06°. A four-mirror Rotating Telescope Assembly (RTA) is used for scanning combined with a Half Angle Mirror (HAM) that directs light exiting from the RTA into the aft-optics. It has 14 Reflective Solar Bands (RSBs), seven Thermal Emissive Bands (TEBs) and a panchromatic Day Night Band (DNB). There are three internal calibration targets, the Solar Diffuser, the BlackBody and the Space View, that have fixed scan angles within the internal cavity of VIIRS. VIIRS has calibration requirements of 2% on RSB reflectance and as tight as 0.4% on TEB radiance that requires the sensor’s gain change across the scan or Response Versus Scan angle (RVS) to be well quantified. A flow down of the top level calibration requirements put constraints on the characterization of the RVS to 0.2%–0.3% but there are no specified limitations on the magnitude of response change across scan. The RVS change across scan angle can vary significantly between bands with the RSBs having smaller changes of ~2% and some TEBs having ~10% variation. Within a band, the RVS has both detector and HAM side dependencies that vary across scan. Errors in the RVS characterization will contribute to image banding and striping artifacts if their magnitudes are above the noise level of the detectors. The RVS was characterized pre-launch for both S-NPP and JPSS-1 VIIRS and a comparison of the RVS curves between these two sensors will be discussed.

Preliminary Assessment of Suomi-NPP VIIRS On-orbit Radiometric Performance

The Visible-Infrared Imaging Radiometer Suite (VIIRS) is a key instrument on-board the Suomi National Polarorbiting Partnership (NPP) spacecraft that was launched on October 28th 2011. VIIRS was designed to provide moderate and imaging resolution of the planet Earth twice daily. It is a wide-swath (3,040 km) cross-track scanning radiometer with spatial resolutions of 375 m and 750 m at nadir for imaging and moderate bands, respectively. It has 22 spectral bands covering the spectrum between 0.4 μm and 12.5 μm, including 14 reflective solar bands (RSB), 7 thermal emissive bands (TEB), and 1 day-night band (DNB). VIIRS observations are used to generate 22 environmental data record (EDRs). This paper will briefly describe NPP VIIRS calibration strategies performed by the independent government team, for the initial on-orbit Intensive Calibration and Validation (ICV) activities. In addition, this paper will provide an early assessment of the sensor on-orbit radiometric performance, such as the sensor signal to noise ratios (SNRs), dual gain transition verification, dynamic range and linearity, reflective bands calibration based on the solar diffuser (SD) and solar diffuser stability monitor (SDSM), emissive bands calibration based on the on-board blackbody calibration (OBC), and cross-comparison with MODIS. A comprehensive set of performance metrics generated during the pre-launch testing program will be compared to VIIRS early on-orbit performance, and a plan for future cal/val activities and performance enhancements will be presented.