The Suomi National Polar Orbiting Partnership (S-NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) employs a large number of temperature and voltage sensors (telemetry points) to monitor instrument health and performance. We have collected data and built tools to study telemetry and calibration parameters trends. The telemetry points are organized into groups based on locations and functionalities. Examples of the groups are: telescope motor, focal plane array (FPA), scan cavity bulkhead, radiators, solar diffuser and Solar Diffuser Stability Monitor (SDSM). We have performed daily monitoring and long-term trending studies. Daily monitoring processes are automated with alarms built into the software to indicate if pre-defined limits are exceeded. Long-term trending studies focus on instrument performance and sensitivities of Sensor Data Record (SDR) products and calibration look-up tables (LUTs) to instrument temperature and voltage variations. VIIRS uses a DC Restore (DCR) process to periodically correct the analog offsets of each detector of each spectral band to ensure that the FPA output signals are always within the dynamic range of the Analog to Digital Converter (ADC). The offset values are updated based on observations of the On-Board Calibrator Blackbody source. We have performed a long-term trend study of DCR offsets and calibration parameters to explore connections of the DCR offsets with onboard calibrators. The study also shows how the instrument and calibration parameters respond to the VIIRS Petulant Mode, spacecraft (SC) anomalies and flight software (FSW) updates. We have also shown that trending studies of telemetry and calibration parameters may help to improve the instrument calibration processes and SDR Quality Flags.
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.
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 Aerospace Corporation has developed a testbed for studying pointing, acquisition, and tracking systems for
lasercom terminals. The testbed consists of two configurable terminals that are currently set up to represent a GEO-to-
GEO link. Each terminal has the ability to point open-loop, execute scan patterns, and track a received beam. The system
operates in small-beam space and consists of a far-field space simulator and two lasercom terminals operating at 473 nm
and 633 nm with representative hardware (fast steering mirrors, optical detectors, etc.). This paper discusses the software
developed for the testbed and the characterization of its performance, which includes open-loop pointing accuracy and
residual tracking error in the presence of applied disturbances. Analytical predictions are compared to experimental
results. Each terminal has the ability to progress from acquisition to tracking mode and the two terminals together
demonstrate the cooperative acquisition process.