Lunar calibration is a commonly used method to track a climate satellite sensor’s long-term radiometric stability. We present a modeling approach to examine the satellite sensor lunar observation uncertainties due to several important aspects related to the lunar image acquisition by the satellite sensor: lunar pixel shift, point spread function (PSF), lunar orientation, pitch, and oversampling rates. Our analyses can be summarized as follows. (1) The sensor observed lunar irradiance can vary due to small lunar pixel shift if the PSF is less than ideal. (2) During lunar calibration, an unstable oversampling rate due to spacecraft control will result in errors in observed lunar irradiance. A drift in oversampling rate would result in a bias in observed lunar irradiance and a random variation in oversampling rate would cause random error in lunar irradiance. Increasing the overall oversampling rates can reduce random error in observed lunar irradiance but would not change the biases in the observation. (3) Furthermore, the biases can vary when the Moon is observed at different orientations. Our results show impacts on observed lunar irradiance are on the order of 0.1%, which is a significant part of the overall uncertainty for a lunar irradiance measurement of a climate satellite sensor.
Hawkeye is an ocean color instrument that is part of the SeaHawk satellite developed for SOCON, the Sustained Ocean Color Observations using Nanosatellites program funded by the Gordon and Betty Moore Foundation and managed by the University of North Carolina – Wilmington (UNC-W). HawkEye has spectral characteristics similar to SeaWiFS, but with 8 times finer resolution and a smaller field of view more appropriate for lakes, rivers, and near-shore terrestrial environments. With a volume of only 10 × 10 × 10 cm (a CubeSat 1U), it can produce 8 bands of image data in a single pass, each with 1800 × 6000 pixels, with a resolution of 120 meters per pixel. This paper will present a short summary of instrument design, the spacecraft interface, and "lessons learned" during this effort. Scientists considering using linear arrays in a pushbroom mode for remote sensing will find this useful. Much of the discussion will center on optical performance, such as flat field calibration, polarization effects, stray light, out-of-band response, and exposure linearity. Images from field tests will be shown.
This study presents a modeling approach to improve solar diffuser (SD) degradation determination from SD stability monitor (SDSM) measurements. The MODIS instrument uses a SD to calibrate its reflective solar bands (RSBs) on-orbit. Due to the imperfectly designed SDSM sun view screen, the SD reflectance tracked by SDSM has large noise. The SDSM measurements noise is spectrally coherent and can be minimized by normalizing measurements to the least degraded detector 9 (936 nm). In this study, a SD degradation model is used to determine the SD degradation’s wavelength dependency and the detector 9 degradation is estimated by the model solution. The results show the SD degradations measured at 6 SDSM detectors (554 - 936 nm) have stable relationships, where the degradation is inversely proportion to 1/wavelength^4. The model estimated SD degradation at SDSM detector 9 wavelength (936 nm) is ~0.9% from 2002 to 2018. Based on the SD degradation model solution, the SD degradation at short/mid wave bands are estimated to improve short/mid wave bands calibration. The model can also be used to improve interpolating SD degradation at SDSM detectors to RSB wavelengths. Compared to linear interpolation, bands 9 and 10 show the largest differences of up to 0.3 and 0.4% respectively. These differences directly impact the calibration coefficients of these bands.
Hawkeye is an ocean color instrument designed, manufactured and characterized at Cloudland Instruments, CA. It is a push broom instrument that has 8 spectral bands similar to SeaWiFS and a spatial resolution of 120 m. Each spectral band has 1800 detectors (pixels) and all 14,000 detectors (pixels) need to be calibrated independently. This paper describes the preliminary design of on-orbit calibration method to correct for the instrument response’s temperature sensitivity, scan angle dependency in radiometric sensitivity, relative spectral response (RSR), nonlinearity, and polarization sensitivity. We will provide a brief description on how each of the calibration parameters are used to address the instrument characteristics and how the calibration parameters are derived from instrument test data and use to retrieve ocean color products.
The accuracy of pre-launch VIIRS response versus scan angle (RVS) characterization is vitally important to the quality of calibrated radiance products. The RVS correct the optical system’s scan angle dependency with respect to the calibrator scan angle. In this document, we describe the methodology used in JPSS-1 RSB RVS characterization and the efficacy of MODTRAN5 in water vapor correction. The RVS is characterized using a 2<sup>nd</sup> order polynomial fit over the measured scan angles. The results show high fitting accuracy for all bands except for M9 due to water vapor absorption. To improve M9 RVS, MODTRAN®5 is used to correction water vapor absorption variation during the RVS measurements. This correction greatly reduced M9 RVS characterization uncertainty with a RVS that is up to 0.4% difference compared with RVS prior to water vapor correction.
During JPSS-1 VIIRS testing at Raytheon El Segundo, a larger than expected radiometric response nonlinearity was discovered in Day-Nigh Band (DNB). In addition, the DNB nonlinearity is aggregation mode dependent, where the most severe non-linear behavior are the aggregation modes used at high scan angles (<~50 degree). The DNB aggregation strategy was subsequently modified to remove modes with the most significant non-linearity. We characterized the DNB radiometric response using pre-launch tests with the modified aggregation strategy. The test data show the DNB non-linearity varies at each gain stages, detectors and aggregation modes. The non-linearity is most significant in the Low Gain Stage (LGS) and could vary from sample-to-sample. The non-linearity is also more significant in EV than in calibration view samples. The HGS nonlinearity is difficult to quantify due to the higher uncertainty in determining source radiance. Since the radiometric response non-linearity is most significant at low dn ranges, it presents challenge in DNB cross-stage calibration, an critical path to calibration DNB’s High Gain Stage (HGS) for nighttime imagery. Based on the radiometric characterization, we estimated the DNB on-orbit calibration accuracy and compared the expected DNB calibration accuracy using operational calibration approaches. The analysis showed the non-linearity will result in cross-stage gain ratio bias, and have the most significant impact on HGS. The HGS calibration accuracy can be improved when either SD data or only the more linearly behaved EV pixels are used in cross-stage calibration. Due to constrain in test data, we were not able to achieve a satisfactory accuracy and uniformity for the JPSS-1 DNB nighttime imagery quality. The JPSS-1 DNB nonlinearity is a challenging calibration issue which will likely require special attention after JPSS-1 launch.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on-board the first Joint Polar Satellite System (JPSS) completed its sensor level testing on December 2014. The JPSS-1 (J1) mission is scheduled to launch in December 2016, and will be very similar to the Suomi-National Polar-orbiting Partnership (SNPP) mission. VIIRS instrument was designed to provide measurements of the globe twice daily. It is a wide-swath (3,040 km) cross-track scanning radiometer with spatial resolutions of 370 and 740 m at nadir for imaging and moderate bands, respectively. It covers the wavelength spectrum from reflective to long-wave infrared through 22 spectral bands [0.412 μm to 12.01 μm]. VIIRS observations are used to generate 22 environmental data products (EDRs). This paper will briefly describe J1 VIIRS characterization and calibration performance and methodologies executed during the pre-launch testing phases by the independent government team, to generate the at-launch baseline radiometric performance, and the metrics needed to populate the sensor data record (SDR) Look-Up-Tables (LUTs). This paper will also provide an assessment of the sensor pre-launch radiometric performance, such as the sensor signal to noise ratios (SNRs), dynamic range, reflective and emissive bands calibration performance, polarization sensitivity, bands spectral performance, response-vs-scan (RVS), near field and stray light responses. A set of performance metrics generated during the pre-launch testing program will be compared to the SNPP VIIRS pre-launch performance.
The Day-Night Band (DNB) on Suomi-NPP VIIRS is a visible/near-Infrared panchromatic band capable of observing the earth during both daytime and nighttime. The VIIRS DNB is the first in its class to have on-orbit radiometric calibration using on-board calibrators. This paper describe the DNB calibration methodology used by the NASA VIIRS Characterization Support Team (VCST), and the process to provide consistent calibration Look-Up-Tables (LUTs). The radiometric calibration processes include (1) using the Solar Diffuser observation of the Sun to determine the gain of the DNB’s low gain stage; (2) using the Solar Diffuser signals outside of direct Sun illumination to compute the gain ratios between DNB’s low-to-mid and mid-to-high gain stages; and (3) using the nighttime calibrator observations to track the dark offset drift. Time dependent modulated Relative Spectral Responses (RSRs) are used to correct the optical throughput change due to mirror darkening. The pitch maneuver’s deep space view is used to compute the airglow free HGS offset. DNB stray light is corrected based on a correction LUT estimated from dark surface during each month’s new moon.
Electronic and optical crosstalk is one of the major challenges facing space-based Earth observing sensors, the effects of which could pose serious risks to the successful retrieval of geophysical information. There was an extensive effort during the SNPP VIIRS design and testing phase to characterize the on-orbit VisNIR crosstalk and its impact on environmental products. This paper describes an approach to assess the level of optical and electronic crosstalk on the measured radiance, and thereafter the retrieved geophysical products. During SNPP VIIRS pre-launch testing, a set of electronic and optical cross-talk influence coefficients was derived from measurements, which represent the amount of signal contamination received by each detector when other detectors on the same focal plane were illuminated. These coefficients were used to assess the potential crosstalk and its uncertainty on typical SNPP VIIRS land, atmosphere and ocean scenes. The simulation results show SNPP VIIRS crosstalk contamination is very small, less than 0.3 % for the stressing scenes, except for bands M7 and I2 over the dark ocean regions. These results are encouraging and constitute further evidence that SNPP VIIRS produces high quality imagery. The simulation approach presented in this paper could also be used for early crosstalk impact assessments for future VIIRS instruments.
The on-orbit radiometric response calibration of the VISible/Near InfraRed (VISNIR) and the Short-Wave InfraRed
(SWIR) bands of the Visible/Infrared Imager/Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting
Partnership (NPP) satellite is carried out through a Solar Diffuser (SD). The transmittance of the SD screen and the
SD’s Bidirectional Reflectance Distribution Function (BRDF) are measured before launch and tabulated, allowing
the VIIRS sensor aperture spectral radiance to be accurately determined. The radiometric response of a detector is
described by a quadratic polynomial of the detector’s digital number (dn). The coefficients were determined before
launch. Once on orbit, the coefficients are assumed to change by a common factor: the F-factor. The radiance
scattered from the SD allows the determination of the F-factor. In this Proceeding, we describe the methodology and
the associated algorithms in the determination of the F-factors and discuss the results.
The Visible/Infrared Imager Radiometer Suite (VIIRS) contains six dual gain bands in the
reflective solar spectrum. The dual gain bands are designed to switch gain mode at pre-defined thresholds
to achieve high resolution at low radiances while maintaining the required dynamic range for science.
During pre-launch testing, an anomaly in the electronic response before transitioning from high to low
gain was discovered and was characterized. This anomaly has been confirmed using MODIS data
collected during Simultaneous Nadir Overpasses (SNOs). The analysis of the Earth scene data shows that
this dual gain anomaly can be characterized using sensor earth-view observations. To help understand
this dual gain artifact, the anomaly region and electronic offsets were tracked during the first 8 months of
VIIRS operation. The temporal analysis shows the anomaly region can drift ~20 DN and is impacted by a
detector’s DC Restore. The estimated anomaly flagging regions cover ~2.5 % of the high gain dynamic
range and are consistent with prelaunch analysis and the on-orbit flagging LookUp Table. The prelaunch
results had a smaller anomaly range, likely due to more stable electronics over a shorter data collection
time. Finally, this study suggests future calibration efforts to focus on the anomaly’s impact on science
products and a possible correction method to reduce uncertainties.
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