The NASA Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission Ocean Color Instrument Team has completed the prelaunch radiometric characterization of the thermal response of the Ocean Color Instrument (OCI). The radiometric performance of the ultraviolet to visible (UVVIS) and visible to near-infrared (VISNIR) grating spectrographs and the shortwave-infrared (SWIR) filter spectrograph of OCI were characterized during the thermal vacuum testing of the instrument conducted in September and October of 2022. The thermal characterization test program will be outlined, along with the derived radiometric dependencies on temperature. For the UVVIS and VISNIR spectrographs, the change in radiometric response with temperature is consistent with theoretical models of the measured detector performance and is on the order of 0.15% per °C. For the SWIR spectrograph, the change in radiometric response with temperature in on the order of 0.04% per °C. For the UVVIS spectrograph, uncertainties in the radiometric measurements as the detector temperatures varied by ∼10° C were less than 0.15% for wavelengths of 350 − 593 nm. For the VISNIR spectrograph, uncertainties were less than 0.11% for wavelengths of 625 − 867 nm. For the SWIR spectrograph, the typical uncertainties were less than 0.15% for all bands. Since the expected temperature range for the instrument on orbit is 0.5° C, OCI meets the design goals for upper limits on radiometric uncertainties due to thermal effects.
The NASA Ocean Biology Processing Group (OBPG) has continued monitoring the SNPP VIIRS on-orbit calibration for bands M1-M11 over its mission to optimize the calibration for ocean color applications. The OBPG has recently implemented several changes to the calibration scheme: using solar-derived f-factors to detrend the lunar observations; using long-term exponentials of time as basis vectors (along with libration angles) for radiometric fits to any resulting lunar temporal drifts; deriving gain adjustments to the solar f-factors from these exponentials; and deriving gain adjustments due to modulated RSRs outside of the solar/lunar calibration using TOA reference spectra. These calibration changes minimize the impact of uncertainties in any one component of the calibration on the derived f-factors. The final f-factors incorporate VIIRS solar diffuser measurements, h-factor BRDF corrections, lunar-derived gains, and modulated RSR gains. The combined BRDF corrections, lunar gain adjustments, and mRSR gain adjustments define effective h-factors for each band. The improvements in the on-orbit calibration are validated by evaluation of globally-derived anomaly plots of remote sensing reflectance for the ocean color bands. The ultimate goal of the OBPG calibration effort is incorporation of a consistent SNPP VIIRS ocean color data set into the NASA multi-mission ocean color climate data record.
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.
The United States Geological Survey (USGS) has developed an empirical model, known as the Robotic Lunar Observatory (ROLO) Model, that predicts the reflectance of the Moon for any Sun-sensor-Moon configuration over the spectral range from 350 nm to 2500 nm. The lunar irradiance can be predicted from the modeled lunar reflectance using a spectrum of the incident solar irradiance. While extremely successful as a relative exo-atmospheric calibration target, the ROLO Model is not SI-traceable and has estimated uncertainties too large for the Moon to be used as an absolute celestial calibration target. In this work, two recent absolute, low uncertainty, SI-traceable top-of-the-atmosphere (TOA) lunar irradiances, measured over the spectral range from 380 nm to 1040 nm, at lunar phase angles of 6.6° and 16.9° , are used as tie-points to the output of the ROLO Model. Combined with empirically derived phase and libration corrections to the output of the ROLO Model and uncertainty estimates in those corrections, the measurements enable development of a corrected TOA lunar irradiance model and its uncertainty budget for phase angles between ±80° and libration angles from 7° to 51° . The uncertainties in the empirically corrected output from the ROLO model are approximately 1 % from 440 nm to 865 nm and increase to almost 3 % at 412 nm. The dominant components in the uncertainty budget are the uncertainty in the absolute TOA lunar irradiance and the uncertainty in the fit to the phase correction from the output of the ROLO model.
The NASA Ocean Color calibration team continued to reanalyze and improve on their approach to the on-orbit calibration of the Visible Infrared Imaging Radiometer Suite (VIIRS), aboard the Suomi National Polar-orbiting Partnership (NPP) satellite, now five years into its Earth Observation mission. As the calibration was adjusted for changes in ocean band responsitivity with time, the team also observed the variance and autocorrelation properties of calibration trend fit residuals, which appeared to have a standard deviation within a few tenths of a percent. Autocorrelation was observed to be different between bands at the blue end of the spectrum and bands at the red/NIR end, which are affected by significant changes in responsitivity stemming from mirror contamination. This residual information offered insight into the effect of small calibration biases, which can cause significant trend uncertainties in regional time series of surface reflectance and derived products. This work involves modeling spurious trends that are inherent to the calibration over time and that also arise between reprocessing efforts because of extrapolation of the time-dependent calibration table. Uncertainty in calibration trends was estimated using models of instrument and calibration system trend artifacts and correlated noise models using Monte Carlo techniques. Combined table reprocessing and extrapolation biases are presented for the first time. Calibration trend uncertainty is then propagated through to ocean color remote sensing reflectance and chlorophyll-a concentration time series. The results quantify the smallest trend observable in these oceanic parameters. This quantification furthers our understanding of uncertainty in measuring regional and global biospheric trends in the ocean using VIIRS, and better defines the roles of records in climate research.
The NASA Ocean Biology Processing Group (OBPG) has continued monitoring the SNPP VIIRS on-orbit calibration since the derivation of the calibration for Reprocessing 2014.0 of the VIIRS ocean color data set. This paper examines four changes to the on-orbit calibration data processing scheme: the prelaunch counts-toradiance conversion; residual solar beta-angle effects in the solar calibration time series; the impact of additional lunar observations on the solar/lunar time series comparisons; and the necessity of putting calibration epochs into fits of the radiometric time series. Updating the prelaunch counts-to-radiance conversion from a linear function of instrument counts to a temperature-dependent, quadratic function of counts had the primary effect of reducing the observational scatter in the lunar calibration time series. The RMS errors due to residual solar beta angle effects are 0.1% for bands M1 (412 nm), M2 (445 nm), and M5 (672 nm) and less for the other bands. The additional lunar observations show that the slopes of the differences in the lunar and solar radiometric trends change nonlinearly over time. VIIRS bands M1–M11 all show changes in radiometric response trends between late 2014 and early 2015, which can be mitigated with an epoch boundary in the fits to the radiometric response on 1 January 2015. The updated solar calibration time series show RMS residuals per band of 0.05–0.22%. The updated lunar calibration time series shows RMS residuals per band of 0.08–0.27%. The solar and lunar time series show RMS differences of 0.10–0.20%.
KEYWORDS: Calibration, Data modeling, Visible radiation, Radiometry, Infrared imaging, Reflectivity, Climatology, Short wave infrared radiation, Monte Carlo methods, Error analysis
During the first few years of the Suomi National Polar-orbiting Partnership (NPP) mission, the NASA Ocean Color calibration team continued to improve on their approach to the on-orbit calibration of the Visible Infrared Imaging Radiometer Suite (VIIRS). As the calibration was adjusted for changes in ocean band responsitivity, the team also estimated a theoretic residual error in the calibration trends well within a few tenths of a percent, which could be translated into trend uncertainties in regional time series of surface reflectance and derived products, where biases as low as a few tenths of a percent in certain bands can lead to significant effects. This study looks at effects from spurious trends inherent to the calibration and biases that arise between reprocessing efforts because of extrapolation of the timedependent calibration table. With the addition of new models for instrument and calibration system trend artifacts, new calibration trends led to improved estimates of ocean time series uncertainty. Table extrapolation biases are presented for the first time. The results further the understanding of uncertainty in measuring regional and global biospheric trends in the ocean using VIIRS, which better define the roles of such records in climate research.
The NASA Ocean Biology Processing Group (OBPG) has continued monitoring the SNPP VIIRS on-orbit calibration since the derivation of the calibration for Reprocessing 2014.0 of the VIIRS ocean color data set. That calibration was based on solar and lunar observations through July 2014. Updates to the R2014.0 calibration include: 1) the addition of solar and lunar observations through May 2015; 2) the extension of the lunar libration corrections to incorporate sub-solar point corrections in addition to sub-spacecraft point corrections; 3) the implementation of a shortwave infrared (SWIR) band lunar and solar calibration; and 4) the absolute calibration of the solar observations using solar diffuser measurements. The SWIR band lunar calibration shows residual libration effects. Comparison of the lunar and solar time series yields lunar-derived adjustments to the solar calibration. The solar calibration time series show RMS residuals per band of 0.066–0.17%. The lunar calibration time series show RMS residuals per band of 0.072–0.23%. The solar and lunar time series show RMS differences per band of 0.10–0.23%. The VIIRS on-orbit calibration stability is comparable to that achieved for heritage instruments (SeaWiFS, Aqua MODIS). The quality of the resulting ocean color products is sufficient for incorporation of the VIIRS data into the long-term NASA ocean color data record.
The radiometric stability requirements for ocean color climate data records place tight constraints on the onorbit calibration of ocean color instruments. A major component of the on-orbit calibration methodology for NASA ocean color sensors is the normalization of lunar observations for variations in observing geometry by the USGS ROLO photometric model of the Moon. SeaWiFS made 204 lunar observations over its 13-year mission. 145 radiometric trending observations were made at low phase angles (-8° to -6° and +5° to +10°). 59 additional observations were made at high phase angles (-49° to -27° and +27° to +66° degrees). The NASA Ocean Biology Processing Group has undertaken a reanalysis of residual geometric effects in the SeaWiFS lunar observations. Ratios of SeaWiFS observations to ROLO model predictions were fit with quadratic functions of phase angle and linear functions of sub-spacecraft point and sub-solar point libration longitude and latitude angles. The resulting phase and libration fit coefficients have been used as additional geometric corrections for the SeaWiFS lunar observations. For the low phase angle observations, the phase corrections are 0.16% and the libration corrections are 0.18%. For the low and high phase angle observations, the phase corrections are 1.8% and the libration corrections are 0.22%. These geometric corrections have reduced the overall scatter in the lunar observations, bringing the high phase angle data into family with the low phase angle measurements without impacting the radiometric response in the low phase angle observations.
Remotely sensed ocean color products require highly accurate top-of-atmosphere (TOA) radiances, on the order of 0.5% or better. Due to incidents both prelaunch and on-orbit, meeting this requirement has been a consistent problem for the MODIS instrument on the Terra satellite, especially in the later part of the mission. The NASA Ocean Biology Processing Group (OBPG) has developed an approach to correct the TOA radiances of MODIS Terra using spatially and temporally averaged ocean color products from other ocean color sensors (such as the SeaWiFS instrument on Orbview-2 or the MODIS instrument on the Aqua satellite). The latest results suggest that for MODIS Terra, both linear polarization parameters of the Mueller matrix are temporally evolving. A change to the functional form of the scan angle dependence improved the quality of the derived coefficients. Additionally, this paper demonstrates that simultaneously retrieving polarization and gain parameters improves the gain retrieval (versus retrieving the gain parameter only).
KEYWORDS: Calibration, MODIS, Near infrared, Satellites, Sensors, Environmental sensing, Data modeling, Climatology, Infrared sensors, Monte Carlo methods
Launched in late 2011, the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (NPP) spacecraft is being evaluated by NASA to determine whether this sensor can continue the ocean color data record established through the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) and the MODerate resolution Imaging Spectroradiometer (MODIS). To this end, Goddard Space Flight Center generated evaluation ocean color data products using calibration techniques and algorithms established by NASA during the SeaWiFS and MODIS missions. The calibration trending was subjected to some initial sensitivity and uncertainty analyses. Here we present an introductory assessment of how the NASA-produced time series of ocean color is influenced by uncertainty in trending instrument response over time. The results help quantify the uncertainty in measuring regional and global biospheric trends in the ocean using satellite remote sensing, which better define the roles of such records in climate research.
The NASA VIIRS Ocean Science Team (VOST) has developed two independent calibrations of the SNPP VIIRS moderate resolution reflective solar bands using solar diffuser and lunar observations through June 2013. Fits to the solar calibration time series show mean residuals per band of 0.078–0.10%. There are apparent residual lunar libration correlations in the lunar calibration time series that are not accounted for by the ROLO photometric model of the Moon. Fits to the lunar time series that account for residual librations show mean residuals per band of 0.071–0.17%. Comparison of the solar and lunar time series shows that the relative differences in the two calibrations are 0.12–0.31%. Relative uncertainties in the VIIRS solar and lunar calibration time series are comparable to those achieved for SeaWiFS, Aqua MODIS, and Terra MODIS. Intercomparison of the VIIRS lunar time series with those from SeaWiFS, Aqua MODIS, and Terra MODIS shows that the scatter in the VIIRS lunar observations is consistent with that observed for the heritage instruments. Based on these analyses, the VOST has derived a calibration lookup table for VIIRS ocean color data based on fits to the solar calibration time series.
Following the launch of the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polarorbiting
Partnership (NPP) spacecraft, the NASA NPP VIIRS Ocean Science Team (VOST) began an evaluation of
ocean color data products to determine whether they could continue the existing NASA ocean color climate data record
(CDR). The VOST developed an independent evaluation product based on NASA algorithms with a reprocessing
capability. Here we present a preliminary assessment of both the operational ocean color data products and the NASA
evaluation data products regarding their applicability to NASA science objectives.
The NASA VIIRS Ocean Science Team (VOST) has the task of evaluating Suomi NPP VIIRS ocean color data
for the continuity of the NASA ocean color climate data records. The generation of science quality ocean color
data products requires an instrument calibration that is stable over time. Since the VIIRS NIR Degradation
Anomaly directly impacts the bands used for atmospheric correction of the ocean color data (Bands M6 and
M7), the VOST has adapted the VIIRS on-orbit calibration approach to meet the ocean science requirements.
The solar diffuser calibration time series and the solar diffuser stability monitor time series have been used to
derive changes in the instrument response and diffuser reflectance over time for bands M1–M11. The lunar
calibration observations have been used, in cooperation with the USGS ROLO Program, to derive changes in
the instrument response over time for these same bands. In addition, the solar diffuser data have been used to
develop detector-dependent striping and mirror side-dependent banding corrections for the ocean color data. An
ocean surface reflectance model has been used to perform a preliminary vicarious calibration of the VIIRS ocean
color data products. These on-orbit calibration techniques have allowed the VOST to produce an optimum timedependent
radiometric calibration that is currently being used by the NASA Ocean PEATE for its VIIRS ocean
color data quality evaluations. This paper provides an assessment of the current VIIRS radiometric calibration
for the ocean color data products and discusses the path forward for improving the quality of the calibration.
Ocean color climate data records require water-leaving radiances with 5% absolute and 1% relative accuracies
as input. Because of the amplification of any sensor calibration errors by the atmospheric correction, the 1%
relative accuracy requirement translates into a 0.1% long-term radiometric stability requirement for top-of-theatmosphere
radiances. The rigorous on-orbit calibration program developed and implemented for SeaWiFS by
the NASA Ocean Biology Processing Group (OBPG) Calibration and Validation Team (CVT) has allowed the
CVT to maintain the stability of the radiometric calibration of SeaWiFS at 0.13% or better over the mission.
The uncertainties in the resulting calibrated top-of-the-atmosphere (TOA) radiances can be addressed in terms of
accuracy (biases in the measurements), precision (scatter in the measurements), and stability (repeatability of the
measurements). The calibration biases of lunar observations relative to the USGS RObotic Lunar Observatory
(ROLO) photometric model of the Moon are 2-3%. The biases from the vicarious calibration against the Marine
Optical Buoy (MOBY) are 1-2%. The precision of the calibration derived from the solar calibration signal-tonoise
ratios are 0.16%, from the lunar residuals are 0.13%, and from the vicarious gains are 0.10%. The long-term
stability of the TOA radiances, derived from the lunar time series, is 0.13%. The stability of the vicariouslycalibrated
TOA radiances, incorporating the uncertainties in the MOBY measurements and the atmospheric
correction, is 0.30%. These results allow the OBPG to produce climate data records from the SeaWiFS ocean
color data.
For several years, the NASA/Goddard Space Flight Center (GSFC) NPP VIIRS Ocean Science Team (VOST) provided
substantial scientific input to the NPP project regarding the use of Visible Infrared Imaging Radiometer Suite (VIIRS) to
create science quality ocean color data products. This work has culminated into an assessment of the NPP project and
the VIIRS instrument's capability to produce science quality Ocean Color data products. The VOST concluded that
many characteristics were similar to earlier instruments, including SeaWiFS or MODIS Aqua. Though instrument
performance and calibration risks do exist, it was concluded that programmatic and algorithm issues dominate concerns.
Scanning radiometers on earth-orbiting satellites are used to measure the chlorophyll content of the oceans
via analysis of the water-leaving radiances. These radiances are very sensitive to the atmospheric correction
process, which in turn is polarization dependent. The image created by a scanning radiometer is usually composed
of successive scans by two mirror sides and one or several detectors. The Moderate Resolution Imaging
Spectroradiometer (MODIS) has 10 detectors for each ocean color band. If the polarization sensitivities are
different among detectors and this is not taken account of in the atmospheric correction process, striping will
occur in different parts of the images. MODIS polarization parameters were derived using ground truth data
from another earth-orbiting sensor (Sea-viewing Wide Field-of-view Sensor, SeaWiFS), allowing a comparison
of the on-orbit characterization and the prelaunch characterization. This paper presents these comparisons for
the MODIS instruments on the Aqua and Terra satellites. The detector dependency is clearly different in the
prelaunch characterization. This paper also describes the detector dependency of the vicarious corrections to
the radiometric calibration coefficients. During the first four years of each mission, the only correction needed
to minimize striping in the ocean color products is a constant offset, there is indication of a temporal trend or a
view angle dependency for these offsets. The offsets are similar for both instruments, but larger in Terra.
Observations of the Moon provide one technique for the cross calibration of Earth remote sensing instruments.
Monthly lunar observations are major components of the on-orbit calibration strategies of the SeaWiFS and
MODIS instruments. SeaWiFS has collected more than 132 low phase angle and 59 high phase angle lunar
observations over 12 years, while Terra MODIS has collected more than 82 scheduled and 297 unscheduled
lunar observations over 9 years and Aqua MODIS has collected more than 61 scheduled and 171 unscheduled
lunar observations over 7 years. The NASA Ocean Biology Processing Group's Calibration and Validation
Team (OBPG CVT) and the NASA MODIS Characterization Support Team (MCST) use the U.S. Geological
Survey's RObotic Lunar Observatory (ROLO) photometric model of the Moon to compare these time series
of lunar observations. In addition, the Moon was observed simultaneously by SeaWiFS and Terra MODIS
on 14 April 2003 as part of the Earth Observing System (EOS) Lunar Calibration Experiment, allowing
a direct comparison of one set of lunar measurements. The OBPG CVT and MCST use residuals of the
lunar observations from the ROLO model to cross calibrate SeaWiFS and the two MODIS instruments.
The cross calibration results show that Terra MODIS and Aqua MODIS agree, band-to-band, at the 1-3%
level, while SeaWiFS and either MODIS instrument agree at the 3-8% level. The main implication of these
cross-calibration results is that the operations concepts for upcoming remote sensing instruments should be
designed to maximize the number of lunar observations over the mission time frame, while minimizing the
phase angle range of the observations.
MODIS has 20 reflective solar bands (RSB), covering the VIS, NIR, and SWIR spectral regions. They are calibrated on-orbit
using a solar diffuser (SD) panel, made of space-grade Spectralon. The SD bi-directional reflectance factor (BRF)
was characterized pre-launch by the instrument vendor with reference to the NIST reflectance standard. Its on-orbit
degradation is tracked by an on-board solar diffuser stability monitor (SDSM). The SeaWiFS on-orbit calibration
strategy uses monthly lunar observations to monitor the long-term radiometric stability of the instrument and applies
daily observations of its solar diffuser (an aluminum plate coated with YB71 paint) to track the short-term changes in the
instrument response. This paper provides an overview of MODIS and SeaWiFS SD observations, applications, and
approaches used to track their on-orbit degradations. Results from both sensors are presented with emphasis on the
spectral dependence and temporal trends of the SD degradation. Lessons and challenges from the use of SD for sensor
on-orbit calibration are also discussed.
The Moon plays an important role in the radiometric stability monitoring of the NASA Earth Observing System's (EOS)
remote sensors. The MODIS and SeaWIFS are two of the key instruments for NASA's EOS missions. The MODIS
Protoflight Model (PFM) on-board the Terra spacecraft and the MODIS Flight Model 1 (FM1) on-board the Aqua
spacecraft were launched on December 18, 1999 and May 4, 2002, respectively. They view the Moon through the
Space View (SV) port approximately once a month to monitor the long-term radiometric stability of their Reflective
Solar Bands (RSB). SeaWIFS was launched on-board the OrbView-2 spacecraft on August 1, 1997. The SeaWiFS
lunar calibrations are obtained once a month at a nominal phase angle of 7°. The lunar irradiance observed by these
instruments depends on the viewing geometry. The USGS photometric model of the Moon (the ROLO model) has been
developed to provide the geometric corrections for the lunar observations. For MODIS, the lunar view responses with
corrections for the viewing geometry are used to track the gain change for its reflective solar bands (RSB). They trend
the system response degradation at the Angle Of Incidence (AOI) of sensor's SV port. With both the lunar observation
and the on-board Solar Diffuser (SD) calibration, it is shown that the MODIS system response degradation is
wavelength, mirror side, and AOI dependent. Time-dependent Response Versus Scan angle (RVS) Look-Up Tables
(LUT) are applied in MODIS RSB calibration and lunar observations play a key role in RVS derivation. The
corrections provided by the RVS in the Terra and Aqua MODIS data from the 412 nm band are as large as 16% and
13%, respectively. For SeaWIFS lunar calibrations, the spacecraft is pitched across the Moon so that the instrument
views the Moon near nadir through the same optical path as it views the Earth. The SeaWiFS system gain changes for
its eight bands are calibrated using the geometrically-corrected lunar observations. The radiometric corrections to the
SeaWiFS data, after more than ten years on orbit, are 19% at 865 nm, 8% at 765 nm, and 1-3% in the other bands. In
this report, the lunar calibration algorithms are reviewed and the RSB gain changes observed by the lunar observations
are shown for all three sensors. The lunar observations for the three instruments are compared using the USGS
photometric model. The USGS lunar model facilitates the cross calibration of instruments with different spectra
bandpasses whose measurements of the Moon differ in time and observing geometry.
The NASA Ocean Biology Processing Group's Calibration and Validation Team uses SeaWiFS on-orbit lunar
calibrations to monitor the radiometric response of the instrument over time. With almost eleven years of
lunar measurements (more than 124 monthly observations) available for this analysis, the Cal/Val Team
has undertaken an investigation of the optimum function to use in fitting the time series and the fidelity of
resulting radiometric corrections that are applied to the ocean data. Two aspects of the on-orbit behavior
of SeaWiFS show changes over time: the long-term radiometric response for each band and the dependence
of the individual detector response in each band on the varying focal plane temperatures. Since band 8 (865
nm) shows the greatest changes in response over time, the analysis has concentrated on that band.
The initial goal of the SeaWiFS on-orbit calibration effort has been to use a single function to fit the
mission-long lunar time series. To date, that goal has been met by using a pair of simultaneous decaying
exponential functions with short-period and long-period time constants. As late mission observations were
added to the time series (beyond seven years into the mission), the long-term radiometric trend has been
approaching a linear function of time. Consequently, the long-term trend is starting to bias the fit for the
first three years of the mission. The Cal/Val team has addressed this issue by introducing a radiometric
epoch into the time series fitting functions, where the best fit for the early mission is provided by exponential
functions with periods of 200 and 2500 day and the best fit for the late mission is provided by an exponential
with a 400-day time constant and a linear function (or an exponential with a 40,000-day time constant). A
complication in optimizing these fits is that the dependence of the detector response on varying focal plane
temperatures began changing approximately seven years into the mission.
Analyses of periodic residuals in the lunar calibration time series in the latter part of the mission show
that either the temperature-dependence of the detector response or the overall thermal environment of the
instrument is changing over time. The Cal/Val Team has used correlations between these residuals and
the focal plane temperatures to evaluate revisions to the temperature corrections for the detector response.
Complications in computing these revised temperature corrections are that the behavior of the temperature
corrections is not readily described by an analytical function and that the long-term radiometric fits
compensate, to an extent, for changes in the temperature corrections.
In order to develop an improved calibration model for SeaWiFS, the Cal/Val Team has developed a
methodology for simultaneously fitting the long-term radiometric trend of each band and the change in
the temperature-dependence of the individual detector responses. This work shows the increased fidelity of
the calibration derived simultaneously for the long-term radiometric trend and the focal plane temperature
response compared to the sequential derivations of these corrections.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Earth Observing System (EOS) Aqua
platform has 9 spectral bands with center wavelengths from 412nm to 870nm that are used to produce the
standard ocean color data products. Ocean color products require a stability of the radiometric calibration on
the order of 0.2%, which surpasses the official requirements for the MODIS reflective solar bands. The primary
calibration source for the MODIS reflective solar bands is the on-board solar diffuser. For the ocean color bands,
the SD calibration is performed with an attenuation screen to prevent saturation. The ocean color products are
calculated using supplemental sun beta angle corrections (with a magnitude of about 0.5%) for the MODIS Aqua
solar diffuser measurements in the ocean color bands. The initial corrections were derived using a three-year
time series of solar diffuser measurements. This paper presents an update to these corrections for Aqua using a
six-year time series, and describes the effect of these new corrections on the resulting calibration coefficients. The
corrections are also described for the MODIS on Terra. The magnitude of the corrections for Terra is significantly
less than for Aqua, and the sign of the response to the beta angle in Terra is opposite to that of Aqua.
The NASA Ocean Biology Processing Group's Calibration and Validation (Cal/Val) Team has used SeaWiFS onorbit
lunar and gain calibration data, in conjunction with mission-long trends of global ocean color data products,
to diagnose and correct recently emergent residual drifts in the radiometric response of the instrument.
An anomaly analysis of the time series of global mean normalized water-leaving radiances, the atmospheric
correction parameter ∈, and chlorophyll show significant departures from the mission-long trends beginning in
January 2006. The lunar time series trends for the near infrared (NIR) bands (765 nm and 865 nm) show
significant periodic departures from mission-long trends beginning at the same time. ∈ is dependent on the ratio
of these two bands; trends in this parameter would propagate through the atmospheric correction algorithm to
the retrieved water-leaving radiances. An analysis of fit residuals from the lunar time series shows that the focal
plane temperature dependencies of the radiometric response of the detectors for these two bands have changed
over the 9+ year mission. The Cal/Val Team has used these residuals to compute a revised set of temperature
corrections for data collected starting 1 January 2006. The lunar calibration data and a mission-long ocean color
test data set have been reprocessed with the revised temperature corrections. The reprocessed data show that
the trends in the NIR bands have been minimized and that the departures of the water-leaving radiances, ∈, and
chlorophyll from the mission-long trends have been greatly reduced.
The anomaly analysis of the water-leaving radiances in the 510 nm band still shows a residual drift of -2.9%
over the mission. The anomaly analysis of the ∈ time series shows a residual drift of +2.8% over the mission. A
corresponding drift is not observed in the lunar calibration time series for the NIR bands. The lunar calibration
data are obtained at a different set of instrument gains than are the ocean data. An analysis of the mission-long
time series of on-orbit gain calibration data shows that the gain ratios for the NIR bands change -0.76% (765 nm)
and +0.56% (865 nm) over the mission, corresponding to a -1.3% drift in the band ratio. The lunar calibration
time series for the NIR bands have been corrected for this gain drift, and the change in radiometric response over
time has been recomputed for each band. The mission-long ocean color test data set has been reprocessed with
these revised corrections for the NIR bands. The anomaly analysis of the reprocessed water-leaving radiances
at 510 nm shows the drift to have been essentially eliminated, while the anomaly analysis of epsilon shows a
reduced drift of +2.0%.
These analyses show the sensitivity of ocean color data to small drifts in instrument calibration and demonstrate
the use of time series of global mean geophysical parameters to monitor the long-term stability of the
instrument calibration on orbit. The two updates to SeaWiFS radiometric calibration have been incorporated
into the recent reprocessing of the SeaWiFS mission-long ocean data set.
The NASA Ocean Biology Processing Group's Calibration and Validation Team has used monthly lunar calibrations
of SeaWiFS to establish and maintain the on-orbit radiometric stability of instrument at the 0.1% level
over its 9-year mission. The Cal/Val Team has compared the SeaWiFS lunar observations with the USGS ROLO
photometric model of the Moon to verify the long-term stability of the SeaWiFS radiometric calibration. This
stability has allowed the Team to apply a system-level vicarious calibration of the sensor/atmospheric calibration
algorithm that is independent of time, yielding a single gain per band. SeaWiFS bands 1-6 (412-670 nm)
are calibrated against water-leaving radiances measured by the Marine Optical Buoy (MOBY) that have been
propagated to the top of the atmosphere. Band 7 (765 nm) is calibrated relative to band 8 (865 nm) so that
the atmospheric correction algorithm selects maritime aerosol models over open ocean scenes. The long-term
radiometric stability of SeaWiFS allows the Cal/Val Team to directly compare the mean residuals of the lunar
observations from the ROLO model with the vicarious gains. A linear fit of the vicarious gains vs - (mean
ROLO residual) for bands 1-6 gives a slope of 1.084 with a correlation of 0.980. The predicted mean ROLO
residual for band 7, computed from the observed mean residual for band 8 and the vicarious gain for band 7,
agrees with the observed mean residual for band 7 to within 0.5%. The radiometric stability of SeaWiFS allows
the comparison of the prelaunch calibration of SeaWiFS, the calibration of MOBY, and the calibration of the
USGS ROLO model. Such a comparison is of interest to other Earth-observing instruments which use the Moon
as a calibration reference, such as MODIS, VIIRS, and ABI.
KEYWORDS: Diffusers, Signal to noise ratio, Reflectivity, Calibration, Bidirectional reflectance transmission function, Satellites, MODIS, Aluminum, Radio optics, Space operations
The NASA Ocean Biology Processing Group's Calibration and Validation (Cal/Val) Team implemented daily
solar calibrations of SeaWiFS to look for step-function changes in the instrument response and has used these
calibrations to supplement the monthly lunar calibrations in monitoring the radiometric stability of SeaWiFS
during its first year of on-orbit operations. The Team has undertaken an analysis of the mission-long solar
calibration time series, with the lunar-derived radiometric corrections over time applied, to assess the long-term
degradation of the solar diffuser reflectance over nine years on orbit. The SeaWiFS diffuser is an aluminum
plate coated with YB71 paint. The bidirectional reflectance distribution function of the diffuser was not fully
characterized before launch, so the Cal/Val Team has implemented a regression of the solar incidence angles and
the drift in the node of the satellite's orbit against the diffuser time series to correct for solar incidence angle
effects. An exponential function with a time constant of 200 days yields the best fit to the diffuser time series.
The decrease in diffuser reflectance over the mission is wavelength-dependent, ranging from 9% in the blue (412
nm) to 5% in the red and near infrared (670-865 nm). The degradation of diffuser reflctance is similar to that
observed for SeaWiFS radiometric response itself from lunar calibration time series for bands 1-5 (412-555 nm),
though the magnitude of the change is four times larger for the diffuser. Evidently, the same optical degradation
process has affected both the telescope optics and the solar diffuser in the blue and green. The Cal/Val Team has
developed a methodology for computing the signal-to-noise ratio (SNR) for SeaWiFS on orbit from the diffuser
time series. The on-orbit change in the SNR for each band over the nine-year mission is less than 7%. The
on-orbit performance of the SeaWiFS solar diffuser should offer insight into the long-term on-orbit performance
of solar diffusers on other instruments, such as MODIS, VIIRS, and ABI.
The Ocean Biology Processing Group (OBPG) at NASA's Goddard Space Flight Center is responsible for the processing and validation of oceanic optical property retrievals from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A major goal of this activity is the production of a continuous ocean color time-series spanning the mission life of these sensors from September 1997 to the present time. This paper presents an overview of the calibration and validation strategy employed to optimize and verify sensor performance for retrieval of upwelling radiances just above the sea surface. Substantial focus is given to the comparison of results over the common mission lifespan of SeaWiFS and the MODIS flying on the Aqua platform, covering the period from July 2002 through December 2004. It will be shown that, through consistent application of calibration and processing methodologies, a continuous ocean color time-series can be produced from two different spaceborne sensors.
The Moon has served as a reference for several satellite instruments including SeaWiFS and MODIS, both of which provide design innovations for NPP VIIRS. However, as yet, the Moon is not a formal part of the calibration baseline for NPP VIIRS. In particular, the lunar measurements by the MODIS instruments require on-orbit maneuvers (spacecraft rolls of up to 20 degrees) to maintain a constant lunar phase angle. Here, we use a simulated set of NPP VIIRS lunar measurements to demonstrate the quality of the Moon as a reference for long-term measurements by VIIRS. With nine lunar comparisons (1 year of VIIRS measurements), it is possible to detect linear changes over time in the calibration of the VIIRS reflective solar bands at the 0.1% per year level or better. In addition, the surface of the Moon does not change over periods of a million years or more. As a result, the Moon can act as a cross-calibration reference for NPP VIIRS and the Terra MODIS instrument that precedes it, even with a time gap between the operation of the two sensors. The quality of this cross-comparison reference is estimated to be significantly better than 1%. However, to accomplish both of these functions, NPP VIIRS must make measurements at the same lunar phase angle as Terra MODIS, that is, at 55 degrees after full phase. This requires periodic spacecraft maneuvers.
The SeaWiFS Project uses monthly lunar calibrations to monitor the on-orbit radiometric stability of SeaWiFS over the course of its mission. Ongoing analyses of the steadily increasing lunar calibration data set have led to improvements in the calibration methodology over time. The lunar measurements must be normalized to a common viewing geometry for the calibration time series to track the radiometric stability of the instrument. Corrections computed from the time and geometry of the observations include Sun-Moon and instrument-Moon distances, oversampling of the lunar image, and variations in the lunar phase angles. The Project has recently implemented a correction for lunar libration that is computed from regressions of the libration angles of the observations against the lunar radiances. Decaying exponential functions of time are fit to the geometry-corrected calibration time series. The observations for bands 1,2,and 5-8 are fit to two simultaneous exponential functions of time, while bands 3 and 4 are fit to single exponential functions of time. The corrections to the radiometric response of the instrument over time are the inverses of these fits. The lunar calibration methodology provides top-of-the-atmosphere radiances for SeaWiFS that are stable to better than 0.07% over the course of the mission, with residual time drifts that are smaller than -0.004% per thousand days. The resulting water-leaving radiances are stable to better than 0.7%, allowing the Project to implement a vicarious calibration of the water-leaving radiances that is independent of time. The calibration methodology presented here will be used to generate the calibration table for the fifth reprocessing of the SeaWiFS global ocean data set.
The SeaWiFS Project uses daily on-orbit detector and gain calibrations to address issues which arise from the bilinear gain in the SeaWiFS radiometric response function. The bilinear gains provide high sensitivity over the ocean while preventing saturation over clouds or land. The bilinear gains are implemented by averaging the output from three high-sensitivity ocean detectors and one low-sensitivity cloud detector for each band. The calibration issues are: 1) the applicability of time corrections computed from lunar data obtained at one set of instrument gains to ocean data obtained at a different set of gains; and 2) the applicability of time corrections computed from data obtained with all four detectors in each band to the cloud detector alone. The Project uses the gain calibration to monitor the SeaWiFS gain ratios over time. The gain ratios for each detector are computed from measurements at each gain of a constant voltage injected into the post-detector electronics. The gain ratios are stable to within 0.1% over the course of the mission. The Project uses the detector calibrations to compare the response of individual detectors within each band to the response of the four detectors in that band. The detector response is monitored during a modified solar calibration where measurements are obtained from each detector. The departure of the cloud detectors from the four detectors is less than 0.5% for all eight bands. These analyses show that, for SeaWiFS, the time corrections derived from lunar calibrations are applicable to both ocean and land data.
SeaWiFS was launched onboard the OrbView-2 satellite on 1 August 1997. On 4 September 1997, the day of first light for the instrument, SeaWiFS global images were processed automatically using the instrument’s prelaunch calibration and distributed on the World Wide Web. With the first reprocessing of SeaWiFS data in January 1998, the radiometric calibration coefficients for the SeaWiFS visible bands were linked to the water-leaving radiances measured by the Marine Optical Buoy (MOBY). In addition, the calibration coefficient for the 765 nm SeaWiFS infrared band was adjusted to give values consistent with those for an atmosphere with the maritime type of aerosol found in the vicinity of the MOBY buoy. Since the infrared bands were designed to allow the inference of aerosol type for the SeaWiFS atmospheric correction algorithm, this vicarious calibration forces their agreement with the conditions for a known aerosol type. With the second reprocessing in August 1998, temporal changes in the radiometric sensitivities of the SeaWiFS near infrared bands were corrected using lunar and solar measurements. The third SeaWiFS reprocessing in May 2000 introduced small time dependent calibration corrections to some visible bands. Future SeaWiFS reprocessings are scheduled to occur on an annual to biennial basis. With the third reprocessing, the emphasis of the instrument calibration program has shifted to the assessment of the surface truth comparisons used by SeaWiFS, principally those with MOBY.
Measurements of the lunar surface in the visible and near infrared wavelength regions provide a new and intriguing method of determining changes in the sensitivities of Earth observing radiometers. Lunar measurements are part of the calibration strategy for the instruments in the Earth Observing System (EOS) and part of the calibration strategy for the Sea Viewing Wide Field of View Sensor (SeaWiFS). SeaWiFS was launched on August 1, 1997. The first SeaWiFS images of the Earth were taken on September 4, 1997, and the first lunar measurements were made on November 14, 1997. We describe the results from the initial nine lunar measurements by SeaWiFS, extending to July 10, 1998. The time series for the lunar images show changes in the sensitivities of SeaWiFS bands one through five (412 to 555 nm) to be very small over the eight month interval. For band 6 (670 nm), the decrease in sensitivity over seven months is 1/2%. For bands 7 and 8 (765 and 865 nm), the decreases are 11/2% and 5% respectively. These changes, with reduced scatter in the results, are also found in the band ratios. The instrument changes can be seen in the SeaWiFS data products. Using the lunar time series, plus data from diffuser and ocean surface measurements, a time-dependent correction for the changes in the sensitivities of bands 6, 7, and 8 has been applied in the SeaWiFS data processing algorithm.
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