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%.
Sustained ocean color monitoring is vital to understanding the marine ecosystem. It has been identified as an Essential Climate Variable (ECV) and is a vital parameter in understanding long-term climate change. Furthermore, observations can be beneficial in observing oil spills, harmful algal blooms and the health of fisheries. Space-based remote sensing, through MERIS, SeaWiFS and MODIS instruments, have provided a means of observing the vast area covered by the ocean which would otherwise be impossible using ships alone. However, the large pixel size makes measurements of lakes, rivers, estuaries and coastal zones difficult. Furthermore, retirement of a number of widely used and relied upon ocean observation instruments, particularly MERIS and SeaWiFS, leaves a significant gap in ocean color observation opportunities This paper presents an overview of the SeaHawk mission, a collaborative effort between Clyde Space Ltd., the University of North Carolina Wilmington, Cloudland Instruments, and Goddard Spaceflight Center, funded by the Gordon and Betty Moore Foundation. The goal of the project is to enhance the ability to observe ocean color in high temporal and spatial resolution through use of a low-cost, next-generation ocean color sensor flown aboard a CubeSat. The final product will be 530 times smaller (0.0034 vs 1.81m3) and 115 time less massive (3.4 vs 390.0kg) but with a ground resolution 10 times better whilst maintaining a signal/noise ratio 50% that of SeaWiFs. This paper will describe the objectives of the mission, outline the payload specification and the spacecraft platform to support it.
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.
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.
The first Visible Infrared Imager Radiometer Suite (VIIRS) instrument was successfully launched on-board the Suomi
National Polar-orbiting Partnership (SNPP) spacecraft on October 28, 2011. Suomi NPP VIIRS observations are made
in 22 spectral bands, from the visible (VIS) to the long-wave infrared (LWIR), and are used to produce 22 Environmental
Data Records (EDRs) with a broad range of scientific applications. The quality of these VIIRS EDRs strongly
depends on the quality of its calibrated and geo-located Sensor Date Records (SDRs). Built with a strong heritage to the
NASA’s EOS MODerate resolution Imaging Spectroradiometer (MODIS) instrument, the VIIRS is calibrated on-orbit
using a similar set of on-board calibrators (OBC), including a solar diffuser (SD) and solar diffuser stability monitor
(SDSM) system for the reflective solar bands (RSB) and a blackbody (BB) for the thermal emissive bands (TEB). Onorbit
maneuvers of the SNPP spacecraft provide additional calibration and characterization data from the VIIRS instrument
which cannot be obtained pre-launch and are required to produce the highest quality SDRs. These include multiorbit
yaw maneuvers for the characterization of SD and SDSM screen transmission, quasi-monthly roll maneuvers to
acquire lunar observations to track sensor degradation in the visible through shortwave infrared, and a driven pitch-over
maneuver to acquire multiple scans of deep space to determine TEB response versus scan angle (RVS). This paper provides
an overview of these three SNPP calibration maneuvers. Discussions are focused on their potential calibration and
science benefits, pre-launch planning activities, and on-orbit scheduling and implementation strategies. Results from
calibration maneuvers performed during the Intensive Calibration and Validation (ICV) period for the VIIRS sensor are
illustrated. Also presented in this paper are lessons learned regarding the implementation of calibration spacecraft
maneuvers on follow-on missions.
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.
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.
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
The VIIRS Ocean Science Team (VOST) has been developing an Ocean Data Simulator to create realistic
VIIRS SDR datasets based on MODIS water-leaving radiances. The simulator is helping to assess instrument
performance and scientific processing algorithms. Several changes were made in the last two years
to complete the simulator and broaden its usefulness. The simulator is now fully functional and includes
all sensor characteristics measured during prelaunch testing, including electronic and optical crosstalk influences,
polarization sensitivity, and relative spectral response. Also included is the simulation of cloud and
land radiances to make more realistic data sets and to understand their important influence on nearby ocean
color data. The atmospheric tables used in the processing, including aerosol and Rayleigh reflectance coefficients,
have been modeled using VIIRS relative spectral responses. The capabilities of the simulator were
expanded to work in an unaggregated sample mode and to produce scans with additional samples beyond the
standard scan. These features improve the capability to realistically add artifacts which act upon individual
instrument samples prior to aggregation and which may originate from beyond the actual scan boundaries.
The simulator was expanded to simulate all 16 M-bands and the EDR processing was improved to use these
bands to make an SST product. The simulator is being used to generate global VIIRS data from and in
parallel with the MODIS Aqua data stream. Studies have been conducted using the simulator to investigate
the impact of instrument artifacts. This paper discusses the simulator improvements and results from the
artifact impact studies.
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.
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.
One of the roles of the VIIRS Ocean Science Team (VOST) is to assess the performance of the instrument and scientific processing software that generates ocean color parameters such as normalized water-leaving radiances and chlorophyll. A VIIRS data simulator is being developed to help aid in this work. The simulator will create a sufficient set of simulated Sensor Data Records (SDR) so that the ocean component of the VIIRS processing system can be tested. It will also have the ability to study the impact of instrument artifacts on the derived parameter quality. The simulator will use existing resources available to generate the geolocation information and to transform calibrated radiances to geophysical parameters and visa-versa. In addition, the simulator will be able to introduce land features, cloud fields, and expected VIIRS instrument artifacts. The design of the simulator and its progress will be presented.
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 MODIS (Moderate Resolution Imaging Spectroradiometer) scanner makes subframe measurements in some
of its bands to increase the spatial resolution from its standard 1km resolution to 500m or 250m. This is achieved
by sampling a detector of a high resolution band at twice (or four times) the sampling rate of the 1km bands.
This paper shows that a calibration equation nonlinear with radiance and specific to the individual subframes will
reduce striping in the images. The effects are significant for low radiance levels like those encountered over ocean
scenes. A preliminary calibration correction is derived with two approaches: first from prelaunch measurements,
then from on-orbit data. The results of the two methods are qualitatively similar.
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 (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 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 Moderate Resolution Imaging Spectroradiometer (MODIS) on the
Earth Observing System (EOS) Aqua platform uses biweekly solar diffuser measurements for the radiometric calibration of the ocean
color bands. The solar angle relative to the spacecraft changes
throughout the year. This document describes correlations in the solar diffuser measurements of the ocean color bands to the sun yaw angle. The functional form of the correlations depends on the position of the respective band and detector on the focal plane. The proposed corrections often exceed 0.5% (peak-to-peak). The most likely source of the correlations is the radiometric characterization of the solar diffuser screen. These results show the importance of a complete prelaunch characterization for spaceborne sensors regarding the radiometric calibration.
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 United States Geologic Survey (USGS) has developed a lunar irradiance model for comparison with on-orbit sattelite instruments. The comparisons are given as the percent difference of the satellite measurements from the model-generated lunar irradiances. For users of the USGS lunar model, details of the inner operation of the model are unknown. That operation is not examined here. Rather, we examine the outputs of the model. We treat the model-generated lunar irradiances as independent data sets, with each covering only a limited range of phase angles in the comparison with an individual satellite instrument. We correct the model irradiances to standard Sun-Moon and Moon-instrument distances and to fixed values for the phase and libration angles. These same corrections can be applied to the measuements by the comparison satellite instrument. For the model outputs examined here, the time trends in the corrected irradiances differ from zero change by less that 0.0014% per thousand days, and the residual scatter in the results is 0.013% (1σ). There are no apparent oscillations in the time series. At this level, the lunar model should provide an adequate reference for instruments that measure long-term climate change. The model outputs here correspond to the bands from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) instrument and to the phase angle range for the SeaWiFS lunar measurements, which is from 5° to 10°, both before and after full phase. Similar results have been obtained for model outputs corresponding to lunar measurements from the Moderate Resolution Imaging Sectroradiometer (MODIS) instruments onboard the Terra and Aqua spacecraft. This technique is applicable to other instruments that measure at wavelengths from 400 nm to 2500 nm since, in addition to the relative spectral responses of the instrument's bands, the only requirements for the model are the time and location of the instrument's lunar measurements.
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.
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.