Frederick S. Patt

Changes in the radiometric sensitivity of SeaWiFS determined from lunar and solar-based measurements

We report on the lunar and solar measurements used to determine the changes in the radiometric sensitivity of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Radiometric sensitivity is defined as the output from the instrument (or from one of the instrument bands) per unit spectral radiance at the instruments input aperture. Knowledge of the long-term repeatability of the SeaWiFS measurements is crucial to maintaining the quality of the ocean scenes derived from measurements by the instrument. For SeaWiFS bands 1-6 (412-670 nm), the change in radiometric sensitivity is less than 0.2% for the period from November 1997 through November 1998. For band 7 (765 nm), the change is approximately 1.5% and for band 8 (865 nm) approximately 5%. The rates of change of bands 7 and 8, which were linear with time for the first eight months of lunar measurements, are now slowing. The scatter in the data points about the trend lines in this analysis is less than 0.3% for all eight SeaWiFS bands. These results are based on monthly measurements of the moon. Daily solar measurements using an onboard diffuser show that the radiometric sensitivities of the SeaWiFS bands have changed smoothly during the time intervals between lunar measurements. Because SeaWiFS measurements have continued past November 1998, the results presented here are considered as a snapshot of the instrument performance as of that date.

Assessment of tilt capability for spaceborne global ocean color sensors

The importance of tilt capability for Sun glint avoidance for future global ocean color missions was analyzed. The analyses focused on Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission, because its radiometric, orbital, and sensor characteristics are well defined. The analyses concentrated can two major questions: 1) does tilting to avoid Sun glint increase or decrease the total ocean coverage, and 2) at high latitudes far from the region of maximum Sun glint, should the sensor be tilted or untilted? For ocean coverage maximization, if the sensitivity of ocean color algorithms to Sun glint is of the same order as the error in the atmospheric correction algorithms, then a tilted sensor produces nearly 20 percent better coverage than an untilted one after 2 d in the absence of clouds, and 12 percent after 4 d including clouds. Thus, the tilt capability can improve the ocean coverage of future ocean color missions. At high latitudes differences in transmitted water-leaving radiance between tilted and untilted sensors were well within the algorithm errors. Furthermore, Sun glint radiances exceeding the algorithm errors occurred at high wind speeds as far as 70/spl deg/ from the solar declination, suggesting that sensors should remain in the tilted mode up to this limit. >

SeaWiFS measurements of the moon

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.

Development of a simulated data set for the SeaWiFS mission

A realistic simulated data set is essential for mission readiness preparations and can potentially assist in all phases of ground support for a future mission. Such a data set was created for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), a global ocean color mission due for launch in 1997. This data set incorporates a representation of virtually every known aspect of the flight mission. Thus, it provides a high fidelity data set for testing most phases of the ground system, including data processing, data transfers, calibration and validation, quality control, and mission operations. The data set is constructed for a seven-day period, March 25-31, 1994. Specific features of the data set: it includes Global Area Coverage (GAC), recorded Local Area Coverage (LAC), and real-time High Resolution Picture Transmission (HRPT) data for the seven-day period; it includes a realistic orbit which is propagated using a Brouwer-Lyddane model with drag; the data correspond to a command schedule based on the orbit for this seven-day period; it includes total (at-satellite) radiances for ocean, land, clouds, and ice; it utilizes a high-resolution land/sea mask; it includes actual SeaWiFS spectral responses; it includes the actual sensor saturation responses; it is formatted according to current onboard data structures; and it includes corresponding telemetry (instrument and spacecraft) data. The methods are described and some examples of the output are given.

Residual correlations in the solar diffuser measurements of the MODIS Aqua ocean color bands to the sun yaw angle

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.

Initial analysis of ocean color data from the ocean color and temperature scanner. II. Geometric and radiometric analysis

We assessed the geometric and radiometric performance of the ocean color and temperature scanner (OCTS) using data acquired over the United States. Initial results indicated a geometric offset in the along-track direction of 4-5 pixels that was attributed to a tilt bias. OCTS radiometric data appeared to suffer from near-field and possibly far-field scatter effects. Analysis of radiometric stability was inconclusive because of daily variability and the absence of a full seasonal cycle. Comparison of OCTS-computed water-leaving radiances with colocated in situ measurements showed that the prelaunch calibration required adjustment from -2% to +13%. Minor modification of OCTS data processing based on these results and avoidance of near-field scatter effects can enable improved and more-reliable OCTS data for quantitative scientific analyses.

Calibration of SeaWiFS on orbit

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 instruments 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.

Sun beta angle residuals in solar diffuser measurements of the MODIS ocean bands

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.

SeaWiFS calibration: status after two years on orbit

The SeaWiFS Project has corrected the instrument calibration for drifts in the response of bands 1, 2, and 5-8 using lunar calibration data and has performed a vicarious calibration of SeaWiFS using in situ data from the Marine Optical Buoy deployed off of Lanai, Hawaii. This vicarious calibration has been validated by comparing SeaWiFS data with in situ data from various ship cruises. The calibration has also been validated through a global clear water analysis. Since the validation data come from regions of the ocean other than the MOBY site, the agreement between the SeaWiFS data in the validation data show that the vicarious calibration of SeaWiFS is a global calibration.

SeaHawk: an advanced CubeSat mission for sustained ocean colour monitoring

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.