Robert A. Barnes

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.

On-orbit radiometric calibration over time and between spacecraft using the Moon

The Robotic Lunar Observatory (ROLO) project has developed a spectral irradiance model of the Moon that accounts for variations with lunar phase through the bright half of a month, lunar librations, and the location of an Earth-orbiting spacecraft. The methodology of comparing spacecraft observations of the Moon with this model has been developed to a set of standardized procedures so that comparisons can be readily made. In the cases where observations extend over several years (e.g., SeaWiFS), instrument response degradation has been determined with precision of about 0.1% per year. Because of the strong dependence of lunar irradiance on geometric angles, observations by two spacecraft cannot be directly compared unless acquired at the same time and location. Rather, the lunar irradiance based on each spacecraft instrument calibration can be compared with the lunar irradiance model. Even single observations by an instrument allow inter-comparison of its radiometric scale with other instruments participating in the lunar calibration program. Observations by SeaWiFS, ALI, Hyperion and MTI are compared here.

Comparison of spectral radiance calibrations at oceanographic and atmospheric research laboratories

This report describes the first Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Radiometric Intercomparison (SIMRIC-1). The purpose of the SIMRIC-1 is to ensure a common radiometric scale among the calibration facilities that are engaged in calibrating in situ radiometers used for ocean colour-related research and to document the calibration procedures and protocols. SIMBIOS staff visited the seven participating laboratories for at least two days each. The SeaWiFS Transfer Radiometer (SXR-II) measured the calibration radiances produced in the laboratories. The measured radiances were compared with the radiances expected by the laboratories. Typically, the measured radiances were higher than the expected radiances by 0 to 2%. This level of agreement is satisfactory. Several issues were identified where the calibration protocols need to be improved, especially the reflectance calibration of the reference plaques and the distance correction when using the irradiance standards at distances greater than 50?cm. The responsivity of the SXR-II changed from 0.3% (channel 6) to 1.6% (channel 2) from December 2000 to December 2001. Monitoring the SXR-II with a portable light source showed a linear drift of the calibration, except for channel 1, where a 2% drop occurred in summer.

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.

Intercomparison of ETM+, MODIS, and SeaWiFS using a land test site

The Remote Sensing Group at the University of Arizona has recently shown the successful use of a cross-comparison approach between the high spatial resolution sensor Enhanced Thematic Mapper Plus (ETM+) on the Landsat-7 platform and the 1-km footprint of the Moderate Resolution Imaging Spectroradiometer (MODIS). This work showed that these two sensors compared to better than 5% in solar reflective bands not affected by atmospheric absorption. In this paper, the work is extended to include the 1-km spatial resolution sensor SeaWiFS as part of a three-way comparison. The method described here relies on data ground-based reflectance data to estimate the surface reflectance in the MODIS and SeaWiFS bands based on surface reflectance data derived from ETM+. The cross-comparison takes into account changes in solar zenith angle due to the time separation in overpass times of the three sensors and the differing view angles between SeaWiFS and the other two sensors (MODIS and ETM+, while 40 minutes apart view the test site with the same view angle). Also included are corrections due to the spectral differences between the sensors. Results show that the at-sensor reflectance agrees to better than 4% in the solar reflective for bands not affected by atmospheric absorption and view angles out to 35 degrees.

Characterization and calibration results from the Visible and Infrared Scanner (VIRS) for the Tropical Rainfall Measuring Mission (TRMM)

The visible and infrared scanner (VIRS), one of three primary sensors on the Tropical Rainfall Measuring Mission (TRMM), has completed its development and test phase at Santa Barbara Remote Sensing and has ben delivered to the Goddard Space Flight Center where it has been integrated on the TRMM spacecraft. VIRS is a five band imaging radiometer with bandpasses similar to those of the Advanced Very High Resolution Radiometers that have flown on the NOAA series of satellites for the last 18 years. VIRS will can a +/- 45 degree swath with a 2.11 kilometer IFOV at nadir from the non-sun-synchronous 350 kilometer TRMM orbit. All five bands will be cooled to 107K at mission start using a passive radiative cooler. The two reflected solar bands will be calibrated on orbit using a solar diffuser. This paper discusses ground calibration and characterization results and proposed post-launch radiometric calibration procedures for the VIRS data.

Radiometric calibration of the Sea-viewing Wide-Field-of-View Sensor using ground-reference techniques

The Sea-viewing Wide Field of View Sensor (SeaWiFS) was launched during the summer of 1997. While its primary purpose was to provide quantitative data on ocean bio-optical properties at a global scale, its bi-linear gain design allows it to provide data over land as well. Thus, there has been greater interest in understanding the radiometric calibration of the sensor for both gain levels. The Remote Sensing Group of the Optical Sciences Center at the University of Arizona has been using vicarious calibration techniques that rely on ground-based test sites to calibrate a variety of sensors since the mid-1980s. The results of applying these techniques to SeaWiFS are presented here. Three ground-reference data sets are presented, the first from White Sands Missile Range in October 1997, the second from Railroad Valley Playa, Nevada in June 1998, and the third from Railroad Valley Playa in April 2000. The technique used here is a modified version of the reflectance-based method. In this technique, results from ground-based measurements of the surface and atmosphere are used in a radiative transfer code to determine the calibration coefficients for SeaWiFS. The results for all three cases are compared with calibration coefficients derived from the onboard calibration and vicarious calibration approaches used for SeaWiFS as well as to results.

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.

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.