Calibration of SeaWiFS. I. Direct techniques.

We present an overview of the calibration of the Sea-viewing Wide Field-of View Sensor (SeaWiFS) from its performance verification at the manufacturer's facility to the completion of its third year of on-orbit measurements. These calibration procedures have three principal parts: a prelaunch radiometric calibration that is traceable to the National Institute of Standards and Technology; the Transfer-to-Orbit Experiment, a set of measurements that determine changes in the instrument's calibration from its manufacture to the start of on-orbit operations; and measurements of the sun and the moon to determine radiometric changes on orbit. To our knowledge, SeaWiFS is the only instrument that uses routine lunar measurements to determine changes in its radiometric sensitivity. On the basis of these methods, the overall uncertainty in the SeaWiFS top-of-the-atmosphere radiances is estimated to be 4-5%. We also show the results of comparison campaigns with aircraft- and ground-based measurements, plus the results of an experiment, called the Southern Ocean Band 8 Gain Study. These results are used to check the calibration of the SeaWiFS bands. To date, they have not been used to change the instrument's prelaunch calibration coefficients. In addition to these procedures, SeaWiFS is a vicariously calibrated instrument for ocean-color measurements. In the vicarious calibration of the SeaWiFS visible bands, the calibration coefficients are modified to force agreement with surface truth measurements from the Marine Optical Buoy, which is moored off the Hawaiian Island of Lanai. This vicarious calibration is described in a companion paper.

Calibration of SeaWiFS. II. Vicarious techniques.

We present an overview of the vicarious calibration of the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS). This program has three components: the calibration of the near-infrared bands so that the atmospheric correction algorithm retrieves the optical properties of maritime aerosols in the open ocean; the calibration of the visible bands against in-water measurements from the Marine Optical Buoy (MOBY); and a calibration-verification program that uses comparisons between SeaWiFS retrievals and globally distributed in situ measurements of water-leaving radiances. This paper describes the procedures as implemented for the third reprocessing of the SeaWiFS global mission data set. The uncertainty in the near-infrared vicarious gain is 0.9%. The uncertainties in the visible-band vicarious gains are 0.3%, corresponding to uncertainties in the water-leaving radiances of approximately 3%. The means of the SeaWiFS/in situ matchup ratios for water-leaving radiances are typically within 5% of unity in Case 1 waters, while chlorophyll a ratios are within 1% of unity. SeaWiFS is the first ocean-color mission to use an extensive and ongoing prelaunch and postlaunch calibration program, and the matchup results demonstrate the benefits of a comprehensive approach.

SeaWiFS transfer-to-orbit experiment.

We present the results of an experiment designed to measure the changes in the radiometric calibration of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) from the time of its manufacture to the time of the start of on-orbit operations. The experiment uses measurements of the Sun at the manufacturer's facility to predict the instrument outputs during solar measurements immediately after launch. Because an onboard diffuser plate is required for these measurements, the experiment measures changes in the instrument-diffuser system. There is no mechanism in this experiment to separate changes in the diffuser from changes in the instrument. For the eight SeaWiFS bands, the initial instrument outputs on orbit averaged 0.8% higher than predicted with a standard deviation of 0.9%. The greatest difference was 2.1% (actual output higher than predicted) for band 3. The estimated uncertainty for the experiment is 3%. Thus the transfer-to-orbit experiment shows no changes in the radiometric sensitivities of the SeaWiFS bands--at the 3% level--from the completion of the instrument's manufacture to its insertion into orbit.

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 instrument's 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.

Killing and preserving nematodes in soil samples with chemicals and microwave energy.

Three basic procedures for treating nematode-bearing soil samples for international shipment or from areas under quarantine were tested for their killing effect and recovery of nematodes by sugar flotation for diagnostic and advisory purposes. These were: fumigation with methyl bromide followed by storage at -15 C; microwave treatment (2450 MHz, 630 w, 2-5 min) followed by addition of FAA + picric acid or 5% Formalin; and adding chemical preservatives (FAA + picric acid, 5% Formalin, NAN, and 2-phenoxyethanol) directly to the soil. Larvae of Heterodera glycines in eggs within cysts were stimulated to hatch by 2-rain exposure to microwaves, and an exposure of 5 rain was required to kill them. Soil type and moisture significantly affected microwave effectiveness. Direct saturation of soil samples with preservative chemical solutions (FAA + picric acid or 5% Formalin) was most effective, and often increased the number of nematodes recovered. The high concentration (2%) of NaN a required for soil sterilization is too hazardous for routine work. NaN, therefore, is not recommended for this purpose.