An Overview

Sensor Description

The SeaWiFS instrument consists of an optical scanner and an electronics module (line drawing). Below is a listing of the central wavelengths and bandwidths for SeaWiFS.

The SeaWiFS instrument has been modified to produce a bilinear response; the original sensitivity is maintained up to about 80% of the digital output range, and then changed discontinuously to extend the dynamic range substantially; the net result is no expected saturation over clouds (or bright sand, ice, etc.). For example, in the original design, Band 1 saturation (1023 counts) corresponded to an input radiance of about 13.6 mW per (cm um sr), with a linear response; now the response is linear up to radiance about 10.9 (about 817 counts), and changes slope above that point so that saturation is reached at about 60.1. The complete set of gain responses has been published in the SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization Report in the SeaWiFS Project Technical Report Series.

The instrument has scanning mechanisms to drive an off-axis folded telescope and a rotating half-angle mirror that is phase-synchronized with, and rotating at half the speed of, the folded telescope. The rotating scanning telescope, coupled with the half-angle scan mirror arrangement, provides a design configuration that permits a minimum level of polarization to be achieved, without field-of-view rotation, over the maximum scan angle requirement of 58.3 degrees. The absence of field-of-view rotation permits the use of a multichannel, time-delay and integration (TDI) processing in each of the eight spectral bands to achieve the required SNR. This capability, in turn, allows a relatively small sensor collecting aperture and, therefore, results in a smaller and lighter instrument than would otherwise be required. Incoming scene radiation is collected by the folded telescope and reflected onto the rotating half-angle mirror. The collected radiation is then relayed through dichroic beam splitters to separate the radiation into four wavelength intervals---each wavelength interval encompassing two each of the eight SeaWiFS spectral bands. The radiation in the four separate wavelength intervals is directed by four corresponding aft-optics assemblies through two separate spectral bandpass filters that further separate the radiation into the eight required SeaWiFS spectral bands. The aft-optics assemblies also image each of the resultant defined bands of radiation onto four detectors that are aligned in the scan direction. The detected radiation signals are then amplified by preamplifiers for TDI processing in the electronics module. The off-axis scanning telescope rotates at six revolutions per second in the cross-track direction, for HRPT format compatibility, to provide contiguous scan coverage at nadir from a 705 km (380 nmi) orbital altitude with the SeaWiFS spatial resolution of 1.6 mrads (1.13 km or 0.6 nmi at nadir). A scanner tilt mechanism enables the instrument to be oriented in the along-track direction to +20, 0 -20 degrees to avoid sun glint from the sea surface. Tilting the entire scanner, rather than only a section of the optical train, assures that the SeaWiFS calibration, polarization, and angular scanning characteristics will be identical for all tilt positions and, thereby, simplifies the ground processing of in-flight data. Monitoring of sensor calibration over periods of a few orbits, to several months or years, is accomplished using solar calibration for the former and lunar calibration for the latter. Solar calibration uses a solar radiation diffuser and an input port located in a fixed position outside of the 58.3 degrees SeaWiFS scene-scan interval. The diffuser is located on the inside of a baffle pointed in the +y (minus velocity vector) direction. The diffuser will be covered with an aperture plate with numerous small holes that will adjust the diffuser system output to the required level and minimize diffuser surface degradation from contamination or ultraviolet exposure. The diffuser is located so calibration will take place near the southern terminator. Lunar calibration is accomplished by a spacecraft maneuver to view the moon when the spacecraft is in the nighttime part of the orbit. The spacecraft is oriented such that the SeaWiFS scene-scan interval is 180 degrees from the normal Earth oriented position, i.e., looking outward. The lunar observation can, therefore, be accomplished under nearly full moon conditions through the identical SeaWiFS optical path as that for Earth scenes. The detected and amplified signals are routed from the scanner to the electronics module where they are further amplified and then filtered to limit the noise bandwidth. The filtered signals are digitized by a 12 bit analog-to-digital converter and the digitized signals directed to a commandable processor where the TDI operation is performed in real time as data are generated. The resultant summed signals are divided by four and rounded to 10 bit numbers, and then sent from the processor to the spacecraft data system at 1.885 Mbps during the data acquisition period of each scan line. The instrument angular momentum will be compensated by the angular momentum wheel. This is necessary to avoid nutation coupling when the instrument is tilted. Implementation will consist of a brushless DC motor driven synchronously at approximately 2,000 rpm. The accurately controlled frequency derived from the instrument clock will ensure compliance to the 1 oz-in-sec uncompensated angular momentum requirement for the spacecraft attitude control system.

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