NOAA KLM User's Guide

Appendix J.4

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Introduction Page, NOAA KLM TOC, Acronyms
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APPENDIX J.4: SBUV/2

The SBUV/2 instrument is a nadir pointing nonspatial scanning instrument sensitive to radiation in the 160 to 400 nm ultraviolet spectrum. The overall radiometric resolution is approximately 1 nm in this spectral band. The SBUV instrument optical hardware and main electronics are carried in two modules. The Sensor Module (SM) contains the optical elements and detectors while the Electronics Module (ELM) houses the main electronics and power supplies.

The use of a deployable diffuser in the SM gives the instrument the versatility of selecting between solar and earth measurements. With the diffuser stowed, the instrument views the earth directly. The data from this configuration corresponds to earth radiance. With the diffuser deployed into the "Sun" position, the detector output measurements correspond to solar irradiation data. Ground and in flight calibration data are used to convert the detector data and diffuser mode data to solar irradiation or earth radiance units.

The SM houses the monochromator optical hardware (see Figure J.4-1) which uses a movable grating to select the wavelength where measurements will be made. The grating mechanism can be commanded to any one of 8,192 positions giving the monochromator approximately 0.1 nm wavelength resolution. Commands which correspond to grating positions come from a Read Only Memory (ROM). Data read from the ROM correspond to 12 discrete wavelength positions in the "Discrete" mode. In the "Sweep" mode, the ROM data is simply a grating position corresponding to the wavelength where the sweep will start.

Figure showing simplified optical path of SBUV/2 instrument

The photo multiplier tube (PMT) in the monochromator has a very large dynamic range (greater than 120 db). This range is transmitted in 3 ranges requiring a total of .75 seconds for stepping and settling of the grating to a new position and 1.25 seconds of integrating time before transmission, when the instrument is in this "Discrete" grating mode.

In the "Sweep" mode, the grating is stepped every 50 ms and the PMT signal is integrated (while the stepping continues) for 100 ms before transmission.

The Cloud Cover Radiometer (CCR) has a fixed 379 nm filter for wavelength selection and is co-aligned to the monochromator; therefore, it views the same scene as the monochromator. The output of the CCR represents the amount of cloud cover in a scene, as the name implies, and is used to remove the effects of clouds in the monochromator data. CCR data is transmitted once per second in both Discrete and Sweep modes.


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