NOAA KLM User's Guide
The purpose of the SBUV instrument is to measure the Solar irradiance and Earth radiance in the near ultraviolet spectrum. From these data, the following atmospheric properties can be deduced:
Usable data can only be collected by the SBUV when it is integrated onto an afternoon spacecraft due to solar angle requirements.
Two optical radiometers form the heart of the SBUV instrument: a monochromater and a small but very important Cloud Cover Radiometer (CCR). The instrument contains four mechanisms: a movable grating for wavelength selection in the monochromater, a deployable diffuser which selects solar or Earth radiation measurements, a deployable Mercury lamp for wavelength calibration and an optical chopper mechanism which converts the steady incoming radiation to pulses of UV light which can be readily processed by the SBUV detectors and electronics.
The optical detectors consist of a photo multiplier tube (PMT) in the monochromater and a vacuum photo diode (VPD) in the CCR. The PMT operates from a high voltage supply of about -700 volts DC; the VPD in the CCR operates from a bias of about -10 volts derived from the instrument's low voltage power supply (LVPS).
The SBUV/2 is a nadir pointing nonspatial scanning instrument sensitive to radiation in the 160 nm 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 monochromater optical hardware 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 monochromater 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.
The PMT has a very large dynamic range (greater than 120 dB). This range is transmitted in three ranges requiring a total of 0.75 seconds for stepping and settling of the grating to a new position and 1.25 seconds of integration 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 CCR has a fixed 379 nm filter for wavelength selection and is co-aligned to the monochromater; therefore, it views the same scene as the monochromater. 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 monochromater data. CCR data is transmitted once per second in both discrete and sweep modes.
The ELM houses the low voltage power supplies and the electrical (Command and Data) interface to and from the spacecraft.
The SBUV/2 mechanical configuration is shown in Figure 3.8.2-1.
The SBUV/2 provides in-flight measurement of changes in gain of the detector(s), including the preamplifier stage. The measurements are sensitive enough to determine a gain change of 0.5% or less in the spectral range 300 to 340 nm. Averaging 100 measurements is permissible.
The SBUV/2 provides the means of in-flight wavelength calibration. The onboard calibration is sensitive enough to detect 0.1 nm shift in the indicated wavelength with a measurement precision of 0.01 nm.
The ratio of the radiance calibration to irradiance calibration at the same wavelength is determined to an accuracy of 2.35% in the spectral range 200 nm to 250 nm and to an accuracy of 1.53% at 250 nm; 1.57% at 300 nm and 1.82% in the spectral range 340 nm to 400 nm. The ratio is determined from radiance and irradiance measurements made with a minimum time separation; i.e., at each Discrete Mode wavelength, the spectral drive is stopped and both measurements made before continuing. During the time of ratio measurement, the instrument response does not vary by more than 0.5%.
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