SBUV/2 Information

Solar Backscatter UltraViolet Instrument (SBUV/2)

At NOAA satellite observations of total ozone and the vertical profile of ozone are done using the Solar Backscatter UltraViolet Instrument (SBUV/2). The usage of backscattered ultraviolet (BUV) measurements began with the BUV instrument on board the NASA NIMBUS-4 satellite launched in April 1970. Improvements were made and the next instrument: SBUV was launched on board the NIMBUS-7 spacecraft launched in October 1978. Slight modifications were made to this instrument (deemed SBUV/2) and NOAA has been using it as their ozone monitoring instrument since the launch of NOAA-9 in December 1984. Since NOAA-9 (an afternoon orbiting satellite) three additional satellites have been launched in to earth orbit with a SBUV/2 instrument on board: NOAA-11 (launch date September 1988), NOAA-14 (December 1998), and NOAA-16 (September 2000). Unlike the NIMBUS satellites, NOAA-9, NOAA-11, and NOAA-14 all precessed towards later ascending equator crossing times. NOAA-16 has a static equator crossing time. NOAA-9 and NOAA-11 had precessed such that the descending orbits were used to collect ozone data. Figure 1 shows the life span of each satellite and the portion of which the SBUV/2 data is used for ozone studies.

The SBUV/2 instrument differs from the NASA TOMS instrument in that the TOMS is a scanning radiometer which measures UV radiation at 6 distinct wavelengths. The TOMS can provide global coverage of the sunlite portion of the earth. The SBUV/2 does not scan. Instead it only looks down in the nadir direction. However, it measures UV radiation at twelve(12) variable wavelengths. This allows for not only the determination of total ozone but also the determination of ozone concentrations at various altitudes throughtout the stratosphere. These concentrations (in g/g) are determined for 12 Umkehr layers (see Figure 2).

Purpose of the SBUV Instrument

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:

• The global and vertical distribution of stratospheric ozone.
• The structure and dynamics of stratospheric ozone.
• Photochemical processes and the influence of "trace" constituents on the ozone layer.
• Long-term solar activity in the Ultraviolet spectrum.
• Usable data can only be collected by the SBUV when it is integrated onto an afternoon spacecraft due to solar angle requirements.

Instrument Description and Data Characteristics

The SBUV/2 is an operational remote sensor designed to map total ozone concentrations and the vertical distribution of ozone in the earth's atmosphere on a global scale. The purpose of the SBUV/2 instrument is to provide data on an operational basis, from which the distribution of ozone can be determined on the ground. The SBUV/2 system was chosen by NOAA because of the precision and reliability demonstrated by its predecessors, the SBUV and BUV, developed by NASA and flown on the NIMBUS-7 and NIMBUS-4 satellites, respectively.

The SBUV/2 contains a scanning double monochromator and a cloud cover radiometer (CCR) designed to measure ultraviolet (UV) spectral intensities. In its primary mode of operation, the monochromator measures solar radiation Backscattered by the atmosphere in 12 discrete wavelength bands in the near-UV, ranging from 252.0 to 339.8 nanometers, each with a bandpass of 1.1 nm. The total-ozone algorithm uses the four longest wavelength bands (312.5, 317.5, 331.2 and 339.8 nm), whereas the profiling algorithm uses the shorter wavelengths. The cloud cover radiometer operates at 379 nm (i.e., outside the ozone absorption band) with a 3.0 nm bandpass and was designed to measure the reflectivity of the surface in the IFOV. The SBUV/2 also makes periodic measurements of the solar flux by deploying a diffuser plate into the FOV to reflect sunlight into the measurement.

The monochromator and the cloud cover radiometer are mounted so that they look in the nadir direction with coincident FOV's of 11.3 by 11.3 degrees. As the satellite moves in a sun synchronous orbit, the FOV traces 160 km wide paths on the ground. The earth rotates approximately 26 degrees during each orbit. The satellite footprint moves at a speed of about 6 km/sec. In discrete mode, a set of 12 measurements, one for each discrete wavelength band, is taken every 32 seconds. The order of measurements is 252.0 to 339.9 nm and the integration time is 1.25 seconds per measurement. For each monochromator measurement, there is a cloud cover radiometer measurement.

The SBUV/2 instrument can also measure the solar irradiance or the atmospheric radiance with a continuous spectral scan from 160 to 400 nm in increments of nominally 0.148 nm.