This section describes the instrument and data characteristics of the Solar Backscattered Ultraviolet system/Version 2 (SBUV/2). Section 4.4.1 contains a description of the instrument and data characteristics. Section 4.4.2 contains general information about the actual data for the three types of SBUV/2 data archived by SSB: 1) Level 1b, 2) Historical Instrument File, and 3) Product Master File. Tape formats are not included in this document but are thoroughly covered in a separate document entitled: Solar Backscattered Ultraviolet Radiometer Version 2 (SBUV/2) User's Guide (November 15, 1990).

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.

Magnetic Tape Formats

Three operational products based on the data collected from the SBUV/2 instrument (on NOAA-9 and NOAA-11) are archived and available through SSB. All products are archived on IBM 3480 cartridges. The NOAA-9 SBUV/2 data (Level 1b) have been reprocessed for data from 1985 to the present. For NOAA-9, SSB archives the reprocessed Level 1b data and the original Level 1b, and HIF data (no PMF). The NOAA-11 Level 1b data has been reprocessed and is available in the archive (original Level 1b not available).

SSB has no selection software for any type of SBUV/2 data. Tape-to-tape copies of the data is the only option available at this time. Each of the three types of SBUV/2 data is stored on one IBM 3480 cartridge per month of data.

Level 1b Data Set

The Level 1b data set contains: 1) all SBUV/2 sensor data and support data necessary for the derivation of atmospheric ozone and solar flux; 2) instrument in-flight calibration data and housekeeping functions for monitoring post-launch instrument changes; and 3) prelaunch calibration factors, and computed current-day instrument calibration and albedo correction factors to adjust the ozone algorithm for actual instrument performance.

The SBUV/2 sensor data consists of radiance and irradiance measurements taken in both the discrete mode (12 wavelengths) and the sweep mode (1680 wavelengths at approximately 2 Angstrom intervals). The support data includes cloud and temperature data from TOVS, ancillary data to initialize the algorithm, surface pressure data, and snow/ice data.

Product Master File

The Product Master File (PMF) contains the ozone information derived by the algorithm, located in space and time, other meteorological information developed in support of the ozone computation, parameters indicating the validity of the individual ozone retrievals, and the radiance information derived from the SBUV/2 measurements.

Historical Instrument File

The Historical Instrument File (HIF) is a collection of data files, created by the Instrument Support Subsystem which provide the data to characterize the instrument performance and albedo correction over time.