The Nimbus 7 spacecraft was in a south-to-north, sun-synchronous polar orbit so that it was
always close to local noon/midnight below the spacecraft. Thus, ozone measurements were taken
for the entire world every 24 hours. SBUV is a nadir-viewing instrument with a 200 kilometer
square field of view at the sub-satellite point. One measurement is made every 32 seconds along
the orbital track, approximately every 1.8 degrees in latitude, from 80 degrees south to 80 degrees
north.
SBUV directly measures the ultraviolet sunlight scattered by the Earth's atmosphere at 12 wavelengths from 255 nm to 340 nm. Total column ozone is inferred using wavelengths identical to TOMS - 312.5 nm, 317.5 nm, 331.2 nm and 339.8 nm - wavelengths that penetrate to the ground. Total ozone is calculated by taking the ratio of two wavelengths (312 nm and 331 nm, for example), where one wavelength is strongly absorbed by ozone while the other is absorbed only weakly.
The ozone vertical profile is inferred from measurements from 255 nm through 306 nm, wavelengths which do not penetrate to the ground. The ozone cross section is higher at the shorter wavelengths and penetrations is less - 288 nm penetrates to about 38 km, while 255 nm penetrates only to 50 km. Thus, a wavelength scan is equivalent to an altitude scan. The profile inversion uses an optical statistical method [Rodgers, 1976] to obtain ozone profiles from the measured radiances and from a priori information. For details see Bhartia et al. [1996]. The profile inversion is done in 12 Umkehr layers (~5 km thick layers) although the actual vertical resolution of the SBUV instrument is approximately 8 km in the upper stratosphere and drops significantly in the lower stratosphere as a result of broader low-level contribution functions and multiple scattering contamination. The data in the lowest layers mostly reflect the difference between total column ozone and profile amounts above ~25 km and provide no real information about the altitude dependence of ozone in the lower stratosphere and troposphere.
The SBUV data were reprocessed (Version 6) with an updated calibration to correct for time-dependent instrument errors. The correction for diffuser degradation for the total ozone wavelengths was derived using the pair justification technique [Herman et al., 1991]. The profiling wavelengths were corrected using a "Langley" method [Bhartia et al., 1994], which uses measurements made at the same latitude at different solar zenith angles. If average ozone at a given latitude is assumed invariant, the requirement of internal consistence can be used to estimate relative errors in the shorter wavelengths. Errors in the profile data after this correction was applied were estimated to be +/-5% per decade at 1 mb and +/-2% per decade at 10 mb.
Finally, a correction was applied to the SBUV data after Feb. 1987 to account for instrument
synchronization problems, which caused an apparently random error of ~3% in the radiance
measurements. A scan-by-scan correction was applied using data from the 343 nm photometer
channel, which should vary smoothly, to correct the simultaneous monochromator measurements
[Gleason et al., 1994]. There was no apparent bias in the corrected data at the 1-2% level. After
June 1990 the synchronization error became much worse and SBUV data could no longer be
used.
Bhartia, P.K., S. Taylor, R. D. McPeters, and C. Wellemeyer, Applications of the Langley Plot Method to the Calibration of SBUV Instrument on Nimbus-7 Satellite, J. Geophys. Res., 100, 2997-3004, 1995.
Bhartia, P.K., R.D. McPeters, C.L. Mateer, L.E. Flynn, and C. Wellemeyer, Algorithm for the estimation of vertical ozone profile from the backscattered ultraviolet (BUV) technique, J. Geophys. Res., 101, 18793-18806, 1996.
Gleason, J. F. and R. D. McPeters, Corrections to the Nimbus 7 solar backscatter ultraviolet data in the "nonsync" period (February 1987 to June 1990), J. Geophys. Res., 100, 16873-16877, 1995.
Rodgers, C.D., Retrieval of atmospheric temperature and composition from remote measurements of thermal radiation, Rev. Geophys. Sp. Phys., 14, 609-624, 1976.