SSBUV (Shuttle Solar Backscatter Ultraviolet Spectrometer)
SSBUV is a NASA/GSFC instrument using UV backscatter observations in nadir to measure vertical profiles of ozone in the stratosphere and in the lower mesosphere in the spectral range from 200 to 405 nm. The objective is to fly the SSBUV payload on numerous Shuttle missions to provide complementary calibration data for long-term satellite ozone data sets. 1) 2) 3) 4) 5) 6) 7)
The first flight with SSBUV instrumentation occurred on October 19, 1989 on the Shuttle Atlantis (STS-34). Throughout this Shuttle flight coincident observations were taken with the SBUV on Nimbus-7 and the SBUV/2 on NOAA-9 and NOAA-11 satellites.
Table 1: Survey of Shuttle flights with the SSBUV payload
The SSBUV instrument and its flight support electronics, power, data and command systems are mounted in the Shuttle's payload bay in two flight canisters (total mass of 410 kg). The SPAIM (Small Payload Accommodations Interface Module) interfaces SSBUV instrument to the Shuttle's avionic systems. The instrument canister holds the SSBUV instrument, its aspect sensors and inflight calibration system. Once in orbit, a motorized door assembly opens the canister, allowing the SSBUV to view the sun and the Earth. The canister closes, providing contamination protection, while SSBUV performs inflight calibrations.
The SSBUV instrument (PI: E. Hilsenrath) is in fact the SBUV/2 engineering model now flying on NOAA satellites (see POES series of NOAA). The SBUV/2 sensor is a spectrally scanning UV radiometer (nadir-viewing sensor) measuring solar irradiance and scene radiance (backscattered solar energy) over a spectral range from 200 to 405 nm (note: SSBUV actually measures from 160-405 nm; however, there is no calibration below 200 nm because a vacuum is needed for this region). Resolution = 50 km.
Table 2: Specification of the SSBUV instrument 8)
The nadir-viewing SBUV/2 contains a scanning double monochromator and a CCR (Cloud Cover Radiometer) 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 . It 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. 9)
The monochromator and the CCR are mounted so that they look in the nadir direction with coincident FOV's of 11.3º x 11.3º. As the satellite moves in a sun synchronous orbit, the FOV traces 160 km wide paths on the ground. The earth rotates approximately 26º during each orbit. The satellite footprint moves at a speed of about 6 km/s. 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 s/measurement. For each monochromator measurement, there is a CCR 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.
The SSBUV program has pursued a vigorous laboratory calibration effort using NIST-traceable standards. Of particular importance are the direct comparisons conducted with the instruments for the TOMS-EP (TOMS), ERS-2 (GOME), and NOAA-14 (SBUV/2) missions. These instruments were calibrated using SSBUV standards. The SSBUV flight on STS-72 provided a first opportunity to compare space observations from instruments which have been intercalibrated on the ground.
Figure 1: Photo of the SBUV/2 instrument (image credit: NOAA, NASA)
1) “Calibration of Long Term Satellite Ozone Data Sets Using the Space Shuttle,” E. Hilsenrath, in Optical Remote Sensing of the Atmosphere, 1990 Technical Digest Series of the Optical Society of America, Vol. 4, pp. 409-412
2) R. P. Cebula, E. Hilsenrath, “SSBUV Middle Ultraviolet Solar Spectral Irradiance Measurements,” N95- 11129, URL: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950004716_1995104716.pdf
3) R. P. Cebula, E. Hilsenrath, P. W. DeCamp, K. Laamaan, S. Janz, K. McCullough, ”The SSBUV Experiment Wavelength Scale and Stability: 1988 to 1994,” Metrologia (Bureau International des Poids et Mesures), No 6, Vol. 32, Dec. 1995, pp. 633-636
4) “Shuttle Solar Backscatter Ultraviolet (SSBUV) Experiment,” NASA documentation, URL: http://disc.sci.gsfc.nasa.gov/ozone/documentation/publications/sensor/ssbuv_pubs.shtml
5) R. P. Cebula, L. K. Huang, E. Hilsenrath, “ SSBUV sensitivity drift determined using solar spectral irradiance measurements,” Metrologia, Volume 35, Number 4, Aug. 1, 1998 , pp. 677-683
6) M. T. DeLand, R. P. Cebula, E. Hilsenrath, “ SSBUV and NOAA-11 SBUV/2 Solar Variability Measurements,” Studia Geophysica et Geodaetica, Vol. 42, No 2, April 1998, pp.83-91
7) E. Hilsenrath, R. P. Cebula, M. T. Deland, K. Laamann, S. Taylor, C. Wellemeyer, P. K. Bhartia, “Calibration of the NOAA-11 Solar Backscatter Ultraviolet (SBUV/2) Ozone Data Set from 1989 to 1993 using In-Flight Calibration Data and SSBUV,” Journal of Geophysical Research, Vol. 100, 1995, pp. 1351-1366
8) Information provided by Ernest Hilsenrath of NASA/GSFC, Greenbelt, MD
9) NOAA Polar Orbiter Data User's Guide, Section 4.4, June 17, 1999, URL: http://www2.ncdc.noaa.gov/docs/podug/html/c4/sec4-4.htm
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.