Products - LEO-LEO Intercalibration - Ultraviolet Instruments

NOAA Solar Backscattered Ultraviolet (SBUV/2)

There are three operating NOAA Solar Backscattered Ultraviolet (SBUV/2) instruments. Each instrument uses three gain ranges to cover the full measurement range. In its typical earth-view measurement mode, the SBUV/2 makes 12 sequential measurements at wavelengths ranging from 250 nm to 340 nm over 24 seconds with a field of view (FOV) of approximately 200 km by 200 km. An additional sensor component makes coincident measurements at 380 nm. In a second mode, called continuous scan mode, the SBUV/2 makes full spectral measurements from 200 nm to 400 nm sweeping through 1500 wavelengths over two minutes. For the purpose of instrument calibration, the SBUV/2 makes corresponding solar irradiance spectral measurements, both discrete and continuous, by deploying a diffuser into the FOV. The instrument also contains an on- board Hg-Lamp for calibration measurements.

There is a new operational UV instrument, courtesy of EUMETSAT, called Global Ozone Monitoring Experiment Series-2 (GOME-2). It uses linear arrays of detectors to obtain simultaneous hyperspectral measurements from 240 nm to 790 nm. It gets close to full global coverage by using a mirror to scan cross track. The FOVs for the total ozone wavelengths have a resolution of 40 km by 80 km. It has a solar diffuser and an on-board lamp for onboard calibration.

The NASA Earth Observing System (EOS) Aura satellite has a UV instrument flying on it called the Ozone Monitoring Instrument (OMI). OMI uses a two dimensional array detectors to get hyperspectral measurements from 270 nm to 500 nm with 12 km by 24 km FOVs for total ozone at nadir. For calibration purposes, it uses solar diffusers and an on-board lamp. There is an additional instrument called SCIAMACHY on the European Space Agencies (ESA) ENVISAT and the Ozone Mapping and Profiler Suite (OMPS) is planned to fly on NPOESS satellites.

Ozone profiles and total column amounts are derived from the ratio of the observed backscattered spectral radiance to the incoming solar spectral irradiance. This ratio is referred to as the backscattered albedo. The only difference in the optical components between the radiance and irradiance observations is the instrument diffuser used to make the solar irradiance measurement; the remaining optical components are identical. Therefore, a change in the diffuser reflectivity will result in an apparent trend in ozone.

Ozone and other atmospheric products are primarily derived using ratios of earth radiance to solar irradiance, which cancel many instrument throughput and sensitivity changes. The only difference in the optical components between the radiance and irradiance observations is the instrument diffuser used to make the solar irradiance measurement; the remaining optical components are identical. On instruments designed to estimate ozone, the notable exception to this is the solar diffuser, which degrades with time. Therefore, a change in the diffuser reflectivity will result in an apparent trend in ozone. The main calibration effort related to UV instruments in thus tracking solar diffuser degradation/changes. As mentioned earlier, the SBUV/2 has the capability to view an on-board Hg- Lamp directly of illuminating the diffuser to help to track these changes. Some other instruments use multiple diffusers to monitor the changes.

Image used with permission from the Stratospheric Ozone group at NOAA.