| 1 |
|
DMSP SSM/I- Microwave Imager
|
The SSM/I is a seven-channel, four frequency, linearly-polarized, passive microwave
radiometric system which measures atmospheric, ocean and terrain microwave brightness
temperatures at 19.35, 22.235, 37.0 and 85.5 GHz. The data are used to obtain synoptic
maps of critical atmospheric, oceanographic and selected land parameters on a global
scale. The SSM/I archive data set consists of antenna temperatures recorded across
a 1,400 km conical scan, satellite ephemeris, earth surface positions for each pixel
and instrument calibration. Electromagnetic radiation is polarized by the ambient
electric field, scattered by the atmosphere and the Earth's surface and scattered
and absorbed by atmospheric water vapor, oxygen, liquid water and ice. The SSM/I instrument
consists of an offset parabolic reflector that is 24 x 26 inches fed by a seven- port
horn antenna. The reflector and feed are mounted on a drum which contains the radiometers,
digital data subsystem, mechanical scanning subsystem and power subsystem. The drum
assembly rotates about the axis of the drum. A small mirror and a hot reference absorber
are mounted on the assembly. The instrument sweeps a 450 cone around the satellite
velocity vector so that the Earth incidence angle is always 540. Data are recorded
during the 102.40 of the cone when the antenna beam intercepts the Earth's surface.
The channel footprint varies with channel energy, position in the scan, along scan
or along track direction and altitude of the satellite. The 85 GHz footprint is the
smallest with a 13 x 15 km and the 19 GHz footprint is the largest at 43 x 69 km.
Because the 85 GHz footprint is so small, it is sampled twice as often, i.e., 128
times a scan. One data cycle consists of 4 85 GHz scans and 2 scans of the 19, 22
and 37 GHz channels. The complete cycle takes 28 seconds and it must be complete to
process the data. The SSM/I processor is queried once a second by onboard computer
and the data are placed into the "TS SSP" data field. Data is ingested as sent from
AFWA on a T-1 line. At the NOAA National Centers for Environmental Information (formerly
National Geophysical Data Center), the "TS SSP" data are decommutated, deinterleaved,
bit flipped, reordered and restructured into orbits beginning with the equatorial
crossing as the satellite travels from south to north. Satellite ephemeris are computed
using a physically-based, orbital mechanics model. SSM/I pixels are geolocated using
the satellite ephemeris and satellite attitude corrections. Antenna temperatures are
computed from instrumental counts by a linear equation, i.e., the conversion is reversible.
In the decommutation step, we encountered bit reversals that occurred 1.8 - 3.4% of
the time and are probably caused by ionospheric scintillation. These are identified
through careful checking procedures and corrected. Archive files contain metadata
by orbit and geolocated antenna temperatures.
|