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The Special Sensor Microwave Imager (SSM/I) Sensor is carried aboard Defense
Meteorological Satellite Program (DMSP) satellites
DMSP F-8 ,
DMSP F-10 ,
DMSP F-11,
DMSP F-12,
and DMSP F-13.
This document will discuss the mission objectives, principles of operation, sensor specifications and
calibration information of the SSM/I.
- Document Information
- Instrument Information
- Instrument Layout, Design, and Measurement Geometry
- Manufacturer of Instrument
- Calibration
- Glossary of Terms
- List of Acronyms
-
-
- July 13, 1994
-
- July 18, 1994
-
-
- Special Sensor Microwave Imager, SSM/I
guides.in
-
- The SSM/I is flown aboard Defense Meteorological Satellite Program (DMSP) satellites
DMSP F-8 ,
DMSP F-10 ,
DMSP F-11,
DMSP F-12,
and DMSP F-13.
-
- The primary mission of the SSM/I instrument is to support Department of Defense (DoD)
operations. The release of the data to the scientific community is a by-product of DoD support.
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- 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 SSM/I rotates continuously about an axis parallel to the local spacecraft vertical and
measures the upwelling scene brightness temperatures. The absolute brightness temperature of
the scene incident upon the antenna is received and spatially filtered by the antenna to produce an
effective input signal or antenna temperature at the input of the feedhorn antenna. The passive
microwave radiometer output voltages are transmitted to both the Air Force Global Weather Central (AFGWC),
Offutt Air Force Base, Nebraska and the Fleet Numerical Meteorology and Oceanography Center (FNMOC),
Monterey California.
At both locations, the radiometer output voltages are converted to sensor counts.
The AFGWC sensor counts are relayed to the National Environmental Satellite, Data, and Information
Service (NESDIS), reformated into the NESDIS Level 1b format and used by NESDIS in generating temperature
sounding data sets from another instrument.
FNMOC converts their sensor counts into antenna temperatures(TDR), brightness temperatures(SDR), and
derived geophysical parameters(EDR). The TDRs, SDRs, and EDRs are sent to NESDIS for archival. The
FNMOC antenna temperatures are used as the basis for the SSM/I antenna temperature and geophysical
parameter data sets produced by Remote Sensing Systems(Wentz data set).
-
-
- Passive Microwave Radiometer
-
- The SSM/I instrument consists of an offset parabolic reflector of dimensions 24 x 26 inches, fed by
a corrugated, broad-band, 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 reflector-feed-drum assembly is rotated about the axis of the drum by a coaxially
mounted bearing and power transfer assembly (BAPTA). All data, commands, timing and telemetry
signals, and power pass through the BAPTA on slip ring connectors to the rotating assembly.
A small mirror and a hot reference absorber are mounted on the BAPTA and do not rotate with
the drum assembly. They are positioned off axis such that they pass between the feed horn and the
parabolic reflector, occulting the feed once each scan. The mirror reflects cold sky radiation into the
feed, thus serving, along with the hot reference absorber, as calibration references for the SSM/I. This
scheme provides an overall absolute calibration which includes the feed horn. Corrections for spillover
and antenna pattern effects from the parabolic reflector are incorporated in the data processing
algorithms.
The SSM/I rotates continuously about an axis parallel to the local spacecraft vertical at 31.6 rpm
and measures the upwelling scene brightness temperatures over an angular sector of 102.4 deg about
the sub-satellite track. The scan direction is from the left to the right when looking in the forward (F10,F11) or aft (F8) direction of
the spacecraft with the active scene measurements lying 51.2 deg about the forward (F10,F11) or aft (F8) direction. This results
in a swath width of approximately 1400 km. The spin rate provides a period of 1.9 sec during which the spacecraft
sub-satellite point travels 12.5 km. Each scan 128 discrete uniformly spaced radiometric samples are
taken at the two 85 GHz channels and, on alternate scans, 64 discrete samples are taken at the
remaining 5 lower frequency channels. The antenna beam intersects the Earth's surface at an incidence
angle of 53.1 deg (as measured from the local Earth normal).
-
- A total-power radiometer configuration is employed in the SSM/I. The signal from the output of the
feedhorn is down-converted by a balanced mixer, amplified, and converted to a video voltage with a
square-law detector. The bandpass filter is used to define the receiver passband and to improve
out-of-band rejection. The detected video signal is then amplified and offset to remove part of the
component of receiver output due to receiver noise. The output of the video amplifier is integrated by an
integrate and dump filter for 3.89 msec at 85 GHz and 7.95 msec for the remaining channels and
delivered to the data processing system. The time between radiometer output samples is 4.22 msec at
85 GHz and is the same time required for the antenna beam to scan 12.5 km in the cross-track direction.
The time between samples at the remaining frequencies is 8.44 msec.
The data processor multiplexes the seven radiometer output signals with an analog multiplexer
and samples and holds the signals before being digitized into 12-bit words. In addition, 12 channels are
multiplexed with the radiometer data. These channels contain three hot target temperature
measurements, two temperature sensor measurements within the radiometer, reference voltage, and
reference return data. A microprocessor supervises instrument timing, control, and data buffering with
the DMSP Operational Line Scanner (OLS) instrument (collocated on the satellite) which records all
SSM/I data. The average data rate of the SSM/I including zeros required to match the OLS interface
is 3276 bps.
The SSM/I sensor weighs 107 lbs. A high speed momentum wheel weighing 16 lbs is mounted
inside the spacecraft. The SSM/I system consumes 45 watts.
-
-
- Hughes Aircraft Company
-
-
- The SSM/I instrument consists of an offset parabolic reflector of dimensions 24 x 26 inches, fed by
a corrugated, broad-band, 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 reflector-feed-drum assembly is rotated about the axis of the drum by a coaxially
mounted bearing and power transfer assembly (BAPTA). All data, commands, timing and telemetry
signals, and power pass through the BAPTA on slip ring connectors to the rotating assembly.
A small mirror and a hot reference absorber are mounted on the BAPTA and do not rotate with the
drum assembly. They are positioned off axis such that they pass between the feed horn and the parabolic
reflector, occulting the feed once each scan. The mirror reflects cold sky radiation into the feed, thus
serving, along with the hot reference absorber, as calibration references for the SSM/I. This scheme
provides an overall absolute calibration which includes the feed horn every 1.9 seconds. Corrections for
spillover and antenna pattern effects from the parabolic reflector are incorporated in the data processing
algorithms.
-
- Not available
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- Once every 1.9 seconds
-
- None
-
-
- TBD
-
-
- BAPTA - Bearing and Power Transfer Assembly
- DAAC - Distributed Active Archive Center
- DMSP - Defense Meteorological Satellite Program
- GSFC - Goddard Space Flight Center
- NASA - National Aeronautics and Space Administration
- NESDIS - National Environmental Satellite, Data, and Information Service
- OLS - Operational Line Scanner
- SSM/I - Special Sensor Microwave Imager
Change History
- Version 2.0
- Version baselined on addition to the GES Controlled Documents List, July 13
, 1994.
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