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Instrument Description

Special Sensor Microwave Imager (SSM/I)


Abstract

The Special Sensor Microwave Imager (SSM/I) is carried aboard Defense Meteorological Satellite Program (DMSP) satellites F8, F10, F11, F12, F13, and F15. The next generation SSM/I instrument, the Special Sensor Microwave Imager/Sounder (SSMI/S), is carried aboard the DMSP F17. This document discusses the mission objectives, principles of operation, sensor specifications and calibration information of the SSM/I.

Table of Contents

  1. Document Information
  2. Instrument Information
  3. Instrument Layout, Design, and Measurement Geometry
  4. Manufacturer of Instrument
  5. Calibration
  6. Acronyms and Abbreviations

1. Document Information

Revision Date MSFC: July 13, 1994; NSIDC: September 25, 1996

Review Date MSFC: July 18, 1994; NSIDC: September 25, 1996

2. Instrument Information

Instrument Long Name, Acronym

Special Sensor Microwave Imager, SSM/I

Instrument Introduction

The SSM/I is flown aboard Defense Meteorological Satellite Program (DMSP) satellites F8, F10, F11, F12, F13, and F15. The next generation SSM/I instrument, the Special Sensor Microwave Imager/Sounder (SSMI/S), is carried aboard the DMSP F17.

Instrument Mission Objectives

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.

Key Variables

The SSM/I is a seven-channel, four-frequency, orthogonally polarized, passive microwave radiometric system that measures atmospheric, ocean and terrain microwave brightness temperatures at 19.35, 22.2, 37.0, and 85.5 GHz.

Scanning or Data Collections Concept/Principles of Operation

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 feed horn 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), reformatted 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).

3. Instrument Layout, Design, and Measurement Geometry

Sensor Description

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 that 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 that 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 degrees 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 degrees 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. During 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 five lower frequency channels. The antenna beam intersects the Earth's surface at an incidence angle of 53.1 degrees (as measured from the local Earth normal).

Sensor/Detector Specifications - Optics and Spacing

A total-power radiometer configuration is employed in the SSM/I. The signal from the output of the feed horn 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 multiplexor 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, which records all SSM/I data, supervises instrument timing, control, and data buffering with the DMSP Operational Linescan System (OLS) instrument (collocated on the satellite). 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 pounds. A high speed momentum wheel weighing 16 pounds is mounted inside the spacecraft. The SSM/I system consumes 45 watts.

4. Manufacturer of Instrument

Hughes Aircraft Company

5. Calibration

Specifications

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 that 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 that 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.

Frequency of Calibration

Once every 1.9 seconds.

6. Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

AFGWC Air Force Global Weather Central
BAPTA Bearing and Power Transfer Assembly
DAAC Distributed Active Archive Center
DMSP Defense Meteorological Satellite Program
FNMOC Fleet Numerical Meteorology and Oceanography Center
GHz GigaHertz
Msec Millisecond
NASA National Aeronautics and Space Administration
NESDIS National Environmental Satellite, Data, and Information Service
OLS Operational Linescan System
RPM Rotations Per Minute
SSM/I Special Sensor Microwave Imager