Special Sensor Microwave Imager/Sounder (SSMIS)


The Special Sensor Microwave Imager/Sounder (SSMIS) continues the legacy of passive microwave instruments carried aboard Defense Meteorological Satellite Program (DMSP) satellites. Beginning with the launch of the DMSP F-16 satellite on 18 October 2003, the SSMIS marks the commencement of a new series of passive microwave conically scanning imagers and sounders planned for launch over the next two decades (Sun and Weng 2008). SSMIS improves upon the surface and atmospheric retrievals of the Special Sensor Microwave Imager (SSM/I), and upon the atmospheric temperature and water vapor sounding capabilities of both the Special Sensor Microwave Temperature Sounder (SSM/T-1) and the Special Sensor Microwave Humidity Sounder (SSM/T-2). Furthermore, the SSMIS imaging and sounding sensors share the same viewing geometry, thereby allowing surface parameters to be retrieved simultaneously (Yan and Weng 2008). The SSMIS instrument is able to estimate atmospheric temperature, moisture, and surface parameters from data collected at frequencies ranging from 19 to 183 GHz over a swath width of 1707 km. SSMIS is currently carried aboard DMSP-F16, -F17, and -F18 satellites, and is slated for future missions aboard DMSP-F19 and -F20. This document discusses the mission objectives, principles of operation, sensor specifications, and calibration information of the SSMIS instrument.

Click on image for a high-resolution image.
From top to bottom, the image displays main reflector (top), cold- and warm-load calibration reflectors (middle), and feedhorns (top of lower section).

ssmis instrument picture
Figure 1. The SSMIS Instrument
Image courtesy of Colorado State University

Table of Contents

  1. Instrument Information
  2. Instrument Layout, Design, and Measurement Geometry
  3. Manufacturer of Instrument
  4. Calibration
  5. References and Related Publications
  6. Document Information

1. Instrument Information

Instrument Long Name, Acronym

Special Sensor Microwave Imager/Sounder, SSMIS

Instrument Platform

The SSMIS instrument was first launched aboard the DMSP-F16 platform. The instrument is currently flown aboard the DMSP-F16, -F17 and -F18 platforms, with follow-on SSMIS instruments planned for launch on two additional DMSP platforms: F-19 and F-20.

Instrument Mission Objectives

The primary mission of the SSMIS instrument is to combine and extend the imaging and sounding capabilities of three previously separate DMSP microwave sensors: the SSM/T-1 temperature sounder, the SSM/T- 2 moisture sounder, and the SSM/I. With improved temperature sounding capabilities, the SSMIS is capable of profiling the mesosphere, or 10 mb to 0.03 mb. As such, the SSMIS is currently the only operational passive microwave sensor that can collect temperature measurements in the 40 km to 80 km altitude range. In addition, SSMIS offers capabilities associated with radiometer channels having common fields of view, uniform polarizations, and fixed spatial resolutions across the active scene scan sector, making it thus far the most complex and unique operational satellite passive microwave imager/sounder flown (Kunkee et al. 2008).

A comprehensive calibration/validation (cal/val) study for the first SSMIS sensor was conducted under joint sponsorship by the DMSP and the Navy Space and Warfare Systems Command. Detailed information regarding the cal/val objectives and results are provided in Kunkee et al. 2008.

Key Variables

SSMIS is a 24-channel, passive microwave radiometer designed to obtain a variety of polarized atmospheric temperature, moisture, and land variables under most weather conditions. Channel frequencies range from 19 GHz to 183 GHz and are obtained over a swath width of approximately 1707 km. Detailed channel characteristics are provided in Table 1. As indicated in Table 1, the SSMIS channels correspond to four main categories of measurement parameters (Bell 2006):

  • Lower Atmospheric Sounding (LAS)
  • Upper Atmospheric Sounding (UAS)
  • Environmental (ENV)
  • Imaging (IMA)
Table 1. Channel Characteristics of the SSMIS Instrument (Yan and Weng 2009)
Channel Center Frequency (GHz) 3-db Width
Stability (MHz)
Polarization NEΔT (K)I Sampling
Interval (km)II
1 50.3 380 10 V 034 37.5 LAS
2 52.8 389 10 V 0.32 37.5 LAS
3 53.596 380 10 V 0.33 37.5 LAS
4 54.4 383 10 V 0.33 37.5 LAS
5 55.5 391 10 V 0.34 37.5 LAS
6 57.29 330 10 RCPIV 0.41 37.5 LAS
7 59.4 239 10 RCP 0.40 37.5 LAS
8 150 1642(2)III 200 H 0.89 12.5 IMA
9 183.31 ± 6.6 1526(2) 200 H 0.97 12.5 IMA
10 183.31 ± 3 1019(2) 200 H 0.67 12.5 IMA
11 183.31 ± 1 513(2) 200 H 0.81 12.5 IMA
12 19.35 355 75 H 0.33 25 ENV
13 19.35 357 75 V 0.31 25 ENV
14 22.235 401 75 V 0.43 25 ENV
15 37 1616 75 H 0.25 25 ENV
16 37 1545 75 V 0.20 25 ENV
17 91.655 1418(2) 100 V 0.33 12.5 IMA
18 91.655 1411(2) 100 H 0.32 12.5 IMA
19 63.283248 ± 0.285271 1.35(2) 0.08 RCP 2.7 75 UAS
20 60.792668 ± 0.357892 1.35(2) 0.08 RCP 2.7 75 UAS
21 60.792668 ± 0.357892 ± 0.002 1.3(4) 0.08 RCP 1.9 75 UAS
22 60.792668 ± 0.357892 ± 0.0055 2.6(4) 0.12 RCP 1.3 75 UAS
23 60.792668 ± 0.357892 ± 0.016 7.35(4) 0.34 RCP 0.8 75 UAS
24 60.792668 ± 0.357892 ± 0.050 26.5(4) 0.84 RCP 0.9 37.5 LAS
INEΔT for instrument temperature (0°C) and calibration target (260 K) with integration times of
8.4 msec for Channels 12–16; 12.6 msec for Channels 1–7, 24; and 25.2 msec for
Channels 19–23 and 4.2 msec for Channels 8–11, 17-18.

IIAlong-scan direction sampling based on 833 km spacecraft altitude.

IIINumber of sub-bands is indicated by (n) next to individual 3-db width.

IVRCP denotes right-hand circular polarization.

Scanning or Data Collection Concept/Principles of Operation

The following has been adapted from the NGES Algorithm and Data User Manual (ADUM) for the Special Sensor Microwave Imager/Sounder (SSMIS) (Grumman 2002):

The SSMIS system scans at a constant 45-degree angle from nadir and intersects the surface of the Earth at a constant incidence angle of 53.1 degrees. During the morning ascending node spacecraft orbit, the instrument collects data to the aft of nadir and, during the morning descending node spacecraft orbit, collects data forward of nadir. One scan is produced every 1.9 seconds by rotating the system counter-clockwise at 31.6 rpm. Earth scene data for 24 channels are collected at 180 sample positions along the active portion of the scan, at an angle of 143.2 degrees. At the nominal orbital height of 833 km, this produces a swath width on the ground of 1707 km with 12.5 km scene spacing. The subsequent swath width applies uniformly to all channels of the SSMIS. The inter-scan period of 1.9 seconds provides along-track sample spacing of 12.5 km equivalent to the along-scan spacing.

The SSMIS collects microwave energy from the Earth’s surface and atmosphere with a rotating 24-inch parabolic reflector. This reflector focuses the energy on an assembly consisting of six feedhorns, which provide initial frequency multiplexing for the 24 channels. The reflector and the six feedhorns rotate with the entire sensor canister. A cold calibration reflector and a warm calibration source are located at the top of the canister and do not rotate with the canister. The feedhorns view a fixed cold calibration reflector and a fixed warm calibration source for each revolution of the sensor. These calibration data are used to convert the sensor output to absolute radiometric brightness temperatures.

The feedhorn data are input to the receiver subsystem where frequency multiplexing occurs to produce 24 channels of data. The receiver outputs are converted to the video spectrum, digitized and formatted, and sent to the Operational Linescan System (OLS) under control of the sensor signal processor and the flight software. The SSMIS data are then transmitted to the ground by the OLS. Ground processing is performed at Air Force Weather Agency (AFWA) and Fleet Numerical Meteorological Oceanography Center (FNMOC) to convert the sensor data into calibrated and Earth-located Sensor Data Records (SDRs) and finally into a variety of Environmental Data Records (EDRs).

2. Instrument Layout, Design, and Measurement Geometry

Sensor Description

The SSMIS measures the same frequencies and polarizations as the SSM/I, with the exception of the 85.5 GHz channel, which has been replaced by a 91.655 GHz channel to reduce hardware complexity. The LAS channels closely resemble those used by SSM/T, and the imaging channels are nearly identical to those used by SSM/T-2. Based on Environmental Data Record (EDR) performance analyses conducted by Northrop Grumman, horizontal polarization was selected for the sounding channels that sense the surface emissions.

In addition, the SSMIS employs the same conical scan geometry and external calibration scheme as the heritage SSM/I instrument. By employing the conical scan geometry of the SSM/I, the SSMIS maintains uniform spatial resolution, polarization purity, and common concentric fields of view (FOV) for all channels across the swath (Kunkee et al. 2008). Figure 2 displays the details of the SSMIS scan geometry.

Though the antenna reflector design is also the same as for SSM/I, a multiple-horn frequency multiplexing scheme was selected in order to accommodate the wide range of frequencies, obtain desired spatial resolutions, and obtain high main beam efficiencies with low feedhorn spillover loss and cross-polarization coupling (Kunkee et al. 2008). Frequency multiplexing occurs as the main reflector is illuminated by six broadband corrugated feedhorns, grouping Channels 1–5; Channels 6, 7, 19–24; Channels 12–14; Channels 15–16; Channels 8–11; and Channels 17–18. Table 1 provides channel characteristics for the SSMIS instrument.

The SSMIS instrument is comprised of the following six major subsystems:

  • Antenna/Calibration Subsystem:
    Includes main reflector, six feedhorns, a broadband warm-load calibration target, and a small reflector (a cold-space calibration target).
  • Receiver Subsystem:
    Receives the energy from the six antenna feeds and provides amplification, filtering, and additional frequency multiplexing to output 24 discrete frequency bands to the signal processing subsystem.
  • Signal Processing Subsystem:
    Digitizes the integrated outputs of the 24 channels using a 16-bit analog-to-digital converter. Also compresses the 16-bit data to 12 bits using the average warm- and cold-space calibration radiometric observations to insure that the data rate is compatible with the maximum allowable data rate of 14.22 KB/s.
  • Scan Drive Subsystem:
    Provides the mechanical interface between the rotating sensor and its fixed mount and consists of a motor, position encoder, and slip rings. The slip rings are used to transmit power and signals between the stationary and rotating portions of the sensor. The scan drive electronics controls the sensor spin rate to within 0.05 percent.
  • Deployment Subsystem:
    Supports rotation of the main reflector, rotation of the cold-space reflector, and the main sensor body. The main sensor body deployment consists of a 90-degree rotation and 26.38 cm (10.387 in) translation of the sensor body away from the spacecraft.
  • Structural/Thermal Subsystem:
    Is a canister structure that houses all other subsystems, instrument temperature sensors, thermal control surfaces, and heaters. The thermal design is a cold bias configuration with radiated area and active heater power to provide temperature margins over the range of expected orbital conditions.
Image of SSMIS Scan Geometry
Figure 2. SSMIS Scan Geometry (Poe et al. 2001)

Sensor/Detector Specifications - Optics and Spacing

A total-power radiometer configuration is employed in the SSMIS. 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 91 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 91 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 SSMIS 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 SSMIS is 14.2 KB/s. The SSMIS sensor weighs approximately 96 kilograms.

3. Manufacturer of Instrument

The SSMIS was built by GenCorp Aerojet Electronics System Division under a subcontract with Northrop Grumman Electronic Systems (NGES), and in cooperation with the Air Force DMSP.

4. Calibration


The SSMIS instrument consists of an offset 24-inch parabolic reflector, 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 feedhorn 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 SSMIS. This scheme provides an overall absolute calibration that includes the feedhorn 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

On-orbit calibration is completed once every 1.9 seconds.

5. References and Related Publications

The information in this document was derived from the following sources and related publications (Table 2):

Amlien, Jostein. 2008. Remote Sensing of Snow with Passive Microwave Radiometers: A Review of Current Algorithms. Report No 1019. Norsk Regnesentral: Oslo, Norway.

Bell, W. 2006. A Preprocessor for SSMIS Radiances Scientific Description. NWPSAF-MO-UD-014, Version 1.0. Met Office, United Kingdom: EUMETSAT.

Bommarito, J. J. 1993. DMSP Special Sensor Microwave Imager Sounder (SSMIS). Proceedings of SPIE. 1935, 230 (1993). doi:10.1117/12.152601.

Boucher, D., G. Poe, et al. 2005. Defense Meteorological Satellite Program Special Sensor Microwave Imager Sounder (F-16) Calibration/Validation. Final Report, November 2005.

Deblonde, G. 2001. Stand-Alone 1D-Var Scheme for the SSM/I, SSMIS and AMSU. Meteorological Service of Canada. NWPSAF-MO-UD-001 Version 1.0.

Gloersen, P. and F. T. Barath. 1977. A Scanning Multichannel Microwave Radiometer for Nimbus-G and SeaSat-A. IEEE Journal of Oceanic Engineering 2:172-178.

Gloersen, P., W. J. Campbell, D. J. Cavalieri, J. C. Comiso, C. L. Parkinson, and H. J. Zwally. 1992. Arctic and Antarctic Sea Ice, 1978-1987: Satellite Passive-Microwave Observations and Analysis. National Aeronautics and Space Administration Scientific and Technical Information Program. Washington, D.C. USA.

Gloersen, P and L. Hardis. 1978. The Scanning Multichannel Microwave Radiometer (SMMR) experiment. The Nimbus 7 Users' Guide. C. R. Madrid, editor. National Aeronautics and Space Administration. Goddard Space Flight Center, Maryland.

Hollinger, J.P., J. L. Peirce, and G. A. Poe. 1990. SSM/I Instrument Evaluation. IEEE Trans. Geosci. Remote Sens., Vol. 28, No. 5, pp. 781–790, Sep. 1990.

Kunkee, D. B., G. A. Poe, D. J. Boucher, S. D. Swadley, Y. Hong, J. E. Wessel, and E. A. Uliana. 2008. Design and Evaluation of the First Special Sensor Microwave Imager/Sounder. IEEE Trans. Geosci. Remote Sens. 46, no. 4: 863–883.

Kunkee, D. B. and K. St. Germain. 2008. Foreword to the Special Issue on the DMSP SSMIS. IEEE Transactions on Geoscience and Remote Sensing. 46 (4, 1): 859-861. doi: 10.1109/TGRS.2008.919866.

Northrop Grumman. 2002. Algorithm and Data User Manual (ADUM) for the Special Sensor Microwave Imager/Sounder (SSMIS). Report 12621. Northrop Grumman Corporation: Space Systems Division. Azusa, California.

Poe, G., K. St. Germain, J. Bobak, et al. 2001. DMSP Calibration/Validation Plan for the Special Sensor Microwave Imager Sounder (SSMIS). Naval Research Laboratory: Washington, DC, USA. 2001.

Sun, N. and F. Weng. 2008. Evaluation of Special Sensor Microwave Imager/Sounder (SSMIS) Environmental Data Records. IEEE Transactions on Geoscience and Remote Sensing. 46, no. 4.

Yan, B. and F. Weng. 2009. Assessments of F16 Special Sensor Microwave Imager and Sounder Antenna Temperatures at Lower Atmospheric Sounding Channels. Advances in Meteorology. 2009, Article ID 420985, 18 pages. doi:10.1155/2009/420985.

Yan, B. and F. Weng. 2008. Intercalibration Between Special Sensor Microwave Imager/Sounder and Special Sensor Microwave Imager. IEEE Transactions on Geoscience and Remote Sensing. 46 (4, 1): 984-995.

Yan, B., F. Weng, N. Sun, and H. Xu. 2006. Assessments of F16 Special Sensor Microwave Imager and Sounder Data for NOAA Operational Applications. In Proceedings of the 9th Specialist Meeting on Microwave Radiometry and Remote Sensing Applications (MicroRad '06), San Juan, Puerto Rico, February-March 2006: 18-23.

Table 2. Related Publications
Document Description URL
Algorithm and Data User Manual (ADUM) for the Special Sensor Microwave Imager/Sounder (SSMIS) A detailed user manual by the manufacturer of the SSMIS instrument, Northrop Grumman http://lwf.ncdc.noaa.gov/oa/rsad/ssmi/adum-ssmis-description.pdf
Special Sensor Microwave – Imager/Sounder (SSMIS) The World Meteorological Organization (WMO) SSMIS instrument Web page http://www.wmo-sat.info/oscar/instruments/view/536
Special Sensor Microwave Imager and Sounder (SSMIS) Antenna Brightness Temperature Data Record (TDR) Calibration and Validation User Manual A detailed user manual by the NOAA/NESDIS Center for Applications and Research http://www.ncdc.noaa.gov/oa/rsad/ssmi/star-ssmis-tdr-calval-user-manual.pdf
SSMIS Calibration/Validation Final Report (November 2005) PowerPoint presentation of comprehensive cal/val findings http://lwf.ncdc.noaa.gov/oa/rsad/ssmi/ssmis-cal-val.pdf

6. Document Information

Acronyms and Abbreviations

Table 3 lists the acronyms and abbreviations used in this document.

Table 3. Acronyms and Abbreviations
AFGWC Air Force Global Weather Central
BAPTA Bearing and Power Transfer Assembly
dB decibel
DAAC Distributed Active Archive Center
doi Digital Object Identifier
DMSP Defense Meteorological Satellite Program
EDR Environmental Data Record
EDRP Environmental Data Record Processor
EIA Earth Incidence Angle
ENV Environmental
FNMOC Fleet Numerical Meteorology and Oceanography Center
FOV Field of View
GDPS Ground Data Processing Software
GHz Gigahertz
IMA Imaging
KB/s Kilobytes per second
LAS Lower Atmospheric Sounding
Msec Millisecond
MHz Megahertz
NASA National Aeronautics and Space Administration
NEΔT Noise-Equivalent Difference Temperature
NESDIS National Environmental Satellite, Data, and Information Service
NGES Northrop Grumman Electronic Systems
NOAA National Oceanic and Atmospheric Administration
NRL Naval Research Laboratory
NWP Numerical Weather Prediction
OLS Operational Linescan System
RCP Right-hand Circular Polarization
RPM Rotations Per Minute
SDR Sensor Data Record (brightness temperature)
SDRP Sensor Data Record Processor
SPAWAR Space and Naval Warfare Systems
SSM/I Special Sensor Microwave Imager
SSMIS Special Sensor Microwave Imager/Sounder
SSM/T-1 Special Sensor Microwave Temperature Sounder
SSM/T-2 Special Sensor Microwave Humidity Sounder
TDRP Temperature Data Record Processor
UAS Upper Atmospheric Sounding