Scanning Multi-channel Microwave Radiometer (SMMR)

The information in this document has been derived from the following sources:

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, 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.

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.


The Scanning Multichannel Microwave Radiometer operated on NASA's Nimbus-7 satellite for more than eight years, from 26 October 1978 to 20 August 1987, transmitting data every other day. Intended to obtain ocean circulation parameters such as sea surface temperatures, low altitude winds, water vapor and cloud liquid water content on an all-weather basis, the SMMR is a ten channel instrument capable of receiving both horizontally and vertically polarized radiation. A parabolic antenna 79 cm in diameter reflected microwave emissions into a five-frequency feed horn. The antenna beam maintained a constant nadir angle of 42 degrees, resulting in an incidence angle of 50.3 degrees at Earth's surface. The antenna was forward viewing and rotated equally ± 25 degrees about the satellite subtrack. The 50 degree scan provided a 780 km swath of the Earth's surface. Scan period was 4.096 seconds.

Table of Contents

1. Document Information
2. Instrument Information
3. Instrument Layout, Design, and Measurement Geometry
4. Manufacturer of Instrument
5. Calibration
6. Glossary of Terms (section not used)
7. List of Acronyms

1. Document Information

Document Type: Instrument Document
Revision Date: December 1995

2. Instrument Information

Instrument Long Name, Acronym

Scanning Multi-Channel Microwave Radiometer; SMMR

Instrument Introduction

The scanning multichannel microwave radiometer represents an advance in passive microwave observation technologies. Developed to obtain sea surface temperatures, wind stress, sea ice coverage and terrain parameters, the instrument made it possible for the first time to positively differentiate between ice types using satellite observations. The SMMR originated with ideas by C. R. Laughlin and K. Richter at Goddard Space Flight Center, and drew on the experience gained from a wide variety of experiments carried out in the laboratory, in the field, onboard air- and spacecraft using microwave radiometers over a wide wavelength range. Final design and fabrication was accomplished at the Jet Propulsion Laboratory by a team led by J. Johnston.

Several microwave radiometers have flown on previous Nimbus satellites. The Electrically Scanned Microwave Radiometer (ESMR) operated at 1.55 cm wavelength on Nimbus 5 and at 0.81 cm on Nimbus 6, and provided very useful surface observation data. The Nimbus E Microwave Spectrometer (NEMS) on Nimbus 5 and the Scanning Microwave Spectrometer (SCAMS) on Nimbus 6 were significant atmospheric-observing experiments. Much of the radiometer hardware technology developed for the NEMS and SCAMS was directly applied to the SMMR. The earliest attempt at the SMMR concept was the Passive Multichannel Microwave Radiometer (PMMR) proposed for the Earth Observatory Satellite. That instrument consisted of ten channels and used phased arrays as antennae.

Earlier work has demonstrated that variations of sea surface wind give rise to variations in the observed microwave brightness temperatures - even at wind speeds of less than seven meters per second where foam is not present. More recently, the spectral nature of this variation has been studied and found to be separable from microwave brightness temperature changes caused by atmospheric and sea surface temperature variations. These studies provide a basis for extracting sea surface winds and temperatures from the SMMR data.

Other geophysical parameters are also extracted from the data, including sea ice parameters, a mesoscale soil wetness index, snow accumulation rates over continental ice sheets, subsurface physical temperatures in snow cover, and atmospheric parameters over open ocean water of total water vapor, total non-precipitating liquid water and rainfall rate.

For more information on Nimbus 7 platform and SMMR, please see the Platform and Sensor Description and History web page.

Instrument Mission Objectives

The SMMR mission objective was to obtain ocean circulation parameters such as sea surface temperatures, low altitude winds, water vapor and cloud liquid water content, sea ice extent, sea ice concentration, snow cover, snow moisture, rainfall rates, and differentiation of ice types.

Key Variables

To conserve power, the scan is sinusoidal. Part of the time spent at the scan extremities is used for reading the radiometer internal and space horn references. The dwell time of all the SMMR channels are integral multiples of synchronous with the 0.81 cm channel dwell time of 32 ms. Concurrent dwell time facilitates multispectral data analyses on various geometric scales.

Conversion of the raw radiometric readings to microwave brightness temperatures involves correcting for actual antenna patterns, including sidelobe effects, as well as separating out the horizontal and vertical polarization components of each of ten channels of radiometric data.

Scanning or Data Collection Concept/Principles of Operation

The SMMR is a ten-channel instrument delivering orthogonally polarized antenna temperature data at five microwave wavelengths, 0.81, 1.36, 1.66, 2.8 and 4.54 cm.

Six conventional Dicke-type radiometers are used. Those operating at the four longest wavelengths measure alternate polarizations during successive scans of the antenna; the others, at the shortest wavelength, operate continuously for each polarization. A two point reference signal system is used, consisting of an ambient RF termination and a horn antenna viewing deep space. A switching network of latching ferrite circulators selects the appropriate polarization or calibration input for each radiometer.

The most novel feature of the instrument is the antenna subsystem: a 42 degree offset parabolic reflector focuses the received power into a single feedhorn covering the entire range of operating wavelengths and provides coaxial antenna beams for all channels.

Feed design features a ridge-loaded corrugated conical horn with peripheral slot couplers, mode transducers, and filters. Scanning is achieved by oscillating the reflector about an axis coincident with the axis of the feedhorn. The instrument is installed on the spacecraft in such a manner that this axis is parallel to the local vertical, resulting in a conical scan pattern with the angle of incidence constant on the surface of the earth near 50 degrees. The reflector is supported on a hexapod attached to a ring surrounding the feed horn. This ring, in turn, is supported on three peripheral rolling bearings and is driven through two cogged belts by a preprogrammed servo system with position and velocity feedback. The entire mechanism is caged during launch; release is by redundant pyrotechnic devices.

The SMMR data stream processing was separated into three categories: initial flight data was received by the Meteorological Operations Control Center (MetOCC). The user formatted output tape from MetOCC is then transferred to and processed by the Science and Applications Computer Center (SaCC). SaCC derives the required geophysical parameters from the radiometric data.

3. Instrument Layout, Design, and Measurement Geometry

Figure 1. SMMR Instrument

The SMMR instrument consists of five hardware elements:

  • The antenna assembly consisting of the reflector, fabricated of graphite epoxy, and the feedhorn.
  • The scan mechanism, including momentum compensation devices
  • An RF module containing the input and reference switching networks, the mixer-IF preamplifiers, and the Gunn local oscillators
  • An electronics module containing the main IF amplifiers, all the post-detection electronics, and the power supplies for the scan and data subsystems
  • A power supply module which contains the dc-to-dc converters and regulators for the rest of the instrument

The antenna, scan mechanism, RF module, and sky horn cluster are mounted on a bridge-like platform which is then installed as a preassembled, aligned and calibrated unit on the spacecraft. The electronics and power supply modules are mounted separately and are cabled to the instrument and spacecraft through connectors. The elliptical antenna reflector is approximately 110 cm x 80 cm. Total instrument weight is about 50 kg, its power consumption is 60 watts, and its digital data output rate is 2 kbs.

On Nimbus 7, the SMMR scan pattern is forward viewing and scans equally to either side of the orbital track so the swath is centered on that track. With a subsatellite velocity of about 6.5 km per second and a scan period of 4.096 seconds, overlap coverage is provided at all wavelengths.

4. Manufacturer of Instrument

Final design and fabrication was accomplished at the Jet Propulsion Laboratory by a team under the direction of J. Johnston.

5 Calibration

The instrument is calibrated by the following equation for each wavelength and polarization:

where aiJK will be supplied by launch based on thermal-vacuum chamber and laboratory tests and updated after launch during the validation period.


Frequency of Calibration

After launch, the prelaunch constants will be updated by checking against earth targets of known properties - open, calm sea water with clear skies or light clouds, and consolidated first-year sea ice. The Tb's will be verified by comparison with Tb's obtained with an airborne radiometer with all SMMR channels during Nimbus 7 underflights. The underflights are particularly important, since extrapolation from the laboratory cold reference of 100 degrees Kelvin to the postlaunch value of 30 degrees K cannot be done with complete confidence.

6. Glossary of Terms

No further information at this time.

7. List of Acronyms

SMMR: Scanning Multichannel Microwave Radiometer

Please see the SMMR Acronym List.