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Infrared Interferometer Spectrometer (IRIS)

NSSDCA ID: 1970-025A-03

Mission Name: Nimbus 4
Principal Investigator:Dr. Rudolf A. Hanel


The Nimbus 4 Infrared Interferometer Spectrometer (IRIS) experiment was designed to provide information on the vertical structure of the atmosphere and on the emissive properties of the earth's surface by measuring the surface and atmospheric radiation in the 6.25- to 25-micrometer range using a modified Michelson interferometer. Radiation from a cone of the atmosphere, whose base on the surface of the earth was a circle about 94 km in diameter for a nominal satellite altitude of approximately 1100 km, was received and reflected by a mirror. The reflected radiation was split into two approximately equal beams by a beamsplitter. After reflection on a fixed and moving mirror, respectively, the two beams interfered with each other with a phase difference proportional to the optical path difference between both beams. The moving mirror traveled about 3.6 mm in 13 s to give an output signal from the bolometer. This signal, an interferogram, was recorded on tape. The interferograms were transmitted to a ground receiving station, where a Fourier transform was performed to produce a thermal emission spectrum of the earth. From these spectra, vertical profiles of temperature, water vapor, and ozone were derived, as well as other parameters of meteorological interest. The instrument had a field of view of 5 deg and a spectral resolution of less than 0.4 micrometer (nominally 1.4 reciprocal centimeters). For a complete description of the IRIS experiment, see Section 4 in "The Nimbus IV User's Guide" (TRF B06861), available from NSSDC. The IRIS experiment was successful in spite of a transmission conflict with the Real-Time Transmission System (RTTS) that resulted in some periods of lost data after November 28, 1970. The IRIS experiment was turned off on January 25, 1972 to conserve spacecraft power.

Alternate Names

  • IRIS
  • Nimbus4/IRIS

Facts in Brief

Mass: 17 kg
Power (avg): 12 W
Bit rate (avg): 3.75 kbps

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Earth Science: Atmospheric Dynamics
  • Earth Science: Land Surface Processes

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Dr. Rudolf A. HanelPrincipal InvestigatorNASA Goddard Space Flight Center
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