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Solar Flux Radiometer (LSFR)

NSSDCA ID: 1978-078D-07

Mission Name: Pioneer Venus Large Probe
Principal Investigator:Dr. Martin G. Tomasko


The objective of the Solar Flux Radiometer was to determine the regions in Venus' atmosphere where solar energy is deposited. Narrow field-of-view detectors were used to measure the intensity of scattered solar light. As the probe descended through the atmosphere, the difference between upward-looking and downward-looking detectors indicated the net downward flux. The data were used to determine the deposition of solar energy in the atmosphere of Venus between 67 km and the surface along with upward and downward fluxes and radiances. The Solar Flux Radiometer continuously measured the difference in atmospheric radiance above and below the horizon of the probe as a function of altitude, to determine how much sunlight was absorbed by the clouds and how much reached the surface. The results allow for more accurate modelling of the radiation balance of the atmosphere than previously possible, to help determine the role of the greenhouse effect on the heating of the Venus atmosphere.

The instrument consisted of five 3-mm diameter fused silica lenses inside five flat sapphire windows. The lenses pointed in different directions, one basically upward (zenith) pointing at 27 degrees, one basically downward (nadir) pointing at 142 degrees, and three roughly azimuthally pointed, at 60, 83, and 102 degrees from vertical up. The lenses had a focal length of 11.45 mm and a focal ratio of F/3.8. The lenses focused the light down five quartz light pipes to an array of 12 photovoltaic detectors with filters, covering the spectral range from 0.4 to 1.8 microns. (This covers approximately 83% of the total solar energy.) Two broad and flat spectral channels were included for each lens and light pipe, one filtered from 400 to 1000 nm, and one from 1000 to 1800 nm, using glass absorption type band-pass filters. Additionally a narrow filter, from 600 to 650 nm, was used for the 60 and 142 degree lenses. This was a Fabry-Perot filter with metallic reflectors and a 50-nm half-maximum bandwidth centered at 625 nm. This channel was designed to get information on the single scattering albedo and optical depth of the clouds during descent. The field of view for each lens was purposely narrow, 5 degrees (full-width half-maximum), to avoid interference from the probe body or parachute, by aluminizing the receiving ends of the light pipes and leaving a 1-mm diameter aperture.

The instrument was housed in a gold-finished titanium can, with the five sapphire windows for viewing. The 400 to 1000 nm detectors were photovoltaic silicon, the 1000 to 1800 nm were germanium. Thermal control, maintaining the radiometer detectors at approximately 30 C, was achieved by foil-wrapped min-K and multilayer insulation and filling a heat sink with lithium salts (LiNO3-3H2O, melting point 29 C, heat of fusion 70 cal/gm). The heat sink was coupled to the array by closely meshed aluminum fins. It had a mass of 1.65 kg and used 3.5 W power.

The analog detector output currents were processed with twelve logarithmic transimpedance converters and a 12-channel multiplexer. There was also input from two analog thermometers. It was digitized with an 11-bit A/D converter and stored. The data rate averaged 20 bps, included housekeeping (sync, spin period, sample timing, and mode).

It had two modes of operation. It started by detecting the peak at solar azimuth and used the time between successive peaks to establish the rotation period and initiate mode-1 sampling. This involved subdividing the rotation period to take samples at 0, 12, 25, 61, 119, and 137 degrees azimuth as the spacecraft turned. If a period of 16 seconds passed without a peak being detected, the instrument automatically switched to mode-2, in which samples were collected at each zenith angle every 8 seconds, the maximum rate allowed by the telemetry. If peaks were then detected with a period of less than 16 seconds it would switch back to mode-1. At 8 minutes after entry, only Mode-2 would be used. A single sample consisted of a sequential reading from all channels within 6 ms. It had an altitude resolution of about 300 meters. The radiometer performed well and provided data from 67 km altitude to the surface.

Alternate Names

  • LSFR
  • PioneerVenusLargeProbe/LSFR
  • urn:nasa:pds:context:instrument:pvmp.lp.lsfr

Facts in Brief

Mass: 1.65 kg
Power (avg): 3.5 W

Funding Agency

  • NASA-Office of Space Science (United States)


  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Prof. William L. WolfeOther InvestigatorUniversity of
Mr. A. ClementsOther InvestigatorUniversity of Arizona
Dr. Martin G. TomaskoPrincipal InvestigatorUniversity of

Selected References

  • Tomasko, M. G., et al., Preliminary results of the solar flux radiometer experiment aboard the Pioneer Venus multiprobe mission, Science, 203, No. 4382, 795-797, doi:10.1126/science.203.4382.795, Feb. 1979.
  • Tomasko, M. G., et al., Pioneer Venus sounder probe solar flux radiometer, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 93-96, doi:10.1109/TGRS.1980.350288, Jan. 1980.
  • Pollack, J. B., et al., Distribution and source of the UV absorption in Venus' atmosphere, J. Geophys. Res., 85, No. A13, 8141-8150, doi:10.1029/JA085iA13p08141, Dec. 1980.
  • Tomasko, M. G., et al., Measurements of the flux of sunlight in the atmosphere of Venus, J. Geophys. Res., 85, No. A13, 8167-8186, doi:10.1029/JA085iA13p08167, Dec. 1980.
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