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Net Flux Radiometer (SNFR)

NSSDCA ID: 1978-078G-04

Mission Name: Pioneer Venus Small Probe (Day)
Principal Investigator:Dr. Verner E. Suomi


The objectives of the Small Probe Net Flux Radiometers (SNFR) were to locate regions of radiative convergence and divergence as a function of altitude and to indicate the height at which solar energy is absorbed by the atmosphere. The North and Night Probes, both entering the nightside of Venus, measured net infrared flux only, while the Day Probe also measured net solar flux. The instrument was designed to cover the spectral band from 0.2 to 50 microns with a resolution of 1 km.

The primary measurement was the total (solar plus longwave) net planar radiative flux density, the difference between the total radiative power per unit area upward and downward, to understand the energy balance in the atmosphere. This helps define the radiative driving forces to aid in understanding Venus atmospheric circulation, testing the greenhouse explanation for the high temperatures, and constraining composition and cloud structure.

The SNFR comprised two modules, an electronics module with a mass of 0.640 kg housed inside the probe pressure vessel and a sensor assembly (0.360 kg) mounted externally. The instrument used a total of 2.2 W power. The electronics module consisted of five circuit boards housed in a gold-plated titanium box The modules housed the amplifier, voltage-controlled oscillator, max/min electronics, digital electronics, heater and solenoid drivers, temperature monitors, and power conversion unit.

The sensor assembly contained protective windows, occulting mirrors, temperature sensor, heater, and a flux plate detector inside a disk-shaped titanium housing mounted on the end of a 10 cm long titanium shaft. The assembly was 1.5 cm in diameter. The flux plate detector is a flat (approximately 1 mm thick) ceramic plate, situated normal to the zenith and nadir. It is blackened on both sides with a coating consisting of a spectrally flat mixture of carbon particles and potassium silicate. The response is fairly constant over the range from 0.35 microns to beyond 100 microns, covering both visible and relevant infrared wavelengths. When exposed to net radiation flux, the plate heats up on one side more than the other. A differential thermopile (a wire wound thermocouple) produces a voltage proportional to the temperature gradient across the plate, which provides a measure of the net radiation flux. The response time for the plate to reach temperature gradient equilibrium is about 0.12 seconds.

The titanium shaft and sensor assembly was deployed from behind a protective cover when the entry deceleration fell below 5.5 g. The titanium shaft is flipped 180 degrees once per second so that the radiometer looks alternately "up" and "down". It used a two-way magnetically latched linear solenoid system. The sensor plate was in the horizontal up (UP) or down (DN) position at all times except for the 20 to 50 ms time it took to flip. The fast response time allowed the sensor to measure the instantaneous net flux each second, regardless of whether the flux was upward or downward directed. The SNFR used two measurement periods, one of 8 seconds when the probe was above about 30 km, and one of 32 seconds when the probe was below this altitude, and measured the maximum, minimum, and average net flux difference over the given measurement period.

The measurements were made such that, for a positive net flux, an UP measurement was positive and a DN measurement was negative. The measurement periods were determined by the minor frame transmission cadence. Above about 30 km altitude, a minor frame was transmitted every 8 seconds, corresponding to 8 (4 UP, 4 DN) SNFR measurements. A set of 2 UP and 2 DN measurements over 4 seconds had their absolute values averaged to give an 8-bit AVG reading, there were two AVG readings per minor frame. For each measurement period the highest (MAX) and lowest (most negative) reading would also be transmitted, each 8-bits. The minor frames below 30 km altitude were transmitted every 32 seconds, each also contained two AVG, one MIN, and one MAX reading. Four ranges were available for these readings, if a measurement exceeded the selected range, the 8-bit register would overflow, giving a reading equal to the true count mod 256. For each minor frame, the detector housing temperature was measured, and once each 16 minor frames the amplifier temperature, status of solenoid, heater, and bit rate was transmitted.

Alternate Names

  • SNFR
  • urn:nasa:pds:context:instrument:pvmp.sp-day.snfr
  • PioneerVenusSmallProbeDay/SNFR

Facts in Brief

Mass: 1 kg
Power (avg): 2.2 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
Dr. Lawrence A. SromovskyOther InvestigatorUniversity of
Dr. Jacqueline LenobleOther InvestigatorUniversite
Mr. G. Edward Danielson, Jr.Other InvestigatorCalifornia Institute of
Dr. Maurice HermanOther InvestigatorUniversite
Dr. Alain L. FymatOther InvestigatorNASA Jet Propulsion
Dr. Verner E. SuomiPrincipal InvestigatorUniversity of Wisconsin-Madison

Selected References

  • Sromovsky, L. A., et al., Pioneer Venus small probes net flux radiometer experiment, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 117-121, doi:10.1109/TGRS.1980.350293, Jan. 1980.
  • Suomi, V. E., et al., Preliminary results of the Pioneer Venus small probe net flux radiometer experiment, Science, 205, No. 4401, 82-85, doi:10.1126/science.205.4401.82, July 1979.
  • Suomi, V. E., et al., Net radiation in the atmosphere of Venus: Measurements and interpretation, J. Geophys. Res., 85, No. A13, 8200-8218, doi:10.1029/JA085iA13p08200, Dec. 1980.
  • Colin, L., Ed., and D. M., Ed. Hunten, Pioneer Venus experiment descriptions, Space Sci. Rev., 20, No. 4, 451-525, doi:10.1007/BF02186463, June 1977.
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