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Atmospheric Structure (SAS)

NSSDCA ID: 1978-078F-01

Mission Name: Pioneer Venus Small Probe (Night)
Principal Investigator:Dr. Alvin Seiff


The Atmospheric Structure Experiment was flown on all four Pioneer Venus Probes (slightly different versions on the large probe from the three small probes) to characterize the atmosphere of Venus down to the surface at the four different entry sites. The instruments for this experiment on the small probes included a single-axis accelerometer, pressure sensors, and temperature sensors. The measurements were used to construct a profile of atmosphere state properties for the probe trajectory from the surface to approximately 140 km altitude. They were also used to determine vertical wind velocity, horizontal wind velocity, and turbulence.

They were based on the technology demonstrated by the PAET rocket vehicle (Planetary Atmosphere Experiment Test R7106-2001). The instruments weighed about 1.2 kg and consumed about 3.5 W of power. By comparing atmospheric conditions along the large probe and small probe trajectories, circulation models of the atmosphere were determined. At the hypersonic velocities at entry, direct temperature and pressure measurements were not feasible, these sensors operated effectively below about 65 km altitude.


A single-axis accelerometer, developed from highly accurate guidance accelerometers (Systron Donner 4841), was mounted near the center of gravity of the small probes such that it measured along the probe vertical axis. It used a pendulous mass containing a current carrying coil of wire suspended in a null position in a permanent magnetic field. The force caused by acceleration on the mass was proportional to the current necessary to maintain the mass in the null position. Position of the test mass was sensed inductively, and a servo loop regulated the coil current to hold the test mass within microradians of the null position. The current in the coil is passed through a load resistor, whose value is switched to change sensor ranges, and the voltage drop across the load resistor is the acceleration signal. The sensor could be switched between four ranges, with full scale values of 100 micro-g, 10 milli-g, 1.5 g, and 600 g. The full range covered 0.4 micro-g to 600 g.

During the entry phase, the velocity change was output every 0.25 s with 8-bit resolution. During the descent phase (below 65 km), the data output was 12 bits each 8 or 16 seconds, dependent on the spacecraft bit rate. An electronics problem resulted in the instrument failing to acquire the two high sensitivity ranges so no data above 120 km altitude was returned.

Pressure Sensors

The pressure sensors were silicon diaphragm sensors, an array of 12 of these sensors covered the full pressure range from 30 mbar to 100 bar. Each covered a partial range of pressure, but overlapped so that if one failed, data from that pressure regime could still be returned by other sensors.

Kulite sensors were used, with an active element about 3 mm in diameter and 5 mm long. The diaphragm thickness varied from 75 microns to 250 microns to accommodate the full pressure range. A very shallow vacuum reference space was sealed beneath the diaphragm. Metallic strain gauges were vacuum deposited onto the diaphragm outer surface to form a Wheatstone bridge, with two opposite legs being deformable and two constant.

Range switching occurred automatically as the pressure increased. The telemetry resolution was 0.2-percent full scale (9-bit words) for the pressure readings.

Temperature Sensors

The temperature sensors were held on a paddle-shaped frame of thin walled (130 micron), 1.5 mm diameter, platinum-rhodium tubing. The frame was 2.8 cm wide with a fine wire resistance thermometer being the primary sensing element. 17.5 turns of this 0.1 mm thick wire were wrapped around the frame. The wire resistance was 1.9 ohms at 0 degrees C.

A secondary bonded sensor, for redundancy and comparison, consisted of approximately 6 cm of 25 micron thick resistance wire on a thin glass insulator bonded onto the leading surface of the sensor frame. The resistance of this wire was 11.4 ohms at 0 degrees C. The frame extended from the equator of the probe, held on a support tube made of 0.5 mm thick type 304 stainless-steel. The tube held the sensor frame roughly 3.5 cm beyond the probe boundary layer, to avoid thermal convection from the probe surface. The change in resistance in the wire with temperature was measured.

The sensors were powered by 10 mA current, constant to within 20 ppm, and the potential drop across the sensors was output. This was a function of the resistance, which in turn was a function of temperature. The temperature sensor was tested to be accurate over a range from 200 K to 800 K within 0.2 K, which was the resolution of the 12-bit word reserved for the telemetry temperature transmission.

The response time of the sensors was longer in the thinner atmosphere at higher altitudes and lower towards the surface. Response times varied from 150 ms at deployment to 5 ms at the surface for the fine wire sensor on the large probe, and from 2.5 s to 55 ms for the bonded sensor.

An electronics package handled distribution of power to the sensors, sampling of the output from each sensor, changing the ranges as necessary, and storing the data for telemetry. Three separate data formats were used, for high-speed entry, transition to the descent phase, and the descent phase itself, including for use if the probe survived on the surface.

Alternate Names

  • PioneerVenusSmallProbeNight/SAS
  • SAS

Facts in Brief

Mass: 1.2 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
Mr. S. C. SommerOther InvestigatorNASA Goddard Space Flight Center
Dr. Donn B. KirkOther InvestigatorNASA Ames Research Center
Mr. Robert C. BlanchardOther InvestigatorNASA Langley Research
Dr. John S. DerrOther InvestigatorUS Geological
Dr. Richard E. YoungOther InvestigatorNASA Ames Research
Dr. Alvin SeiffPrincipal InvestigatorNASA Ames Research Center

Selected References

  • Seiff, A., et al., Atmosphere structure instruments on the four Pioneer Venus entry probes, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 105-110, doi:10.1109/TGRS.1980.350291, Jan. 1980.
  • Seiff, A., et al., Structure of the atmosphere of Venus up to 110 kilometers: Preliminary results from the four Pioneer Venus entry probes, Science, 203, No. 4382, 787-790, doi:10.1126/science.203.4382.787, Feb. 1979.
  • Seiff, A., et al., Measurements of thermal structure and thermal contrasts in the atmosphere of Venus and related dynamical observations: Results from the four Pioneer Venus probes, J. Geophys. Res., 85, No. A13, 7903-7933, doi:10.1029/JA085iA13p07903, Dec. 1980.
  • Hoffman, J. H., et al., Composition and structure of the atmosphere of Venus, Space Sci. Rev., 20, No. 3, 307-327, doi:10.1007/BF02186468, May 1977.
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