ELECTRIC FIELD SENSORS

Developed by University of California, Berkeley (UCB)

Balloon launched on 8/7/1968, from Fort Churchill Airport, Manitoba, Canada

Purpose of the flight and payload description

The objective of the flight was to measure horizontal and vertical atmospheric electric fields at stratospheric altitudes and use them to infer large-scale ionospheric and magnetospheric electric fields. The payload was developed by scientists at the Physics Department and Space Sciences Laboratory of the University of California, Berkeley.

The image at left shows one of the payloads immediately after launch. The electric-field instrument consisted of three mutually orthogonal pairs of conductive spheres mounted on insulating booms approximately three meters long. Each pair formed an electric-field antenna aligned along one of three perpendicular axes. The spheres were connected to high-input-impedance differential voltmeters that measured the potential difference between opposite spheres. Electric-field strength along each axis was determined by dividing the measured potential difference by the separation distance between the spheres. This configuration allowed simultaneous measurement of three orthogonal components of the ambient electric field. An onboard magnetometer provided orientation data, allowing the horizontal electric-field measurements to be resolved into magnetic northward and eastward components. The payload was suspended approximately 50 yards below the balloon to reduce distortion of the electric field by the balloon.

A principal design requirement was the separation of large vertical electric fields generated by atmospheric weather processes from weaker horizontal fields associated with ionospheric and magnetospheric processes. To accomplish this, the payload incorporated a leveling system based on a carpenterÂ’s level sensor that continuously monitored orientation relative to the vertical. The system maintained the horizontal antennas within approximately 0.5° of horizontal, reducing contamination of the horizontal measurements by the vertical atmospheric electric field. The payload also rotated at an average rate of about six revolutions per hour. As a result, a horizontal external electric field appeared as an alternating signal at the rotation frequency, while residual contributions from the vertical electric field appeared primarily as a direct-current offset.

The design also addressed contact-potential effects between sensor spheres. Small differences in surface work function could produce voltage offsets equivalent to electric fields of several millivolts per meter. To minimize this effect, the spheres were coated with carbon to produce nearly identical surface properties. The instrument used ultra-high-input-impedance electronics to ensure accurate electric-field measurements throughout the flight altitude range.

An additional subsystem measured atmospheric conductivity during flight. Every 256 seconds, antenna elements were momentarily shorted together. After the short was removed, the recovery time constant was measured. Atmospheric conductivity was then calculated from the recovery time and the dielectric constant of free space. These measurements provided conductivity profiles as a function of altitude during ascent and supported analysis of electric-field propagation between the ionosphere and balloon altitude.

The telemetry system used four FM-FM subcarrier channels. Three channels continuously transmitted the measured electric-field components. The fourth channel transmitted multiplexed data from a commutator system, including additional electric-field sensitivity ranges, magnetometer measurements, horizontal-orientation data, and housekeeping parameters such as temperatures, voltages, and currents.

Details of the balloon flight

Balloon launched on: 8/7/1968
Launch site: Fort Churchill Airport, Manitoba, Canada  
Balloon launched by: Raven Industries Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon  
 End of flight (L for landing time, W for last contact, otherwise termination time): 8/??/1968
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 23 h

External references

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