SOLAR CELL CALIBRATION

The purpose of the high altitude solar cell calibration program was to produce air mass zero (AM0) standard solar cells that could be used for accurately setting solar simulator intensities. The program was started in 1962 by the Jet Propulsion Laboratory of the California Institute of Technology in Pasadena and endured for more than 40 years.

Since the output of solar cells is spectrally selective, the electrical power derived from solar cells depends on the total intensity and the spectral distribution of the light source: the Sun. Attempts were made to procure a laboratory light source which has the same spectral character and intensity as the Sun. However, such attempts have been complicated as different types of radiation sensing devices, such as pyrheliometers and thermopiles, have been used which can not be accurately calibrated on either a total intensity or spectral intensity basis. To overcome this was introduced the concept of flying cells on a balloon, to measure their output at altitudes, to recover the cells and to use them as reference standards. The calibrated standard solar cell is placed in the solar simulator beam, and the beam intensity is adjusted until the standard solar cell reads the same as it reads on the balloon.

Althought some modifications were introduced over time, the main components of the balloon flight system were maintained along the entire program: a sun tracker, a helium-filled balloon, a telemetry system, and a battery power supply, as shown in the scheme at left (click for more detail). The sun tracker was mounted on the balloon apex, which is the most stable position of the balloon system. The telemetry transmitter and battery power supply, along with several instruments for measuring altitude, were suspended in a second gondola beneath the balloon. An electrical cable, incorporated into the balloon during manufacture, connected the top and bottom payloads.

The solar tracker was used to position the solar cell payload toward the sun, independent of balloon movements. The tracker was capable of movement in both elevation and azimuth to maintain an "on-sun" condition within +2 deg. A reflection shield attached to the solar tracker was used to prevent unwanted reflected light from reaching the solar cell payload. The tracker and associated electronics boxes were mounted on a plywood disk 6 ft in diameter, which, in turn, was bolted to the balloon top end fitting. The plywood disk permited the tracker to "float" on top of the helium bubble.

Balloon launched on: 8/10/1967 at 8:46 CDT
Launch site: University of Minnesota Airport, New Brighton, US  
Balloon launched by: Litton Industries Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon 77-1-2 3DRS816 (1.5 mil)
Flight identification number: LITTON 3045
End of flight (L for landing time, W for last contact, otherwise termination time): 8/10/1967 at 17:03 CDT
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 8 h 20 m
Landing site: 10 miles NW of Austin, Minnesota, US

The balloon was launched by dynamic method from the Litton Flight Facility located near New Brighton, Minnesota at 8:46 CDT on August 10, 1967 as Flight No. 3045. The balloon ascended slowly to the southeast. Over Millville, Minnesota at 70,000 ft, the system turned westward. For the remaining ascent period and during the float period the system moved west to a point approximately 5 miles south of Faribault, Minnesota at the initiation of descent.

The descending balloon moved south to impact 10 miles northwest of Austin, Minnesota at 17:03 CDT. The lower payload touched down in an alfalfa field and although the destruct device actuated on impact the gondola was dragged about 30 yards to a barbed wire fence where the balloon and tracker laid over into a soybean field. Actual damage to the property and crops was minor. The equipment (solar cells, sun tracker and instrumentation, except for the parachute which was ripped on the fence) suffered no damage. The recovery crew was able to load the equipment, clean up the area, discuss damages with the property owner and return to the Litton plant that evening without incident.

• Suntracker balloon flights 3042, 3043, 3044, and 3045, (1967) Final report Technical Report - Litton Systems, Inc.; Minneapolis, MN, United States