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: 7/14/1967 at 8:35 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 1DRS816 (1.5 mil)
Flight identification number: LITTON 3042
End of flight (L for landing time, W for last contact, otherwise termination time): 7/14/1967 at 17:18 CDT
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 8 h 45 m
Landing site: 3 miles SW of Adrian, Minnesota, US

The balloon was launched by dynamic method from the Litton Flight Facility located near New Brighton, Minnesota at 8:35 CDT on July 14, 1967 as Flight No. 3042. The balloon took a southerly course during ascent to a point 5 miles southwest of Faribault, Minnesota at 10:19 CDT, the beginning of the float period. During float the system moved due west to a point 10 miles south of Marshall, Minnesota at 15:00, where the onboard flight timer initiated descent by opening a helium gas port.

The system continued west during initial descent to a point 10 miles from the South Dakota border at 55,500 ft altitude at 16:20 CDT. The remainder of descent was in a south-southeasterly direction to an impact point 3 miles southwest of Adrian, Minnesota, at 17:18 CDT. Touchdown was in an open pasture and the recovery crew was at the site. They were able to observe impact and detonation of the balloon destruct device.

Cord destruct device opened the balloon but a light surface wind twisted the bubble to trap the remaining lift gas before it had escaped. The system was dragged across the pasture a short distance by the surface wind until the recovery crew drove up and secured the lower payload to the recovery vehicle. The crew then manually split open the balloon to release the remaining gas and packed up the undamaged flight equipment. The crew returned to Minneapolis late that evening with all gear in excellent condition.

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