Purpose of the flight and payload description
The objective of the flight was to perform calibration of solar cells using a specially developed platform transported to the top of the atmosphere by a stratospheric balloon.
When designing solar cell energy systems for space missions, it is necessary to obtain precise performance parameters of solar cells under standard sunlight. Especially for deep space exploration the weight of the energy system carried is very limited and the calculation needs to be very precise. This information can't be obtained from the ground due to the presence of the earth's atmosphere which filters part of the solar incoming radiation.
The purpose of any high altitude solar cell calibration program is to produce air mass zero (AM0) standard solar cells that can be used for accurately setting solar simulator intensities. The concept is to fly the cells on a balloon, to maintain the sun rays as perpendicular as possible to the cells, to measure their output at altitude, to recover the cells and to use them as reference standards.
The CNES program of Solar Cell Calibration
In late 1970's decade the french space agency CNES started a program for solar cell calibrations under the generic denomination of CASOLBA (CAlibration de cellules SOLaires sur vol BAllon). In the image at left we can see a detail of the platform configuration used since the inception of the program between 1998 and 2001 (click to enlarge).
The standard flight used a 100.000 m3 balloon which assured an altitude of flight between 34 km and 40 km. The structure of the gondola was formed by aluminum alloy tubes assembled together by hemispheres. It counted with pointing capability to direct the cells to face the sun. A special baseplate housed the solar cells which also were covered with a sunshield that acted as collimator to avoid the parasitic light coming from the earth limbo and from the sun reflexion on the balloon envelope. A passive thermal control system cooled the cells during the ascent to ensure that they were at a reasonable temperature at the time of starting the measurements. In the lower part of the gondola were housed the onboard electronics and batteries and cardboard crush pads to soften the impact of the landing. The size of the gondola was slighty bigger than the used until 1996 to carry more TTC (tracking, telemetry and control) and
localisation equipment. Total weight was 185 kg.
The measuring electronics were improved in late 1990's. The temperature was recorded continuously during the entire flight allowing the calibration of 30 cells by performing measurements of the I-V characteristics from short circuit to open circuit at each temperature.
Details of the balloon flight
Balloon launched on: 6/25/1998 at 6:07
Launch site: Aérodrome de Gap-Tallard, Haute Alpes, France
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon model 100sf Zodiac - 100.000 m3
Balloon serial number: 100 SF Nº 26
End of flight (L for landing time, W for last contact, otherwise termination time): 6/25/1998 at ~ 15:10
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 9 h 4 m
Payload weight: 353 kgs
Gondola weight: 143 kgs