Constant volume balloons (CVBs) are operated by the French Space Agency (Centre National d’Etudes Spatiales).

figure VI.6-a: Constant volume balloons

They are vertical, cylindrical balloons, 5m in length and 0.6m in diameter, with half-spheres at the extremities. They are composed by two thin (20 mm) mylar envelopes, which are over-pressurized by up to 100hPa. The volume inside the envelope is about 2 m³, filled by Helium (He). The maximum payload is about 1kg.

For Carbo-Europe experiment, CVBs will fly at altitudes of about 1000m. The excess of pressure inside the balloon would be 40hPa on average, which would allow vertical excursions of up to 500m above or below the the mean flight level without bursting of the balloon. For flight periods of some hours, the He leakage is weak (<1/1000).

The flight level is defined before the filling of the balloon, according to atmospheric profiles observed on radiosonde profiles. The air density at the flight level allows to compute the mass of the ballast below the balloon, and the He quantity inside the envelope in order to the internal pressure overpasses the atmospheric pressure by about 40hPa at the flight level.

The radiosondes are identical to those used for the profiles (soundings) : Vaisala RS92SGP. They measure atmospheric pressure, temperature and moisture along the balloon trajectory. In order to avoid spurious heating of the sensors because of the weakness of the wind relative to the balloon, the temperature and moisture sensors are put inside a small screen equipped with a fan. The position of the balloon is measured by GPS.

The balloons will be launched from the site “La Cape Sud (CS)”, at the same location of the radiosonde soundings. Only one Vaisala system will be used to receive the balloon data via telemetry (403MHz). It is expected to track the balloon for at least 50km.