Description of the payload

A photo-polarimeter developed in 1982 that measures, at two near-infrared wavelengths, the radiance and the degree of polarization of sunlight scattered from the stratospheric aerosol at various angles.

A schematic of the polarimeter optical system can be seen at left (click to enlarge). The entrance radiance is spectrally filtered by a filter wheel (W), then passed through a rotating analyzer (A), and finally focused on detector (D). The objective lens has a diameter of 6.1 cm; the field stop (S) in its focal plane provides a 2º field of view; the lens aperture is f/7. The condensed beam passes through the filters just behind (S) baeing its diameter 6 mm. There are two filters: one for the 850nm wavelength and the other for the 1650 nm wavelength with a diameter of 50 mm. Both filters are mounted on the rotating wheel (W). Given the respective diameters of the wheel, the filters, and the beam, each filter is full-beam positioned within a 48º rotation of (W). Since the detector sensitivity is quite constant regardless of the polarization state of the received light, the polarization analysis is provided by a simple rotating analyzer (Infrared H.C. Polaroid).

The double condenser (C) finally images the field stop into detector (D) a Judson germanium photodiode of 3-mm diameter. Given radiance computations for stratosphere standards on the one hand and the previous choices for the field of view, the entrance lens, and the spectral bandwidths on the other hand, the derived power budget shows that the degree of linear polarization could be measured within ±0.01 at 25-km altitude.

The germanium photodiode is operated in the photovoltaic mode at a temperature of -27.40 C, with the thermal regulation being provided by a cooling Peltier device. An inverting amplifier used for the first amplification stage brings the signal up to a usable level. A second amplification stage may impose three different gain levels: X1, X10, and X100. Then the signal is taken by an analog-to-digital conversor resulting in a 12-bit digitized stream which is treated by an onboard Intel 8085 microprocessor; thus, after a full revolution of the filter wheel the onboard telemetry system transmits the data stream along with several housekeeping parameters and positioning information to the ground station.

The entire instrument is packaged in three distinct units: the polarimeter with its detector amplification system, the data processing electronics, and the battery supply. These unpressurized units are thermally insulated by polystyrene housings. Moreover, a heater is incorporated in the polarimeter unit, so when temperature reaches -7°C, the cooling Peltier device of the detector is activated.

The polarimeter is fixed on a stratospheric platform carefully leveled before the launch whose angular scan is provided by a nominal spin rate of 1 rpm. The horizontality of the observation direction is checked from an onboard inclinometer while the azimuth is obtained from an onboard magnetometer. Finally, the solar elevation and azimuth are obtained from the balloon geographical position along with the time of the measurement.

No onboard calibration device has been included, so the system stability must be determined from preflight and postflight calibrations.

Details of the balloon flight and scientific outcome

Launch site: Centre de Lancement de Ballons CLBA, Aire Sur L'Adour, Landes, France  
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Open balloon model 35SF Zodiac - 35.000 m3

After ceiling reached followed a controlled and slowly descent phase from 24 to 14 km

External references and bibliographical sources

Images of the mission

Main balloon inflation RADIBAL payload detail RADIBAL launch hanging from auxiliar balloons