RADIBAL (RADIomètre BALlon) was a photo-polarimeter developed in 1982 by the Laboratoire d'Optique Atmosphérique (LOA), Université des Sciences et Technologies de Lille in France. Its goal was to measure 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 and a view of the gondola on which it was mounted(click to enlarge) . The entrance radiance was spectrally filtered by a filter wheel (W), then passed through a rotating analyzer (A), and finally was focused on detector (D). The objective lens had a diameter of 6.1 cm; the field stop (S) in its focal plane provided a 2º field of view; the lens aperture was f/7. The condensed beam passed through the filters just behind (S) being its diameter 6 mm. There were two filters: one for the 850nm wavelength and the other for the 1650 nm wavelength with a diameter of 50 mm. Both filters were mounted on the rotating wheel (W). Given the respective diameters of the wheel, the filters, and the beam, each filter was full-beam positioned within a 48º rotation of (W). Since the detector sensitivity was quite constant regardless of the polarization state of the received light, the polarization analysis was provided by a simple rotating analyzer (Infrared H.C. Polaroid).
The double condenser (C) finally imaged 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 showed that the degree of linear polarization could be measured within ±0.01 at 25-km altitude.
The germanium photodiode was 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 bringed the signal up to a usable level. A second amplification stage imposed three different gain levels: X1, X10, and X100. Then the signal was taken by an analog-to-digital conversor resulting in a 12-bit digitized stream which was treated by an onboard Intel 8085 microprocessor; thus, after a full revolution of the filter wheel the onboard telemetry system transmited the data stream along with several housekeeping parameters and positioning information to the ground station.
The entire instrument was packaged in three distinct units: the polarimeter with its detector amplification system, the data processing electronics, and the battery supply. These unpressurized units were thermally insulated by polystyrene housings. Moreover, a heater was incorporated in the polarimeter unit, so when temperature reached -7°C, the cooling Peltier device of the detector was activated.
The polarimeter was fixed on a stratospheric platform carefully leveled before the launch whose angular scan was provided by a nominal spin rate of 1 rpm. The horizontality of the observation direction was checked from an onboard inclinometer while the azimuth was obtained from an onboard magnetometer. Finally, the solar elevation and azimuth were obtained from the balloon geographical position along with the time of the measurement.
Two other instruments were also onboard the gondola for this mission. Both were developed also at LOA: BALLAD (BALloon Limb Aerosol Detection) and BOCCAD (Balloon OCCultation for Aerosol Detection)
BALLAD was aimed at studying the scattering properties of the stratospheric aerosol by measuring the intensity (at 450, 600 and 850 nm) and the polarization (at 850 nm) of the Earth's limb, for a wide range of scattering angles. An inversion method of the radiances only, obtained in forward scattering, has been developped to retrieve the vertical profiles of the extinction coefficient and of an asymetry parameter of the aerosol. The objective of the instrument formed the Earth's limb image on a linear charge-coupled device (CCD) detector of 1728 pixels, the vertical FOV of the system was about 6°. A filter wheel, located between the objective and the detector, allowed multispectral radiance and polarization measurements to be performed. Two interference filters at 850 and 450 nm were dedicated to aerosol studies and a third channel centered at 600 nm was deployed for ozone detection. Three other filters centered at 850 nm were equipped with polaroids rotated by 60° with respect to one other to evaluate the linear degree of polarization of scattered light.
BALLAD was installed on board of the gondola with its optical axis, pointing approximately 2° below the balloon horizon. Combined with its large vertical FOV allowed observation of all the stratospheric layers, from the tropopause up to the balloon level, simultaneously with one exposure. The vertical resolution was about 350 m for a balloon ceiling at 30 km and a tangent height equal to 15 km. Measurements were carried out for solar elevation ranging from 2 to 10° and the rotation of the gondola around the vertical axis (1/3 rpm) permited BALLAD to observe Earth's limb for various azimuth angles (angular sampling 16°).
BOCCAD was built by LOA in 1994 to observe solar occultation through the atmosphere. It was planned to provide useful information for RADIBAL, and to provide a direct comparison with satellite measurements SAGE II and a POAM II. The objective formed the Sun's image on a CCD matrix of 244 by 550 pixels. The vertical and horizontal FOV of the instrument were 10° and 7.5° respectively. A filter wheel, located between the objective and the detector, allowed multispectral measurements to be performed. Three interference filters centered at 850, 780 and 443 nm were used for aerosol studies, and a fourth channel at 600 nm was dedicated to ozone detection. BOCCAD was installed on the same gondola as BALLAD and also operated at the balloon ceiling level. The gondola stoped rotating after the BALLAD measurements and the instrument pointed 2° below the balloon horizon so that the large FOV allowed observation of the whole occultation event. The measurements started when the solar elevation reached ca. 2° and the Sun's image could be obtained almost outside the atmosphere to provide a reference image in each channel. Typically about 300 images were formed successively in each channel during a sunset at high latitudes.
A fourth instrument provided by the University of Wyoming was also onboard an Optical Particle Counter (OPC). It was a white light counter measuring aerosol scattering at 25º (hence it was also denominated OPC25) in the forward direction and using Mie theory to determine aerosol size. An incandescent lamp and light controller supplied constant illumination of the scattering region. Light scattered from particles passing through the beam was collected over a solid angle of ~30º and focused onto a photomultiplier tube (PMT) for pulse height detection. Two symmetrical independent photon paths were used to limit noise and the influence of cosmic rays by coincidence counting. Integral particle concentrations were measured by counting coincident PMT pulses which exceeded preset discriminator levels.
Balloon launched on: 3/2/1995 at 12:10 utc
Launch site: European Space Range, Kiruna, Sweden
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon model 100z Zodiac - 100.000 m3
Balloon serial number: 100Z Nº 69
End of flight (L for landing time, W for last contact, otherwise termination time): 3/2/1995 at ~ 16:30
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 4 h 21 m
Campaign: SESAME (Second European Stratospheric Arctic and Midlatitude Experiment)
Payload weight: 510 kgs
Gondola weight: 276 kgs