Description of the payload
The LPMA instrument (Limb Profile Monitor of the Atmosphere) is a high spectral resolution Fourier transform spectrometer operating in absorption against the sun. The objective is to record limb atmospheric spectra in selected intervals from the thermal infrared to the near-infrared.
The instrument is composed by a commercial BOMEM DA2 spectrometer customized for balloon operations. A specially designed two detectors output optics enables to cover simultaneously two spectral regions making it possible to sample simultaneously the two interferograms detected during the same scan of the moving mirror and allowing to collect information on atmospheric species absorbing in widely different spectral regions.
In order to reach a good signal to noise ratio spectra performance, the interferometer must be feeded with a stable solar beam exactly aligned along the optical axis of the instrument. To be able to do this, two sub-systems of the gondola are needed:
- a primary pointing system initially developed by the Observatory of Geneva for astronomical payloads, controling the azimuth of the gondola and maintaining the sun in the field of view of the instrument even during ascent in the dense layers of the upper troposphere or lower stratosphere where this is a rather difficult achievement.
- a suntracker (also known as a heliostat) developed in cooperation with the Institut d'Electronique el de Micro-electronique du Nord performing the fine pointing of the acquisition mirror through 2 axes servo-controlled gimbal compensating for the residual motions of the payload and maintaining a jitter of the solar beam direction at the input of the interferometer of less than 1 arc min.
In addition it is possible to accomodate on the gondola another instrument of the class 30-50 kg wich can benefit from their good pointing capabilities. In this flight also was part of the scientific payload onboard the gondola the DOAS (Differential Optical Absorption Spectroscopy) consisting of 2 light intake telescopes for simultaneous Nadir and scanning Limb observations (the latter being mounted on an automated elevation scanner). The incoming light is conducted into two Ocean Optics USB-2000 spectrometers which are mounted into an evacuated and thermo-stated housing and controlled by a single board computer for data handling and storage. The instrument was created to help in the validation of the SCHIAMACHY instrument onboard ENVISAT.
Details of the balloon flight and scientific outcome
Launch site: European Space Range, Kiruna, Sweden
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Open balloon model 100z 100.000 m3
Balloon serial number: 100Z Nº 107
Campaign: THESEO 2000
Payload weight: 728 kgs
Gondola weight: 464 kgs
The balloon was launched by dynamic method assited by auxiliar balloon at 12:15 UT on 18 Feb. 2000 from the ESRANGE balloon launch pad in Kiruna. After an initial ascent phase, the balloon reached float at 14:15 UT for an altitude of about 30 km.
After near 4 hours of flight the balloon was separated from the payload wich landed in Finland.
This was the flight 16 of LPMA and the first time that the LPMA/DOAS was operating with a smaller mechanical structure (for reducing weight) with a mass of 460 kg.
All the equipment operated in a nominal manner with the sun-tracker feeding both the FTIR (parallel beam) and the UV-vis (fibre optics) spectrometers with a reasonably stable solar beam. Slightly more perturbed conditions were observed, however, as compared to other Arctic flights of the same LPMA/DOAS payload. This translates into slightly noisier spectra for the FTIR instrument.
Measurements were possible from an altitude of about 2.5 km during the ascent with only a small number of loss of sun periods in the dense troposphere and around the tropopause when the stability of the pointing (azimuth control of the gondola in the direction of the sun) is always difficult. Measurements were continued at float until loss of sun just above the horizon for a tangent altitude of about 13 km and allowed to sample air masses inside the Arctic vortex.
External references and bibliographical sources