SALOMON (Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et Nox)

SALOMON is the acronym of Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et Nox. Its a instrument developed by the Laboratoire de Physique et Chimie de l'Environnement (LPCE) with funds from the Centre National d'Etudes Spatiales (CNES). The instrument is a balloon-borne UV-visible spectrometer designed to acquire vertical profiles of O3, NO2, NO3, OClO and OBrO as well as the extinction coefficient of aerosols, at altitudes between 15 and 40 km. It was developed based on the experience acquired by the scientific teams that developed the AMON and SAOZ spectrometers, that used stars and sun as light source respectively.

At left we can see an scheme of the instrument (click to enlarge). SALOMON is made from three boxes. The top two boxes, linked by an optical fiber, include a spectrometer and the moon-tracker system. The third, below the two others, contains the CNES telemetry system. The boxes are made from polystyrene, which is well suited for protecting the onboard instruments at landing. The pivot (also known as the primary pointing system) is located above the boxes and links the gondola to the flight chain. The total weight of SALOMON is about 80 kg, which permits the use of balloons in the 10,000-65,000-m3 range to reach float altitudes of 27 to 38 km at mid-latitudes.

The measurements are performed by the lunar occultation method. This method consists of recording spectra affected by atmospheric absorption during a moonset or a moonrise. The elevation of the Moon decreases from a few degrees above the gondola's horizon to 24°, which implies a duration at float of about 40 minutes. A reference spectrum is recorded when the Moon is as high as possible and the balloon is at float.

The hearth of the system is the spectrometer, the detector, and the electronics which are the same as those used for the SAOZ instrument. The spectrometer is a Jobin-Yvon Model CP200, with a wavelength domain shifted to 350~700 nm to cover the NO3 absorption band at 662 nm. The detector is a Hamamatsu photodiode array of 1024 pixels, with a theoretical spectral resolution of 0.34 nm, which is 2.5 times the theoretical resolution of AMON. A dark current exposure is performed after every ten exposures, and its value is subtracted automatically from the recorded spectra by the onboard processor.

The moon tracker consists of a 25 mm x 76 mm plane mirror that can move on elevation mounted upon a turret that turns on azimuth. A portion of the light focused by a lens in the turret is sent to a position sensor, which determines the spot position. The error in elevation and the error in azimuth are calculated, and the calculated correction is sent directly to the motors by the onboard processor. The system is designed to achieve a precision below 30 arc/sec when the gondola is affected by only small perturbations. In the case of strong oscillations of the gondola, as can be encountered during ascent of the balloon, the error on azimuth can be as much as 1 arc min, and the error on elevation can reach a few arc minutes because this axis is not controlled by the pivot. The flux collected by the mirror is focused onto an optical fiber to send the light to the spectrometer. This device homogenizes the flux and eliminates sensitivity to the variations of the Moon's albedo induced by its nonuniform ground composition.

The pivot or stabilization unit is designed to stabilize the gondola and to perform rough azimuth control. It is composed of a magnetic damping device to stabilize the flight chain and a mechanical unit with a velocity sensor to perform the gondola rotation. The Moon's location in the sky is determined automatically by two photodiodes mounted 45° from each other. The pivot induces a rotation of the gondola until the fluxes on the two photodiodes are identical.

SALOMON operates automatically for on-off switching, pointing, data acquisition, and telemetry. The housekeeping and spectroscopic data are telemetered in real time to the ground.

Balloon launched on: 10/31/1998 at 00:06
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: Zero Pressure Balloon model 12sf Zodiac - 12.000 m3
Balloon serial number: 12SF Nº 99
End of flight (L for landing time, W for last contact, otherwise termination time): 10/31/1998 at 2:15 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 2 h 19 m
Payload weight: 209 kgs
Gondola weight: 59 kgs

This was the first flight of SALOMON. The balloon was launched from Aire Sur L'Adour at 00:06 utc on October 31, 1998. The mission was devoted to testing all the technical parameters of the instrument. Flight conditions allowed for measurements to be performed during the ascent of the balloon and during the whole moonset. Inasmuch as all the parameters of the pointing system, the
moon tracker, and the spectrometer were nominal, good scientific results could be extracted from the spectra.

Launch occurred at 00:06 UT, the ascent lasted 75 min, and the float altitude of the balloon was 26.65 km with fluctuations of 6150 m. This altitude was suitable for the observation of O3 but was
problematic for observation of NO2 and NO3 because the maximum concentrations of these species are above 27 km. The measurements started at 00:52 UT, just after crossing the tropopause, and stopped at 01:46 with a total of 43 spectra recorded. Because of strong winds and the fast ascent of the balloon, some oscillations of the gondola occurred during the observations and stopped only a few minutes before the end of the measurements at float altitude. Without affecting the spectra, this situation allowed for testing of the pointing system both in perturbed and in quiet conditions.

The phase of the Moon was 68% of full. Inasmuch as the flux was strong enough for both the spectrometer and the pointing system, this flight demonstrated that measurements can be performed less
than two days after the first quarter. Thus flights can occur at least 10 days per month, not including the day of the full Moon for which moonrise and moonset occur too close to sunset and sunrise.

• SALOMON Instrument Website Laboratoire de Physique et Chimie de l'Environnement etde l'Espace
• SALOMON: a new, light balloonborne UV-visible spectrometer for nighttime observations of stratospheric trace-gas species Appl. Opt. 39, 386-392 (2000)