Definition of an Uncooled Sub-Millimeter/THz Limb Sounder for Measuring Middle Atmospheric Winds.
Baron, Philippe¹; Murtagh, Donal²; Urban, Joachim²; Sagawa, Hideo¹; Eriksson, Patrick^2
1NICT, JAPAN; ²Chalmers University of Technology, SWEDEN

Winds observations in the middle atmosphere, i.e. from the upper troposphere to the mesosphere are sparse. Previous observations were obtained from ground based active sensors (radar and lidar), and recently from a ground-based microwave spectrometer. Satellite observations have been successfully conducted by the WINDII and HRDI instruments onboard the UARS satellite and TIDI onboard the TIME satellite . Atmospheric winds have also been retrieved from the sub-millimeter limb sounder AURA/MLS. These observations are based on the measurements of the Doppler shift of lines and were mostly sensitive to the upper atmosphere (altitudes >70 km). For the future, space agencies are focusing on the use of spaceborne lidar instrument with a high sensitivity in the troposphere but with limited performances in the middle atmosphere ([1] contains references for the cited instruments).
Recently winds were retrieved between 30 and 80 km with a theoretical lower limit of 20-25 km from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) onboard the Japanese Experiment Module (JEM) of the ISS. The advantage of SMILES was its low measurement noise, about 10 times lower than concurrent systems such as Odin/SMR and AURA/MLS, thanks to the use of 4K-cooled SIS mixers. Noise level is the key issue to be able to measure the small wind-induced Doppler shift of the sub-millimeter spectral lines (the frequency shift is smaller than the spectral resolution of the instrument). However, SMILES was not designed for wind observation and suffered from serious limitations due to the choice of the spectrometer and of the spectral lines. Further a 4-K cooled receiver has severe limitations in terms of mass, power consumption and life time.
We investigate the theoretical performances of a stratospheric winds instrument based on the limb observation from space of atmospheric lines below 3 THz (λ>100 µm). Only uncooled or mechanically cooled (as for the Odin/SMR) radiometers are considered. We are interested in estimating if the increase of noise compared to the one of SMILES could be partly compensated by a better optimisation for winds retrieval. Information about atmospheric temperature and various constituents relevant for dynamics or chemistry studies will also be derived from the molecular lines within the measured spectral band. Special focus is placed on designing a lightweight instrument that could be carried by a micro or mini satellite.
[1] Baron et al., Observation of horizontal winds in the middle-atmosphere between 30S and 55N during the northern winter 2009-2010, doi:10.5194/acpd-12-32473-2012