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27 April 2012 EGU General Assembly 2012 PowerPoint Presentation
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27 April 2012 EGU General Assembly 2012

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27 April 2012 EGU General Assembly 2012

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  1. Introducing VESPA-22: a ground-based microwave spectrometer formeasuring middle atmospheric water vapour at polar latitudes Pietro Paolo Bertagnolio, Giovanni Muscari, Irene Fiorucci and Massimo Mari 27 April 2012 EGU General Assembly 2012 IstitutoNazionale di Geofisica e Vulcanologia, Rome, Italy Department of Earth Sciences, University of Siena Distributed under Creative Commons Attribution 3.0

  2. Our goal To observe changes in the water vapour concentration profile in the stratosphere and mesosphere in the polar regions Long-term (decadal trends) Short-term (diurnal cycle) With a new ground-based microwave spectrometer to measure the 22.235 GHz transition of water vapour as part of the NDACC network

  3. Main ideas • Balancing technique (signal-reference) • Front-end: • Parabolic reflector • Uncooled heterodyne receiver • chopper with dielectric sheet • Wobbler for λ/4 shift • Back-end: • Acqiris FFT spectrometer • Calibration: • Noise diodes for on-line calibration • LN2 for absolute calibration

  4. Functional scheme

  5. VESPA-22 (water Vapor Emission Spectrometer for Polar Atmospheres at 22 GHz) Parabolic mirror Receiver Quarter-wavelength shift Chopper mirror

  6. Parabolic antenna • Feedhorn from University of Navarra • Same design as IMK Karlsruhe • Length: 20 cm • HPBW: 12.5° • Parabolic reflector from Thomas Keating Ltd. • Our design • Long axis: 60 cm • Total HPBW: 3.5°

  7. Radiation pattern Half-PowerBeamWidth (HPBW) = 3.5° Sidelobes < -40 dB below main lobe Cross-polarization < -24 dB below main polarization

  8. Diagrammi di radiazione e fase fra 21.2 e 23.2 GHz Campo lontano (4 m) Antenna + specchio parabolico Campo vicino (0.43 m) Antenna

  9. Chopper mirror With small absorber bar With dielectric (DELRIN) sheet

  10. Chopper performance

  11. Wobbler performance

  12. Setup di test Amplifier 2IF Mixer To Back-end Antenna LO 2IF IF filters LO 1IF LNA Sidebandfilter Amplifier 1IF Mixer

  13. Noise diode calibration Hot body Cold body (LN2) Calibration sources

  14. Noise diode calibration Trec = 312 K

  15. No baseline subtracted

  16. 2 sine waves subtracted

  17. Future work (now the fun starts…) Conclusions • Long-term monitoring of polar stratospheric water vapour is needed • We designed and built a new 22-GHz spectrometer for polar observations • We measured the first atmospheric spectra (“first light”) • Improve baseline flatness: • λ/4 wobbler instead of fixed shift • Delrin compensating sheet • Front-end optimization • Improve sensitivity and Trec • Test single-sideband mixer • Test with longer integration times from an high-altitude observatory (Gran Sasso) • Set up inversion algorithm