Measurement Principle


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MIAWARA - Measurement Principle

The instrument measures the intensity of the pressure broadened emission line of water vapour at 22.235 GHz. Due to the exponential decrease of atmospheric pressure with altitude, the broadening of the measured line spectrum contains information from the H2O emission intensity at different altitudes. Therefore we are able to retrieve middle atmospheric water vapour profiles in the range of 25 - 80 km from the measured microwave spectrum using a retrieval algorithm based on Rodgers optimal estimation technique.

Calibration technique Combined tipping curve and balancing calibration
Operational mode Single sideband (SSB) 50 dB sideband suppression
Mirror Plane mirror (gauss beam optimised shape)
Antenna Corrugated horn (HPBW 6 deg)
Receiver noise-temperature 133 K SSB
Radio-frequency range 21.735 - 22.735 GHz
Narrowband spectral analysis FFT spectrometer "BEAM" (channels: 2048)
Broadband spectral analysis FFT spectrometer "Acqiris" (channels: 16385, f: 0-1 GHz)
Retrieval ARTS2/QPack2
Key parameter of the instrument.
Measured Spectrum at 22 GHz Calibration

We determine the water vapour spectrum by performing a balancing-calibration using the sky as reference signal (zenith) and cold calibration target (60 deg elevation). The use of the sky as calibration target and the correction of the tropospheric attenuation on the middle atmospheric emission line require knowledge of the tropospheric properties. We determine these properties with the same instrument (tipping curve calibration). This calibration technique is validated with periodic liquid nitrogen calibrations. The results of both calibration techniques agree within +/-0.6%. A detailed description of the calibration technique can be found in: Beat Deuber:  MIAWARA - Middle Atmospheric Water Vapour Radiometer: Calibration Technique, Research Report 2002-09, Institute of Applied Physics, University of Bern, (2002).
Measured balanced spectrum by the Acqiris FFTS in Mai 2011 (blue) with a fitted spectrum (red) used in the retrieval process. The spectrum was corrected for tropospheric attenuation.