Ground-based microwave spectroradiometer for thermal sounding of atmosphere

Abstract

Studying of thermal balance of terrestrial atmosphere is substantially connected with results of measurements of tropospheric and stratospheric temperature profile. Now in the Russian Federation the system of quasi continuous monitoring of thermodynamic characteristics of atmosphere is carried out thanks to discrete start-up of radiosondes, their top working border seldom reaches stratospheric heights. However, continuous aerologic sounding may be realized by means of the remote methods using microwave atmosphere thermal radiation. Particularly, presence of strong lines and band of molecular oxygen in the atmospheric millimeter wave radiation spectrum make it possible to use them for thermal sounding from ground up to stratopause. For measurement of troposphere temperature profile in Institute of Applied Physics Russian Academy of Sciences 8-channel spectroradiometer is developed. It operates in a frequency range of 50-55 GHz which corresponds to a low-frequency slope of a 5-millimeter wave-band of molecular oxygen spectrum. The device represents heterodyne receiver with low-noise HEMT front end amplifier (LNA). The amplifier has in a working wave-band a gain about of 16 dB and a noise figure no more than 4 dB. The following LNA Shottky diode mixer works on the first harmonic of a local oscillator signal (fLO = 48 GHz) and converts RF input signal to the 2 - 7 GHz intermediate frequency band (IF). Conversion loss of the mixer is 8-10 dB. Local oscillator signal (48 GHz) is formed by means of a signal synthesizer (8 GHz) having output power of 50 mW by multipliers and band-pass filters circuit. Power level of local oscillator signal 48 GHz on a mixer input is not less than 10 mW with suppression of harmful frequencies not less than 50 dB.