A 22 GHz Pseudo-correlation Water Vapor Radiometer.

Abstract

The radiation that comes from celestial objects is affected by different atmospheric components, mainly by water vapor, which is a highly variable component. This molecule can scatter, absorb and re-emit radiation and, therefore, attenuate it, affecting astronomical observations. Water vapor radiometry is an accurate method to measure the water vapor content in the atmosphere and, at altitudes 4000 masl, the 22 GHz emission line is chosen to estimate the amount of water vapor present in the at the time of the astronomical observation. This instrument, which is under development at CePIA UdeC laboratory, will provide a notable improvement in the measurement of the line profile of water vapor and will allow atmospheric characterizations that can be validated with other instruments in places where this quantity is relevant. The architecture designed for this project corresponds to a self-calibrated instrument, heterodine and is based on the pseudo-correlation principle. It consists of three parts: frontend, where incoming the RF signal is at a frequency range of 20 - 26 GHz; analog backend, where the RF signal is converted to a manageable frequency range of 0 - 6 GHz, called Intermediate Frequency (IF); and finally, the digital backend where the IF signal is processed in real-time through FFT, with a high spectral resolution of 62.5 kHz; the spectrum is divided intro three bands of 2 GHz each, in different Nyquist zones. Each output of the spectrometer is directly proportional to the brightness temperature of the load or the input, that provides a stable output measurement over time.