[A-AS07] Atmospheric Chemistry

[AAS07-09] Development of frequency-modulated millimeter-wave receiver for improving stratospheric ozone observation efficiency

*Takumi Hikosaka¹, Tac Nakajima¹, Akio Taniguchi², Masato Hagimoto², Tomoo Nagahama¹, Kanta Tanigawa¹, Kazuji Suzuki¹, Kenichi Harada³, Masato Fujii³, Koichiro Shimoyama³, Kosuke Takezawa³ (1.Institute for Space-Earth Environmental Research, 2.Department of Physics, Graduate School of Science, Nagoya University, 3.Elecs)

Keywords:atmospheric ozone observation, millimeter spectroscopy

In radio astronomy observations in the millimeter- and submillimeter-wave bands, a new observation method with a frequency-modulating local oscillator (FMLO) has been proposed (Taniguchi et al. 2020). This method can significantly improve the observation efficiencies by modulating the frequency (FM) of the local oscillator (LO) of a heterodyne receiver. This method samples and stores the time-series spectrometer output while modulating the LO frequency at a high frequency (10 Hz). Then the separates discrete molecular emission lines and quasi-continuum wave components as correlated noise on the time series data can be separated using a statistical analysis method called principal component analysis. This method does not need to obtain any reference direction (off-point) spectra, unlike conventional switching observations, in which the observation direction (on-point) and off-point are sequentially switched. However, there is a problem that not only interstellar molecular emission lines but also emissions from the atmospheric molecules are detected at the same time. We have taken this in reverse and devised an application of FMLO for observation of atmospheric molecules. By using FMLO in millimeter-wave radiometers, it is expected to monitor stratospheric ozone concentration distributions with more than twice the time resolution of existing instruments because it eliminates the need for off-point observations. In that case, it is difficult to correctly obtain the spectral shape of atmospheric ozone with the modulation parameters of FMLO used for astronomical observations because their line widths are broader (a baseline width of ∼1 GHz) than those from typical interstellar medium. Therefore, it is important to consider the optimal modulation parameters for atmospheric observations.
We first carried out FMLO observation simulations for an atmospheric model spectrum of ozone at 110.85 GHz to investigate optimal modulation parameters that would recover the original spectrum. As a result, we found that the ozone line spectrum can be fully reproduced by setting the maximum modulation frequency to be comparable to or wider than its baseline width (~1 GHz). To achieve such wide frequency modulation, we developed a new receiver system using a digital spectrometer (Direct RF Sampler with DSP 4th generation: DRS4, Elecs Industry) with a frequency bandwidth of 0.1-4 GHz and a frequency resolution of 75 kHz. The spectrometer can also output a 10-Hz trigger signal synchronized with the start time of data acquisition. This enables to synchronize it with the frequency modulation of a signal generator, the source of the LO, without any other control devices. For precise time synchronization, the spectrometer can manually set a slight (±20 ms) time offset on the trigger signal. In our laboratory measurement system, we achieved the time synchronization of less than 0.1 ms by setting an optimal time offset of 4.4 ms, which was derived by an FMLO measurement of a test signal. We finally demonstrated precise frequency modulation and demodulation in our system by an FMLO observation of a test signal that simulates a molecular emission line. Now, preparations are underway for the FMLO test observations of the ozone emission line at 110.85 GHz on the Nagoya University campus.