Frequency-Agile FFT Spectrometer for Microwave Remote Sensing Applications

Abstract

We report on a Fast Fourier Transform Spectrometer (FFTS) that provides larger bandwidth by fast local oscillator switching of the base-band converter. We demonstrate that this frequency scanning technique is suited for atmospheric remote sensing and conduct measurements of atmospheric ozone using the WIRA-C (WInd RAdiometer for Campaigns) Doppler wind radiometer. The comparison of our measurements to an adjusted atmospheric and instrumental model exposes no systematic biases due to the switching procedure in the measured spectra. It further shows that the combination of high spectral resolution with large bandwidth yields good measurement response to stratospheric and mesospheric ozone from approximately a 20 km to 70 km altitude with a resolution of 7 km in the lower stratosphere to 20 km in the mesosphere. We conclude that low-cost, low-power software-defined radio hardware designed for communications applications is very well suited for a variety of spectroscopic applications, including ozone monitoring. This allows the design of low-cost, multi-purpose instruments for atmospheric remote sensing and thus has a direct impact on future radiometer developments and their adoption in remote sensing campaigns and networks.

Comparison of Three High Resolution Real-Time Spectrometers for Microwave Ozone Profiling Instruments

“…For this study, it was used with a customized FFT spectrometer firmware, processing a 200 MHz bandwidth around the ozone line frequency. As an option, the USRP bandwidth can be extended by frequency switching to observe a broader spectral region, and therefore to get profiling capabilities at lower altitudes [21]. For instance, this spectrometer is now operated in our two wind monitoring radiometers observing the ozone emission line at 142 GHz [22].…”

“…Following the formalism of [5], the Optimal Estimation Method is now integrated within the ARTS package itself, which enables the inversion of the integrated spectra directly within ARTS. This setup was used successfully in [21] for ozone retrievals at 142.175 GHz. Fig.…”

Development of a Polarimetric 50-GHz Spectrometer for Temperature Sounding in the Middle Atmosphere

“…The intermediate frequency bands are analyzed using a digital FFT spectrometer / correlator implemented on the "Universal Software Radio Peripheral" device (USRP X310) 1 . A similar USRP with two UBX-160 daughterboards was first used for atmospheric Ozone measurements in [14]. For TEMPERA-C we are using two TwinRX daughterboards.…”

Comparison of three high resolution real-time spectrometers for microwave ozone profiling instruments

“…For this study, it was used with a customized FFT spectrometer firmware, processing a 200 MHz bandwidth around the ozone line frequency. As an option, the USRP bandwidth can be extended by frequency switching to observe a broader spectral region, and therefore to get profiling capabilities at lower altitudes [21]. For instance, this spectrometer is now operated in our two wind monitoring radiometers observing the ozone emission line at 142 GHz [22].…”

“…Following the formalism of [5], the optimal estimation method is now integrated within the ARTS package itself, which enables the inversion of the integrated spectra directly within ARTS. This setup was used successfully in [21] for ozone retrievals at 142.175 GHz.…”