Results and discussion

A double side band heterodyne receiver front-end designed at Onsala Space Space Observatory, optimized for retrieval of mesospheric O3 and CO, is used to test the GNU-Radio and Ettus USRP Dicke-switched back-end. An overview of the full receiver chain is provided by the flow graph in Fig. 3. Details of the receiver front-end and it’s performance is outlined in [8]. The back-end consist of the Ettus USRP X310 with the SBX daughterboard and a host computer providing the GNU Radio interface. The system is used in frequencyswitched mode where the external LO 1 in Fig. 3 is rapidly switched, approximately 1 Hz, between two frequencies corresponding to the signal and reference states. A sync bus transmits signals to the GPIO of the Ettus USRP X310 indicating the state of the received RF, signal or reference, as well as the transition state of the RF. Fig. 4 shows uncalibrated spectral data of a simultaneous measurement of frequency switched O3 at 110.84 GHz and CO at 115.27 GHz averaged over approximately one week. The bandwidth is 60 MHz with a resolution of about 15 kHz and the frequency throw between signal and reference is 8 MHz. The O3 line corresponds to the lower side band of the IF-output and the narrow CO line corresponds to upper side band. The vertical distribution of the observed mesospheric CO and O3 can be retrieved by applying the Optimal Estima- tion Method (OEM) described in [7]. A Hot/Cold calibration procedure is outlined and described in [8].

Figure 3. Flow graph describing the receiving chain i.e. a front-end optimized for CO and O3 retrieval [8]. The back-end consists of the Ettus USRP with host software interface GNU Radio providing a general purpose Dicke-switched FFT spectrometer.

Figure 4. Uncalibrated frequency-switched measurement of CO at 115.27 GHz and O3 at 110.84 GHz using Ettus USRP and GNU Radio as back-end.

The results show that a high performing Dicke-switched back-end can be achieved using the Ettus USRP and a simple GNU Radio application based on free and open source software. The back-end can easily be integrated in existing front-ends as shown in the results where a state of the art COO3 front-end was combined with the back-end. Thus providing both the radio-aeronomy and astronomy society a back-end that is easily integrated in new or existing systems with minimal efforts and investments. With simple, cost effective and high performing solutions, like the one proposed, a greater distribution of microwave radiometer systems performing quality Dicke-switched measurements can be expected. This opens for e.g atmospheric monitoring on a whole new scale. Due to the simple and elegant nature of SDR platforms the back-end solution would also serve as a great educational resource and thus be a part in spreading the knowledge and importance of atmospheric and interstellar measurements.