[P-EM12] Coupling Processes in the Atmosphere-Ionosphere System

[PEM12-P26] Development of a Compact HF Doppler Receiver Using a Software-Defined Radio Receiver

*Koichi Noguchi¹, Hiroyuki Nakata¹, Keisuke Hosokawa², Hiroyo Ohya¹ (1.Chiba University, 2.The University of Electro-Communications)

Keywords:Ionosphere, Software-defned radio

In the altitude range from 60 to 1000 km above the Earth’s surface, a region known as the ionosphere exists due to ionization caused by the absorption of solar ultraviolet and X-rays. The ionosphere interacts with the magnetosphere and neutral atmosphere, resulting in both periodic and sudden variations. One of the methods used to observe these ionospheric variations is HF Doppler sounding, which estimates the vertical velocity of the ionosphere at the reflection point by transmitting HF-band radio waves to the ionosphere and measuring the frequency shifts caused by the Doppler effect. Currently, HF Doppler observations are conducted by four institutions, led by the University of Electro-Communications, with a total of 11 observation sites nationwide. In HF Doppler observation, the replacement of analogue receivers[1] with digital receiver was proceeded because of some problems such as aging receivers, using the USRP N210, which is a software-defined radio (SDR) device developed by Ettus Research. This has enabled us to develop a high-performance digital receiver that surpasses traditional analog receivers in performance while significantly reducing the device’s size. However, the price of the USRP has been rising due to the weak yen and inflation. With the rapid advancement of SDR technology, more compact and affordable SDR receivers are now available. Therefore, this study focuses on developing a receiving system capable of performing measurements equivalent to those of the USRP-based system by using the RSPdx receiver from SDRplay, known for its affordability and compact size. As of January 2025, the price of the RSPdx is about 1/15 of the USRP’s cost, Additionally, the size has been reduced to about 1/5 of the original system. As a result, we successfully developed a receiver with performance comparable to the current observation system. Additionally, we implemented an automated system capable of continuous 24-hour operation to acquire data. However, the Doppler frequency and received signal intensity were offset by about 0.5 Hz and 40 dB, respectively, but the time variation of the data was consistent, and by calibrating the difference, the data can be treated as the same as the USRP data. The remaining challenges include occasional data loss during data acquisition. Furthermore, unlike the USRP-based system capable of simultaneous HF Doppler observations at four frequencies, the current setup only supports dual-frequency measurements due to processing limitations. Further improvements are needed to enhance system performance and expand multi-frequency observation capabilities.

References
[1] H.Nakata et al. ,“Software-defined radio-based HF doppler receiving system ”, Earth Planets and Space, 73, 209, 2021.