To examine the dynamics of the ionosphere, observations of both the steady and disturbed states of the ionosphere are important. Various observation systems have been utilized to observe the structure of the ionosphere and many types of disturbances occurring in the ionosphere. One useful ground-based observation system is high-frequency Doppler (HFD) sounding. HF band radio waves transmitted from the ground are reflected by ionized plasmas at ionospheric altitudes. When the reflection point of the radio wave in the ionosphere moves vertically, the length of the phase path varies and the frequency of the radio wave shifts owing to the Doppler effect. The frequency variation of the radio wave at the receiving point, , is given by:
where f is the frequency of the transmitted radio wave, l is the distance between the transmitting and receiving points along the ray path, and is the group velocity of the radio wave (Ogawa 1958). Using Equation (1), the relationship between the frequency variation (Doppler frequency) and the vertical variation of the reflection point,, or the vertical speed of the reflection point, , is given by
where θ is the incident angle of the radio wave to the ionosphere. From this Doppler frequency, we can estimate the vertical motion of ionospheric plasma. Because the phase path of the radio wave is also affected by the variation of the group velocity on the propagation path, the temporal variation of the electron density also introduces the frequency variation of the radio waves through the variation of the refractive index. Assuming that the ionospheric plasmas are dragged by the neutral atmospheric particles due to collisions, we can estimate the speed of the atmospheric wave using HFD observations.
Since the basics of HFD sounding were invented by Ogawa (1958), the initial HFD observation in Japan was carried out by researchers at Doshisha University. Ichinose and Ogawa (1974) examined the vertical motion of the ionosphere using HFD observations and showed that the derivative of the horizontal component of the geomagnetic field varied in phase with the Doppler frequency during a sudden commencement (SC). Concerning the internal gravity waves caused by a solar eclipse, Ichinose and Ogawa (1976) showed that fluctuations with a period of 22 min appeared on the day of a solar eclipse. The University of Electro-Communications (UEC) and the Radio Research Laboratory (RRL, now the National Institute of Information and Communications Technology) have also promoted HFD sounding in Japan. At UEC, the ionospheric disturbances in the lower atmospheres have been extensively examined, such as acoustic gravity waves (AGWs) (Shibata and Okuzawa 1983; Shibata 1986), earthquakes (Okuzawa et al. 1983), and typhoons (Okuzawa 1986). At RRL, Kikuchi (1985) and Kikuchi et al. (1986) focused on the storm-time variations in the electric field in the ionosphere.
In these earlier observations, standard radio waves (JJY) at frequencies of 5000 and 8000 kHz were used. However, the transmission of JJY shifted from the HF band to the LF band at the end of March 2001, and the transmission of JJY at 5000 and 8000 kHz was terminated. To maintain the HFD observations in Japan, since 2001 UEC has started transmitting alternate HF radio waves at two frequencies (5006 kHz and 8006 kHz) (Tomizawa et al. 2003) and installed receivers in several places in Japan. Later, the HFD receiving systems at several observation points were upgraded to receive two additional radio waves for commercial broadcasting of Radio NIKKEI (6055 kHz and 9595 kHz) transmitted from the Nagara transmitter. By receiving radio waves at four different frequencies, we were able to observe ionospheric fluctuations at four different altitudes (the transmission of radio waves at 9595 kHz stopped on 01 October 2018).
Even in recent years, HFD data provided by this network have been used in many studies. Kikuchi et al. (2016, 2021) examined the response of the ionosphere to SC and showed that the vertical motion of the ionosphere is well explained by the potential electric field transmitted from the high-latitude ionosphere. Hashimoto et al. (2020) also examined the prompt penetration of electric fields during an intense geomagnetic storm on 22 June 2015. In combination with the HFD data obtained by the Institute of Atmospheric Physics (IAP) in the Czech Republic, Hashimoto et al. (2020) showed the penetration of an eastward electric field in evening and a westward electric field during night during the main phase of the storm. Taking advantage of the ability to receive radio waves at four different frequencies, Nakata et al. (2021) examined coseismic ionospheric disturbances observed at different altitudes in association with the foreshock of the Tohoku Earthquake and demonstrated that coseismic disturbance was caused by the vertical propagation of the acoustic mode wave.
HFD sounding systems have been developed worldwide. A group at the IAP has constructed a network of HFD sounding systems globally (Buresova et al. 2007). Over hundreds of square kilometers, transmitter/receiver networks have been constructed in the Czech Republic, South Africa, Taiwan, and France. The details and outcomes of this system were reviewed by Laštovička and Chum (2017). In the case of this HFD system, because the transmitters and the receiver are closely located, the vertical speed of the motion of the reflection point can be determined accurately. Farges et al. (2003) installed an HFD sounding system in the north of France and carried out continuous observations using three receivers at a distance of 50 to 80 km. The generation of atmospheric waves in association with a solar eclipse was examined using this system. This system has also been used to study the ionospheric signatures of Rayleigh waves in association with over-the-horizon radar (Occhipinti et al. 2010). Mainly to observe the ionospheric disturbances associated with earthquakes, Doppler observations have been conducted by several universities in Taiwan, covering the entire island of Taiwan (Liu et al. 2016). In India, a single frequency HFD radar was installed in 1982, and a multi-frequency Doppler radar (2.5 MHz, 3.5 MHz, and 4.5 MHz) was developed in 2003 (Simi et al. 2020). The data obtained by this system has proven useful for many types of research, including studies of plasma drift in low-latitude regions.
Several other radio instruments can observe frequency variations as well as HFD sounding. Reinish et al. (2009) improved the hardware and software of the digisonde to observe the angles of arrival (AoA), time of flight, and Doppler frequency of reflected radio signals. Reinish et al. (2018) constructed an oblique ionosonde network and specified the wave parameters of traveling ionospheric disturbances (TIDs) using the AoA and Doppler frequency obtained by the ionosonde observation of the bistatic digisonde network. The SuperDARN radars (Greenwald et al. 1995) are capable of obtaining many types of ionospheric data, including Doppler frequency (Chisham et al. 2007; Nishitani et al. 2019). Basically, the SuperDARN radar observes coherent scatter from field-aligned irregularities (FAI) in the ionosphere. The Doppler velocity of such ionospheric echoes corresponds to the electric field perpendicular to the line of sight of the beam and magnetic field. The ground scatter echoes reflect the vertical motion and incident angle variation of the bottom side of the ionosphere, which is equivalent to two-hop propagation. In the case of sea scatter, an additional surface wave spectrum may be supplied by the Doppler shift (Anderson 1990; Skolnik 2018).
As long as stable and accurate transmitters are prepared, the HFD sounders have the advantage of observing the motion of the ionosphere with high temporal resolution (e.g., 10 s) at a fixed point. The frequency variation of the HF band radio wave is very small (usually less than several Hz) in actual observations. To detect such small frequency variations in HFD soundings, it is necessary to transmit continuous high-frequency waves accurately and stably.
Recently, software-defined radio (SDR) has become widely used for radio observations in the ionosphere. Bostan et al. (2019) developed an inexpensive SDR-based ionospheric sounding system. The system, which was mostly constructed with commercial off-the-shelf products and open-source software, performed satisfactorily. Ivanov et al. (2015) demonstrated that the SDR HF band frequency modulated continuous wave (FMCW) oblique ionosonde receiver based on SDR has higher noise immunity and reliability than the analog receiver over a propagation distance of 3000 km. SDR-based receivers, such as universal software radio peripheral (USRP), have become more reliable. They are now being deployed as synthesizers and receiver systems in SuperDARN radars, as reported by Bristow (2019).
Our current HFD observation system has been operating since 2001, without any significant hardware changes. Recently, however, continuous operation and maintenance of the system have become difficult owing to the aging of transmission and reception systems. Because of the progress of SDR, a new receiving system for HFD soundings can be developed as a digital receiver. As will be detailed later, the advantages of the use of SDR are: 1) flexible selection of observation frequency and/or filter bandwidth; 2) commercial-off-the-shelf components and relatively lower cost of equipment development by using open-source software; 3) much smaller equipment size; and 4) development of a new system without sufficient knowledge of analog circuits. While the conventional analog receiver is controlled by the performance of the analog devices, the performance of the digital receiver depends on the data processing capability of the personal computer (PC) incorporated in the SDR. The decreasing cost and size of the processing PC are also the advantages of SDR, which enable the new receiving system to be transported as hand luggage.
Based on the foregoing, we developed a new digital receiving system for HFD observations. As will be described later, we succeeded in developing a new receiving system with a higher performance than conventional analog receivers. Guo et al. (2019) developed a similar observation system using USRP, in which dedicated software was installed to receive radio waves at multiple frequencies simultaneously. Unfortunately, the detailed characteristics of the receiver using USRP have not yet been clarified. Therefore, we examined the characteristics of the new digital receiver in detail.
In this paper, we describe the details of the newly developed digital receiving system in comparison with a conventional analog system. We also introduce simultaneous observations made using both analog and digital systems in Awaji, Japan, to evaluate the performance of the new system. The results of this simultaneous observation confirm that the new digital system has sufficient performance for HFD observations of ionospheric phenomena and is quite useful for actual observations.