Software-Dened-Radio Based HF Doppler Receiving System

High-frequency Doppler (HFD) sounding is a major remote sensing technique for monitoring the ionosphere. Conventional systems for HFDs mainly utilize analog circuits. However, existing analog systems have become di�cult to maintain as the number of individuals adept at working with analog circuits has declined. To solve this problem, we developed an alternate HFD receiver system based on digital signal processing. The software-dened radio (SDR) technique enables the receiver to be set up without the knowledge of analog circuit devices. This approach also downsizes the system and reduces costs. A highly stabilized radio system for both the transmitter and receiver is necessary for stable long-term observations of various phenomena in the ionosphere. The global positioning system disciplined oscillator with an accuracy of H compensates for the frequency stability required by the new receiving system. In the new system, four frequencies are received and signal-processed simultaneously. The dynamic range of the new system is wider (> 130 dB) than that of the conventional system. The signal-to-noise ratio signi�cantly improved by 20 dB. The new digital system enables radio waves to be received with much smaller amplitudes at four different frequencies. New digital receivers have been installed at some of the stations in the HFD observation network in Japan and have already captured various ionospheric phenomena, including medium-scale traveling ionospheric disturbances and sudden commencement induced electric �eld �uctuations, which indicates the feasibility of SDR for actual ionospheric observations. The new digital receiver is simple, inexpensive, and small in size, which makes it easy to deploy new receiving stations in Japan and elsewhere. These advantages of the new system will help drive the construction of a wide HFD observation network.


Introduction
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 re ected by ionized plasmas at ionospheric altitudes.When the re ection 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 re ection point, , or the vertical speed of the re ection 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 eld 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 uctuations 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 eld 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 uctuations 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. (2016Kikuchi et al. ( , 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 eld transmitted from the high-latitude ionosphere.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 re ection 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 ( Our current HFD observation system has been operating since 2001, without any signi cant hardware changes.Recently, however, continuous operation and maintenance of the system have become di cult 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) exible selection of observation frequency and/or lter 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 su cient 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 clari ed.
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 con rm that the new digital system has su cient performance for HFD observations of ionospheric phenomena and is quite useful for actual observations.

Observation Systems
In this section, we introduce both a conventional receiving system using analog circuits and the newly developed digital system.As described above, both systems receive four different radio waves with carrier frequencies of 5006, 6055, 8006, and 9595 kHz.operational throughout the day.The radio wave at 9595 kHz was terminated on 30 September 2018.In the new digital receiving system, public broadcaster JOZ4 transmitted at a frequency of 3925 kHz from Nemuro, Hokkaido (43.297N, 145.563E) with a power of 10 kW was selected as a new target radio wave.
The list of receiving stations utilizing both analog and digital systems in the present system is given in Table 2. Figure 1 shows the locations of transmitters and receivers.The transmitters and analog/digital receivers are denoted in the gure by red triangles and green/red circles, respectively.The blue crosses denote the mid-points between the transmitter and receivers, at which the radio waves are assumed to be re ected.

Conventional analog receiving system
Radio waves arriving at remote sites were received by a 1 m diameter loop antenna (Wellbrook Communications ALA-1530) directed to the JG2XA transmitting station.A pre-ampli er was installed at the base of the antenna to amplify the received signal.The analog receiving system consists of single superheterodyne receivers for the corresponding frequencies of 5006, 6055, 8006, and 9595 kHz, as shown in Fig. 2. The received radio signal is down-converted to a baseband signal, and its intensity is recorded.The dynamic spectrum of the signal is obtained by applying the fast Fourier transform (FFT) to the time series of the raw signal intensity.The frequency with the largest signal intensity at a certain time was derived as the Doppler frequency.Because the received raw signal intensity in the analog receiver is not complex but is a real signal, the Doppler spectrum shows a symmetrical spectral distribution centered at 0 Hz when the FFT is applied.This makes it impossible to distinguish between the positive and negative Doppler frequencies.To distinguish between positive and negative Doppler frequencies, the frequency is deliberately shifted by 8 Hz when down-converting the received radio waves.This means that the analog receiving system can observe the Doppler frequencies from − 8 Hz to 8 Hz.The speci c distribution of spectral intensity will be discussed later.

New digital observation system
We have developed a new digital receiving system equipped with a commercially available SDR device (Ettus Research USRP N210).An LFRX daughterboard is incorporated in model N210 to receive radio waves in the HF band together with the GPS-disciplined oscillator (GPSDO) for a 10 MHz reference signal.
In the N200 series of USRP, signal processing that included modulation and demodulation was performed on the PC side.The data segments were transferred to the host PC through a local area network.
The USRP operation can be performed using the GNU Radio free software.Using the software libraries, we created a radio by connecting signal processing blocks.A user can easily build a radio by connecting the inputs and outputs of multiple blocks.Such a connection can be made using the GNU Radio Companion GUI software, which allows the system to be graphically con gured inside the USRP. Figure 3 provides a ow graph used for the HFD observations.The observation procedure of the new receiver is basically the same as that of the analog receivers.In the current system, the received signal ows into four block groups for each frequency (3925, 5006, 6055, and 8006 kHz) from the source (the signal from the antenna).The USRP receives signals from the antenna with a center frequency of 6 MHz at a sampling frequency of 5 MHz (USRP Source).Therefore, the system can capture signals with a range of ± 2.5 MHz (i.e.The procedure from the frequency multiplying FIR lter is executed on the PC side.The signal processing ow is illustrated schematically in Fig. 4. Because the USRP captures signals as in-phase and quadraturephase (IQ) values, positive and negative Doppler frequencies can be distinguished using FFT, which is di cult to achieve in the analog system.The signal intensities of the carrier frequencies can thus be captured directly by the digital receiver.The properties of both systems are listed in Table 3.

Table 3
The comparison of the speci cations of the analog system with those of the digital system

Cross-evaluation of analog and digital receivers
Prior to the actual observation, we evaluated the performance of the new digital receiver.A schematic illustration of the evaluation system is shown in Fig. 5.A GPS-disciplined monochromatic signal at a frequency of 8006 kHz was used as a signal input of the antenna coupler so that the same signals were supplied to both the analog and digital receivers simultaneously.The output levels of both receivers are shown in Fig. 6(a).We examined the variations in the gains of both systems for the same input signal intensity because these output values are delivered by a built-in software whose internal processing coe cients are not available.The black line in Fig. 6(a) shows the output variation of an ideal linear response.In both systems, the output level varied linearly with respect to the input level, indicating the possibility to observe the power variations according to the reception power of the radio waves.In the analog system, however, the output level slightly deviated from the linear response when the input was weaker than − 100 dBm.Concerning the dynamic range, the output level of the analog receiver was saturated at an input level of − 40 dBm, while the digital receiver responded linearly to 0 dBm.The ndings indicated that the dynamic range of the digital receiver is larger than 130 dB.This in turn indicated that the digital receiver is capable of handling observations with a wide dynamic range.
Next, we examined the ltering characteristics of both receivers for the baseband signal.In this examination, the test signal at a frequency of 8006 kHz was input to both receivers by shifting the frequency of the test signal from − 40 to + 40 Hz.The results are presented in Fig. 6(b).In the usual observation, the Doppler frequency is derived within the range of ± 4 Hz (a light-blue shaded area), as shown in the Quick-Look (QL) of the project website.The second-order high-pass and low-pass lters incorporated in the analog circuit have broad lter characteristics.However, the digital system has steep shoulder characteristics because the signals are processed numerically.This leads to a reduction in the unwanted contribution from noise and interference of the digital system.We discuss this point later in this paper.The analog receiver is shown in the red line of Fig. 6(b) and has a dip at − 8 Hz because the DC component of the baseband signal is cut due to AC coupling between individual modules in the analog system.
Figure 7 shows the spectra of both the digital and analog receivers.To obtain these data, a test signal at a frequency of 5006 kHz was input to both receivers by changing the frequency of the test signal from − 40 to + 40 Hz.The rectangles located in the center of Figs.7(a) and 7(b) indicate the frequency range of the QL (from − 4 Hz to + 4 Hz).The output signal at a frequency of − 8 Hz in Fig. 7(a) is a leakage of the local oscillator signal from the analog receiver.In a digital system, the frequency of the output signal varies corresponding to the frequency of the input signal.The output outside ± 10 Hz is suppressed due to the low-pass lter with steep shoulder characteristics, as described previously.On the other hand, in the analog system, a mirror-image output symmetrical to − 10 Hz appeared to be caused by the FFT signal processing.Noise signals with uctuating frequencies also appeared in the output frequencies below − 30 Hz and above 10 Hz.The noise was also suppressed in the digital system owing to the better characteristics of the low-pass lter.

Side-by-side comparison between analog and digital receivers
As described in the previous section, the digital receiver has su cient performance for actual Doppler observations.Accordingly, we commenced actual test Doppler observations using both analog and digital receivers located at the Minami-Awaji Shichi Campus of Kibi International University (AWJ).The location of the AWJ station is shown in Fig. 1(a).The ground distances from the AWJ station to the transmitters are 463 km for JG2XA and 528 km for JOZ.Thus, only the sky waves, which are re ected in the ionosphere, are received, and the ground wave can be neglected in the observation at AWJ.As shown in Fig. 8, both receivers are installed onsite.We recognize that the size of the digital receiver becomes quite small compared to the analog receiver.The signals received by the loop antenna are fed to both systems simultaneously through the antenna coupler.The data obtained from the two systems were processed through a common procedure involving the sampling rate, FFT, and other aspects, and thus could be directly compared.
Figure 9 depicts the dynamic spectrum from both systems at a frequency of 5006 kHz on 18 January 2021.On this day, the radio wave was received with relatively strong signal intensity during the daytime (8-21 JST).The Doppler frequency was determined for both systems.However, the analog system suffered considerable noise throughout the day and showed apparent mirror-image contamination, as shown in Fig. 9(b).Because the digital system provides sharp ltering to the baseband signal, as shown in the section of Cross-evaluation of analog and digital receivers, the digital system eliminates such interference near the receiving frequency.Figures 9(c) and (d) display the received signal intensity versus the Doppler frequency at 12 JST.These gures correspond to the cross-sections at 12 JST in Figs.9(a) and (b), respectively.As seen in Figs.9(a) and 9(b), the background noise was signi cantly attenuated in the digital receiver.Figure 9(c) also shows that the noise in the digital receiver is strongly suppressed compared to the analog receiver.The superposition of Figs.9(c) and 9(d) is shown in Fig. 10.The signalto-noise ratio of the analog receiver was approximately 10 to 20 dB, while that of the digital receiver reached 40 dB.These ndings indicate the improvement of the signal-to-noise ratio in the digital system by more than 20 dB compared to the analog in actual observations.

Case examples
The results presented in the previous subsection support the conclusion that the digital receivers are su cient for actual Doppler observations.Therefore, we replaced several analog receivers in our observation system with digital receivers.Here, we show two examples of actual HFD observations to demonstrate the ability of digital receivers to measure ionospheric perturbations.

Observations of MSTID during daytime in winter
At mid-latitudes, medium-scale traveling ionospheric disturbances (MSTIDs) are usually observed during daytime in winter and during nighttime in summer (Hunsucker 1982).Here, we introduce an MSTID event during daytime in winter, which was obtained on 27 November 2020. Figure 11 shows the 5006 kHz Doppler frequency data from four stations in the vicinity of the transmitter (Sugadaira, Sugito, Chiba, and Oarai; see Fig. 1(b) for the detailed location of these stations).Because the re ection points between these stations and the 5006 kHz transmitter (JG2XA) at UEC are close to each other, the Doppler frequency data show very similar wavy features throughout the interval.The features correspond to the variations in altitude of the re ection points induced by the passage of MSTIDs across the sensing area.The receiving systems at Oarai and Chiba are newly developed digital receivers, and those at Sugito and Sugadaira are conventional analog receivers.A direct comparison between the data from Chiba and Sugito, whose re ection points were very close (approximately 30 km separation), demonstrated that the Doppler frequency variations from the new system were very similar to those from Sugito.The ndings con rmed the feasibility of using the USRP-based system for the digital receiver.
The observations shown in Fig. 11 were obtained during the daytime.Thus, the signatures of MSTIDs are manifestations of AGWs in winter (Shibata and Okuzawa 1983;Shibata 1986).Recently, Chum et al. (2021) employed multi-point HFD observations in the Czech Republic to derive the propagation of AGWs at thermospheric/ionospheric altitudes.The HFD observation network in Japan, which partially includes newly developed digital receivers, can also be used to estimate the propagation characteristics, possibly in three dimensions, because most of the Rx stations record signals at four different frequencies re ected at different altitudes.While the details are not provided in this paper, the newly developed digital receivers were also able to detect the signatures of MSTIDs during the night in the summer months.These signals were produced by the plasma instability in the electric coupling between the E and F regions of the ionosphere.The multi-point HFD observations, which will be enabled by the development of the digital receiver, will allow us to investigate the propagation characteristics of such MSTIDs in both winter and summer.
Psc event observed simultaneously with magnetometer data Changes in the magnetic eld and dynamic pressure of the solar wind cause magnetospheric disturbances such as geomagnetic storms, substorms, magnetic SCs, and geomagnetic pulsations (Pc).The electric elds generated by the magnetospheric disturbances propagate to the polar ionosphere via eld-aligned currents and propagate almost instantaneously to the global ionosphere in the earthionosphere waveguide (Kikuchi and Araki 1979;Kikuchi et al. 1996).The propagated electric elds cause the ionospheric plasma to move perpendicular to the magnetic eld lines owing to the ExB drift.As a result, the altitude of the re ection point of the HF radio wave changes.Thus, the electric eld strength can be estimated from the Doppler frequency obtained with the HFD sounding system, as has been done for the SC (Davies et al. 1962;Kikuchi et al. 1985Kikuchi et al. , 2016Kikuchi et al. , 2021;;Sastri et al. 1993) Figure 12(a) shows the Pc5 magnetic oscillations observed at the Kakioka magnetic observatory during an SC that began at 1058UT on 16 September 2020.This Pc5 event featured damped oscillations with a period of approximately 9 min, which can be classi ed as Psc5 (Kato and Saito 1958).During Psc5, periodic oscillations in the HFD frequency were observed by both the analog and digital receivers in AWJ (Fig. 12(b)), indicating that the motion of the ionosphere is caused by the electric eld of the Psc5.
Because the oscillations in the Doppler frequency were out of phase with those in the geomagnetic pulsations, the Psc5 observed at Kakioka may not have been caused by the ionospheric currents driven by the ionospheric electric eld, but rather by magnetospheric currents, such as magnetopause currents and eld-aligned currents (Kikuchi and Hashimoto 2016).The electric eld observed by the HFD sounding at low latitudes is propagated from the polar ionosphere, while the geomagnetic uctuations are caused by direct propagation from the magnetospheric currents (Kikuchi et al. 2016).
The Doppler shifts of the new digital receiver agree well with those of the analog receiver in terms of time evolution and amplitude.Because the digital receiver is simple and inexpensive, the new receiving system makes it easy to deploy receiving points overseas and maintain continuous operation.Electric elds that originate from the magnetosphere can be observed simultaneously on a global scale.The global deployment of the new system would enable the observation of the electric elds originating from the magnetosphere in the global ionosphere.The new system would contribute to the research of the midand low-latitude ionosphere, in particular, coupling from the polar ionosphere and magnetosphere.

Discussion
In this paper, we introduce a new digital receiving system for HFD observations in Japan.We also provide several case examples of actual observations obtained with the new digital receiver in comparison with those from the conventional analog receiver.There are several advantages of the new digital system: 1.The system has a wide dynamic range compared with the analog system, which enables the detection of weak signatures of ionospheric re ection, often seen during nighttime.
2. The Doppler spectrum from the digital system is free from image contamination in the FFT process using IQ coherent detection.
3. The shoulder characteristics of the frequency response are improved by the use of digital lters, which reduces interference.
4. There is no need to assemble an analog circuit to produce the receiving system.The digital system can be prepared without any knowledge of analog devices, which makes it much easier to reproduce the system and install the receivers into multiple stations.It is easy to construct a receiving system by connecting only the blocks of each speci c function on the GUI ow graph. 5.The digital system is much smaller and less expensive than the conventional analog receiver, which contributes to expanding the network observations in the framework of the distributed array of small instruments. .Even if other radio waves are received, or the transmission system is changed in the future, it can be easily handled by simply changing the process in the ow graph.
The frequencies of the JG2XA transmitters and receivers are referenced to 10 MHz signals from the Rb and GPSDO frequency standards, respectively, whose frequency accuracies are on the order of .
Therefore, it is expected that the accuracy of the Doppler frequency determined by the digital receiving system is less than 1 mHz.To check this, we examined the Doppler frequency obtained at several digital receiving points, as shown in Fig. 13.To obtain this data, the FFT was performed using a time series of 1000 s duration.Thus, the Doppler frequency was derived with a frequency resolution of 1 mHz.These Doppler frequency values were sampled at approximately 03 UT (12 JST).At this time, the propagation of the radio waves should be stable and the Doppler frequency should be minimal.In Fig. 13, the Doppler frequency for 5006 kHz and 8006 kHz is indicated in red and green, respectively.Because the distance between AWJ and JG2XA is longer than the distance between AWJ and CHB/FJS, the Doppler frequency at AWJ uctuates more than at CHB and FJS.Only sky waves are received in the path between JG2XA and AWJ, while ground waves are received together with sky waves in CHB and FJS.The frequency biases for 5006 kHz and 8006 kHz were approximately 3 mHz and 30 mHz, respectively, regardless of the location of the Rx stations (Fig. 13(a)).Most of the receiving systems were tested at the laboratory within a distance of 100 m from the JG2XA transmitting antenna in the Chofu Campus (CHF) before deployment to the remote site.Figure 13(b) shows the ne Doppler shifts obtained at CHF.The format was the same as that in Fig. 13(a).The Doppler shift values again demonstrated the existence of identical biases.Furthermore, we checked whether this Doppler shift was due to the difference in frequency accuracy between the transmitter and receiver.To do so, signals at frequencies of 5006 kHz and 8006 kHz from the function generator, referenced to the GPSDO incorporated in the USRP receiver, were input to the digital receivers.Although the Doppler frequency should be zero in this experiment, the frequency bias, as shown in Fig. 13, still appeared.Therefore, the biases in the Doppler frequency seem to be attributed to the frequency references used in the transmitters/receivers and also to the data processing in the A/D sampling, frequency conversion, and decimation processes incorporated within the     Schematic of the evaluation system of the new digital receiver (USRP).In this system, the 10-MHz reference signal output from the GPS-Disciplined oscillator (GPSDO) was supplied to the signal generator.
Using a coupler, the signal from the signal generator was supplied to both analog and digital receivers simultaneously.The outputs from both receivers were processed by separate PCs.
Hashimoto et al. (2020) also examined the prompt penetration of electric elds 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 eld in evening and a westward electric eld 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

, 3 . 5
MHz to 8.5 MHz).Each frequency multiplying nite impulse response (FIR) lter shifts the signal to the desired frequency.The low-pass lters pass the signals within the ± 10 Hz band centered at the carrier frequency and decimate signals to 100 Hz sampling.The signals are stored in the le-sink.

Figure 2 Block
Figure 2

Figure 3 Flow
Figure 3

Figure 5
Figure 5 (a) Output responses of the analog and digital receivers to input signal level (blue: digital, red: analog).The left-hand and right-hand vertical axes show the digital and analog output level, respectively.The black line shows the ideal linear response.(b) Transmission characteristics of the lters of analog and digital receivers using the 8006 kHz channel.The blue shaded region shows the bandwidth of the frequency range of the provided Doppler frequency data in the web page of the project (from -4 Hz to 4 Hz).

Figure 7 Dynamic
Figure 7

Figure 10 The
Figure 10

Figure 11 Observations
Figure 11 (Liu et al. 2016.2010).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).

Table 1
shows a list of transmitters used in the HFD sounding in Japan.The monochromatic signals at frequencies of 5006 kHz and 8006 kHz (call sign: JG2XA) with a power of 200 W are regulated at the transmitter side by a Rubidium (Rb) oscillator and transmitted from the Chofu campus of UEC (35.657N, 139.543E) (Tomizawa et al. 2003).The radio waves at frequencies of 6055 kHz and 9595 kHz are transmitted from the Nagara transmitter of commercial broadcasting Radio NIKKEI (35.465N, 140.359E) with the call sign JOZ, which is not always

Table 1
All the data obtained at the remote sites are transferred to the central data archive server at the UEC through the Internet.The Doppler frequencies are then calculated at the server-side every 10 s by applying the FFT to 4096 samples in approximately 40 s time window.Quick look (QL) plots of the derived Doppler frequencies and dynamic spectra of the raw data (termed the f-t diagram) are provided in near real-time at the website of the project (http://gwave.cei.uec.ac.jp/~hfd/plt.html) Local frequencies, shifted by 8 Hz from the observation frequencies, were generated by direct digital synthesizers stabilized by an Rb frequency reference at a frequency of 10 MHz.The baseband signals go through second-order active low-pass lters (cutoff: 17 Hz) and high-pass lters (cutoff: 0.5 Hz) before the analog/digital (A/D) conversion.After ltering, the baseband signals were sampled by an A/D converter with 16-bit integer values at a sampling rate of 100 Hz.The time accuracy of the data is determined by a PC whose time is synchronized using the network time protocol.