The sporadic-E (Es) layer is an ionospheric layer which appears occasionally near 100-km heights with extremely high electron density. The Es layer may reflect very high-frequency radio signals when the incident angle is shallow (Es layer anomalous propagation [EsAP]). It is known that radio signals with frequencies above 100 MHz sometimes reach distant locations, more than 600 km apart from the transmitters, due to EsAP. Since air-navigation radio channels are allocated on frequencies between 108 and 118 MHz, EsAP may interfere with directly propagating wave (ground wave) signals. However, as the occurrence and strength of EsAP on these frequencies have not been studied well, it is difficult to assess the impact of EsAP on the air navigation. In this paper, we report the statistics of the occurrence and strength of EsAP based on a 3-year continuous monitoring of very high-frequency air-navigation radio signal strength on the ground. The statistics show that strong EsAPs occur frequently in summer. The occurrence pattern of EsAP is generally consistent with ionosonde observations of Es layer: Most Es layers appear during summer, primary peak in daytime, with a second peak in the evening. During an extreme EsAP event, in a particular channel, an EsAP signal was superposed on a ground wave signal which resulted in a fluctuation of ground wave signal by more than ±10 dB. Our statistical results suggest that the Es layer has a potential impact on air-navigation applications.
- A system to continuously monitor VHF air navigation radio (VHF-NAV) signals was developed to study the sporadic-E (Es) radio propagation
- Statistics of anomalous propagation of VHF air navigation radio signals due to Es were studied with 3-year data
- Strength of anomalous propagation signals was shown to be strong enough to potentially cause interference to avionics receivers
The sporadic-E (Es) layer is a layer in the ionospheric E region with a dense electron density and thin altitudinal thickness at altitudes around 100 km (Haldoupis, 2011; Mathews, 1998; Whitehead, 1989). It has been known that the Es layer could cause anomalous propagation of very high-frequency (VHF) radio waves by reflecting radio waves which normally penetrate all the way through the ionosphere (Davis et al., 1959). Figure 1 illustrates anomalous long-distance propagation of VHF radio waves by the Es layer (here we abbreviate this type of propagation as EsAP). It has been pointed out that the Es layer could cause anomalous propagation in the frequency band of aeronautical navigation and communication. In that sense, information on the Es layer could be a part of space weather information.
Radio frequencies ranging from 108 to 118 MHz are assigned to aeronautical navigation systems such as VHF omni range (VOR) and instrumental landing system (ILS) localizer (LOC; ICAO, International Civil Aviation Organization, 2014). Furthermore, in addition to above conventional navigation systems, a new system called Ground-Based Augmentation System VHF ground-air Data Broadcast, which has started operations in the world, will also share the same frequency range. This frequency range is called navigation band (NAV band); in this paper we refer to these navigational systems as NAV systems and their transmitting stations as NAV stations.
A particular frequency (channel) within the NAV band is usually assigned to multiple NAV stations which are geographically separated such that VHF radio waves from one station do not propagate to the other stations using the same frequency under normal conditions. However, as illustrated in Figure 1, when Es layers appear, radio signal from a distant station may reach the service area of a local station and may interfere the desired signal from the local station. In NAV systems, such an interference may affect the operation of onboard NAV devices and might be a potential cause of errors in flight operation.
Although EsAP has long history of research, its influence on air navigation systems has not been studied well to date. One of the reasons is that there has not been statistical data which show the occurrence rate and signal strengths of anomalous propagation. In this study, we have monitored the entire VHF NAV band for 3 years at two locations on the ground in Japan. This paper shows the results of our observation and proposes further study of this subject. Although our focus is mainly on VOR channels in this paper because VOR waves are transmitted omnidirectionally, we also examine events occurred on ILS LOC channels.
2 Experimental Setup
2.1 VOR Stations
In this study we performed continuous monitoring of VHF NAV band signals at two separate locations, Chofu, Tokyo, and Kure, Hiroshima, as shown in Table 1a. VHF radio waves propagate straight in the air and can be received within the radio horizon in nominal conditions. Since the frequencies of ground navigation aids are high enough to penetrate all the way through the normal ionosphere, those radio waves are not received beyond the radio horizon. On the ground, direct propagation waves can be observed within a distance of 200 km from radio sources. However, when the Es layer appears with a density high enough to reflect those waves with greater incident angles, those waves can be received beyond a certain distance (anomalous propagation). It is known that anomalous propagation occurs in a distance range of 600–2,000 km from the radio source. Between 200 and 600 km, radio waves are observed by neither normal nor anomalous propagation, and the area is called skip zone (Davies, 1969).
|Location||Chofu, Tokyo||Kure, Hiroshima|
|(a) Geographic coordinates|
|Latitude||35.66° N||34.25° N|
|Longitude||139.54° E||132.53° E|
|Type||Log periodic||Log periodic|
|Product name||Create Design CLP-5130-1||Create Design CLP-5130-3|
|Frequency range||50–1,300 MHz||80–250 MHz|
|Gain||10–12 dB (10 dB at 110 MHz)||12–13 dB (12 dB at 110 MHz)|
|Product name||AOR ABF128||(Not used)|
|Frequency range||108–136 MHz|
|Insertion loss||4 dB|
|Product name||AOR LN4000||TV booster|
|Frequency range||100–3,000 MHz||70–250 MHz|
|Gain||20 dB||30 dB|
|Product name||Anritsu MS2683A||Anritsu MS2711E|
|(c) Data availability|
|From||12 May 2014||01 April 2014|
|To||30 September 2016||30 June 2016|
|Annual statistics||2014–2016||2014 and 2015|
For the direct propagation, the radio wave strength received by a receiver on the ground may be considerably different from that received by an airborne receiver which is located at higher altitudes. For EsAP, in contrast, the radio wave strength received on the ground and in the air (up to about 12 km) at the same horizontal location can be considered very similar, because the radio signal propagates over a large distance (600–2,000 km) and the propagation geometry for them would be nearly the same. In this study, based on the above feature of EsAP waves, we suppose that the ground-observed EsAP-wave strength represents the strength of EsAP waves observed on an aircraft at cruise altitudes.
Figure 2 shows the locations of VORs around Japan observable at Chofu and Kure by either direct propagation (within 200-km distance) or anomalous propagation (600- to 2,000-km distance). Information on VOR locations can be obtained from Aeronautical Information Service, or equivalent services, provided by following countries: Japan, Korea, Taiwan, China, and Philippines (e.g., https://aisjapan.mlit.go.jp).
About 200 channels are distributed within a 10-MHz NAV band between 108 and 118 MHz. To detect anomalous propagation events, we have recorded the signal intensity of these channels continuously and simultaneously. In order to achieve this continuous observation, we employed a spectrum analyzer, instead of tuned receivers, at each monitoring station. Figure 3a shows the radio receiver part of observation system used in Chofu receiving site, which includes an antenna, an air-band filter, a preamplifier, a spectrum analyzer, and control software. The data processing part, Figure 3b, which processes data from both Chofu and Kure, will be discussed in the next section. Table 1b summarizes the specifications of radio system components in Chofu and Kure.
In this study, a log-periodic antenna is used to receive VHF radio waves in each monitoring station. The antenna direction is adjusted based on geographical distribution of VOR stations. The polarization of ionosphere-reflected waves is usually different from that of transmitted waves due to Faraday rotation effect (Davies, 1990). For this reason, although NAV radio utilizes the horizontally polarized waves, adjustment of polarization in receiving antennas is not critical in this study. A band-pass filter with a passband from 108 to 136 MHz is inserted between the antenna and preamplifier to eliminate the interference (i.e., saturation of the next-stage amplifier) caused by strong radio signals, mainly from commercial FM broadcast stations, in the adjacent frequency band. The filter-output is fed to a preamplifier to amplify the received signal by 20 dB and passed to a spectrum analyzer. The spectrum analyzer is controlled by instrumentation software which retrieves spectrum information and writes it to a database. The database contains the signal strength information, which is sampled every 10 s, for each NAV channel. The radio system in Kure is similar to that in Chofu, but the air-band filter is omitted since there is no strong interference source around Kure, and the gain of the amplifier is 30 dB.
2.3 Data Set
The observation database contains signal strength records since 2012. In this study we analyzed a 3-year set of records obtained between 2014 and 2016, the period in which the receiver-gain information is readily available. Table 1c summarizes the periods of available data used in this study. In the database, a 1-day observation set for each location consists of 193 separate files corresponding to NAV channels. A channel, which corresponds to a specific frequency in the NAV band, is identified by a unique number, from 1 to 193, in this study. It should be noted that our channel-numbering scheme is independent of any other official schemes. For a particular day and a channel, the corresponding file consists of sequential records of observed signal strengths with 10-s interval, which constitute 8,640 records a day. The recorded signal strength is the observed signal level, in decibel-milliwatts (dBm) per channel, measured with the spectrum analyzer; this value is converted to the antenna input value (electric field strength) in the data processing phase based on the gain-correction factors derived from the calibration data at the time of installation.
3 Analysis Methods
3.1 Event Example
We first show an example of EsAP event, Figure 4, before proceeding to the analysis methods in detail. Figure 4 is created from the observation record of 16 July 2015 at Chofu. This figure illustrates a typical EsAP event.
Figure 4a shows the critical frequencies of Es layer (foEs) observed by four ionosonde stations in Japan: Wakkanai (W; 45.16° N, 141.75° E), Kokubunji (K; 35.71° N, 139.49° E), Yamagawa (Y; 31.20° N, 130.62° E), and Okinawa (O; 26.68° N, 128.15° E), which are mutually separated by about 1,000 km. Between 9 JST and 18 JST, Kokubunji station recorded high foEs (∼10 MHz). In particular, around 13 JST, foEs exceeded 12 MHz, which shows that an intense Es layer existed during this period. Yamagawa station also recorded high foEs on that day. These facts indicate the presence of Es layer over Kanto (central Japan) and Kyushu (southwest part of Japan) area on that day. Note, however, that ionosondes observe the ionosphere vertically so that the Es layer detected by an ionosonde may not necessarily coincide with the Es layer that causes a particular EsAP event.
Figure 4b shows the time series of signal intensity on channels 120–149, which correspond frequencies between 112.1 and 114.4 MHz, observed at Chofu. These frequencies are allocated exclusively for VOR stations. In this figure, only samples with signal intensity higher than 120 dBm are plotted with corresponding color (see the color bar on the right). Two channels, 121 and 131, received relatively strong signals continuously throughout the day. Referring to a list of NAV stations, channels 121 and 131 are identified as Haneda VOR (35.56° N, 139.76° E) and Oshima VOR (34.71° N, 139.41° E), respectively. Both stations are located within the range of 150 km from Chofu, so these traces correspond to direct (ground) waves from these VOR stations. Around 12 JST, and 16 JST as well, several channels, which are almost quiet in other times, show traces of strong signals. It is likely that these traces represent EsAP since the period coincides with the period of high foEs at Kokubunji shown in Figure 4a.
Electric field strengths of four selected channels are plotted in Figure 4c. In this paper, a channel at which no ground waves are observed during a quiet (no EsAP) period is referred to as a null channel. Two ground wave channels, 121 and 131, and two null channels, 126 and 146, are plotted in this figure. The electric field strengths of ground waves were roughly −110 dBm with noticeable fluctuations in channel 121. Electric field strengths observed on null channels, around −130 dBm in this case, are considered to be the background noise level. However, channel 146 recorded quite strong electric fields for 2 hr between 11 and 13 JST, which is regarded as an EsAP event as already seen in Figure 4b. During this period, the electric field strength exceeded the normal ground wave strength and reached a peak vaue of −95 dBm. Furthermore, the strength of channel 131, which is a ground wave channel, also increased significantly during this period and fluctuated around −100 dBm after 12 JST, as also shown in Figure 4b. This example shows that the electric field strength of EsAP waves (unwanted waves) may exceed the strength of desired ground waves, at least on the ground.
3.2 Identifying EsAP Events
To see the field-strength characteristics of ground waves, null channels, and EsAP waves, the distribution of electric field strength of representative channels are plotted in Figure 5. The selected channels are the same as those in Figure 4c. Samples are extracted during a period between 9 JST and 15 JST, which includes the period of EsAP event, on the same day as shown in Figure 4. Figure 5a shows the distribution of field strength observed on channel 121, which is considered to include ground waves from Haneda VOR. The field strengths are distributed between −121 and −103 dBm, and samples within the range between −115 and −107 dBm are most frequent. Another ground wave channel, channel 131 with Oshima VOR signals, is shown in Figure 5c. In this case, the distribution is narrower than that in Figure 5a. A probable cause of this difference is the difference in propagation path conditions (e.g., the Haneda-Chofu path passes through the urban area with many high-rise buildings, while the Oshima-Chofu path does not). A complete null channel is shown in Figure 5b, which indicates that the samples are confined in a narrow range between −135 and −125 dBm. Figure 5d shows another null channel which received EsAP waves for more than 1 hr. As in Figure 5b, the distribution shows a narrow peak near −130 dBm, which we regard the characteristic feature of null channel. However, considerable samples are distributed between −120 and −94 dBm with a peak around −110 dBm.
From these examples, we have drawn the following basic criteria for extracting EsAP events from the archived data: (1) Data samples with less than −125 dBm should not be regarded as signals. (2) Channels which continuously receive signals of which field strength exceeds −120 dBm (i.e., ground wave channels) should be excluded, and (3) a data sample with more than −120 dBm is a candidate of an EsAP signal. However, further considerations are needed to deal with natural and artificial noises.
3.3 Noise Considerations
In the previous paragraphs, we have considered relatively quiet conditions to illustrate the characteristics of ground waves and EsAP waves. Frequently, noises are superposed on those signals, which make the EsAP identification difficult. We have surveyed archived data and found that two types of noise are important: one from lightning and the other from power-electronics equipment. Examples are shown in Figure 6. The format of the figure is the same as that of Figure 4. In Figure 6b, a large number of spiky impulses are recorded on almost all channels during early morning and daytime. Referring to the lightning record provided by Japan Meteorological Agency (http://www.data.jma.go.jp/obd/stats/etrn/index.php), we suppose that the noises in the early morning are from lightning strikes. The daytime noises are supposed to be come from nearby solar power equipment. Note that in Figure 4, there are no traces of such noise because of rainy weather on the day. In Figure 6c, field strengths of selected channels are plotted. Lightning noises are more discrete and spiky than solar-panel noises, which are more continuous.
Above features are clearly illustrated in Figure 7. This figure compares three types of null channels with EsAP signal (blue, Figure 7b), solar-panel noise (green, Figure 7c), and lightning noise (red, Figure 7d). Figure 7a shows the time series of field strength of each channel extracted from different day/time data and superposed in a 1-hr frame. The blue line in Figure 7a shows that EsAP field strength is roughly −115 dBm and usually higher than the null channel with artificial noise (green line) by at least a few decibels. This is evident by comparing Figures 7b and 7c. Figure 7c shows that artificial noise rarely exceeds −110 dBm, whereas EsAP signal (Figure 7b) extends to −93 dBm. This facts suggest that artificial noise can be isolated from EsAP signal by setting a certain threshold level, allowing that infrequent overlap may inevitably happen.
The electric field strength distribution of lightning noise, Figure 7d, is significantly different from that of artificial noise. The range of the field strength extends up to −97 dBm, which overlaps the range of EsAP field strength. As shown in the red line in Figure 7a, the receiver input consists of strong discrete impulses when lightning happens. These impulses may be eliminated by smoothing, or low-pass filtering, the recorded samples. The black lines in Figure 7a are the examples of smoothed data. The thin black histograms in Figures 7b–7d correspond to smoothed data. Although this technique is effective for eliminating spike noises, it also reduces the peak value of EsAP strength samples and affects the statistical analysis of EsAP strength. To avoid this problem, we have used the following features of lightning noise and EsAP signals to isolate the target EsAP events. Our preliminary study, by manual inspection, showed that most lightning noises are confined in a short period which corresponds to one or two sampling cycles. In contrast, the EsAP signals are continuous such that a series of modest or high amplitude samples are observed consecutively. Accordingly, we have discarded large amplitude data which (1) rises from the background level and (2) composed of less than four consecutive samples. However, even single large-amplitude data are regarded as an EsAP signal if its preceding samples are at relatively high level compared with the background level.
3.4 Quantifying EsAP Occurrence
3.4.1 Event Criterion
Based on the preliminary study mentioned above, we have set the following criteria to extract data samples to be counted as an EsAP sample. To quantify the intensity of EsAP, we used two numbers, namely, number of EsAP samples and field strength of a sample. Since we are interested in the influence of EsAP to NAV applications, weak EsAP signals were disregarded. We have defined the lower limit of field strength that might cause undesired effect on NAV systems at −98 dBm, based on several standards of aeronautical NAV systems (ICAO, International Civil Aviation Organization, 2014; RTCA, Radio Technical Commission for Aeronautics, 1986a, 1986b, 2008). Adding a margin of −6 dB to this threshold, we have picked up data samples of which strength are more than or equal to −104 dBm. With this criterion, samples from null channels and samples with artificial noise are automatically eliminated. Lightning noises are eliminated by the technique mentioned in the previous paragraph.
3.4.2 Data Processing and Statistics
A block diagram of the data processing part of the observation system, which performs EsAP event extraction mentioned above, is illustrated in Figure 3b. It consists of the observation database, programs for EsAP event extraction, the EsAP event database, and a set of programs which perform statistics. The observation database keeps receiver output data recorded at both Chofu and Kure. This database contains following information: receiver location (Chofu/Kure), date and time, channel number (1–193), and receiver output level (in dBm) for each sample recorded at every 10 s. The receiver output level is converted to electric field strength using gain calibration data, which have been gathered at the receiver site at major maintenance events. To eliminate ground wave channels, such channels are summarized in a table and kept in the analysis system. Using this information, data representing ground wave signals are discarded, and remaining data, null channel data, are fed into EsAP-event extraction program. The EsAP-event extraction program performs noise-separation and event-extraction processing explained above, and the output is stored in the EsAP event database. The EsAP event database holds same data items as the observation database, but only those samples relevant to statistical study are stored in this database. In this study, we performed basic statistics about EsAP such as monthly occurrence, local-time variations, and field-strength distribution.
4 Statistical Results
4.1 Overview of EsAP Occurrence
In this section, we present key statistical results generated from the EsAP-event database records. Before investigating detailed statistics, an overview of the occurrences of EsAP is presented here. Top two panels of Figure 8 show the occurrences of EsAP in the season and local time, in 2015. The left panel shows the Chofu data, and the right panel shows the Kure data. In these panels, a color represents the maximum electric field strength observed on VOR channels during a 15-min period. These panels suggest that EsAPs occurred between late April and mid-August in 2015. They also suggest that most EsAPs occurred during the period between 6 JST and 21 JST. Data collected in other years show similar trends. Furthermore, EsAPs on ILS channels showed similar trends. To compare the EsAP occurrences with Es-layer appearance detected by ionosondes, the critical frequencies of the Es layer (foEs) are plotted in the bottom two panels in Figure 8. In these panels, foEs values larger than 8 MHz, which corresponds to an electron density of ∼8×1011 m−3, are plotted. As these plots show, most Es layers appear between April and August, with a very few exceptions in other months. They also show that Es layer rarely appears in morning hours. Accordingly, EsAP events are recorded in the same period, both seasonally and daily, as mentioned above. Although Es-layer appearance detected by the ionosondes and EsAP occurrence in these data sets are roughly consistent, it is not likely that these two parameters show a strong correlation with each other within a limited time since the Es layer is a localized phenomenon. For example, even though Chofu and Kokubunji are very close to each other, Kokubunji ionosonde observes the ionosphere vertically, but the VHF receiver at Chofu detects EsAP caused by the Es layer at least 300 km away from Chofu. As a result, a particular Es layer that is observed by Kokubunji ionosonde at a particular time may not necessarily directly cause the EsAP at Chofu in the same period, for example, within an hour.
4.2 Monthly Variation
Figure 8 shows that EsAP occurrence has clear seasonal dependence. To see this more precisely, the observed EsAP samples are counted, and monthly occurrence at each location is summarized in Figure 9. In this figure, the top panels show the VOR EsAP counts, and the bottom panels show the ILS LOC EsAP counts. Samples in EsAP event database are sorted by field strength, put into 6-dB size bins, and counted. The left panels show Chofu, 2014–2016, data and the right panels show Kure, 2014–2015, data. The height of each bar represents the total count of EsAP samples recorded in the corresponding month. A bar is divided into subbars by the received signal power; each subbar corresponds to a particular bin. A subbar is painted with a color which corresponds to the range of electric-field strength of the bin.
In Figure 9, EsAP samples with electric field at least −104 dBm are plotted. As explained in the last section, we have set the criterion of potentially hazardous EsAP field strength at −98 dBm. Note that the range of vertical axes is different from panel to panel. The EsAP count of VOR channels recorded at Chofu (top left) peaked in June, and a significant number of EsAP samples are recorded in July. During these 2 months, almost a quarter of samples were in the range above −98 to −92 dBm, and a substantial number of samples were in the range from −92 to −86 dBm. In May and August, the occurrences were less than 25% of that of the peak month, but there are quite a few samples that exceeded −98 dBm. In September, no EsAP samples are found. This trend generally agrees with the seasonal variation of high-foEs count derived from ionosonde observations (Figure S1 in supporting information). Similar trend is observed in ILS LOC channels (bottom, left), but the total number of EsAP samples are about one tenth of that in VOR channels.
At Kure, the annual peak was in May with more than 10,000 EsAP samples, the number far above the peak count in Chofu. Furthermore, the electric field strength of more than 100 samples exceeded −74 dBm on ILS LOC channels. This is due to an extreme EsAP event occurred on 15 May 2015. This event influenced the statistics significantly. Another feature found in Kure data is that the count in July is about the half of the count in August; this trend is different from Chofu.
4.3 Local Time Variation
As we have seen in Figure 8, most Es layers appeared between 7 JST and 21 JST, and EsAP occurred in daytime. Figure 10 summarizes the local time dependences of EsAP observed Chofu (left) and Kure (right), which indicates that EsAP occurred between 7 JST and 20 JST, with a very few exceptions. It is interesting to note that there are two peaks, around 12 JST (stronger) and around 18 JST (weaker), and this pattern is similar to the result obtained by Maeda and Heki (2015). However, it is difficult to determine the general trend of daytime EsAP occurrence from a few years of observation because, as mentioned above, these distributions are largely affected by a few significant events.
4.4 Distribution of Signal Amplitude
To assess the influence of EsAP on NAV system, we need to answer two questions: How often does it occur and how strong can it be? Basic data answering these questions are summarized in Figure 11. The layout of panels in Figure 11 is similar to the one in Figures 9 and 10: The top half is for VOR, and the bottom half is for ILS LOC; the left half is for Chofu, and the right half is for Kure. The format of each panel is as follows. The horizontal axis represents electric field strength in dBm, starting from −104 dBm up to −62 dBm which is well above the minimum receivable level of aeronautical navigation receivers. The vertical axis represents the number of samples. A small window inside the panel shows an expanded view of stronger-signal and fewer-sample region. The color of a bar represents electric field strength; the correspondence of color to field-strength is defined in the color bar on the right. The number of samples within a 6-dB bin is indicated next to the color bar.
Each histogram in Figure 11 shows roughly exponential decrease. In VOR statistics, a large number of EsAP are recorded at Chofu and Kure, but most of them are less than −80 dBm. On the other hand, Kure's ILS LOC statistics shows a large number of samples with strong signal, two of which exceeded −68 dBm. However, the total number of ILS LOC EsAP samples in Chofu is small, and there has not been exceptionally strong signals. The different statistics at Chofu and Kure for the ILS LOC EsAP are left for future studies.
- The occurrence date/time of high foEs and that of EsAP are generally consistent. However, due to the localized nature of the Es layer, exact coincidence may not be expected. This implies that the ionosonde monitoring with separations of about 1,000 km alone is not sufficient for EsAP prediction or forecast.
- Similar to high-foEs events, EsAP events occur between April and September with peak occurrence in June, neglecting one extreme event in May 2014. As for local time distribution, EsAP events occur between 07 JST and 20 JST, which is also similar to the local time distribution of high-foEs events.
- For a particular channel, the EsAP occurrence rate depends on (1) the geographical distribution of the NAV stations which use that particular channel and (2) the geographical probability distribution of the Es layer occurrence. This results in a fact that certain channels are more likely to observe EsAP. Assessment of the influence of EsAP to NAV systems should be based on the data obtained on those channels.
- A case of an extreme EsAP event, occurred on 15 May 2014, showed that the received amplitude of ground wave NAV station was fluctuated by more than ±10 dB compared with the quiet-day values (a related figure, Figure S2, is included in supporting information). This is considered as the results of interference in our receiving system. Similar interference might occur in aeronautical receivers.
5.2 Future Application
In this paper we have investigated the temporal distribution of EsAPs with good time resolution. From the space-weather point of view, this is not sufficient because the spatial distribution of Es layers has not been identified. Since the Es layer is a localized phenomenon with a scale size of ∼100 km, mapping the Es layer is important for space weather applications. Although there are several methods for Es layer mapping such as active radio sounding (Mathews, 1998), GPS techniques (Maeda & Heki, 2014; Wu et al., 2005), and passive radio network (Rice et al., 2011), these methods do not meet the requirement of Es layer detection applicable to NAV radio, either in spatial resolution or radio frequencies. Using our observation method, it is possible to trace back the reflection point of an EsAP signal because the frequency and location of a VOR station is readily available. Since several VOR stations share the same frequency, it is not easy to determine the reflection point of a particular EsAP signal. However, as shown in Figure 4, EsAPs are detected over several channels, and this helps us determine the most probable reflection point(s). As an example, a midpoint map which corresponds to the data shown in Figure 4 is included in Figure S3. Combining this method with GPS Es layer mapping technique (such as Maeda & Heki, 2014), EsAP can be mapped geographically. For this purpose, information can be obtained from detrended total electron content map at 100-km altitude, which is provided by National Institute of Information and Communications Technology (http://seg-web.nict.go.jp/GPS/GEONET/MAP_ES/; available data are limited to those taken in 2015, 2016, and 2017 before 3 July). Another possibility is to use interferometric synthetic aperture radar technique to observe the Es patches (Furuya et al., 2017; Maeda et al., 2016). We expect that this mapping method will help us improve our understanding of the mesoscale morphology of the Es layer.
This work is supported by JSPS Kakenhi (18H04437). Ionosonde's foEs data were provided by WDC for Ionosphere and Space Weather, Tokyo, National Institute of Information and Communications Technology (http://wdc.nict.go.jp/IONO/index_E.html). Radio observation data used in this study are summarized into a single file and archived in the University of Electro-communications' website (http://gwave.cei.uec.ac.jp/~vor/public/Data_for_Sakai_et_al_2018_RadioScienceSubmission.txt).
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