The use of HF surface wave radar as an effective method for inspecting the environment of the ocean in the beyond-the-horizon area has been developing in recent years. However, because the radiating beam of the transmitting antenna is of a certain width and its electromagnetic radiating power propagates not only along the sea surface but also to the upper space, as a result, after interacting with the ionosphere or a scattering object on its path, the backscattered signal then returns to the radar along the radiative path. Thus, if we analyze the echo signal received by the radar, we can obtain some information about the ionosphere. This paper presents a method of sensing the ionosphere using HF ground wave radar, which can give information about the altitude of the ionosphere, Doppler, meteor showers, etc.
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 High-frequency surface wave radar (HFSWR) uses vertically polarized surface waves that follow the curvature of the Earth. Long ranges can be achieved because of the relatively low attenuation over the highly conductive ocean surface. HFSWR can provide continuous, all-weather, 24-hour coverage in regions up to 200 nautical miles range and is used not only in detecting and tracking targets but also in supplying meteorological and oceanographic data [Sevgi, 1998].
 However, because the ground screen of the transmitting antennas is limited and the ground has attenuation, and the radiating beam of the transmitting antenna is of a certain width in the E plane and consequently turns upward to some degree, the energy of the electromagnetic wave radiates not only along the sea surface but also within the atmosphere, and as a result, after interacting with the ionosphere or a scattering object on its path, the backscattered energy is returned to the radar via the sky wave mode propagation though the ionosphere in the same coming path. Thus, if we analyze the echo signal received by the radar, ionospheric information can be obtained as a by-product of the radar system. This paper presents a method of sensing the ionosphere using HF ground wave radar, which can give information about the altitude of the ionosphere, Doppler, equinox, meteor showers, etc. This method can be useful for ionospheric forecasting, communication channel selecting and optimizing, as well as antijamming in radar systems.
2. High-Frequency Surface Wave Radar System
 The Ocean State Measuring and Analyzing Radar (OSMAR) HF ground wave radar is designed by the Radio Wave Propagation Laboratory of Wuhan University. The radar operates at the frequency of 4–8 MHz and comprises a three-element Yagi-Uda transmitting antenna and a two-row by four-line vertically polarized antenna receiving array, the space between the rows is 8 m and the space between the lines is 13.3 m. The radar system transmits a frequency modulated interrupted continuous waveform. After a few minutes' coherent dwelling of the digitized receiver, the information about range, azimuth, and Doppler can be obtained by Fourier analysis and dedicated array processing. The Radio Wave Propagation Laboratory of Wuhan University has two radar stations with the purpose of wind measurement, wave spectra, and surface current detection at the Zhoushan archipelagoes, Zhejiang. The radar stations are about 100 km apart. Figures 1 and 2show the transmitting and receiving antenna arrays, respectively, at Zhujiajian island, Zhoushan, Zhejiang. Table 1 shows the working parameters of the OSMAR radar system.
 According to classic electromagnetic theory, when the vertically polarized antenna is on a boundless ideal conductive plane, the antenna does not radiate along its axial direction, but if these conditions are not satisfied, that is, the size and density of the ground screen are limited and the attenuation exists in reality, the antenna beam will “turn up” to some degree. Thus the radiation in the axial direction is inevitable. Figures 3 and 4show sketch maps of the vertically polarized transmitting and receiving antenna arrays (and the ground screen), respectively [Sevgi, 2001]. Figure 5 shows the vertical radiation patterns of a vertically polarized transmitting antenna [Sevgi, 2001], which is on an ideal and nonideal conductive plane. Figure 6 shows the vertical radiation patterns of a vertically polarized receiving antenna array [Sevgi, 2001], also on ideal and nonideal conductive planes. (The electrical parameters for the poor ground were σ = 0.003 Ω−1 m−1 and ɛr = 4 and the parameters for the sea were σ = 5 Ω−1m−1 and ɛr = 80 [Sevgi, 2001].) In practical radar engineering, the installation field of the antenna array cannot be built to an ideal boundless conductive plane, so there must be electromagnetic radiation in the axial direction. Also, because the antenna beam is of certain width, there is no doubt that the tilted electromagnetic radiation exists in the elevation direction of the transmitting antenna.
 It is well known [Davis, 1965; Michael, 1989] that part of the atmosphere is ionized by electromagnetic and particled radiation from the Sun which occurs from 60 km height off the ground, forming an ionization region comprising electrons, positively charged ions, negatively charged ions, and neutral molecules. This ionized region is called the ionosphere, which can expand to the exoatmosphere. The ionosphere can be divided into several regions according to the electron density: the D layer (60–90 km), E layer (90–140 km), F1 layer (140–200 km), F2 layer (200–1000 km or 2000 km), and the outer ionosphere (beyond the F2 layer). Figure 7 is the simulation of a typical ionospheric profile of the middle latitude using the finite difference time domain.
 Because of the existence of the ionosphere, when the radiative beam of the transmitting antenna is of a certain width in the E plane or has certain elevation, the electromagnetic wave will radiate not only along the sea surface but also upward. As a result, after interacting with the ionosphere and with the aerial or naval target along its path, the backscattered signal returns to the radar along the outgoing path. Figure 8 shows the single-hop propagation path of the ground wave emitting radiating power between the sea surface and the ionosphere.
 The echo signal received by the radar contains related information about the ionosphere because of the interaction between the radar electromagnetic wave and the ionosphere. Thus analyzing the echo signal can provide some information that is beyond the traditional use of HF ground wave radar, such as moving regularities within the ionosphere.
4. Ionospheric Probing Experiments
 Supposing the operating frequency of the radar is 7.988 MHz, in order to show the ionospheric echo more directly, Figure 9 gives the time domain and frequency domain spectra of the received echo. The stratification of the ionosphere is clear in this experiment.
Figure 10 shows the range spectrum without the ionospheric echo. It is evident that the intensity of the sea clutter decreases with increasing range. Figure 11 shows the range spectrum taken an hour later. Compared with Figure 10, Figure 11 has two extra peaks. According to HF electromagnetic wave propagation theory in the ionosphere, when the E layer exists, the effective range is from 70 to 140 km, while 200–300 km corresponds to the F layer. Therefore we are sure that the two extra peaks in Figure 11 come from the reflection at the height of 90–110 km (E layer) and 200–250 km (F layer), respectively.
 In order to illustrate the regularity of the ionospheric variation, Figures 12 and 13 show the 24-hour results of the received echo on 2 and 3 March 2004 at local time. According to Figures 12 and 13 and other experimental results, we can conclude the following.
4.1. Relationship Between the Sun and the Ionosphere's Appearance and Disappearance Time
 The radar is influenced by the ionosphere most obviously from 0700 LT to 1730 LT in the observation. When the dawn appears, a strong echo wave signal is moving toward the east, and in the highest end of the radar operating frequency (e.g., 8 MHz), the signal time lag is behind that of the lower operating frequency (e.g., 7 MHz); as the dawn progresses, the increase in ionizing radiation impinging on the ionosphere causes rapid changes at progressively lower altitudes. As a result, the ionospheric reflection height at a given frequency moves to progressively lower altitudes, producing a steady decrease in the group path between the radar and any given echolocation on the Earth's surface. The backscatter signal therefore should display a positive Doppler shift. As the Sun rises, the ionosphere appears in the east at first and then moves progressively toward the west. This is basically in accordance with results derived from sky wave radar instead.
4.2. Regularity of the Variation of Height With Time Within the Ionosphere
 According to the observation, the height of the ionosphere changes gradually with time, which is lower in the morning and higher at night, each layer being about 20–100 km thick.
4.3. Regularity of the Doppler Spreading Caused by the Ionosphere's Movement
 The scattered signal's Doppler shift, which is caused by the Es layer reflection, is about 0∼0.1 Hz and about 0.1∼0.4 Hz in the case of the F layer. When the F layer is stationary, the scattered signal's Doppler shift is about 0.1 Hz. The relative stationary time of the F2 layer is about 20 s, while it is 50 s in the case of the E layer. Therefore, in order to diminish the interference on the sea echo waves caused by the ionosphere, the horizontal beam width of the antenna should be reduced, and the radar's coherent cumulative time should not be more than that of the ionosphere.
4.4. Multimodel Propagation Effect of the Sea Echo
 When the E and F layers exist simultaneously, the radio wave is reflected by two different layers of different height. Because the two layers are of different moving speed, their Doppler shifts are also different, which in turn make two Bragg peaks on the radar echo's Doppler spectrum. This is the so-called multimodel propagation effect.
4.5. Multipath Propagation Effect in the Ionosphere
 When the radar is working, each reflecting point corresponds to a particular radio wave propagation path in the region between the sea surface and the radar. This is the multipath propagation effect in ionospheric radio wave propagation. Because the reflecting points are different, the corresponding radials have distinct amplitude, phase, and Doppler shift. At each distance element, the integral radar echo wave is the sum of all the echoes from the different paths. If all the reflecting points' moving states are the same and their velocity is constant during coherent cumulative time, the identical Doppler shifts will emerge. As a result, on the Doppler spectrum there appears a pulse-like spectral line, which is caused by the overlapping power from all the reflecting points. Otherwise, the different moving speeds and Doppler shifts of these reflecting points will lead to the spreading Doppler spectral line.
4.6. Spreading of Doppler Shift Caused by the Wind in the Ionosphere
 When the ionosphere is relatively stationary, a period of about 10 s, it can be perceived as an object with approximately uniform bulk motion. If the coherent cumulative time is longer (e.g., more than 30 s), the ionosphere cannot be treated as uniform. Because of the nonlinear variation of the phase path in the radio wave propagation, the longer the coherent cumulative time lasts, the more the Doppler frequency shift spreads. Therefore reducing the beam width is an effective way to improve the sea echo spectrum's quality. Computer simulation shows that the narrower beam and the shorter coherent time lead to smaller radar Doppler spreading.
4.7. Echo Intensity From the Ionosphere
 The F layer is of great importance to shortwave communication and the HF sky wave radar, and in most cases, it is selected as the reflection layer for long-distance communication. There are two layers in the F region: F1, situated at the height of 150–200 km, and F2, situated from 200 km. Their altitudes change with the season and the period of time in the day. Unlike the other layers, F2 does not disappear after sunset and still keeps ionizing. The echoes from the F layer are sometimes so strong that they can even be comparable with the echoes of the sea wave, as shown in Figure 14. (Regions A and B represent echoes from the sea and the ionosphere, respectively.)
4.8. Scattering Characteristic of the D Layer
 The D layer is also called the absorption layer in some literature [Davies, 1965; Michael, 1989] because it does not reflect any HF electromagnetic waves. In our experiment, however, we do get the ionospheric echo from the range corresponding to this layer in the daytime and even at night. For example, in our observation, the near-vertical incidence clutter from 60 to 70 km can often be found at the same period of daytime. Sometimes they are so strong that they occult the echoes from the sea wave, as Figure 15 shows. In Figure 15, the echo spectrum comes from the range 70 km. The dashed line represents the echo spectrum before eliminating the disturbance of near-vertical incidence clutter, and the solid line represents the spectrum after suppression of the disturbance from near-vertical incidence using adaptive ionospheric clutter suppression based on subarrays. Because of the reflection of the ionosphere, the echoes of the sea wave emerge from the ionospheric echoes, and it is also obvious in Figure 15 that the disturbance comes from the D layer according to its range. That means that the D layer also has the quality of reflecting (or scattering) the HF electromagnetic wave.
4.9. Reflecting Characteristic of the Es layer
 The formation and the origins of the Es layer are very complicated. During the Leonid meteor shower in 2003, observed by radar, it is obvious that the meteor shower has some influence on the formation of the Es layer. The intensive reflection from the Es layer appeared at about the second and the fifth day after the meteor shower. According to the theory of the relation between the Es layer's formation and the meteor shower's appearance [Ellyet, 1976], the intensive Es layer which appeared on the second day corresponds to the Es caused by meteor shower ionization, while the Es layer which appeared on the fifth day corresponds to the Es caused by wind shears wrapping on metallic ions left in the meteor shower. It is identical to Hedberg's conclusion [Hedberg, 1976] that the appearance of the Es layer is lagging behind the breakout of the meteor shower in time. The intense disturbance of the Es layer after the meteor shower, and that of the D, F1, and F2 layers, can severely weaken the radar detection capacity and can even make it disappear. Figure 16 shows the echo wave's Doppler spectrum from two different times at a range of 140 km. The echo spectrum comes from the E layer, the dashed line represents the echo spectrum before eliminating the disturbance of near-vertical incidence clutter, and the solid line represents the spectrum after suppression of the disturbance from near-vertical incidence using adaptive ionospheric clutter suppression based on subarrays. Because of the strong reflection of the ionosphere, the echoes of the sea wave are submerged in the ionospheric echoes.
 Sometimes we can observe the second bounce from the sporadic Es layer. Figure 17 shows the echo's range profile. This range profile indicates that the near-vertical specular reflection from the Es layer is isolated in range, occupying only a couple of range bins. Two elevated levels are observed in the range profile around the range of 100 and 195 km. They are identified as the signal component resulting from the near-vertical specular reflection and the second bounce from the Es layer, respectively. The Doppler maps for two neighbor range bins are shown in Figures 18 and 19, in which the Doppler spectrum for the range of 100 km can be seen and the spectrum for the range of 105 km is superimposed. These range bins are selected because they represent the specular reflection from the Es layer. According to Figures 18 and 19, the magnitude of the Doppler components at 100 km between –0.2 and 0 Hz is more than 20 dB higher. The Doppler characteristics of the specular reflection may be viewed as a large steady component centered on the clutter's “center of gravity.” The lower magnitudes beyond the Doppler region can be seen as the diffuse reflection from the Es layer. The steady component has a small Doppler spread, on the order of a fraction of 1 Hz. The above phenomena can also be seen in Figures 20 and 21 from the second bounce for the Es layer. Comparing this spectrum with Figure 16, we can find that structure, change, and ionospheric Doppler are very complex.
4.10. Characteristic of Meteor Trail Echoes
 These echoes, coming from 80 to 120 km, are usually detected at midnight and midday. Their Doppler spectrum is usually wide, and sometimes their amplitudes are higher than that of the noise by 30–40 dB.
5. Practical Applications
 1. When using HF ground wave radar as a tool for detecting the hydrodynamic coefficient of the sea surface, the ionospheric echoes “contaminate” the sea wave echoes. However, we can observe the movement of the ionosphere, deduce relative parameters, diagnose or forecast its mode, and give suggestions for HF communication channel selecting and HF radar frequency selecting, by analyzing the character of the sea wave and the ionospheric echoes.
 2. The quality of the ionosphere is associated with the activity of the Sun [Mitra, 1974], so it is possible to study the relationship between them in long-term observations.
 3. The structure and quality of the Es layer are complicated, and the factors of its forming are also various; the meteor trail is just one example [Davis, 1989]. It is also possible to study the relationship between the ionosphere and meteor showers in long-term observations.
 4. Analyzing the space-time character of meteor showers can be useful in meteor trail scatter communication and space environment forecasting.
 5. HF electromagnetic radiation from a slightly rough, highly conducting sea surface causes Bragg scattering. Using Doppler spectrum and the first-order and second-order cross sections given by Barrick , kinetic parameters such as the directional surface currents, wave spectra, and wind measurement can be obtained. Because HF radar can receive over-the-horizon echoes reflected from the ionosphere, both long-distance sea state parameters and moving targets can therefore be deduced. This provides a good method for remote sensing.
 E. V. Appleton and M. A. F. Barnett from Great Britain in 1924 and G. Breit and M. A. True from the United States in 1925 adopted the method of continuous wave and pulsed wave, respectively, in their experiments and proved the existence of the ionosphere. They started a new era of detecting and studying the ionosphere with radio waves. For nearly a century, various technology and equipment for ionosphere detecting using radio waves have been developed to a certain extent [Rahert, 1991; Davis, 1989]. With the development of technology, both methods and equipment have been improved and perfected. All of the progress greatly increased the degree of automation, and the parameters we can get from the measurement have increased not only in amount but also in accuracy. This paper showed a new way of using the “leakage” power of the HF ground wave radar as a “carrier” to detect in a close region of the ocean and ground. As a by-product of HF ground wave radar, the method presented in this paper of detecting the ionosphere using HF ground wave radar can provide information about the altitude of the ionosphere, Doppler, equinox, meteor showers, etc. The advantages of this method are that it is economical and it has a large-scale detecting area. This study and observation can be useful in ionospheric forecasting, communication channel selecting and optimizing, and antijamming in radar systems.
 This project is supported by the 863 High Technology Project of China. The authors gratefully acknowledge S. C. Wu, X. B. Wu, and others for the support given to the work. The authors would also like to thank the referees for many helpful comments and suggestions, which have enhanced the quality and readability of this paper.