Design and application of the multi ‐ transmitting and multi ‐ receiving ionospheric detection network

To observe a large area of ionosphere with relatively dense observing points, we design and implement an experimental multi ‐ transmitting and multi ‐ receiving ionospheric detection network in China. Two transmitters located in Wuhan and Mile and two receivers located in Kaiyuan and Luxi are used to form the ionospheric detection network. The radio echoes of the network are reflected at the mid ‐ point ionosphere between the transmitter and receiver. The radio systems in the network work simultaneously and the echoes from the two transmitters can be recorded in one ionogram of a receiver. With the inversion technique, the oblique ‐ incidence ionogram is converted to a vertical ‐ incidence ionogram, and then the ionospheric parameters, such as the critical frequency and peak height of F2 ‐ layer ( f o F2 and h m F2), as wellas the electrondensityprofile,can beobtained. The f o F2 variation recorded by the oblique ‐ incidence ionograms is compared with the same data of the vertical ‐ incidence ionosondes in Qujing and Chongqing. There is a little deviation between the observation results of the two kinds of the ionospheric detection systems. The preliminary experimental results show that the ionospheric oblique ‐ incidence detection network composed of several independent high ‐ frequency transmitters and receivers is an effective and low ‐ cost solution for large ‐ area ionospheric observation.


| INTRODUCTION
About 90 years ago, Sir Edward Appleton used a high-frequency (HF) radio system to detect ionosphere and then ionosondes operating in HF band have been widely applied for ionospheric observation. Along with the development of the electronic technology, ionosondes become smaller, simpler, cheaper and are developed by many science teams. The most famous ionosonde is the Digisonde developed by the University of Massachusetts Lowell [1]. The Dynasonde developed by the National Ocean and Atmospheric Administration is also wildly applied to measure the dynamics of ionosphere [2]. There are also many other kinds of ionosondes in the world, and the Wuhan Ionospheric Sounding System (WISS) is one of them [3]. The WISS is a very distinctive tool for ionospheric observation. It can work as the traditional ionosonde to accomplish the Vertical-Incidence (VI) detection, and its transmitter and receiver can also be divided to realize the Oblique-Incidence (OI) detection.
As a mainstream ionospheric observation method, the VI ionospheric observation can provide many important parameters, such as critical frequency and virtual height of ionospheric E-and F-layers [4,5], and electron density profile of ionosphere, which can be estimated in real time [6][7][8]. A traditional ionosonde consists of a transmitter and a receiver and can only detect the ionosphere overhead. The transmitter and receiver of an OI ionosonde are placed apart and measure the ionosphere in mid-point between the transmitter and receiver [9,10]. Thus, an OI ionosonde can be used to detect the ionospheric region over ocean and desert, where the VI ionosonde cannot be placed. Moreover, several OI ionosondes are much easier to compose the detection network. Theoretically, m transmitters and n receivers can detect m � n ionospheric regions and provide n OI ionograms and m � n echo traces in an observation period (each ionogram contains m echo traces). The parameters, such as propagation mode, Doppler spectrum, multipath information and maximum usable frequency, can be directly gained from an OI ionogram.
With the inversion technology, ionospheric critical frequency, virtual height and even electron density profile can be estimated from an OI ionogram [11].
An experimental multi-transmitting and multi-receiving ionospheric detection network has been set up in China. We want to design a kind of ionospheric detection network with more observation points and less transmitters and receivers. In the Section 2, the hardware and characteristics of the WISS is introduced. In the Section 3, the arrangement and operating mode of the experimental ionospheric detection network are introduced and discussed. In the Section 4, the observed data are presented and analysed. The conclusion and summary are given in Section 5.

| SYSTEM DESCRIPTIONS
WISS is a HF sky-wave detection system characterized by a small size, low power consumption and high performance and can be used for ionospheric detection and short-wave channel management. The system flowchart is shown in Figure 1. The transmitting channel and the receiving channel of the WISS are two independent systems [12]. As shown in Figure 1a, the transmitting system is composed of a system controller, a Digital Signal Processor (DSP) and a frequency source. The frequency source generates the transmitted signal and the local oscillator signal. The centre frequency of the transmitted signals can be selected between 3∼30 MHz. The oven-controlled crystal oscillator is a frequency-tuneable oscillator designed to produce the continuous and repetitive periodic signals. The frequency of the output signal is adjustable by changing the input oscillator voltage. In addition, the Global Positioning System (GPS) receiver is used for time and frequency synchronization between the transmitter and receiver, and it also provides the information of the system position. The structure of the receiving system in Figure 1b is composed of analogue front-end, Analog Digital Conversion (ADC), digital down conversion, system controller, DSP and frequency source circuit. The analogue front-end is in charge of analogue signal processing. The weak signal from antenna is amplified, filtered and frequency-converted, so that the amplitude and frequency of the received signal can meet the input requirements of the subsequent digital circuit. The ADC circuit converts the analogue signal into digital signal and put it in the Digital Down Conversion (DDC) to obtain the digital base-band signal with relatively low sampling rate. At last, the digital signal is sent into a DSP for pulse compression to obtain the channel impulse response. The main function of the frequency source is to provide a standard frequency source to the radio system and a synchronous clock to the AD circuit, the DDC, and the system controller. The control module circuit mainly plays a coordinated control role to guarantee for the data transmission. The processed data are uploaded to a computer via a Universal Serial Bus.
There are two operating modes of the WISS. One is the fixed-frequency mode and the other is the swept-frequency mode. The waveform parameters, such as pulse width, duty cycle, modulating codes and pulse train repetition times, are user-defined for the appropriate pulse compression gain, range and Doppler resolutions [13]. To obtain the electron density profile in the observation experiment, all the WISSs in the network are operated in the swept-frequency mode. The pulse width of the transmitted wave is 25.6 μs. The repetition period of the pulse is 0.013 s. The 255-bit m-sequence is used for the phase modulation. The number of Fast Fourier transform points is 1024. The peak transmitting power can reach 300 W and usually, the transmitter works at 100 W peak power. Thus, the Doppler range determined by the applied waveform is [À 38.3 Hz, þ38.3 Hz]. The Doppler and range resolutions are 0.075 Hz and 3.84 km, respectively. The detailed descriptions of the WISS can be found in the previous report [14].

| DETECTION NETWORK ARRANGEMENT
To improve the observation efficiency, an ionospheric detection network with multi-transmitters and multi-receivers can adopt several kinds of detection modes, such as Time-Division Multiplexing (TDM), Frequency-Division Multiplexing (FDM), Code-Division Multiplexing (CDM) and Space-Division Multiplexing (SDM). In the TDM mode, the transmitters in the network are operated in sequence, and so the observation efficiency is the lowest [15]. For the FDM mode, the system bandwidth is relatively wider and the hardware cost is higher. For the CDM mode, more complex signal processing methods are needed for pulse compression and sidelobe suppression [4]. We select the SDM mode for the experimental multi-transmitting and multi-receiving ionospheric observation [9]. If the transmitters and receivers are carefully arranged, the echoes of each transmitter can appear in different distance range in the OI ionogram of each receiver, thus all the echoes can be recognized and no hardware and software changes of each radio system in the network are required.
The layout of the experiment is shown in Figure 2. Two transmitters and two receivers compose a relatively simple detection network to validate its feasibility for ionospheric observation.  Figure 3. Figure 3a shows the log-period antenna used for the Wuhan transmitter. Figure 3b shows the inverted V-antenna used for the Miler transmitter. The two-wire antenna in Figure 3c is used for the receivers in Kaiyuan and Luxi, respectively. The photo of the WISS, including transmitter, receiver and GPS synchronizing system, is shown in Figure 3d receiver rely on the GPS signal. The log-period antenna in Wuhan points southwest and its wave lobe pattern is simulated by MATLAB and shown in Figure 4. Its antenna beam width (including mainlobe and first sidelobes) on the horizontal plane is ∼60°. With the increase of the operating frequency, the zenith angle of the antenna beam become larger and larger. However, the antenna beam can cover the ionospheric F-layer over the mid-point between Wuhan and Kaiyuan/Luxi, while the operating frequency is between 6 and 30 MHz.

| DATA ANALYSIS
In this experiment, the data were collected in two days on 5 and 6 December 2015. An example of the OI ionograms is shown in Figure 5. The left and right ionograms were recorded by the Kaiyuan and Luxi receivers, respectively, at 9:50 LT on 5 December 2015. The operating frequency band is between 6∼23.1 MHz with 0.3 MHz step. The ionograms contain the echoes transmitted from both Wuhan and Mile. According to the ranges between transmitters and receivers, we can find that the closer echoes between 400 and 600 km group distances come from the Mile transmitter and the farther echoes between 1200 and 1600 km group distances come from the Wuhan transmitter, as the green arrows indicated. The group distance equals the travelling time of the echo from transmitter to receiver multiplied by the speed of light. The front edges of the ordinary waves of the OI echoes are labelled by the black curves. Given that the echoes are reflected in the middle point between the transmitter and receiver, the virtual height of the reflected echoes can be calculated and then the converted VI echo fronts can be obtained, by the group distance of the echoes and the ground distance between the transmitter and receiver.
The fronts of the echoes from Mile to Kaiyuan/Luxi are displayed in Figure 6a as the blue/red curve. The distances between the Mile transmitter and the two receivers are less than 65 km, thus, the echoes from Mile can be looked as VI echoes. The echo front curves are imported into the ARTIST software to gain the ionospheric parameters, such as the critical frequency and peak height of F2-layer (f o F2 and h m F2), as well as the electron density profile [16]. The inverted electron density profile in the middle between Mile and Kaiyuan/Luxi is presented in Figure 6b  The echo fronts from Wuhan are extracted from the OI ionograms in Figure 5. The blue and red curves in Figure 8a present the echo fronts received by the Kaiyuan and Luxi receivers, respectively. Figure 8b displays the converted VI echo fronts from the curves in Figure 8a. The conversion is based on the Martin equivalent path theorem and the assumption that the ionosphere is parallel to the ground [17]. The ground and ionosphere curvatures as well as the geomagnetic fields are also considered [18,19]. We can find that the OI echo traces received by the different receivers in Figure 8a are of great difference in both distance and frequency. After conversion, the converted curves in Figure 8b are almost overlapped. Thus, the estimated electron density profiles in Figure 8c are also F I G U R E 6 (a) Echo fronts of the oblique-incidence echoes from Mile to Kaiyuan (blue curve) and from Mile to Luxi (red curve). (b) Electron density profiles inverted from the echo fronts in plot (a) F I G U R E 7 foF2 variations estimated from the oblique-incidence ionograms of Mile-Kaiyuan (blue curve) and Mile-Luxi (red curve), as well as from the vertical-incidence ionograms of Qujing (green curve). The error bars show the difference between the oblique-and vertical-incidence data. LT ¼ UT þ 8 h similar. The estimated f o F2 and h m F2 of the Kaiyuan receiver are 8.375 MHz and 230 km, respectively. The same parameters of the Luxi receiver are 8.575 MHz and 250 km. Depending on the operating frequency and elevation angle, the transmitted waves will be reflected within a certain range of the ionosphere usually. Therefore, this 'equivalent profile' obtained by us represents a large area of ionosphere other than the profile at the point in the path. In addition, except ignoring the electromagnetic field and the collision assumptions, we only adopt a small number of simplified approximations, so the derived profile is almost the 'true profile' derived from the OI ionograms. The f o F2 variations from 09:45 to 12:00 LT of the Wuhan-Kaiyuan/Luxi path are displayed in Figure 9 as blue/ red curve and compared with the f o F2 recorded by the Chongqing VI ionosonde shown as green curve. Through the mid-point between Wuhan and Kaiyuan/Luxi is 300 km away to Chongqing, the differences between the converted f o F2 and the VI f o F2 are less than 1.5 MHz. This may be due to the quiet background ionosphere and the low Kp index, which never exceeded 4 on the observation day [20]. F I G U R E 8 (a) Echo fronts of the oblique-incidence echoes from Wuhan to Kaiyuan (blue curve) and from Wuhan to Luxi (red curve). (b) Converted vertical-incidence echo fronts of the echoes. (c) Electron density profiles inverted from the converted vertical-incidence echo fronts in plot (b) F I G U R E 9 foF2 variations estimated from the oblique-incidence ionograms of Wuhan-Kaiyuan (blue curve) and Wuhan-Luxi (red curve), as well as from the vertical-incidence ionograms of Chongqing (green curve). The error bars show the difference between the oblique-and vertical-incidence data. LT ¼ UT þ 8 h QI ET AL. An experimental multi-transmitting and multi-receiving ionospheric detection network was designed and operated in December 2015. Two transmitters and two receivers form the network and the ionospheric OI detection is applied. A receiver can receive the echoes from the two transmitters simultaneously and the echoes of different transmitters are designed to appear in different distance range in one ionogram. The ionospheric parameters, such as f o F2, h m F2 and electron density profile, are estimated from the OI ionograms. The f o F2 variations are compared with the same parameter of the VI ionosondes in Qujing and Chongqing, and there are small differences between the OI and VI observation results. This experiment proves the feasibility and utility of the ionospheric detection network with independent transmitters and receivers in different locations. It is an effective and low-cost solution for observation of a large area of ionosphere. In the near future, the detection network will be expanded with more transmitters and receivers to gain more observation points. The algorithm for the network layout will be studied to avoid the overlap of the echo fronts from different transmitters in each OI ionogram.