AG_56.05.13_SHUN_finalonline:Layout 6 ANNALS OF GEOPHYSICS, 56, 5, 2013, A0567; doi:10.4401/ag-6318 A0567 A method for separating O-wave and X-wave and its application in digital ionosonde Wang Shun*, Chen Ziwei, Zhang Feng, Gong Zhaoqian, Li Jutao, Fang Guangyou Key Lab. of Electromagnetic Radiation & Sensing Technology, Inst. of Electronics, Chinese Academy of Sciences, Beijing, China ABSTRACT Separation for O wave and X wave is a very important job in interpreta- tion of ionograms, which is premise for automatic scaling. In this paper, a new digital method for separating O wave and X wave is presented, based on a numerical synthesizing technique, which is different from using image recognition to separate trace O and trace X in the ionograms, and from using the electrical method to synthesize and detect circularly po- larized waves. By replacing analog phase shifters and switches in existing ionosonde with digital phase shifters with different initial phase, 0°, +90°, −90°, circularly polarized waves are synthesized digitally within the range of 1-30 MHz, which eliminates the nonlinearity and expands the bandwidth of the ionosonde, and there is no need to switch the ana- log switches continuously. The new method has been successfully applied to CAS-DIS ionosonde and testing results show that the new digital method is capable of separating O wave and X wave well. 1. Introduction Vertical sounding of the ionosphere is a standard technique to study the electron density profile in the lower ionosphere. By scanning the transmitted fre- quency from 1 MHz to 30 MHz and measuring the time delay of any echoes, a vertically transmitting sounder can provide a profile of electron density vs. height. This is possible because the relative refractive index of the ionospheric plasma is dependent on the density of the free electrons Ne. This simplified prin- ciple is somewhat more complex in practice since the group velocity of the radio wave is slower in the iono- sphere so the height obtained using the time delay of the echo is an overestimation of the actual height of reflection. According to the magnetoionic theory, the propa- gation of radio waves through the ionosphere is signifi- cantly affected by the presence of the Earth’s magnetic field. With regard to this, the Appleton equation shows that the refractive index can assume two values, this results in the splitting of wave incident into the iono- sphere in two components, known as the ordinary wave (O-wave) and extraordinary wave (X-wave). The O and X waves propagate with different velocity and therefore appear as two distinct echoes in ionograms; it is then necessary to distinguish the ordinary mode of reflection for implementing the true height analysis as the one per- formed by the Polan program [Titheridge 1998] or the ARTIST system [Huang and Reinisch 1996]. Currently, there are mainly two kinds of method used for separating the O-wave and X-wave, using image recognition technique and using antenna arrange- ments. Image recognition technique for the separation of O-wave and X-wave has no requirement to the an- tenna of the ionosonde, such methods extract the trace O and trace X in ionograms by digital image process- ing technology. A more detailed review of these meth- ods can be found in Pezzopane and Scotto [2007], Scotto [2009] and Krasheninnikov et al. [2010]. The separation of O-wave and X-wave can be done in an al- ternative way, electrically combining signals from per- pendicularly orientated receiving antennas, in such a way that the two modes of the echoes can be deter- mined. A typical representative of such an equipment is DPS-4 digisonde produced by the University of Mas- sachusetts, Lowell [Reinisch et al. 2009] or the VIPIR ionosonde developed by Scion Associates, supported by the US Air Force Research Laboratory and their Small Business Innovative Research Program [Bullett et al. 2010]. In DPS-4, a cross antenna system, an ana- log ±90° phase shifter (2-32 MHz) and a series of ana- log switches are used to synthesize broadband circularly polarized waves, and consequently recognize the po- larization of the ordinary and extraordinary wave. Anyway, the ultimate effect depends heavily on the lin- earity and bandwidth limitation of analog phase shifters, still, this method needs to switch analog switches be- tween consecutive observations. Article history Received March 22, 2013; accepted June 5, 2013. Subject classification: Ionosonde, Ionogram, Trace O, Trace X, Circularly polarized antenna, Phase shifters. In this paper, a new digital method for separating O-wave and X-wave is presented. Just as it is in DPS-4, two orthogonal antenna arrangements are used for transmitting and receiving, but removing phase shifters and switches in analog domain and introducing ±90° phase shifters in digital domain, implemented in digi- tal down convertor process. Circularly polarized waves are synthesized within the range of 1-30 MHz, which eliminates the nonlinearity and expands the band- width of the whole system. Also there is no need to switch the analog switches continuously. Currently, the method has been successfully applied to the Chi- nese Academy of Sciences-Digital Ionosonde system (CAS-DIS) [Shun et al. 2013] and testing results show that the new digital method can separate O-wave and X-wave well. 2. Principle for separating O-wave and X-wave 2.1. A-H equation In the presence of the Earth’s magnetic field, as re- gards the propagation of high-frequency radio waves, the ionosphere behaves as a bi-refractive medium. On the assumption that the collisions between electrons and heavy particles are negligible, the corresponding refractive index n is given by the well-known Appleton- Hartree (AH) equation: (1-a) where: (1-b) fp is the plasma frequency, f is the radio wave frequency, fH is the electron gyrofrequency, and i is the angle be- tween the Earth’s magnetic field and the propagation vector of radio wave. A wave incident vertically into the ionosphere will be reflected when n = 0, and according to Equation (1-a) this happens when the following re- lations are satisfied: (2-a) (2-b) The relation (2-a) represents the reflection condi- tion of the O mode of propagation, the relation (2-b) represents the reflection conditions of the X mode of propagation. 2.2. Separating O-wave and X-wave Ionospheric echoes usually consist of the super- position of two elliptically polarized waves, correspon- ding to the cited ordinary and extraordinary waves. At the magnetic dip equator the electric field of the ordi- nary wave is North-South and electric field of the ex- traordinary wave is East-West so that the waves can be separated by the use of two linear antennas oriented North-South and East-West. In sufficiently high latitudes, or on sufficiently high radio frequencies, the polarization ellipses are essentially circles [Davies 1990]. The two cir- cularly polarized waves can be separated by two orthog- onal dipoles oriented in the x and y directions. Consider two oppositely rotating waves of amplitude Eo and Ee in- cident on the antennas. The outputs Ex and Ey are (3-a) (3-b) We now introduce a phase shift of ±90° into one of the signals, take Ex for example, (4-a) (4-b) Addition of Ex2 and Ey gives ordinary wave, and ad- dition of Ex1 and Ey gives extraordinary wave. (5-a) (5-b) The addition of the signals can be done numeri- cally, when the data are recorded digitally, or electri- cally. A numeric method is employed in CAS-DIS, different from the electrical method used in DPS-4. 3. Application of the method in digital ionosonde 3.1. The digital ionosonde of CAS-DIS A new ionosonde, called CAS-DIS is developed for the sake of testing the new method for separating O- wave and X-wave. The new design of the ionosonde is based on the thinking of Software Defined Radios, which can be divided into five parts. Specifically, these parts are Timing & Control unit, Transmitting Path, Receiving Path, Antenna System and Display Terminal. In order to SHUN ET AL. 2 , X Y Y X Y X1 1 2 1 4 1 T T L 2 2 2 4 2! n = - - - + -^ ^h h , ,X f f Y f fp H 2 2 = = andcos sinY Y Y YL Ti i= = ,X 1= .X Y1 != cos cos cos cos E E t E t E t E t x o e o e ~ ~ ~ ~ = + - = = + ^ h sin sin sin sin E E t E t E t E t y o e o e ~ ~ ~ ~ = + - = = - ^ h cos cos sin sin E E t E t E t E t 2 2x o e o e 1 ~ r ~ r ~ ~ = + + + = =- - ^ ^h h cos cos sin sin E E t E t E t E t 2 2x o e o e 2 ~ r ~ r ~ ~ = - + - = = + ^ ^h h 2 sinE E E E tO x y o2 ~= + = sinE E E E t2x yX e1 ~= + =- 3 realize the function of polarization separation, two trans- mitting channels, two receiving channels and three DDCs with different initial phase are included in CAS-DIS. Fig- ure 1 shows the block diagram of the whole system. The whole system of CAS-DIS work as follows, the configuration parameters are downloaded to pa- rameter RAM and a corresponding reset signal is gen- erated after power on. Once the Timing & Control unit detects the reset signal, the control logic will generate the timing and control signals according to the config- uration parameters. The DDS module of the Trans- mitting Path generates the carrier signal, and the phase encoded signals are fed to two orthogonal transmitting antennas by two parallel coaxial cable after D/A con- vertor, low pass filter and amplifier. Echo signals re- ceived by the two orthogonal receiving antennas are fed to the Receiving Path: after digitizing and digital down converting processing in three DDCs with different ini- tial phases, a baseband signal is obtained; the coherent integration and the pulse compression processing are implemented in Data Processor unit, and eventually the ionogram is produced by the Display Terminal. CAS-DIS system works in the frequency range of 1-26 MHz with step of 50 kHz; it can detect layers within the range from 90 km to 990 km with the range resolution of 5 km. The 16 bit complementary code pair used in CAS-DIS is modulated onto the odd- numbered pulse and even-numbered pulse, and the maximum transmitted power is about 600 W. The spec- ifications of CAS-DIS are shown in Table 1. 3.2. The timing for separating O-wave and X-wave Just as we have discussed above, when the fre- quency of the transmitted wave is sufficiently high the polarization ellipses of the echoes are essentially circles and two circularly polarized waves can be separated by two orthogonal dipoles. In CAS-DIS, there is a couple of orthogonal antennas for transmission and another couple for reception. According to equations of (3), (4) and (5), when a left-handed circularly polarized wave is transmitted the O-wave can be separated from the echoes by adding the two antenna signals after the sig- nal captured by one of the receiving antennas is phase shifted by −90°. Similarly, when a right-handed circu- larly polarized wave is transmitted the X-wave can be separated from the echoes by adding the two antenna signals after one antenna signal is shifted by 90°. Trans- mitting two pulses, alternatively with left and right cir- cular polarization, it is possible to separate O-wave and X-wave correctly, provided that the interval between the two transmitted pulses is small enough with respect to the typical timescale of ionospheric variations. A METHOD FOR SEPARATING O-WAVE AND X-WAVE Figure 1. Block diagram of CAS-DIS ionosonde. Symbols Description Frequency range: 1-26 MHz Height range: 90 km~990 km Range resolution: 5 km Tx power: 600 W (Max. peak) Frequency point num: 500 (Max) Code type: 16 bits complementary ADC sampling rate: 60 MHz ADC bit number: 14 bits Receiver sensitivity: −120 dBm @ 150 KHz (the useful bandwidth of the baseband signal) Antenna: Crossed delta antenna Table 1. CAS-DIS specifications. Figure 2 shows the timing of the separation process in CAS-DIS. The PRF (Pulse Repetition Frequency) is 20 Hz, corresponding to the pulse repetition interval, PRI, is 50 ms, of which 40 ms is used for transmitting and data sampling, and 10 ms is used for data transfer- ring. For the purpose of separating for O wave and X wave, there are five operating modes switching in a PRI in CAS-DIS system. 3.3. Transmitting Path The Transmitting Path contains DDS unit, Code Generator unit, Phase Encoding unit, DACs unit, Low pass filters and Dual Channel Amplifier. In CAS-DIS, the DDS unit is implemented by the logic resources within the FPGA, which generates the sine and cosine signals from 1 MHz to 26 MHz, and the frequency of the DDS can be tuned to a fixed frequency or be mod- ified real time by the parameters. The initial phase of the DDS should be fixed, for example, 0°, 90°or 270°(which is used to substitute −90°), and this is the premise for separating O-wave and X-wave. The Phase Encoding unit generates the 16 bits complementary code to modulate the carrier signal generated by the DDS unit. After filtered by the low- pass filter with a band of 27 MHz, the modulated sig- nal is converted to analog signals by two DACs. The outputs of DACs can be amplified to 600 W when fed to antennas by the dual channel transmitters in Fig- ure 1. In order to get good performance in separator O-wave and X-wave, the dual channel transmitters should have the same characteristics in amplitude and phase response. 3.4. Receiving Path The Receiving Path is comprised of ADCs, DDCs and Data Processor, as shown in Figure 1. In CAS-DIS, two high performance 14 bits ADCs are used, with 60 MHz sampling rate. Testing results show that the sen- sitivity of the Digital Receiver is −120 dBm. It should be noted that the receiver sensitivity is measured in the baseband and the useful bandwidth with −3dB attenu- ation is 150 kHz. Because of the higher sampling rate, a process of DDC is applied before pulse compression processing. The four parts of the DDC are digital mixers (M1, M2), the cascaded integrator-comb filter (CIC), compensa- tion filter (CFIR) and programmable filter (PFIR). Fig- ure 3 shows the block diagram of DDC. For the sake of improving the SNR of the echo sig- nal, the technique of coherent pulse integration is used in the new system design, the times of integration is in a range of 1-128, which can provide an improvement of more than 20dB to the SNR of the received signal. Also, the digital pulse compression technique is used, adding additional 12 dB to the previous gain, and SHUN ET AL. 4 Figure 2. The timing for separating O wave and X wave in CAS-DIS. Figure 3. Block diagram of DDC in Receiving Path. 5 reaching more than 30 dB for the overall gain in the CAS-DIS system. 3.5. Antenna System In CAS-DIS system, the orthogonal delta antennas or orthogonal dipoles can be used as transmitting or re- ceiving antennas, the loads with the value of 600 ohm are installed on the top or center of antennas, and each has a balun used to match the balanced antennas with the unbalanced coaxial RF cable. 3.6. Implementation for Separating O-wave and X-wave Let SEW be the analog signal received by the an- tenna in EW (East-West) direction after quantization of ADC1, and SSN be the analog signal received by the an- tenna in SN (South-North) direction after quantization of ADC2, as shown in Figures 4 and 5. The process of separation of O wave in CAS-DIS ionosonde is shown in Figure 4. In the process of separation of O wave, the left-hand circular polarization signals are transmitted. In this case, the components of sin and cos from the DDS unit are fed to transmitting antennas in EW di- rection and SN direction, respectively. The received sig- nals of SEW and SSN are fed to DDCs with initial phases of 0 and −90°, being the DDC process linear, the phases added by the DDCs are the same for the two channels, so there is no influence on the output. Sup- pose signals after DDC process are S'EW and S'SN, then (6-a) (6-b) A METHOD FOR SEPARATING O-WAVE AND X-WAVE S I jQ° °EW 0 0= +l S I jQ° °SN 90 90= +- -l Figure 4. Block diagram for separating O wave in CAS-DIS. Figure 5. Block diagram for separating X wave in CAS-DIS. In Equation (6), I0°, Q0°, I−90°, Q−90° are the real and imaginary parts of the signals received by the an- tenna in EW direction and SN direction after DDC. Let SO be the O wave, then (7) Figure 5 shows the process for separation of X wave in CAS-DIS. When the right-hand circular polar- ization signals are transmitted, the components of sin and cos from the DDS unit fed to transmitting anten- nas are interchanged, and the receiving signals of SEW and SSN are fed to DDCs with initial phases of 0° and 90°, respectively. Adding the results of the two DDCs, the X wave can be separated from the received signals. Let SX be the X wave, (8) In Equation (8), I0°, Q0°, I90°, Q90° are the real and imaginary parts of the signals received by the antenna in EW direction and SN direction after DDC. To illustrate the process of separation of O and X waves in CAS-DIS, Table 2 shows a summary to the relationship within different parts of ionosonde, which are transmitting and receiving antennas, ADCs, DDCs and the polarization of transmitted and re- ceived signals. 4. Testing result and Conclusion 4.1. Testing result In order to validate the new design of separating O-wave and X-wave, two experiments have been made made in China, at Ji’an in Jiangxi Province and Wuhan in Hubei Province last year. Figure 6 shows an ionogram observed in Ji’an in the south part of China. In the ionogram, the curve with green is the trace of O wave and red is X wave, the start frequency of E layer is about 2 MHz and the cut-off frequency of the F2 layer is about 18.5 MHz for O-wave. Since the station is located in southern China, the cut- off frequency of the F2 layer is much higher than the ionogram observed in other stations, especially in northern China. The transmitting antennas used in the experiment are two orthogonal Delta antennas with the height of 10 meters. The receiving antennas are two orthogonal dipole antennas with the length of 20 me- ters and are about 2 meters high from the ground. Figure 7 shows an ionogram observed in Wuhan in the center part of China, in which the trace of O- wave and X-wave are separated clearly. The start fre- quency of E layer is about 1.8 MHz and the cut-off frequency of the F2 layer is about 10.6 MHz for O- wave. The transmitting and receiving antennas used in the experiment are orthogonal Delta antennas, with the height of 30 meters and 17 meters, respectively. SHUN ET AL. 6 Categories Antennas Signals ADCs/DACs DDCs initial phase Separation for O wave (left-hand circular polarization signals are transmitted) Transmitting Path (EW) sin DAC1 —— Transmitting Path (SN) cos DAC2 —— Receiving Path (EW) —— ADC1 0° Receiving Path (SN) —— ADC2 −90° Separation for X wave (right-hand circular polariza- tion signals are transmitted) Transmitting Path (EW) cos DAC1 —— Transmitting Path (SN) sin DAC2 —— Receiving Path (EW) —— ADC1 0° Receiving Path (SN) —— ADC2 90° Table 2. The relationship within parts of ionosode for separation of O wave and X wave in CAS-DIS. S S S I jQ I jQ° ° ° °O EW SN 0 0 90 90= + = + + +- -l l S S S I jQ I jQ° ° ° °X EW SN 0 0 90 90= + = + + +l l 7 4.2. Conclusion The method for separating O wave and X wave in CAS-DIS presented in this paper is on the basis of dig- ital technology, which is different from the analog method or by using image processing method. By using digital technology to implement phase shifter of ±90° and synthesizing of circularly polarized waves, the new method eliminates the nonlinearity and ex- pands the bandwidth of phase shifter, and there is no need to switch the analog switch continuously, which improves the reliability of the whole system of the ionosonde. To verify the effectiveness of the new method, an ionosonde called CAS-DIS is developed and experi- ments have been made in 2012, the testing results show that the new method for separating O wave and X wave is effective and reliability in practice. Acknowledgements. This study was supported by the Na- tional High Technology Research and Development program (863 Program) (No.2012AA121005). A METHOD FOR SEPARATING O-WAVE AND X-WAVE Figure 6. Ionogram of CAS-DIS on April 23, 2012, in station of Ji’an (LT). Figure 7. Ionogram of CAS-DIS on November 5, 2012, in station of Wuhan (UT). References Bullett, T., A. Malagnini, M. Pezzopane and C. Scotto (2010). Application of autoscala to ionograms recorded by the vipir ionosonde, Advances in Space Research, 45 (9), 1156-1172. Davies, K. (1990). Ionospheric radio, Peter Peregrinus Ltd., 92 pp. Huang, X., and B.W. Reinisch (1996). Vertical electron density profiles from the digisonde network, Ad- vanced in space research, 18, 121-129. Krasheninnikov, I., M. Pezzopane and C. Scotto (2010). Application of Autoscala to ionograms record by the AIS-Parus ionosonde, Computers & Geo- sciences, 36, 628-635. Pezzopane, M., And C. Scotto (2007). Automatic scall- ing of critical frequency foF2 and MUF(3000)F2: A comparsion between Autoscala and ARTIST 4.5 on Roma data, Radio Sci., 42, RS4003. Reinisch, B.W., I.A. Galkin, G.M. Khmyrov, A.V. Kozlov, K. Bibl, I.A. Lisysyan, G.P. Cheney, X. Huang, D.F. Kitrosser, V.V. Paznukhov, Y. Luo, W. Jones, S. Stel- mash, R. Hamel and J. Grochmal (2009). New Digi- sonde for research and monitoring applications, Radio Science, 44, RS0A24, 1-15. Scotto, C. (2009). Elecron density profile calculation technique for Autoscala ionogram analysis, Ad- vances in Space Research, 44, 756-766. Shun, W., C. Ziwei, Z. Feng and F. Guangyou (2013). The new CAS-DIS digital ionosonde, Annals of Geophysics, 56 (1); doi:10.4401/ag-6203. Titheridge, J.E (1998). The real height analysis of iono- grams: a generalized formulation, Radio Sci., 23, 831-849. *Corresponding author: Wang Shun, Key Laboratory of Electromagnetic Radiation & Sensing Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing, China; e-mail: swang@mail.ie.ac.cn. © 2013 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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