Experimental study on pulse radar target probing in RFS based on interrupted transmitting and receiving

Xiaobin LIU,Jin LIU,Qihua WU,Feng ZHAO,Guoyu WANG

1.Introduction

Target probing and measurement are important tasks for radar system.Field experiment and Radio Frequency Simulation(RFS)1are the two commonly-used methods to conduct the target measurement.In the field experiment,the electromagnetic environment is complex as it includes noise,clutter,noise interference and some other electromagnetic signals.Besides,the motion of target is difficult to control.Therefore,the field experiment for aerial and space target measurement is of low reproducibility and poor controllability.

The measurement system2,3in RFS is constructed to analyze the dynamic scattering properties of space target.In RFS,the whole procedure of radar probing and measurement,i.e.signal generation,radiation,transmission,target re flection,receiving and processing,can be performed in anechoic chamber,4–6which makes it of low cost,high reproducibility and controllability compared with the field experiment.

To date,two major signals are used in RFS:impulse signal7–9and swept-frequency signal.2,3,10However,the impulse signal is rarely used in the real radar system for target probing and its energy is difficult to promote.Meanwhile,the Pulse Repetition Frequency(PRF)of swept-frequency signal generated by the Vector Network Analyzer(VNA)is relatively low.2,3Hence,the measurement for the highly maneuvering target,such as the aerial and spatial targets11–13with high micro-motion rate,is difficult to be simulated with the experiment system in Refs.2,3.

Pulse radar signal with high PRF can be applied to the highly maneuvering target measurement,but it is generally used in field experiment for target measurement.14,15As thefield experiment conditions may affect the measurement results,it is more desirable to evaluate the target probing and measurement performance of pulse radar in RFS.However,since the distance between radar and target is small in RFS,the re flected signal returns before pulse radar signal is fully transmitted.Meanwhile,the transmitter and receiver are close in RFS so that the transmitted and re flected pulse signalsarecoupled and difficultto beseparated atthe receiver.16,17To handle this problem,the Interrupted Transmitting and Receiving(ITR)system is constructed in this paper based on the interrupted sampling.18,19In the ITR system,the pulse signal is transmitted for a short period and then interrupted with receiver switching on to receive the echo.The transmitter and receiver should be alternately turned on and off during the generation of pulse radar signal.And then the target echo can be obtained in RFS.

The remainder of this paper is organized as follows.The principle of ITR and the pulse compression of ITR echo are analyzed in Section 2.In Section 3,the experiment configuration and procedure are presented.The target probing experiment is performed with the ITR system in Section 4.Then,the range pro file is obtained with the ITR echo.The ITR RFS scheme of pulse radar signal is compared with that of the swept-frequency signal.Based on the target probing results,the target measurement performance of pulse radar can be verified in RFS and the highly maneuvering target measurement for space targets can also be performed in the future.Finally,conclusions are drawn in Section 5.

2.Principle of ITR

2.1.Property of ITR control signal

The ideal ITR control signal is a series of rectangle pulses.It can be expressed as

where ‘*”is the convolution operation, δ(·)is the impulse function,Tsis the ITR period,and n is number of the subpulse.In Eq.(1),rect(t/τ)is the rectangular function defined as

where τ is the width of rectangle pulse in ITR.

The waveform of p(t)is shown in Fig.1.τ should be sub-microsecond to ensure that the interrupted signal is entirely transmitted before the echo returns to the receiver.After Fast Fourier Transformation(FFT)operation,the spectrum of p(t)is given by

Fig.1 Waveform of p(t).

where fs=1/Tsis the frequency of ITR,and sinc(x)=sin(πx)/(πx).

2.2.Property of complete echo

The transmitted Linear Frequency Modulation(LFM)signal is

where u(t)=rect(t/Tp)exp(jπμt2)is LFM signal,Tpis pulse width,μ is chirp rate and bandwidth B= μTp;f0is the carrier frequency.

The pulse compression of complete echo can be obtained after Matched Filter(MF)operation,which can be expressed as

where F-1[·]is Inverse Fast Fourier Transformation(IFFT)operation;H0(f)=U*(f)is the frequency response of MF,U(f)is the spectrum of u(t),and U*(f)is the conjugation of U(f);S1(f)is the spectrum of s1(t),and s1(t)=Au(t-Δt)×exp(-j2πf0Δt)is target echo;A is amplitude;Δt=2R/C,C is the electromagnetic wave velocity,and R is the range between the antenna and target.

2.3.Property of ITR echo

where h(t)is impulse response of target.Take FFT operation to the ITR echo,and its spectrum can be obtained as

It can be seen that the spectrum of ITR echo is composed of several sub-spectrums with spectrum width of fs.The pulse compression of ITR echo is obtained by MF operation

From Eq.(8),we can see that the amplitude of the echo is related with τ and Ts.The first sinc(·)is the weight of the amplitude which varies with n.The second sinc(·)denotes that the MF output is the accumulation of several sinc(·)functions.Besides,fs/μ is the time interval between the adjacent peaks.Thus,the range interval is

In anechoic chamber,τ should be smaller than the roundtrip time of electromagnetic wave and ΔR should be larger than target size L to ensure that the target is fully detected.Then,the ITR control parameters should satisfy the following inequalities:

Some conclusions are drawn with the ITR method.

(1)The echo energy is small if Tsis large(fsis small)and τ is small in Eq.(8).As a result,it is difficult to recover target information.Therefore,Tsshould be small enough to increase the echo energy.

(2)According to Eq.(9),ΔR＞L can be guaranteed with a small Ts.Then,the window function can be used to eliminate the fake peaks in the range pro file when n≠0.

3.Experiment configuration and procedure

The ITR is conducted by alternately turning on and off the antennas in the system.There are two key points in the ITR experimental system.One is that the time of turning on and off the antennas should be calculated with the sizes of target and anechoic chamber in Eq.(10).The other is that the generation of complete pulse signal should be continuous to ensure the phase coherence.

3.1.Target model and experiment system configuration

The corner re flectors are used in the experiment to represent the single scattering center and multiple scattering centers in target.The target models are shown in Fig.2.

The experiment is conducted on X band.According to the size of reflector,the Radar Cross Section(RCS)of the single scattering center in Fig.2(a)is about 37.7 m2.In Fig.2(b), five re flectors with different sizes are placed evenly to represent the multiple scattering centers in the target.

The ITR experiment system and radar transmitter and receiver are shown in Fig.3.

Fig.2 Target models.

Fig.3 ITR experiment system and radar transmitter and receiver.

As shown in Fig.3(a),the ITR experiment system consists of Arbitrary Waveform Generator(AWG),Vector Signal Generator(VSG),Down-Conversion(DC)module,intermedi-ate frequency adjustment module and data acquisition module.The LFM signal file is downloaded into the AWG with bandwidth from 0 to 500 MHz.The PRF of pulse signal is 807 Hz.It can be adjusted in different experimental scenes.The VSG is Agilent E8267D with frequency range of 100 kHz–40 GHz.The intermediate-frequency signal can be up-converted to X band by the VSG.And then,the radio-frequency signal is radiated by the transmitter in Fig.3(b).The receiver in Fig.3(b)is connected to the DC module.Then,the down-converted echo can be adjusted in the intermediate frequency adjustment module.Data acquisition module is used to store the ITR echo with the maximum sampling frequency of 1.6 GHz.The sampling time is adjustable in data acquisition module.

3.2.Configuration of anechoic chamber

The experiment configuration in anechoic chamber is shown in Fig.4.The transmitter and receiver are fixed as shown in Fig.3(b).The frequency range of compact range reflector is 0.1–40 GHz.The re flectors are placed on the platform.In the experiment,the ITR is controlled by the experiment system with a fixed period.The pulse compression is carried out on the computer when the ITR echo is obtained.

3.3.Experiment procedure

The detailed experiment procedure of ITR is shown in Fig.5.The LFM signal is generated by the AWG with intermediate frequency being 300 MHz.Then,the VSG up-converts the signal to 9.3 GHz and the antenna radiates the signal to the target.The ITR is performed by alternately turning on and off the antennas.Finally,the ITR echo data is obtained in the data acquisition module after down-conversion and intermediate frequency adjustment.The data processing is then conducted on the computer and the true peaks in the range pro file can be extracted.

4.Experiment results

The transmitted and re flected signals are coupled when pulse radar signal is adopted in RFS.Although the isolator is used between the antennas,20,21the transmitted signal energy is still much larger than the re flected signal energy.Thus,the echo signal is covered because the two signals are coupled.Firstly,the target probing is conducted when the ITR is not adopted in the experiment system.Secondly,the target probing experiment with ITR is conducted according to the experiment procedure in Fig.5.

4.1.Target probing without ITR method

The range between the target and antenna is R=15 m.Tp=20 μs,B=5 MHz.Thecoupledechosignalsare obtained without ITR method.Then,the Hamming window is adopted before pulse compression which suppresses the sidelobe of range pro file.The results are shown in Fig.6.

As revealed in Fig.6(a),the target echo is submerged in the transmitted signal because the transmitted signal energy is much larger than the echo energy.The pulse compression of the coupled echo is presented in Fig.6(b).The true position of target is submerged in the main lobe of pulse compression.Therefore,it is necessary to use ITR to solve the signal coupling problem in RFS.

4.2.ITR target probing with narrow band LFM signal

Because the distance between antenna and target is 15 m,τ should be smaller than 0.1 μs to receive the echo.The single scattering center in Fig.2(a)is adopted for the narrow band signal.Other parameters are listed in Table 1.

The ITR echo and pulse compression results are obtained in Figs.7 and 8 with different ITR parameters.

Fig.7(a)shows the ITR echo obtained by the experiment system when Ts=0.6 μs.The pulse compression results of ITR echo are shown in Fig.7(b).It can be seen that the distance of true peak is 15.03 m in Fig.7(b),which is almost equal to the real range between the antenna and target in the experiment scene.In addition,ΔR is 2 km when Ts=0.6 μs according to Eq.(9).As revealed in Fig.7(b),there are two fake peaks adjacent to the true peak with the range interval being about 2 km,so the experiment results in Fig.7(b)are in agreement with the analysis.

Fig.4 Experiment configuration in anechoic chamber.

Fig.5 Experiment procedure of ITR.

Fig.6 Coupled signal in time domain and pulse compression.

Table 1 Parameters of ITR experiment with narrowband LFM.

In Fig.7(c),the sidelobe of range pro fileis about-15 dB after pulse compression when there ctangular window is adopted.But the sidelobe is suppressed when Hamming window is adopted.Moreover,since the positions of fake peaks can be calculated,the rectangular window is applied to extract the true peak of the target,as shown in Fig.7(d).

When Ts=2 μs and τ=0.08 μs,the experiment results are presented in Fig.8.

Fig.8(a)shows the ITR echo when Ts=2 μs.The range pro file is shown in Fig.8(b)when Hamming window is adopted for sidelobe suppression.Because Tsincreases,the echo energy decreases with τ being fixed.Therefore,the amplitude of true peak is-8.787 dB as shown in Fig.8(b),which is smaller than that in Fig.7(b).

Similarly,the range interval ΔR is 600 m according to Eq.(9).The range pro file in Fig.8(b)indicates that the range interval between the fake and true peak is about 600 m. Therefore, the rectangular window is applied to extract the true peak of target as shown in Fig.8(c).

4.3.ITR target probing with wide band LFM signal

The multiple reflectors in Fig.2 are adopted for the wide band LFM signal.The distance between each reflector is about 1.4 m and L will be 5.6 m.Other experiment parameters are shown in Table 2.

The experiment results are presented in Figs.9 and 10.

Fig.9(a)presents the ITR echo of wideband LFM signal.Fig.9(b)shows the range pro file obtained by the ITR echo.In Fig.9(c),it can be seen that the distance between each peak is about 1.4 m,which is consistent with the distance between each reflector on the platform.Besides,ΔR is 9 m according to the parameters in Table 2.As shown in Fig.9(c),the range interval between the true and fake peaks is about 9 m,which verifies the accuracy of the analysis.Moreover,the rectangular window is used to extract the true peaks in range pro file because ΔR ＞ L.The results are shown in Fig.9(c).

ΔR becomes smaller than L when Tsincreases.Then,the true peaks and fake peaks may be coupled in the range pro file.For example,when Ts=0.6 μs and Ts=0.7 μs,the experiment results are presented in Fig.10.

Fig.10(a)shows the ITR echo when Ts=0.6 μs.And thus, ΔR is 6 m.Because ΔR is slightly larger than L,the true and fake peaks are close after pulse compression as shown in Fig.10(b).Besides,the resolution declines when the Hamming window is adopted in the ITR echo.As a result,the true peaks in the range pro file are difficult to be extracted with the rectangular window in Fig.10(b).The experiment results of Ts=0.7 μs and ΔR=5.14 m are shown in Figs.10(c)and (d).The echo energy in Fig.10(c)is smaller than that in Fig.10(a)because Tsin Fig.10(c)is larger than that in Fig.10(a)when τ is fixed.Besides,the true and fake peaks overlap in Fig.10(d)since ΔR＜L.Then,the window cannot be used to extract the true peaks in the range pro file.Therefore,the ITR period should be small enough to ensure ΔR ＞ L so that the true peaks in the range pro file can be extracted by rectangular window after pulse compression.

According to the principle of ITR RFS scheme and the experiment results,the comparison between the ITR and conventional swept-frequency RFS schemesislisted in Table 3.

As shown in Table 3,the swept-frequency RFS scheme is mainly used in the target RCS and dynamic scattering properties measurement.However,as the pulse signal is appliedin theITR RFSscheme,variousmodulations such as LFM and PCM canbe adopted tosimulate the whole process of real radar system target probing.The performance of pulse radar target measurement can be verified in RFS and the highly maneuvering target measurement for space targets can also be performed.Besides,according to the experiment results,the ITR method can eliminate the coupling between the transmitted and reflected pulse radar signals for target probing in RFS.

Fig.7 ITR echo and pulse compression(Ts=0.6 μs).

Fig.8 ITR echo and pulse compression(Ts=2 μs).

Fig.9 ITR echo and pulse compression(Ts=0.4 μs).

Table 2 Parameters of ITR experiment with wideband LFM.

Fig.10 ITR echo and pulse compression.

Table 3 Comparison of ITR RFS scheme and swept-frequency RFS scheme.

5.Conclusions

The pulse radar target probing is performed in RFS by constructing the ITR experiment system.The target echo and range pro file are obtained with ITR method.Experiment results verify the accuracy and effectiveness of ITR method for eliminating the coupling of transmitted and reflected pulse radar signals at the receiver.Compared with the sweptfrequency RFS scheme,the typical modulations such as LFM and PCM can be used in the ITR experiment system for pulse radar target probing.Therefore,the ITR experiment system is extensible and can be used for the highly maneuvering target measurement,target signature analysis and imaging with pulse radar signal in the future.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China(Nos.61101180,61401491 and 61490692).

1.He WC,Zhang LX,Li NJ.A new method to improve precision of target position in RFS.Proceedings of the international conference on microwave and millimeter wave technology;2007 Apr 18–21;Guilin,China.Piscataway:IEEE Press;2007.p.1–3.

2.Liu J,Wang XS,Ma L,Wang T,Feng DJ,Li YZ,et al.Experimental study on dynamic scattering properties of space precession target.Acta Aeronaut Astronaut Sin 2010;31(5):1014–23[Chinese].

3.Ye TS,Huang PL,Shu CY,Wang Y,Peng YK.Scattering characteristics simulation and experimental analysis of precession cone target.J Beijing Univ Aeronaut Astronaut 2016;42(3):588–95[Chinese].

4.Chiang WY,Wu MH,Wu KL,Lin MH,Teng HH,Ko CC,et al.A microwave applicator for uniform irradiation by circularly polarized traveling waves in an anechoic chamber.Proceedings of 39th international conference on the infrared,millimeter,and terahertz waves;2014 Sep 14–19;Tucson,USA.Piscataway:IEEE Press;2014.p.1.

5.Bazhenov NR.The particularities of antenna phase center measurements in a microwave electromagnetic anechoic chamber.Proceedings ofthe conference on precision electromagnetic measurements;2014 Aug 24–29;Rio de Janeiro,Brazil.Piscataway:IEEE Press;2014.p.196–7.

6.Liu DW,Huang J,Song L,Ji JZ.influence of aircraft surface distribution on electromagnetic scattering characteristics.Chin J Aeronaut 2017;30(2):759–65.

7.Usin VA,Kovalchuk VA,Usina AV.Estimation of the accuracy of the antenna near field amplitude and phase determination in the frequency band by measurements of impulse signals parameters.3rd International conference on ultra wideb and and ultrashort impulse signals.2006 Sep 18–22;Sevastopol,Crimea.Piscataway:IEEE Press;2006.p.367–9.

8.He LW,Xue ZH,Ren W,Li WM.S-band time domain near field planar measurement for RCS insidean anechoic chamber.7thA sia-Pacific conference on environment electromagnetics(CEEM);2015 Nov 4–7;Hangzhou,China.Piscata way:IEEE Press.p.108–11.

9.Liu JB,Mao JJ,Li GS,Liu PG.Study and development of UWB compact range measuring system for radar target of time domain.J Nati Univ Def Technol 2007;29(2):94–100[Chinese].

10.Gao X,Liu Z H,Wu Z.Experimentation on scattering characteristics of serrate gap.Acta Aeronaut Astronaut Sin 2008;29(6):1497–501[Chinese].

11.Huang H,Pan MH,Lu ZJ.Hardware-in-the-loop simulation technology of wide-band radar targets based on scattering center model.Chin J Aeronaut 2015;28(5):1476–84.

12.Ma L,Li Y.Research on simulation of dynamic full-polarization radar echo for spatial targets.J Syst Simul 2012;24(3):628–31[Chinese].

13.Shu T,Tang B,Yin KJ,Sun QY,Chen YC,Yu WX.Development of multichannel real-time hardware-in-the-loop radar environment simulator for missile-borne synthetic aperture radar.IEEE radar conference;2015 May 10–15;Arlington,USA.Piscata way:IEEE Press;2015.p.368–73.

14.Olin ID,Queen FD.Dynamic measurement of radar cross sections.Proc IEEE 1965;53(8):954–61.

15.Peng G,Tian JJ,Wang ZR.Research on two-channels pulse to pulse frequency shifting outdoor RCS measurement radar.J Electr Inf Technol 2006;28(9):1582–5[Chinese].

16.Chang WG,Liang DN,Zhou ZM.Research on Rail-UWB-SAR.Acta Electron Sin 2001;29(9):1213–6[Chinese].

17.Hu CF,Xu JD,Li NJ,Zhang LX.Full-polarization synthetic aperture radar hard-in-loop system.J Syst Eng Electron 2010;32(7):1537–9.

18.Wang XS,Liu JC,Zhang WM,Fu QX,Liu Z,Xie XX.Mathematic principles of interrupted-sampling repeater jamming(ISRJ).Sci China Ser E:Inform Sci 2007;50:113–23.

19.Feng DJ,Tao HM,Yang Y,Liu Z.Jamming de-chirping radar using interrupted-sampling repeater.Sci China Ser E:Inform Sci 2011;54(10):2138–46.

20.Michishita N,Chisaka T,Yamada Y.Evaluation of RCS measurementenvironmentin compactanechoic chamber.Internationalsymposium on antennas and propagation;2013 Oct 23–25;Nanjing,China.Piscataway:IEEE Press;2013.p.400–3.

21.Niroo-Jazi M,Denidni TA,Chaharmir MR,Sebak AR.A hybrid isolator to reduce electromagnetic interactions between Tx/Rx antennas.IEEE Antennas Wireless Propaga Lett 2014;13:75–8.