In this paper, the methodological analysis for a solution of early detection of RGPO jamming, by simultaneously using information of AGC voltage surge and variation of Doppler frequency spectrum of target signal is discussed. An algorithm for determination of RGPO rule based on power regression is also discribed.
Research METHODOLOGICAL ANALYSIS FOR DETECTION AND DETERMINATION OF RGPO JAMMING RULE IN FIRE CONTROL RADARS Trinh Ngoc Lam1, Nguyen Truong Son1*, Vu Hoa Tien1, Pham Vinh Tue2 Abstract: Detecting range-gate-pull-off (RGPO) jamming before pulling the range gate off of the target signal and then determining RGPO rule plays a very important role in the development of effective tracking systems in fire control radars In this paper, the methodological analysis for a solution of early detection of RGPO jamming, by simultaneously using information of AGC voltage surge and variation of Doppler frequency spectrum of target signal is discussed An algorithm for determination of RGPO rule based on power regression is also discribed Simulations results conducted on parameters of an actual fire control radar show that the measureddistance error in distance tracking systems can be cut down significantly by detecting RGPO jamming immediately after the moment of its occurence, and the RGPO rule can be determined by utilizing power regression with an insignificant error of about 7% Keywords: RGPO jamming, Distance tracking system, Jamming-to-signal ratio, Power regression INTRODUCTION Range-gate-pull-off (RGPO) is a jamming which degrades the tracking quality of fire control radars significantly It is a deceptive electronic counter measure (ECM) performed by targets to prevent the tracking radar from obtaining true information The radar pulse is obtained and then repeated at the radar with a controlled delay to pull the tracker’s range gate off of the true target Thanks to amplification, the jamming-to-signal ratio (JSR) is becoming higher, thereby causing the radar to track on deception signals [1] Eventually, the range gate is removed from the actual target When the range gate is sufficiently shifted out of the true target, RGPO is turned off, thus there is no signal at the input of tracking system that forcing the radar to reacquire the target Repetitive reacquisition of the target might result to the loss of target or the failure of missile launch [1] In recent years, there had been several researches on influence of RGPO into the tracking system of radars as well as fire control systems [1], [2], [3], [4] A number of detecting algorithms and anti-RGPO methods were proposed For example, characteristics of the SNR (Signal Noise Ratio) surge, AGC (Automatic Gain Control) voltage surge, and difference between Doppler velocity and changing speed of target distance were used for detecting the presence of RGPO jamming as well as analyzing its characteristics However, analyses and evaluations of these methods are performed individually and lack of the investigation of all of them in the same observation condition In this paper, characteristics of the methods mentioned above by establishing appropriate simulations on Matlab using parameters obtained from actual radarswill be investigated Besides, an algorithm for determining RGPO rule based on power regression is also proposed Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 25 Electronics & Automation The simulations are conducted by using state-of-the-art distance tracking system with the time discriminator based on middle frequency narrow band filter The fire control radar is the system which is based on Doppler-effect The investigation is focus on the discriminating characteristics, moment of occurring and changing law of RGPO jamming Analysis results might be used for developing new algorithms which allow to detect and discriminate RGPO jamming in a better manner In this paper, investigating object is a distance tracking system of the pulse-Doppler radar, in which the centripetal velocity of the target Vdl measured by an independent Doppler tracking system, is used to adjust the measurement parameters of target velocity and the change of range gates STRUCTURE OF DISTANCE TRACKING SYSTEM The basic structure of a distance tracking system of fire control pulse-Doppler radars using approximate successive probing pulses is shown in Fig In this structure, the time discriminator is constructed based on a general optimized successive approximation unit with the sum-difference processing [6], [7] There are two target signal channels at middle frequency in the time discrimination Each one is composed of a frequency mixer and a narrow filter One is called a sum channel, and the other is called a difference channel The distance and velocity signals of the target can be discriminated from these channels by using appropriate mixers and filters The difference between these channels is the manner of phase modulation for signals transferred to the mixer In the sum channel, a coherent oscillation signal without phase shifting is used for modulating the distance pulse signal G3 Whereas, in the difference channel, the tracking range gate pulse signal G1 is modulated by the same way as in the sum channel, but the tracking range gate pulse signal G2 is modulated by the coherent oscillation signal with a phase shifted by 1800 Location and travelling speed of tracking range gates are computed based on measurement parameters of target distance determined by tracking filters Fig Basic structure of a distance tracking system 26 T N Lam, , P V Tue, “Methodological analysis for in fire control radars.” Research The value of distance deflection at the output of the time discriminator at the ith cycle is determined by the following formula: Di Di Dins C. Di2 Di1 K ADC Di2 Di1 (1) In this formula, Di, Dins, and ΔDi indicate the real distance, extrapolated distance and deflection between them, respectively Di1, Di2 denote the distance from the range gate pulse G1 and G2 to the center point of target energy at the ith C.0 m is the discrete value of target distance determined by cycle, respectively the width of probing pulse K ADC is the conversion ratio of the AD converter As shown in Fig.1, a Kalman filter is used for estimation with the adjustment element is Doppler velocity of target obtained from velocity tracking system The Kalman filtering algorithm is shown as follows: X i A.X i 1 Pi A.Pi 1 A' Q Ki Pi H' H Pi H ' R ' X i 1 X i Ki Di Pi 1 I K i H Pi X i 1 ( ) K1 X i ( ) K Vdli Where, X i Ri Ri ' (2) (3) (4) (5) (6) (7) is the vector of distance and velocity parameters at the ith cycle computed based on prior information Xi-1; Pi is the extrapolated variance determined based on prior information; Ki is the amplification factor of Kalman filter at the ith cycle; Xi+1, Pi+1 are posterior estimation state and posterior variance, respectively; K1, K2 are adjustment factors for travelling speed of tracking range gate depending on changing speed of the target; Q, R are variances of process noise and measurement noise, respectively; A, H are state matrix and measurement matrix, repsectively, and they are caculated as follows: 1 A 0 1 H 0 T (8) 0 1 (9) SIMULATION RESULT AND OBSERVATION 3.1 Simulation Result For adequate investigation of RGPO jamming effects, in this paper, parameters of an actual fire control radar are used for simulation: maximum pulsed power at Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 27 Electronics & Automation the output of generator Ppk=75KW; carrier frequency fc=10GHz; amplification gain of the transmitter Gt=40dB, and amplification gain of the receiver Gr=100dB It is assumed that the target is equipped the ECM device, and it moves straight forward with a constant velocity: the heigh has not changed (H=5Km), the initial horizontal distance R0=150Km, the velocity along the horizontal direction Vx=200m/s, and useful reflecting area 1m The radar signals are appropximate successive pulse trains: with the pulse width 0.6 s , pulse repeatation period Ti 10 s , width of pulse train grp 5.4ms , and pulse train repeatation period Tgrp 100ms In our simulation, instead of establishing a simulation model for the velocity tracking system, Doppler velocity of the target Vdl shown in formula (7) is computed based on spectrum estimation of signals obtained by the radar In our work, simulation is conducted by using Matlab Phased.array toolbox for scenario of RGPO jamming generated at the instant t1 1s , capture time of tracking system tcapture 1s , range gate pull-off time t pull _ off 3s with different pull-off accelerations and JSRs (Jamming-to-Signal Ratios) The simulation results are shown in figures from Fig.2 to Fig.6 Fig.2 Range gate is pulled off by RGPO jamming when JSR=0.79dB Fig.3 Variation of AGC voltage effected by RGPO jamming 28 T N Lam, , P V Tue, “Methodological analysis for in fire control radars.” Research (a) In case of signal inside of the range gate (b) In case of signal outside of the range gate Fig.4 Doppler frequency spectrum of signal (a) Distance error Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 29 Electronics & Automation (b) Location variation in comparison with target and RGPO jamming Fig Distance error of tracking system with RGPO jamming (a) Velocity surge (b) Velocity error Fig Target velocity obtained by distance tracking system vs Doppler velocity 3.2 Observation Simulation result in Fig shows that, distance tracking system without antiRGPO solutions can be forced by RGPO jamming, even in a low J/R of 1.2 times in term of amplitude (JSR=0.79dB in term of power) Therefore, detection threshold established at JSR ≥ 3dB (J/R ≥2 in term of amplitude) for detecting 30 T N Lam, , P V Tue, “Methodological analysis for in fire control radars.” Research RGPO in distance tracking systems is acceptable, because of its fairly high detection probability As shown in Fig 3, the presence of RGPO jamming is started at the instant of occuring AGC voltage surge When RGPO jamming appears, AGC voltage decreases greater than 6dB as compared to that of the target signal without RGPO jamming The results in Fig and Fig indicate that, when RGPO jamming pulls the range gate off of the target signal, i.e there is no target signal inside of the range gate, the value of Dopller velocity will be changed suddenly to zero (Vdl = 0m/s) During the time RGPO jamming pulls the range gate off, when the target signal is still inside of the range gate, the difference between distance changing speed computed by tracking filter and Doppler velocity of the target is not high enough for velocity discrimination ability of the radar Consequently, the discrimination solution based on the analysis of distance changing speed computed by tracking filter and Doppler velocity of the target might not be able to specify whether the RGPO jamming is arised or not, before it pulls the range gate off completely of the target signal By analyzing simulation results in Fig 6, it can be seen that, the anomaly of distance changing speed computed by tracking filter as compared to Doppler velocity of the target Vdl measured by velocity tracking system is only occurred at the instant the target signal starts moving out of the range gate After it is outside of the range gate, the distance changing speed will be adjusted to the Doppler velocity of RGPO jamming(Vdl=0 m/s) Obviously, this sign can be used for detecting the instant of occurrence of the RGPO jamming and then determining the RGPO rule τch τ τ+Δτ1 τ+Δτ2 τ+Δτ3 CFAR level Clock t t Τx t Fig Principle of detection based on CFAR ALGORITHM FOR DETERMINATION OF RGPO RULE Determining the RGPO rule after its detection is very important for the distance tracking system The RGPO detection can be implemented by utilizing CFAR (Constant False Alarm Rate) scheme [8] and FFT (Fast Fourier Transform) as shown in Fig In the figure, is the CFAR’s pulse width without RGPO jamming, i is the CFAR’s pulse width extension at the ith cycle caused by RGPO jamming, Tx is the period of reflected pulse, and ch is the width of pulse train, respectively Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 31 Electronics & Automation Here, i is calculated by the following formula: SCFAR ( ).Tx x ( ch gc ) (10) where, SCFAR (0) is the amplitude of central spectrum after FFT and gc is the delay time of probing pulse, respectively In general, RGPO rule can be expressed by the following formula [2], [3], [9]: G S CFAR ( ),t b0 a t B (11) where, b0 is the factor of RGPO jamming delay time, a is the guidance coefficient, and B is the degree of RGPO jamming (B = indicates linear rule, and B = indicates quadratic rule) Due to the restrained time of jamming strategy is as about 1÷ 2s, b0 can be estimated as follows: q b0 i q i 1 (12) where, q ( ).10 within the range of ms Here, Tch is the period of pulse Tch train To compute we propose a method using the power regression The coefficients a and B of formula 11 are computed as follows: Step 1– Average calculation: (13) Step – Calculation of error correlation coefficients: U ln t S tt ln t u u 1 U S yy S ty u ln y lnt ln y u 1 U u 1 ln t U ln y U u (14) U u 1 U lnt u ln t U ln y U 2 u u 1 ln y U (15) U u ln y ln t u u 1 U ln y u ln tln y (16) Step – Calculation of coefficients a and B: (17) 32 T N Lam, , P V Tue, “Methodological analysis for in fire control radars.” Research Step – Calculation of correlation coefficient: r S tt S tt S yy (18) Guidelines for interpreting correlation coefficient are shown as below 0.7 r strong correlation 0.5 r 0.7 moderate correlation 0.2 r 0.5 weak correlation r 0.2 no correlation The RGPO rules determined by using power regression in case of the manuevering acceleration of target at = -25m/s2 with RGPO jamming acceleration aj = 30m/s2 and aj = 35 m/s2 (in Swerling model 1) are illustrated in Fig Fig RGPO rule computation based on power regression As shown in the figure, in the worst case, the coefficients a and B calculated based on power regression are a = 30.8367 and B= 1.8618 as compared to the actual aj = 30 and Bj = The maximum error of coefficient a is approximate 2.8%, and that of coefficient B is approximate 7% This result is completely acceptable and it shows that, our proposed method can be applied to develop actual antiRGPO jamming systems CONCLUSIONS Detecting RGPO jamming before it pulls the range gate off of the target signal and then, determining RGPO rule plays a very important role for development of Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 33 Electronics & Automation effective tracking systems in the fire control radars By detecting RGPO jamming immediately after the moment of its arising, the measurement distance error in distance tracking systems can be cut down significantly (as illustrated in Fig 5) Besides, early detection of RGPO jamming can be achieved by using information of AGC voltage surge and variation of Doppler frequency spectrum of target signal, simultaneously In addition, direction information of the voltage surge of Doppler velocity (Vdl) and distance changing speed from tracking filter can be used for analyzing characteristics of RGPO jamming It indicates the changing direction of RGPO jamming which is very useful for development of anti-RGPO jamming algorithms Determining RGPO rule after detecting can be performed by the use of power regression The computation result attained by this way is quite exact with an insignificant error of about 7% This fact indicates the ability of proposed algorithm for developing effective anti-RGPO jamming systems in practical area REFERENCES [1] F Neri, “Introduction to Electronic Defense Systems” 2nd Edition, SciTech Publishing: Raleigh, 2006 [2] Haiquan Xu, Guohong Wang, Jing Bai, “A Modified Anti-RGPO Jamming Algorithm”, IEEE 10th International Conference on Signal Processing, pp.2555-2559, 2010 [3] P R Katala, T A Chmielewski, “Range Gate Pull Off (RGPO): Detection, Observability and Alpha-Beta Target Tracking”, IEEE 29th Southeastern Symposium on System Theory, pp.505-508, 1997 [4] Jiang Changyong, Gao Meiguo, Fu Xiongjun, Wang Zongbo, “Anti-RGPO Method Based on Memory Tracking”, IEEE International Conference on Radar, pp.365-370, 2008 [5] Robert N Lothes, Michael B Szymanski, and Richard G Wiley, “Radar Vulnerability to Jamming”, Artech House London, 1990 [6] А.И.Канащенкова, В.И.Меркулова, “Защитарадиолокационных систем от помех ”, Радиотехника (Т1,2,3), Москва, 2003 [7] А.И.Канащенкова и В.И.Меркулова, “Оценвание Дальности и Скорости в Радиолокационных Систем”, Москва, “Радиотехника”, 2004 [8] R B James, D F Ricardo, “A constant-false-alarm-rate algorithm”, Linear Algebra and Its Applications, vol 172, no 92, pp 231-241, 1992 [9] Naval Air Warfare Center Weapons Division, “Electronic Warfare and Radar Systems Engineering Handbook”, 2013 34 T N Lam, , P V Tue, “Methodological analysis for in fire control radars.” Research ABSTRACT PHÂN TÍCH KHẢ NĂNG PHÁT HIỆN VÀ NHẬN DẠNG QUY LUẬT CỦA NHIỄU DẪN CỰ LY TRONG ĐÀI RADAR ĐIỀU KHIỂN HỎA LỰC Phát sớm nhận dạng quy luật nhiễu dẫn cự ly (RGPO) có vai trò quan trọng việc xây dựng hệ bám cự ly đài radar điều khiển hỏa lực Trong báo này, tác giả sâu phân tích dấu hiệu phát nhiễu dẫn cự ly qua đột biến điện áp mạch tự động điều chỉnh hệ số khếch đại (AGC) thay đổi phổ tần số Doppler, khả nhận dạng quy luật nhiễu dựa thuật tốn hồi quy hàm mũ Kết mơ dựa tham số thực đài radar điều khiển hỏa lực sai số cự ly đo giảm đáng kể nhiễu dẫn cự ly phát xuất Bên cạnh đó, luật dẫn nhiễu xác định cách tương đối xác nhờ áp dụng thuật toán hồi quy hàm mũ với sai số nhỏ khoảng 7% Từ khóa: Nhiễu dẫn cự ly, Hệ bám cự ly, Tỷ số tín tạp, Hồi quy hàm mũ Received date, 13th Jul., 2017 Revised manuscript, 27th Aug., 2017 Published, 01st Nov., 2017 Địa chỉ:1Department of Control Engineering, Le Quy Don Technical University; Institute of Science and Technology * Email: sonhuongjp@gmail.com Journal of Military Science and Technology, Special Issue, No 51A, 11 - 2017 35 ... the RGPO jamming and then determining the RGPO rule τch τ τ+Δτ1 τ+Δτ2 τ+Δτ3 CFAR level Clock t t Τx t Fig Principle of detection based on CFAR ALGORITHM FOR DETERMINATION OF RGPO RULE Determining... effected by RGPO jamming 28 T N Lam, , P V Tue, Methodological analysis for in fire control radars. ” Research (a) In case of signal inside of the range gate (b) In case of signal outside of the... antiRGPO jamming systems CONCLUSIONS Detecting RGPO jamming before it pulls the range gate off of the target signal and then, determining RGPO rule plays a very important role for development of