Methodological analysis for detection and determination of RGPO jamming rule 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.

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

1 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

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 V dl measured by an independent Doppler tracking system, is used to adjust the measurement parameters of target velocity and the change of range gates

2 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 1 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 1 Basic structure of a distance tracking system

The value of distance deflection at the output of the time discriminator at the ith

cycle is determined by the following formula:

0

C.







In this formula, Di, Di ns , and ΔDi indicate the real distance, extrapolated distance and deflection between them, respectively Di 1 , Di 2 denote the distance

from the range gate pulse G1 and G2 to the center point of target energy at the ith

cycle, respectively  m

2

C0

is the discrete value of target distance determined by the width of probing pulse 0.KADC 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:













K P H' H P H ' R '

X ( 2 ) K X ( 2 ) K Vdl

Where,      

X R R ' is the vector of distance and velocity parameters at the

ith cycle computed based on prior information X i-1 ; P i is the extrapolated variance

determined based on prior information; K i is the amplification factor of Kalman

filter at the ith cycle; X i+1 , P i+1 are posterior estimation state and posterior variance,

respectively; K 1 , K 2 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 T A

0 1

  

(8)

1 0 H

0 1

 

  

 

(9)

3 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

(2) (3) (4) (5) (6) (7)

the output of generator P pk =75KW; carrier frequency f c =10GHz; amplification gain

of the transmitter G t =40dB, and amplification gain of the receiver G r =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 R 0 =150Km, the velocity along the horizontal direction

V x =200m/s, and useful reflecting area 2

1m

The radar signals are appropximate successive pulse trains: with the pulse width  0.6 s , pulse repeatation period T i 10 s , width of pulse train

grp 5.4ms

  , and pulse train repeatation period T grp 100ms

In our simulation, instead of establishing a simulation model for the velocity

tracking system, Doppler velocity of the target V dl 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 t 1 1s, capture time of tracking system t capture 1s, range gate pull-off time

pull _ off

t 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

(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

(b) Location variation in comparison with target and RGPO jamming

Fig 5 Distance error of tracking system with RGPO jamming

(a) Velocity surge

(b) Velocity error

Fig 6 Target velocity obtained by distance tracking system vs Doppler velocity

3.2 Observation

Simulation result in Fig 2 shows that, distance tracking system without anti-RGPO solutions can be forced by anti-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

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 4 and Fig 6 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 (V dl = 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 V dl 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(V dl =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

Fig 7 Principle of detection based on CFAR

4 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 7 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

τ

τ ch

t

t

t

Τ x

CFAR level Clock

Here, i is calculated by the following formula:

0

x

where, S CFAR (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]:

(11)

where, b 0 is the factor of RGPO jamming delay time, a is the guidance coefficient, and B is the degree of RGPO jamming (B = 1 indicates linear rule, and B = 2

indicates quadratic rule) Due to the restrained time of jamming strategy is as about

1÷ 2s, b 0 can be estimated as follows:

q

i 1

1 b



where, within the range of ms Here, T ch is the period of pulse 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 2 – Calculation of error correlation coefficients:

2

2

tt



2

2

yy



ty

Step 3 – Calculation of coefficients a and B:

(17)

3

ch

( 1 2 ).10

q

T





Step 4 – Calculation of correlation coefficient:

tt

tt yy

S r

Guidelines for interpreting correlation coefficient are shown as below

0.7  r  1 strong correlation

0.5  r  0.7 moderate correlation

0.2  r  0.5 weak correlation

The RGPO rules determined by using power regression in case of the

manuevering acceleration of target a t = -25m/s2 with RGPO jamming acceleration

a j = 30m/s2 and a j = 35 m/s2 (in Swerling model 1) are illustrated in Fig 8

Fig 8 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 a j = 30 and B j = 2 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 anti-RGPO jamming systems

5 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

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 (V dl) 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

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