Study On The Jamming To Synthetic Aperture Radar

Study on the Jamming to Synthetic Aperture Radar Xianfeng Tian Guangyou Fang Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China 1. Introduction The history of synthetic aperture radar (SAR) may retrospect to 50's last century. The main purpose of SAR is to image the terrain of the detected area using microwave [1, 2]. It can work at all time and all weather; moreover it has ability to penetrating foliage. Owing to these features, it finds its way in many civil and military applications. The susceptibility of SAR system to interference is of concern in both military and civil applications [1,2]. Synthetic aperture radars play very important roles in the just passed war. How to protect keystone areas to avoid reconnaissance by opponent SAR becomes key field for the study of ECM [2]. The synthetic aperture radar has special features different from conventional radar, such as high processing gain in range and cross-range, large antenna aperture achieved by movement of radar platform and much wider signal bandwidth [1, 3]. So studying on jamming to SAR is significant to war in the future. According to the relationship of noise interference and echo, there are several jamming types, such as, noise jamming, coherent jamming, part coherent jamming and deceptive jamming [2]. This paper introduces a novel radar equation relating the effectiveness of SAR ECM jamming to the main system parameters. Through analyzing the signal power of the video data (image data), it is found that deceptive jamming to SAR is a good method; and coherent jamming is feasible. 2. SAR ECM Radar Equation An expression for a measure of the susceptibility of a SAR to jamming as a function of system parameters is derived. It is shown to be a simple relationship between average transmitted power of SAR, the jammer’s equivalent isotropic radiated power (EIRP), the incidence angle and the normalized clutter backscatter. The assumption is made that the SAR follows the conventional design constraints [3, 4, 5]. The echo power that radar received is presented by: 

Prs =

PG t tσ A (4π r )

2 2

=

PG t t Gr σλ

2

(4π ) r

3 4



(1)

σ = σ 0 A; A = ρ a ⋅ ρ gr , ρ gr is ground resolution. Taking into account system loss,

loss gene ( K S ) is introduced. Equation(1) can be changed: 

Pr s =

2 PG σ 0 λ 3 ⋅ PRF ⋅ ρ gr 1 λ t ⋅ ⋅ r ⋅ T ⋅ PRF = σ ρ ( ) gr a 0 2 Ta vs (4π )3 r 4 K s 2(4π )3 r 3 vs K s 2 PG t t Gr σλ



(2)

The power intercepted by the radar is represented by: 

Prj =

Pj G j 4π R j 2

⋅ L jγ j

Pj G j B B λ2 λ2 ⋅ Ga (θ ) r Prj = Ga (θ ) r γ L j j 2 Bj Bj 4π 4π 4π R j



(3)

The noise single ratio (SNR) is presented by:

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4π p j G j r 4 Ga (θ ) L j γ j KJ = = ⋅ ⋅ ⋅K S  Pr s Ga Kf Pt Gt σ R j 2 Prj

(4)

The jamming equation is presented by: 

EIRP=G j Pj =

Pr Gr K J σ R j 2 4π r

4

⋅

Kf L jγ j

⋅

Ga ⋅K S  Ga (θ )

(5)

Pj G j is jamming power. Pr Gr is transmitter power. R j is the distance between

radar and jammer. r is the distance between radar and target; K f = B j / Br is receiver bandwidth and jamming signal bandwidth. B j is jamming barrage bandwidth. Br is SAR chirp signal bandwidth. L j is jammer loss. γ j is polarization loss. Ga / Ga (θ ) is the ratio of SAR major lobe gain and minor lobes gain. Basic condition of valid jamming to SAR is J / S ≥ K J , so jammer valid working range can be presented by: Ga (θ )r 4 ≥ K J ⋅



2 PG t t σ R j Gt (θ ) L j γ j ⋅  4π p j G j Kf

(6)

When jammer and target stand on same position, Ga (θ ) = Gt (θ ) and least valid target distance Rt min can be got: 2 L jγ j 1 PG t tσ R j ⋅ )4  4π p j G j K f

(7)

2 PG t t σ R j Gt (θ ) L j γ j ⋅  Kf 4π Rt min 4

(8)

Rt min ≥ ( K J



Least jamming power is presented by: EIRP ≥ K J



3. Jamming Energy Distributing Jamming power is a best way to scale jamming effect. It is better to attain same jamming effect, if the jamming power is less. Jamming equation is deduced from energy distribution at this chapter. The power that jammer needs to transmit is analyzed at different jamming modes [6, 7]. Suppose there is a ground distinguish cell ρ x × ρ g , one echo power E (m, n) is: E ( m, n ) = K ⋅ Pt



Gt ρ x ρ g σ At (4π R 2 ) 2

Δt 

(9)

At is effective receiving area; K is gain of receiver power, Gt = 4π At / λ 2 , Δt is sampling holding time. Equation(9) can be changed: E ( m, n ) = KPt



Gt2 ρ x ρ g σλ 2 (4π )3 R 4

Δt 

(10)

At one echo jamming power E j (m, n) is presented byΚ E j ( m, n ) = KPj



G j At (θ j ) L j γ j ⋅ Δt  Kf 4π R j 2

(11)

From equation(11) and equation(4), K J of one echo is presented by: 

KJ =

E j (m, n) E (m, n)

=

(4π R) 2 R 2 Pj G j At (θ j ) L j γ j ⋅ Pt Gt2 ρ x ρ g σλ 2 K f R 2j



(12)

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At (θ j ) is

valid caliber at jammer direction. After imaging to echo data, NSR of video signal is presented by: KJ =



E j (m, n) J r J a L j γ j J r J a (4π R) 2 R 2 Pj G j At (θ j ) = ⋅  E (m, n) MN Kf MN R 2j Pt Gt2 ρ x ρ g σλ 2

(13)

Jr is gain of jamming signal in range. Ja is gain of jamming signal in cross-range. MN is energy gain; M = τ ⋅ B is range gain; and N = R ⋅ Δ ⋅θ PRF / v ,is azimuth gain. Now, the energy gain of SAR amounts to 60Д70dB, so much as more. From equation(13) and equation(8), EIRP can be get:

Kf

MNPav Gt2 ρ x ρ g σλ 2

R 2j

KJ L j γ j J r J a (4π R) 2 PRF ⋅τ ⋅ At (θ j ) R 2 Kf RΔθ / ( v / PRF ) BPav Gt2 ρ x ρ g σλ 2 R 2j KJ  = L jγ j J r J a R2 ( 4π R )2 PRF ⋅ At (θ j )

EIRP ≥



=

Kf

Δθ ⋅ Pav Gt2 ⋅ ( c / 2 ) Δyσλ 2

R 2j

L j γ j J r J a ( 4π )2 R ⋅ PRF ⋅ v ⋅ At (θ j ) ⋅ sinα R 2

(14)

KJ

ρ x = 1/ B ⋅ ( c / 2 ) / sinα ; Pav is average transmitter power. α is angle of incidence; Δθ

is beam width. is different, if jammer works in different modes. Active noise jamming belongs to barrage jamming. It put noise interference over the echoes of targets to diffuse target information. Noise jamming signal is not coherent to SAR echo, so JaJr = 1 . Coherent jamming signal is mainly coherent to echo in range, and JaJr = N . Deceptive jamming is coherent to echo in range and cross-range, and JaJr = MN . Part-coherent jamming is intervenient, and 1 < JaJr < MN . Fig1 shows the jamming effect of four modes. For the purpose is to express focusing and distributing of jamming energy, coordinate data has not actual meaning. In noise jamming mode, energy distributes averagely in video signal, and jammer power required can amount to 106 watt. So it is too difficulty to obtain valid jamming effect. Deceptive jamming is a good mode, and the main energy focuses one spot. Parameters of SAR need to be reconnoitred accurately. Coherent jamming or part Coherent jamming is easy to implement, and needs not too large power, and energy focuses some area. Jamming effect is similar to noise jamming in jammed region. JaJr

4. Conclusion The paper manly relates jamming power and jamming effect of four jamming modes. Jamming equation associating with the SAR and jammer parameters is got. Jammer’s working distance is got through analyzing the jamming equation. Characteristic of four jamming modes is analyzed through the gain of video signal. Deceptive jamming is a preferred mode, if SAR parameters can be acquired accurately. Coherent jamming or part-Coherent is an important mode. References

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[1] [2] [3]

[4]

[5] [6]

[7]

J C Curlander, R N Mcdonough. Synthetic aperture radar systems and signal processing. John Wiley & Sons, Inc, 1991. Walter W.Gog. “Synthetic-aperture Radar and Electronic Warfare”. Artech House, 1993. Boston London. Christopher J C. “Some system considerations for electronic counter measures to synthetic aperture radar”. Radar, IEEE Colloquium on Electronic Warfare Systems, 1991, vol8.pp 1-7. Per Hyberg, “Assessment of Modern Coherent Jamming Methods against Synthetic Aperture Radar”. European Conference on Synthetic Aperture Radar, 1998.5 pp.391 - 394ʳ K Dumper eta1, “Spaceborne Synthetic Aperture Radar and Noise Jamming ”, Radar. 1997.10, pp. 411-414. Zheng Shenghua, Xu Dazhuan, Jin Xueming. “Study on active jamming to synthetic aperture radar”. International conference on computational electromagnetic and its applications proceedings, 2004. 403-406. Wang Shengli, Yu Li, Ni Jinlin, “A Study on the Active Deception Jamming to SAR”. ActaElectronica Siniea. 2003,31(12):pp.1900-1902. Figure

Fig1: Numeric Simulation Result of Four Jamming Modes

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