A Study On Active Jamming To Synthetic Aperture Radar

ZOO4 3d International Conference on Computational Electromagnetics and Its Applications Proceedings

A Study on Active Jamming &O Synthetic Aperture

Zheng Shenghua”, Xu Dazhuan’, Jin Xueming’, Han Hua’ 1. College of Information Science and Technology, Nanjing University ofAeronautics and Astronautics, Nanjing 210016, CHINA 2. the 38th Institute of CETC, Hefei 230031, CHINA E-maiI: zsh91 @sohu.com very important roles in the just passed war. How

Abstract: Synthetic aperture radar (SAR) has its own special features different from conventional radar. It has high processing gain, so jamming to SAR is more difficult than that to conventional radar. Jamming to SAR finds its important usage in some military scene. Started with the operating features of the SAR, the possibility and the effectiveness of several active jamming methods to SAR are analyzed. Because of high processing gain for S A R , it needs very high barrage power to

to protect keystone areas to avoid reconnaissance

by opponent SAR becomes key field for the study of ECM.

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. The jamming to S A R is different from that to

jamming SAR, and it is a good method to adopt deceptive jamming. The simulated results of

conventional radar, and also the effectiveness of the different jamming mode is different.

generating deceptive signal are presented in this paper. It was founded that the jamming method of barrage plus deception is an effective one. Key words: synthetic aperture radar; active jamming; signal to jamming ratio; clutter to jamming ratio

In this paper, the features and theorem of SAR are reviewed. The simulated results of generating deceptive signal are given.

2. SAR OPERATION FEATlTRES ‘I1 [*I

1. INTRODUCTIOK

The purpose of S A R is to improve the radar’s spatial resohtion and form two-dimensional or three-dimensional image of interested a m . There

The history of syntheric aperture radar (SAR) may retrospect to 50’s last century. The main purpose of SAR is to image the terrain of the

are two types image of SAR: point target and extended target. Normally, point target corresponded to manmade target and extended

detected area using microwave. It can work at all time and all weather, moreover it has ability to penetrating foliage. Owing to these features, it

target related to clutter. The clutter described the reflectivity function of imaged area, so it is a key factor for imaging radar.

finds its way in many civil and military applications. Many countries such as the U.S, Eumpe, Russia, Canada and Japan, launched their

The antenna movement is the basis to gain high cross-range resolution. By regular movement, a very large aperture sntenna will be formed. The

S a satellites. There are also m n y airborne synthetic aperture radars developed and equipped for military use. Synthetic aperture radars play

azimuth resolution of SAR is represented by

0-7803-8562-4/04/$20.0002004 IEEC

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where

L,

A

several jamming types, such as, barrage jamming, spot jamming and scan jamming. The image from the radar jammed by noise

= -r

1

is antenna beam width at

interference will present, speckle noise. n e barrage noise covered the whole bandwidth of

-6dB, r is distance between radar and

S A R radar. The size of noise speckle is the order

target.

of range and azimuth resolution. The large amount

of noise samples added no coherently, so the

;t is wavelength.

multiple looks processing of jamming noise

I!,, is synthetic aperture length.

smoothed the brightness variety of pixels. The effect is just like the thermal noise.

I is real aperture of antenna. The ideal azimuth resolution is

1 A, =-I 2

The radar image contaminated by spot jamming noise is different from that of barrage noise jamming. The Fourier transform of

(2)

After processing of correlation or FFT, the

narrowband jamming noise will enlarge the

processing gain of point target or extended target

speckle, so the speckle size of the image is larger

is obtained as:

than that caused by thermal noise and clutter in the

range. The speckle size in azimuth is the same as

that of the thermal noise or clutter. In order to get good effectiveness of the banage noise jamming, enough barrage power must be transmitted. Owing to the special operation and processing method of S A R , calculation of the barrage jamming power is different from that of the conventional radar. The noise power intercepted by the radar is represented by:

The processing gain of noise interference is:

GL = (tB,)n

(4)

where r is pulse width of transmitted signal, B,

is band width of transmitted signal, n is pulse samples collected, The processing gain of SAR includes range compression gain and azimuth compression gain, and normally it may be up to 70dB. It is much higher than conventional radar.

N, = -ArGJ

47q2

3.JAMMlNGMODE ry141

is ERP of the jammer,

where

The main purpose of electronic warfare is to prevent hostile use electromagnetic spectrum by

= PjGjG,

G j is

jamming antenna gain in the direction of radar,

Gf,is

opponents. There are two types of active jamming to SAR,one is the active noise jamming, and the

jammer,

other is deceptive jamming.

radar antenna gain in the direction of

R, is distance between radar and

jammer.

3.1 ACTIVE NOISE JAMMING Active noise jamming belongs to barrage jamming. It put noise interference over the echoes of targets to diffuse target information. According to the relationship of bandwidth of noise interference and bandwidth of receiiw, lhere are

AAer correlation or FFT processing, the noise power is:

where

K,.

spectrum mismatch factor, rrB,

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timewidth 2nd bandn-idth product.

i!

deceptiw jamming will get 13.1-geprocessin g i n

iniegrsted

samples number. The signs1 to clutter ratio behind prccessiiig

io SBW j?"n~

is:

d.ecepri1.r janming.

power. Repeater jamming

is a kind of xrive

Digite!

radio

frequency

m e m q (DRFM) nlay be used to implement repeare:.jamrtiing. This is done by a system n.hich inkrcepts and retransmits 3 signal on The frequency and modulation mode of the enemy system, the reradiated signal being processed and modified to present erroneoils information on azimuth. range, numbers oftsrgers, and SG on.. If the characteristics of radar signal are

The signal to noise ratio of SAR under free jamming is:

maintained in the trmsponded signal, the signal processor will act on the repeater signals in the

p, T,pme,Za3 ( 4 d ) j aq 2 W 0 Y C where

V,

flying

velocity:

same manner as on SAR itseif, If only the intra-pulse characteristics such as: pulsewidth, bandwidth, chirp rate are piwewed, the pulse will be compressed in range. Without Doppler informstion, the resultant image is a bright line with the width of a compressed rmge resolvable element. If the correct Doppler information also added on the reradiated signal: the tmnsponded signals will be atso compressed in the azimuth. By this processing the deceptive image scene will folded OR the real scene image. The trmsmitted L.FM signal of S.AR is represented by

(8)

W,

azimuth

resolution.

From formula (7) and (X), w e obtain:

,191 The clutter ofS.4R plays an important role in imaging. In order t o cover profile of S.AR imge. a -1OdB clutter to interference ratio is needed. The clutter to interference ratio is represented by


signal. A- is FM rate, Q ( I ) is rectangular

Owing to the jammer and radar my not in the direction of boresighr of radar antenna beam,

R,R,

function. The deceptive jamming

is diRerent.

sigiid in time

domain is

3.2 DECEPTIVE JAMMING

d r > = p d ( ? J s x P [ j ~ & - r,>3

In order to implement deceptive jamming: the main panmeters such as frequency, pulse width. pulse repetition frequency? modulation mode etc. must be estimated. SAR obtained hlgh processing gain after range and azimuth compression. TO barnge strong m n n u d e target, very high barrage noise power is needed. The

n

x e x p l j r K ( r - r,,,) 2 j g j-~t,)

: 11:

where oa{ r m )is reIlecrion function of deceptive scene. I([) is deceprive si@

in range in one PE.

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.

The simulated deceptwe signal of one radar perid is illusbated in figure 1, and t h e compressed result is illustrated in figure 2. The X-axis is tune. The deceptive echoes WIU cover the real echoes of the imaged area to diffuse the irnage of real target and t o protect it.

4. CONCLUSIONS Jamming to synthetic aperture radar is different from that of conventional radar. To Jam SAR effectively, the intra-pulse and mter-pulse characteristics must be preserved. O w q to

specd featmes of SAR, It is key step to intercept the SAR signal and estimate the parameters precisely. Jamming to SAR is possible and meaninghl.

W. Goj, “Synthetic Apmtue Radar and Electronic Warfare”, US: Artech House Inc, 11J Walter

1993.

[2]

JohnC. Curlander, Robert N. McDonough,

“Synthetic Aperhre Radar Systems and Signal Processing”, NY,John Wiley & Sons hc,,1991.

Fig. 1 The decqtive Signal in range (echo in one radar period)

131 WANG Shengh, W b, NI J“, ZHANG Guangyi, “A Study on Active Deception Jamming to SAR”, ACTA ELECTRONlCA SINICA, voL3I ,

No. 12,2003[in Chinese). [4] Yin Chengyou, The Application of Signal Transfer to Countermeasure SAR, Jornnaf of Electronic E n g i n e d g Institute, Vol. 22, No. 4, 2003 (in Cbmese).

Fig. 2 The compressed result of deceptive signal

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