Radar Kavramı. Tipik bir Radar Sistemi

Transcription

1 Ankara Radar Systems and Remote Sensing Research Group TOBB UNIVERSITY OF ECONOMICS AND TECHNOLOGY TOBB ETÜ Turgut Özal - Bilkent DERS Radar Kavramı Yrd. Doç. Dr. Sevgi Zübeyde Gürbüz ELE 465: Radar Sinyal İşleme Temelleri ELE 565: Radar ve Sonar Sistemleri Tipik bir Radar Sistemi 1

2 Radar Konfigürasyonları 3 Monostatik Radar: Aynı antenden sinyal alıp veiliyor. Bistatik Radar: Alıcı ve verici konumları farklı Multistatik Radar: Birden fazla sayıda farklı konumları olan alıcılar ve vericiler Menzil Ölçümü 4 Zaman gecikmesinden menzil hesaplanır. Monostatik radar için ct R ct R

3 Menzil Çözünürlüğü İki hedefi birbirinden ayırt edebildiğimiz maksimum menzil farkı. Yansımaların zaman farkı: Menzil çözünürlüğü: 5 Açısal Çözünürlük 6 3

4 Doppler Etkisi Eğer radar ile hedefin arasında bir relatif hız farkı mevcut ise, Doppler effektinden dolayı gönderilen sinyalin frekansı (Ft) alınan sinyalin frekansından (Fr) farklı olacak. Doppler kaydırması: v: radara doğru hareket eden b,r hedefin hızı c: ışığın hızı F r 1 vc Ft 1 vc 7 Düşük hız için basitleştirelim... Hedefin hızı ışığın hızına göre her zaman çok küçüktür, dolayısıyla binomial açılımıyla basitleştirebiliriz: r F v c v c F 1 1 v c v c v c F t 1 v c v c F t t 8 4

5 boresight direction 5/10/013 Düşük hız yakınsaması... v/c çok küçüktür dolayısıyla Fr v c F t 1 Doppler kaydırması frekansı farkı olduğu için v v FD Ft c t Hedef radara doğru hareket ediyorsa, hız pozitif olarak alınmaktadır. 9 Radar radiyal hızı ölçer... Doppler etkisi hedefin radiyal hız bileşeni tarafından belirlenir = 90º y FD v 0 F D v 10 v cos radar antenna x 5

6 00 m/s 00 m/s 5/10/013 Örnek Doppler Sorusu 11 An airborne radar is traveling north at 00 m/s. A target approaches from the NE, also traveling at 00 m/s. The radar is X band (10 GHz). What is the Doppler shift of the echo? Cevap: 1 An airborne radar is traveling north at 00 m/s. A target approaches from the NE, also traveling at 00 m/s. The radar is X band (10 GHz). What is the Doppler shift of the echo? total closing velocity = cos(45 ) = m/s = c/f= 3x10 8 /1x10 10 = 0.03 m F D = v/ = (341.4)/0.03 =.76 khz 00 m/s 6

7 Bant Genişliğin Doppler Etkisi Radar sinyalleri saf sinüslerden oluşmamakadır, sonlu bir bant genişliğine sahipler. Bant genlişliği genellikle en fazla merkez frekansın %10 u dır. Dolayısıyla önemsenmesi gereken bir etki yaratmamaktadır. Br v c B 1 t 13 Uygulamaya Göre Radar Türleri 14 7

8 Radar Frekansları 15 Radar Bant Tanımları 16 8

9 Darbe Şekline Göre Radar Türleri 17 Darbe Doppler Radar Radar, periyodik olarak bir ötüş sinyali göndermektedir. 18 9

10 Darbe Tekrarlama Aralığı (PRI) 19 Belirsizliği Olmayan Maksimum Menzil 0 10

11 Belirsizlik Olunca menzil hatalı algılanır... Minimum Algılama Menzili Radar sinyali gönderirken genellikle aynı antenden herhangi bir yansımayı alamaycağından dolayı, monostatik bir radarin minimum algılama menzili: R min c 11

12 Alınan Sinyal Sadece Hedeften Değil Received signal is a superposition of several components: target echoes (direct and multipath) clutter surface (ground, sea) weather (clouds, rain) noise external (cosmic noise) internal (shot, thermal, etc.) jamming electromagnetic interference (EMI) TV stations cell phones 3 4 RADAR ERİM DENKLEMİ 1

13 Radar Erim Denklemi 5 Radar Erim Denklemi 6 13

14 Hedeften Saçılım 7 Antenin Alım Alanı 8 Ae is NOT the physical area of the antenna. It is a fictional area that accounts for the amount of incident power density captured by the receiving antenna. 14

15 Kayıplar 9 Received power calculation so far is for an ideal radar in free space with no processing to increase sensitivity Real systems suffer losses in duplexers, waveguide, power dividers, radome, etc. represented by a system loss factor L s Also suffer atmospheric propagation losses function of range with R in meters, loss factor a in db/km, we have La a 5000 R 10 R Atmosferik Kayıplar 30 Source: EW and Radar Systems Low frequencies Engineering Handbook, Naval Air Warfare Center, Weapons Division. propagate further See higher power devices available High frequencies narrower beams give better resolution 15

16 Yağış Kayıpları Etkilemektedir 31 Becomes very significant at MMW frequencies Limits radar range Source: EW and Radar Systems Engineering Handbook, Naval Air Warfare Center, Weapons Division. See Noktasal Hedef İçin Radar Erim Denklemi Adding in loss factors gives: P r PG t R L L R s a W 3 Note for a point target, received power decreases as R 4 : Doubling range while maintaining received power requires 16x (1 db) transmitted power increase, or 4x (6 db) antenna gain increase 4x antenna area increase 16

17 Birimlere Dikkat! 33 P r PG t R L L R s a W All terms in the range equation are in linear units However, parameter values are often provided in db units e.g. antenna gain is 30 dbm, atmospheric loss is 1 db/km, etc. So don t forget to do your unit conversions! Örnek 34 X band (10 GHz) = 3x10 8 /10x10 9 = 3 cm Transmitted power P t = 1 kw Beamwidth (azimuth and elevation) = 1º G = 6,000/(1)(1) = 6,000 = 44 db Jumbo jet aircraft: RCS = 100 m Range R = 10 km What is received power P r? P r 1,000 0, , x10 Watts 1 orders of magnitude! 17

18 Alıcı Gürültüsünü de Ekleyelim 35 Maksimum Sezim Erimi R Pr P max n PG t 4 4 R L L ( R) kt BF 4 s PG t a P r L ( ) 0 sla R kt BF Pn 0 min

19 Distributed Target Form of the Radar Range Equation Range equation so far is for point targets target small compared to resolution cell We are also interested in distributed targets surface clutter: scattering from a homogeneous area ground, sea clutter volume clutter: scattering from a homogeneous volume weather (rain, clouds, hail, etc.) smoke, chaff, etc. 37 Approach Approach based on a differential scattering element differential area or volume is the scatterer have to combine contributions from each differential scattering element Still start with transmitted isotropic power density: 38 P t W/m 4 R 19

20 Transmitted Power Density No longer assume single scatterer in direction of peak antenna gain; so must account for antenna gain in direction of each scatterer of interest to get transmitted power density Q Assume P(0,0) = G t, PP t, W/m 4 R 39 Differential Received Power At every (azimuth,elevation) (,), we get backscatter based on the local differential RCS again assume isotropic re-radiation weighted by the effective aperture (thus the antenna pattern) again on receive account for system and atmospheric losses again dp r,,, R L L R PP t d R s a 40 0

21 Total Received Power Integrate over 3-D space to obtain generalized radar range equation: 3 4 L s V note that integrating power assumes noncoherent combination of scattering element contributions However, scatterers in all of 3-D space don t contribute to receiver output at the same time P, t P Pr d R,, 4 R L R a 41 Total Power from a Resolution Cell Only scattering elements within a resolution cell contribute to receiver output at a given instant: Pt P, Pr 3 d 4 R,, 4 L R L s a R VR,, V(R 0, 0, 0 ) is the differential scattering volume centered at nominal coordinates (R 0, 0, 0 ) Check against point scatterer equation: radians R m 4 radians 1

22 Point Scatterer Point scatterer implies differential scatterer modeled by an impulse function: d R,, R R,, dv D Results in generalized point target range equation: PP t 0, 0 Pr W R L L R 0 s a 0 43 identical to previous version if 0 = 0 = 0 P, G RADAR KESİTİ RADAR CROSS SECTION (RCS)

23 RCS Tanımı Suppose incident power density on a target is Q t W/m ; backscattered power density is Q b W/m ; and backscattered power is P b W Note that Q t and Q b are the measurable quantities P b must satisfy Q b Pb W/m 4 R 45 RCS Tanımı RCS is the fictional area that accounts for P b : Therefore P b Q W Q R Q Thus RCS is the fictional area that accounts for the relative value of backscattered power density in terms of the incident power density assuming isotropic reradiation of backscatter t b 4 t 46 3

24 RCS Tanımı RCS usually defined in terms of electric field amplitude, not power Also take limit as R to remove dependence on range then RCS depends only on scatterer characteristics 4lim R R E E b t Backscattered E-field amplitude Transmitted E-field amplitude 47 Tipik RCS Değerleri 48 Target RCS, m RCS, dbsm Conventional unmanned winged missile Small single-engine aircraft 1 0 Small fighter aircraft or 4-passenger jet 3 Large fighter aircraft 6 8 Medium bomber or jet airliner 0 13 Large bomber or jet airliner Jumbo jet Small open boat Small pleasure boat 3 4

25 RCS Normalized to r (db) 5/10/013 Tipik RCS Değerleri 49 Target RCS, m RCS, dbsm Cabin cruiser Large ship at zero grazing angle 10, Pickup truck 00 3 Automobile Bicycle 3 Human 1 0 Bird Insect Bir Kürenin Radar Kesiti Three regions, depending on relative size of sphere and wavelength For radius r >> r aspect and frequency independence makes this a good calibration target Rayleigh Region Mie (resonance) Region Sphere Circumference in Wavelengths, (r/) Optical Region 50 5

26 Basit Şekillerin Radar Kesiti 51 Gerçek Hedeflerin Radar Kesiti 5 6

27 Stealth Teknoloji 53 F-117 Nighthawk İlk Uçuş: 1981 Adet: 54 B- Spirit Ilk Uçuş: 17 Temmuz 1989, Adet: 1 Nukleer ve Konvansiyonel Bombalar Yeni Nesil Stealth 54 F-35 Joint Strike Fighter (JSF) Üretim aşamasında F- Raptor Ilk Uçuş: 1997 Hizmete 004 yılında girmiş 7

28 Chinese Stealth J İlk Uçuş: 31 Ekim 01 Stealth Yöntemler 56 Şekil Düz yüzey olmayacak daha çok yansıtır Veya F-117 gibi yansımları her yöne saçacak şekilde seçilmeli Kapılar ve panellerin birleştiği hatlar hafif oyulmuştur uzun çizgiler oluşturmamak için Eğimli arka kanatlar uçağın düzlükleri gizliyor 8

29 Şekil Tasarımına Devam 57 Angular Air Intakes The air intakes on the F-117 and B- are carefully shielded since their large scoop-like shape tends to create what radar scientists call corner reflectors--shapes that can reflect radar much more strongly than a flat plane. Hiding the air intake in the structure works to obscure it from enemy radar, but the F- uses a more advanced trick where the intakes are, like the tail fins, arranged at peculiar angles that avoid forming corner reflectors. Stealth Yöntemler 58 Radar Emici Malzemeler Metal yerine karmaşık malzemeler Hem daha dayanıklı hem radar yansıması daha az 9

30 F- Malzeme Tasarımı Materials: Carbon-fiber composites Magnetic ferrite-based substance RAM makes object appear smaller 59 Problems with RAM 60 Must be defect free Risk during in-flight fueling High maintanence Affected by weather Reason for why B- stationed only in US 30

31 Infrared Signature, Supercruise Engines The F-117 and B- hid their engine exhaust from infrared sensors (on missiles and other surveillance systems) by venting the jet plume through louvers and over the body of the aircraft. The F- can't do this trick, as its more powerful supersonic-capable engines make the effect harder. Instead it uses a slightly masked exhausts, and a technique called supercruise, which means it can break the sound barrier without needing the infra-red give away of reheat. This lowers its infra red signature a lot. 61 J-0 versus F- Exhausts Big cylindrical engines give off a lot of infra-red heat, this is BAD 6 31

32 Steath and Detection Range 63 Counterstealth 64 3

33 Kaynakça 65 Mark Richards ve C.J. Baker in sunumları Kitaplar 33