Radar Kavrami

Transkript

Radar Kavrami

5/10/2013
Ankara
Radar Systems and Remote Sensing
Research Group
TOBB ETÜ – Turgut Özal - Bilkent
TOBB UNIVERSITY OF
ECONOMICS AND
TECHNOLOGY
DERS 2
Radar Kavramı
Yrd. Doç. Dr. Sevgi Zübeyde Gürbüz
ELE 465: Radar Sinyal İşleme Temelleri
ELE 565: Radar ve Sonar Sistemleri
Tipik bir Radar Sistemi
2
Ankara Radar Systems and Remote Sensing Research Group © SZG 2012
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Radar Konfigürasyonları
3

Monostatik Radar:


Bistatik Radar:


Aynı antenden sinyal
alıp veiliyor.
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
2 R  c  t  R 
c  t
2
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Menzil Çözünürlüğü
5

İki hedefi birbirinden ayırt edebildiğimiz
maksimum menzil farkı.
 Yansımaların
 Menzil
zaman farkı:
çözünürlüğü:
Açısal Çözünürlük
6
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Doppler Etkisi
7


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:
radara doğru hareket eden b,r hedefin hızı
 c: ışığın hızı
 v:
 1 v c 
Fr  
Ft

 1 v c 
Düşük hız için basitleştirelim...
8

Hedefin hızı ışığın hızına göre her zaman
çok küçüktür, dolayısıyla binomial
açılımıyla basitleştirebiliriz:
1
Fr  1  v c 1  v c  Ft
 1  v c  1   v c    v c  

2
 1  2  v c   2  v c  

2
F
 t
F
 t
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Düşük hız yakınsaması...
9

v/c çok küçüktür dolayısıyla
Fr  1  2  v c   Ft

Doppler kaydırması frekansı farkı olduğu için
FD  

2v
2v
Ft  
c
t
Hedef radara doğru hareket ediyorsa, hız pozitif
olarak alınmaktadır.
Radar radiyal hızı ölçer...
10
y
Doppler etkisi
hedefin radiyal
hız bileşeni
tarafından
belirlenir

v
v

FD  0
boresight
direction
 = 90º
FD  
2v

cos 
x
radar antenna
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5/10/2013
Örnek Doppler Sorusu

An airborne radar is
traveling north at 200
m/s. A target
approaches from the
NE, also traveling at
200 m/s. The radar is
X band (10 GHz).
What is the Doppler
shift of the echo?
200 m/s
11
Cevap:
m
0
20
An airborne radar is traveling north at 200
m/s. A target approaches from the NE, also
traveling at 200 m/s. The radar is X band (10
GHz). What is the Doppler shift of the echo?
 total closing velocity
= 200 + 200cos(45°)
= 341.4 m/s
  = c/F= 3x108/1x1010 = 0.03 m
 FD = 2v/ = 2(341.4)/0.03 = 22.76 kHz
200 m/s

/s
12
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Bant Genişliğin Doppler Etkisi
13
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  1  2  v c  Bt
Uygulamaya Göre Radar Türleri
14
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Radar Frekansları
15
Radar Bant Tanımları
16
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Darbe Şekline Göre Radar Türleri
17
Darbe Doppler Radar
18

Radar, periyodik olarak bir ötüş sinyali
göndermektedir.
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Darbe Tekrarlama Aralığı (PRI)
19
Belirsizliği Olmayan Maksimum Menzil
20
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Belirsizlik Olunca...
21

...menzil hatalı
algılanır...
Minimum Algılama Menzili
22

Radar sinyali gönderirken genellikle aynı
antenden herhangi bir yansımayı
alamaycağından dolayı, monostatik bir
radarin minimum algılama menzili:
Rmin 
c
2
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Alınan Sinyal Sadece Hedeften Değil
23

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
 TV stations
 cell phones
(EMI)
24
RADAR
ERİM
DENKLEMİ
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Radar Erim Denklemi
25
Radar Erim Denklemi
26
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Hedeften Saçılım
27
Antenin Alım Alanı
28
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.
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Kayıplar
29



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
Ls
Also suffer atmospheric propagation losses


function of range
with R in meters, loss factor
a in dB/km, we have
La  R   10a R 5000
Atmosferik Kayıplar
30

Low frequencies



propagate further
higher power devices available
Source: EW and Radar Systems
Engineering Handbook, Naval Air
Warfare Center, Weapons Division.
See http://ewhdbks.mugu.navy.mil/
High frequencies

narrower beams give better resolution
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Yağış Kayıpları Etkilemektedir
31
Source: EW and Radar Systems
Engineering Handbook, Naval
Air Warfare Center, Weapons
Division. See
http://ewhdbks.mugu.navy.mil/


Becomes very significant at MMW frequencies
Limits radar range
Noktasal Hedef İçin Radar Erim
Denklemi

Adding in loss factors gives:
Pr 

32
2 2
PG

t
 4  R 4 Ls La  R 
3
W
Note for a point target, received power decreases
as R4:
 Doubling
range while maintaining received power
requires


16x (12 dB) transmitted power increase, or
4x (6 dB) antenna gain increase  4x antenna area increase
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Birimlere Dikkat!
33
Pr 


2 2
PG

t
 4 
3
R Ls La  R 
4
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)
  = 3x108/10x109 = 3 cm
Transmitted power Pt = 1 kW
Beamwidth (azimuth and elevation) = 1º
 G = 26,000/(1)(1) = 26,000 = 44 dB
Jumbo jet aircraft: RCS = 100 m2
Range R = 10 km
What is received power Pr?
12 orders of
magnitude!
1,000  20,000   0.03 100   5.18x109 Watts
Pr 
3
4
 4  10,000 
2
2
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Alıcı Gürültüsünü de Ekleyelim
35
Maksimum Sezim Erimi
36
Pt G 22
Pr

Pn 4 2 R 4 Ls La ( R)kT0 BF
Rmax




Pt G 2 2






P
2
 4  L L ( R)kT BF  r  
s a
0
P  

 n  min 

1
4
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Distributed Target Form of the
Radar Range Equation
37

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.

Approach
38

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:
Pt
W/m 2
2
4 R
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Transmitted Power Density
39

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
Qt  ,  

PP
t  ,  
4 R 2
W/m 2
Assume P(0,0) = G
Differential Received Power
40

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
dPr 
2
PP
 ,   2 d  R, , 
t
 4 
3
R 4 Ls La  R 
20
5/10/2013
Total Received Power
41

Integrate over 3-D space to obtain
generalized radar range equation:
Pr 
Pt  2
 4 


3

Ls V
P 2  , 
d  R, , 
R 4 La  R 
 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
Total Power from a Resolution Cell
42

Only scattering elements within a resolution cell
 radians
contribute to receiver output
 radians
at a given instant:
R m
Pr 


Pt  2
 4  Ls
3





V  R0 ,0 ,0 
P 2  , 
R 4 La  R 
d   R , ,  
V(R0,0,0) is the differential scattering volume
centered at nominal coordinates (R0,0,0)
Check against point scatterer equation:
21
5/10/2013
Point Scatterer
43
Point scatterer implies differential scatterer
modeled by an impulse function:
d  R, ,    D  R  R0 ,  0 ,  0  dV
 Results in generalized point target range
equation:
2
PP
 0 , 0   2 

t
Pr 
W
3
4
 4  R0 Ls La  R0 

 identical
to previous version if
0 = 0 = 0  P  0 ,0   G
44
RADAR
KESİTİ
RADAR CROSS SECTION (RCS)
22
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RCS Tanımı
45

Suppose
power density on a target is Qt W/m2;
 backscattered power density is Qb W/m2; and
 backscattered power is Pb W
 incident
Note that Qt and Qb are the measurable
quantities
 Pb must satisfy

Qb 
Pb
4 R 2
W/m 2
RCS Tanımı
46

RCS is the fictional area that accounts for Pb:
Pb   Qt
W
  4 R 2
Qb
Qt

Therefore

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
23
5/10/2013
RCS Tanımı
47
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

E
2

  4 lim R
R  
Et

b


2


2
Backscattered
E-field amplitude
Transmitted
E-field amplitude
Tipik RCS Değerleri
48
Target
RCS, m2
RCS, dBsm
Conventional unmanned winged missile
0.5
-3
Small single-engine aircraft
1
0
Small fighter aircraft or 4-passenger jet
2
3
Large fighter aircraft
6
8
Medium bomber or jet airliner
20
13
Large bomber or jet airliner
40
16
Jumbo jet
100
20
Small open boat
0.02
-17
Small pleasure boat
2
3
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5/10/2013
Tipik RCS Değerleri
49
Target
RCS, m2
RCS, dBsm
Cabin cruiser
10
10
Large ship at zero grazing angle
10,000+
40+
Pickup truck
200
23
Automobile
100
20
Bicycle
2
3
Human
1
0
Bird
0.01
-20
Insect
0.00001
-50
Bir Kürenin Radar Kesiti
50

Three regions,
depending on
relative size of
sphere and
wavelength
For radius
r >> 
   r2

10
RCS Normalized to r2 (dB)

0
-10
-20
Mie
aspect and
(resonance)
Rayleigh
Optical
-30
frequency
Region
Region
Region
independence
makes this a
10-1
100
101
102
good calibration
Sphere Circumference in Wavelengths, 2(r/)
target
25
5/10/2013
Basit Şekillerin Radar Kesiti
51
Gerçek Hedeflerin Radar Kesiti
52
26
5/10/2013
Stealth Teknoloji
53
F-117 Nighthawk
İlk Uçuş: 1981
Adet: 54
B-2 Spirit
Ilk Uçuş: 17 Temmuz 1989, Adet: 21
Nukleer ve Konvansiyonel Bombalar
Yeni Nesil Stealth
54
F-35 Joint Strike Fighter (JSF)
Üretim aşamasında
F-22 Raptor
Ilk Uçuş: 1997
Hizmete 2004 yılında girmiş
27
5/10/2013
Chinese Stealth J-31 
55
İlk Uçuş: 31 Ekim 2012
Stealth Yöntemler
56

Şekil
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
 Düz
28
5/10/2013
Şekil Tasarımına Devam
57

Angular Air Intakes

The air intakes on the F-117 and B-2 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-22 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
29
5/10/2013
F-22 Malzeme Tasarımı
59

Materials:
 Carbon-fiber
composites
 Magnetic
ferrite-based
substance

RAM makes
object appear
smaller
Problems with RAM
60

Must be defect free
 Risk
during in-flight fueling
High maintanence
 Affected by weather

 Reason
for why B-2 stationed only in US
30
5/10/2013
Infrared Signature, Supercruise
Engines

61
The F-117 and B-2 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-22 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.
J-20 versus F-22 Exhausts
62

Big cylindrical engines give off a lot of
infra-red heat, this is BAD
31
5/10/2013
Steath and Detection Range
63
Counterstealth
64
32
5/10/2013
Kaynakça
65
Mark Richards ve C.J. Baker’in sunumları
 Kitaplar

33

Radar Kavrami

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