Determining Target Position10

7/29/2019 Determining Target Position10

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Determining Target position

In order to detect a target it must be within the beam of the antenna and its echoes mustbe strong enough

Range measurements

Range depends on

Power of transmitted beam

Fraction of time that the power is transmitted

Size of antenna

Reflecting characteristics of target

Duration of observation Wavelength of radio waves

Strength of background

The power of the transmitted beam depends on the transmitters output and the gain of the

antennas mainlobe.

Two factors reduce the power

Absorption within the atmosphere- increases with wavelength

Spreading proportional to 1/R2


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The amount of power intercepted by a target depends on the power density at the target

and the cross sectional area. A certain of energy is scattered back depending on the

reflectivity and the directivity

Reflectivity

A measure of the efficiency of a radar target in intercepting and returning radio energy

ercepted

scattter

P

P

int

tyReflectivi =

Directivity

The ratio of the power scattered back in the radar's direction to the power that would have

been backscattered had the scattering been uniform.

scatter

rbackscatte

isotropic

rbackscatte

P

P

P

P

4

1

yDirectivit

=

=

Since isotropicP and rbackscatteP are expressed as power per unit angle

The cross sectional area, reflectivity and directivity are often grouped together to form

the radar cross section

powerdintercepte

powerrbackscatte4

4

1

yDirectivitty xReflectivixareasectionalCross

int

Ax

P

Px

P

PAx

scatter

rbackscatte

ercepted

scatter

=

=

=

Radar cross section is the measure of a target's ability to reflect radar signals in the direction of

the radar receiver, i.e. it is a measure of the ratio of backscatter power per steradian (unit solid

angle) in the direction of the radar (from the target) to the power density that is intercepted by the

target.


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Radar Cross section as Function of Angle


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Typical Aircraft ACS


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The Radar Equation

The power density at a distant point from a radar with an antenna gain of Gt

is the power density from an isotropic antenna multiplied by the radar antenna gain.

Power density from radar,

If you could cover the entire spherical segment with your receiving antenna you wouldtheoretically capture all of the transmitted energy. You can't do this because no antenna is

large enough. (A two degree segment would be about a 1km across at 25km from the

transmitter.)

The power scattered by the target with scattering cross section spreads out

24 R

and is intercepted by the receiving antenna with an area Ar

Combining with (1) gives

( ) 424 R

AGPP rtt

=

The power transmitted will not be continuous but an average can be taken Pavg.

If the target is illuminated for a time tot the signal energy (SE) received will be

( ) 424 R

tAGPSE

otrtavg

=

If the losses due to atmospheric attenuation and ambient noise are L

( ) LR

tAGPSE

otrtavg

424

=

Maximum range at which the target will be detected

( )4

min

24 LS

tAGPR

otrtavg

=

(1)


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This is the range equation. It allows trade-off to be done on the system parameters

In practice they are not all independent.

E.g.

If the antenna size is doubled this would increase the area by a factor of 4 allowing an

increase in range. However if the antenna size were doubled the beamwidth would be cutby half so this would have an impact on ot. This could be avoided by slowing down the

antenna

Changing the wavelength would also change the beamwidth and would also effect theattenuation within the atmosphere.


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Stealth

Electronic emission issues:

Reduce use of radar and other active communications

Thermal signature issues:

Elements such as the leading edges of the wings, which become heated by air friction in

flight, can be internally cooled by pumping fuel or hydraulic fluid around and through

them. Engines intakes and exhausts to be mounted above the wings, out of "sight" ofheat-seeking SAM's, MANPADS, etc.

Optical illusion:

Blend lights into the lower surfaces of the aircraft in an attempt to blend the visual

silhouette into the sky background.

Overall aircraft colour:

Scattered light from dust and clouds reflects onto an aircraft's underside. Even blackaircraft below 50,000 ft will be visible due to this phenomenon. Most aircraft for that

reason are painted a low visibility grey to take advantage of low light levels. Blackaircraft are useful at night (hence why their missions are usually nocturnal)

Aerodynamic effects, contrail suppression:

Some aircraft have contrail detectors built in which monitor contrail side effects, these

can be countered by chemicals injected into the exhaust plumes using chlorosulphonic

acids, various alcohol mixtures, etc) for suppression

Radar cross section:

AircraftRCS

[dBsm]RCS[m2]

RCS[ft2]

F-15 Eagle +26 400 4,305

F-4 Phantom II +20 100 1,076

B-52 Stratofortress +20 100 1,076

Su-27 +12 15 161

B-1A +10 10 108

F-16 Fighting Falcon +7 5 54

B-1B Lancer 0 1 11

F-18E/F Super Hornet 0 1 11

Rafale 0 1 11

Typhoon -3 0.5 5.5
http://www.aerospaceweb.org/aircraft/fighter/f15/http://www.aerospaceweb.org/aircraft/fighter/f15/http://www.aerospaceweb.org/aircraft/fighter/f4/http://www.aerospaceweb.org/aircraft/bomber/b52/http://www.aerospaceweb.org/aircraft/fighter/su27/http://www.aerospaceweb.org/aircraft/fighter/su27/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/fighter/f16/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/fighter/f18ef/http://www.aerospaceweb.org/aircraft/fighter/rafale/http://www.aerospaceweb.org/aircraft/fighter/rafale/http://www.aerospaceweb.org/aircraft/fighter/typhoon/http://www.aerospaceweb.org/aircraft/fighter/f15/http://www.aerospaceweb.org/aircraft/fighter/f4/http://www.aerospaceweb.org/aircraft/bomber/b52/http://www.aerospaceweb.org/aircraft/fighter/su27/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/fighter/f16/http://www.aerospaceweb.org/aircraft/bomber/b1/http://www.aerospaceweb.org/aircraft/fighter/f18ef/http://www.aerospaceweb.org/aircraft/fighter/rafale/http://www.aerospaceweb.org/aircraft/fighter/typhoon/


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AGM-86 ALCM -6 0.25 2.5

BGM-109 Tomahawk -13 0.05 0.5

SR-71 Blackbird -18 0.015 0.15

F-22 Raptor -22 0.0065 0.07

F-117 Nighthawk -25 0.003 0.03

B-2 Spirit -28 0.0015 0.02

AGM-129 ACM -30 0.001 0.01

Boeing Bird of Prey -70 0.0000001 0.000008

The radar cross section in dBsm is the RCS relative to 1 square

meter

Example The F-117A

The world's first operational aircraft designed to exploit low-observable stealthtechnology

The skeleton of the F-117 is made mainly of aluminium. The aircrafts skin, by contrast, is

mostly composite RAM (Radar Absorbent Material)

"The effectiveness of F-117A's RAM skin was demonstrated in an unusual manner during

the Gulf War, when ground crews started finding dead bats around the tails of hangared

aircraft. The unfortunate creatures had clearly flown "full tilt" into the Black Jet'stailfins, which their high frequency 'sonar' had been unable to detect."

Cross section

10cm2

RAM coating

Coating contains carbonyl iron ferrite. Radar energy is converted to heat. Coating can bein the form of tiles or paint applied by robotics
http://www.aerospaceweb.org/aircraft/recon/sr71/http://www.aerospaceweb.org/aircraft/fighter/f22/http://www.aerospaceweb.org/aircraft/fighter/f22/http://www.aerospaceweb.org/aircraft/bomber/f117/http://www.aerospaceweb.org/aircraft/bomber/f117/http://www.aerospaceweb.org/aircraft/bomber/b2/http://www.aerospaceweb.org/aircraft/recon/sr71/http://www.aerospaceweb.org/aircraft/fighter/f22/http://www.aerospaceweb.org/aircraft/bomber/f117/http://www.aerospaceweb.org/aircraft/bomber/b2/


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Internal Radar absorbent construction

Structures below the skin of the radar that traps the electromagnetic energy

External Geometry

Flat, angled external panels of F-117A are designed to reflect radar waves in alldirections but the direction of the radar's receiving antenna. Leads to poor aerodynamics

which can make detection of turbulent air possible

IR emission Control

The use of broad flat exhausts to spread jet influx along a large surface area. Thisreduces hot spots but makes the engines less efficient


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F117 Nighthawk Stealth Fighter

F22 Raptor


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F35

B2 Spirit- Stealth Bomber


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Bird of Prey

http://www.goxium.com/xma201.htm

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Determining Target Position10