Study of flight route effects on aircraft RCS signature at VHF frequencies by means of wire grid models

Study of flight route effects on aircraft RCS

signature at VHF frequencies by means of wire

grid models

A. DAVID, C. BROUSSEAU, A. BOURDILLON

CONTACT : A. DAVID

Université de Rennes 1

Laboratoire Structures Rayonnantes/Radiocommunications

Campus de Beaulieu BAT. 22

35042 Rennes Cedex

France

33 2 99 28 62 25

FAX : 33 2 99 28 16 59

E-MAIL : [email protected]

mailto:[email protected]

Introduction

A perspective in future radar systems is to perform target recognition to better control the air

traffic. In a military context, new technologies lead to the development of stealthy target, especially

in conventional radar bands, which are additional difficulties for recognition. The use of the low

frequency band, particularly near the resonance range for the considered targets, is a promising way

to fight against radar cross section (RCS) reduction because stealthy techniques are then difficult to

implement. Indeed, the backscattered field induced by the target is the result of interactions between

its different parts and it is less directive at lower frequency. It can then be assumed that the RCS

signature of a given target is less sensitive to small aspect angle changes. Nevertheless, observations

made with the monostatic multifrequency and multipolarization HF-VHF radar MOSAR show that

small changes in aircraft flight route sometimes result in important RCS signature changes [1].

The aim of this paper is to evaluate the influence of flight route on the RCS signature of

commercial aircrafts at low frequencies by means of computational models. First of all, the

modeling of several aircrafts is described and the accuracy of the modeling is discussed. Then, the

models are used to evaluate the flight route effects in low frequency band by studying different

cases which take into account the flying height and spatial position of the aircraft. It is shown that

an accurate knowledge of the aircraft flight route is required for aircraft identification.

Aircrafts modeling

Modeling aircrafts in the resonance range is the worse situation, since backscattering

phenomena are more complex there than in the other frequency domains. In the optical and in the

Rayleigh range some approximations exist to characterize the RCS of targets, but not in the

resonance range. One possibility is to design a wire grid model of the target and to calculate the

currents induced on the structure. The currents can be obtained by solving the electromagnetic field

integral equation (EFIE) if the conductivity is assumed to be infinite and if the wire radius is small

with respect to the wavelength.

To calculate the induced currents, the electromagnetic code NEC2 has been used [2][3]. It

gives a numerical solution of the currents by means of the moment method. Nevertheless, to

simulate the structure with a good agreement, several conditions must be respected. First of all, the

wire structure must describe the target as closely as possible. Then, due to the method used, the

segment length and the wire radius must be chosen according to the working wavelength.

Reasonable accurate results are generally obtained with a segment length around 0.1 wavelength at

the desired frequency. Although the wire radius cannot be randomly chosen, it should be adjust

heuristically to improve the results. A rule of thumb exist [4], but it does not give satisfying results

in every cases.

Three commercial aircraft models have been designed to be used with the NEC2 software :

the Boeing 747-200, the Airbus A320 and the Boeing 737-200. Their main characteristics and wire

grids are described in figure 1. The number of segments used for each aircraft is between 2000 and

4000. A higher number of segments is used for the Boeing 747-200 due to its dimensions, which

are practically twice the ones of the two other aircrafts. Simulations have been performed in the

20-60 MHz frequency band. The size of aircrafts are then situated between the upper resonance

range and the beginning of the optical region. The wire grid structures have been defined according

to the NEC2 guidelines and by taking into account the previous conditions. To assess the reliability

of the models, simulations have been compared with measurements made in an anechoïc chamber

on a scaled aircraft [5]. Two examples of comparison between simulations and measurements for

the Boeing 747-200 are shown in figures 2 and 3. These figures present the RCS variations with

azimuth at fixed frequency (figure 2) and the synthesized impulse response at broadside incidence

(figure 3). In the two cases a good agreement is obtained between simulations and measurements

showing the validity of the model.

FIG. 1 : Main characteristics and wire grid structures of the commercial aircrafts Boeing 747-200, Airbus

A320, Boeing 737-200.

0 20 40 60 80 100 120 140 160 180

5

10

15

20

25

30

35

Azimuth in degrees

RC

S M

agn

itude

in d

Bm

²

ONERA measurements

NEC Simulations

FIG. 2 : Comparison between simulations and measurements in horizontal polarisation at 20 MHz

Boeing 747-200

Boeing 747-200

Boeing 737-200 Airbus A320

Fuselage length : 68.6 m

Wing span : 59.6 m


Wing span : 28.3 m


Wing span : 34.1 m

-80 -60 -40 -20 0 20 40 60 800

1000

2000

3000

4000

5000

6000

7000

Range in meters

RC

S m

agn

itude

in m

²

Incidentwaveform

ONERA measurements

NEC simulations

FIG. 3 - Synthesized impulse response at broadside incidence in horizontal polarization using the 40-60 MHz

frequency band – Comparison between simulations and measurements.

To evaluate the influence of the flight route on target RCS signature, simulations have been

performed with the three models. The RCS has been determined for several azimuth and site angles

with two degrees and one degree resolution, respectively, and for different positions of the aircrafts

along the flight route.

Flight route influence

In an air lane, the aircraft can fly at different heights and it can be shifted with respect to the

air lane center. This is due to the air security rules. Indeed, aircrafts are allowed to fly inside the air

lane, as long as no other aircraft is flying with the same route. Several observations made by the

MOSAR radar, located near Rennes in France [1], have shown that although aircrafts fly in the

same air lane, the RCS signatures of targets can be largely modified by a change in the flight route.

Figure 4 displays the power received by the radar MOSAR in the case of two Boeing 737-200

flying in the same direction, at the same altitude, in the same air lane. The measurements have been

obtained simultaneously at two frequencies. From figure 4 it is concluded that the power profiles

are very different.

25 30 35 40 45-120

-110

-100

-90

Radar-target range (km)

f1 = 33.36 MHz target 1


25 30 35 40 45-120

-110

-100

-90

Receiv

ed p

ow

er(

dB

m)




Receiv

ed p

ow

er(

dB

m)

FIG. 4 – Received power for two Boeing 737-200 measured in vertical polarization at 33.36 MHz and

40.56 MHz the 12/9/97 et 11/18/97 at 14h12 and 14h10 UT, respectively.

To understand better the influence of the aircraft route, RCS variations have been studied

along flight routes by means of simulations. The two major parameters, the altitude and the spatial

position (along the path) of the aircraft, have been independently considered.

In the first case, it is assumed that the aircrafts are flying along the same path but at different

heights. If the center of the air lane is considered in the line of sight of the radar, aircrafts are shifted

by -2.6 km from the center of the air lane when they are near 20 km away from the radar, and by

-1.1 km when they are at 50 km. The routes followed by the aircrafts are displayed in figure 5. The

altitude is 10.06 km for target 1 and 11.28 km for target 2.

1020

3040

5060

-9-6

-30

36

90

3

6

9

12

15


Air lane width (km)

Fly

ing

Heig

ht (k

m)

Target 1

Target 2

Air lane size

RADAR

FIG. 5 – Aircrafts flying along the same route at different heights.

20 25 30 35 40 45 500

5

10

15

20

25

30

35

40

radar-target range (km)

Site

(d

eg

ree

s)

Target 1Target 2

20 25 30 35 40 45 50-20

-15

-10

-5

0

5

10

15

20


Azi

muth

(d

eg

ree

s)

Target 1Target 2

FIG. 6 – Site and azimuth variations during the aircrafts flight.

0 20 40 600

5

10

15

20

25

30

35

40

45f=33 MHz


RC

S m

ag

nitu

de

(dB

m2)

0 20 40 600

5

10

15

20

25

30

35

40

45f=52 MHz


RC

S m

ag

nitu

de

(dB

m2)

Target 2

Target 1

Target 1

Target 2

0 20 40 600

5

10

15

20

25

30

35

40

45f=33 MHz


RC

S m

ag

nitu

de

(d

Bm

2)

0 20 40 600

5

10

15

20

25

30

35

40

45f=52 MHz


RC

S m

ag

nitu

de

(d

Bm

2)

Target 2

Target 2

Target 1Target 1

0 20 40 600

5

10

15

20

25

30

35

40

45f=33 MHz

RC

S m

ag

nitu

de

(dB

m2)

0 20 40 600

5

10

15

20

25

30

35

40

45f=52 MHz


RC

S m

ag

nitu

de

(dB

m2)


Target 2

Target 1Target 2

Target 1

FIG. 7 – RCS variations at two frequencies, 33 MHz and 52 MHz, in vertical polarization for the

Boeing 747-200, Airbus A320 and Boeing 737-200, along the flight route defined in figure 5.

Boeing 737-200

Airbus A320

Boeing 747-200

During the flight the aspect angles change as shown in figure 6. The altitude affects mainly

the site angle variations. The RCS fluctuations in vertical polarization are presented for the Boeing

747-200, the Airbus A320 and the Boeing 737-200, as a function of radar-target range in figure 7.

Simulations have been performed at two frequencies, 33 MHz and 52 MHz. For all aircrafts, it is

observed that the RCS signature is modified when the altitude changes. The RCS can sometimes

differ by more than 10 dB. The discrepancies between signatures are characterized by a shift in

range, but RCS fluctuations are rather similar.

The second parameter is the path. Now, the aircrafts are assumed to be located at the same

altitude, but they follow different paths. The simulations have again been performed in vertical

polarization and at the two frequencies defined previously. The two targets are flying at a 10.67 km

height. Target 1 path is shifted by – 2.6 km when aircrafts are at 20 km away from the radar, and

shifted by – 1.1 km at 50 km. For the target 2 the shifts are – 4 km close to the radar and – 1.7 km

far away from the radar as shown figure 8. In this case, path changes imply mainly azimuth angle

variations (figure 9).

1020

3040

5060

-9-6

-30

36

90

3

6

9

12

15

Radar-target range (km)Air lane width (km)

Fly

ing

he

ight(

km

)

Target 1

Target 2

Air lane size

RADAR

FIG. 8 – Aircrafts flying along different routes at the same height.

As in the previous example, the RCS signatures are modified by the different paths (figure

10). It is observed that RCS magnitude can differ by more than 15 dB. Now the fluctuations cannot

be characterized by a shift in range as in the previous example. In these examples it is more difficult

to associate a RCS profile to a given aircraft. The influence of the path parameter turns out to be

very important.

20 25 30 35 40 45 500

5

10

15

20

25

30

35

40


Site

(d

eg

ree

s)

Target 1Target 2

20 25 30 35 40 45 50

-20

-15

-10

-5

0

5

10

15

20


Azi

muth

(d

eg

ree

s)

Target 1Target 2

FIG. 9 – Site and azimuth variations during the aircrafts flight.

0 20 40 600

5

10

15

20

25

30

35

40

45

f=33 MHz


RC

S m

agnitude(d

Bm

2)

0 20 40 600

5

10

15

20

25

30

35

40

45

f=52 MHz


RC

S m

agnitude(d

Bm

2)Target 1

Target 2

Target 1

Target 2

0 20 40 600

5

10

15

20

25

30

35

40

45

f=33 MHz

Radar-targe range (km)

RC

S m

agnitude(d

Bm

2)

0 20 40 600

5

10

15

20

25

30

35

40

45

f=52 MHz

RC

S m

agnitude(d

Bm

2)

Target 1

Target 2

Target 2

Target 1


0 20 40 600

5

10

15

20

25

30

35

40

45

f=33 MHz

RC

S m

agnitude(d

Bm

2)

0 20 40 600

5

10

15

20

25

30

35

40

45

f=52 MHz


RC

S m

agnitude(d

Bm

2)


Target 1

Target 2

Target 1

Target 2

FIG. 10 – RCS variations at two frequencies, 33 MHz and 52 MHz, in vertical polarisation for the

Boeing 747-200, Airbus A320 and Boeing 737-200, along the flight route defined in figure 8.

Boeing 737-200

Airbus A320

Boeing 747-200

At 33MHz, discrepancies are weaker for the Boeing 737-200 and the Airbus A320 than for

the Boeing 747-200, very likely because this last aircraft is the biggest one, and at a given frequency

its RCS is more sensitive to azimuth variations. The combination of the two parameters, path and

altitude, emphasizes the discrepancies between RCS signatures.

In conclusion it has been shown that slight changes on the flight route can greatly modify the

RCS signature of the aircraft and this explains the differences between the measurements presented

in figure 4.

Conclusion

The use of the low frequency band for target recognition seems to be a suitable choice against

RCS reduction, but a lot of studies are still to be done and the possibilities are not well-known. At

HF-VHF, all aircraft elements contribute to the RCS, which could produce an original signature

especially faced with stealthy aircrafts. Nevertheless, although the aircrafts are studied near the

resonance range, the results presented here for commercial aircrafts point out that RCS is still

sensitive to the flight route.

Classification techniques are often based on comparisons between unknown aircrafts and a

database composed of known targets. So, this dependence of RCS on flight route becomes a

drawback. The poor angular resolution of the low frequency radar does not allow an determination

of angle aspects. If the recognition method uses features based on RCS fluctuations, a large

database is then needed to take into account all situations correctly. To reduce the size of the

database, one solution could be to use a tracking radar to determine the flight route. Nevertheless,

such a radar works at high frequencies and it is not appropriate to detect stealthy targets.

References

[1] C. BROUSSEAU, A. DAVID, A. BOURDILLON, “ Multifrequency Polarimetric Radar System in

the Low VHF Band ”, Progress in Electromagnetic Research Symposium/JIPR'98 , pp. 229-

239, Nantes, France, July 1998.

[2] G. J. BURKE AND A. J. POGGIO, “ Numerical Electromagnetic Code – Method of moments,

Part I: Program description and theory ”, Tech. Document 116, Naval Electron. Syst.

Command (ELEX 3041), July 1977.

[3] G. J. BURKE AND A. J. POGGIO, “ Numerical Electromagnetic Code – Method of moments,

Part III: User’s guide ”, Tech. Document 116, Naval Electron. Syst. Command (ELEX

3041), July 1977.

[4] A. LUDWIG, “ Wire grid modeling of surfaces ”, IEEE Transaction on Antennas and

Propagation, Vol AP-35, No 9, pp. 1045-1057, Sept. 1987.

[5] A. DAVID, C. BROUSSEAU, A. BOURDILLON, “ Validation of heavy aircraft RCS simulations

at very high frequencies ”, 5th International conference on radar systems - Radar’99, Brest,

France, May 1999.

Documents

Study of flight route effects on aircraft RCS signature at VHF frequencies by means of wire grid models