Is it Possible to Detect the Stealth Flying Objects by the

Millimetre Wave Radiometer?

Jinghui Qiu, Shengchang Lan, Hao Liu, Xinyu Yin and Alexander Denisov

Department of Microwave Engineering, Harbin Institute of Technology, P.R.China

Xiao Qing and Zhao Man

Southwest Institute of Electronic Equipment, Chengdu, P.R.China

Francesco Soldovieri

Istituto per il Rilevamento Elettromagnetico dell'Ambiente Consiglio Nazionale delle Ricerche, Napoli, Italy

Keywords: Microwave radiometer, Passive millimetre wave imaging system, Remote sensing control, Antiradar coating

Abstract: This work evaluates the possibility of using the passive millimeter waves (PMMW) radiometric

discriminator for the remote control and the detection of stealth aircrafts.

1 INTRODUCTION

Microwave radiometry is concerned with

measurements of natural electromagnetic radiation of

an object of physical temperature above 0° K.

A large literature describes the main working

principles of microwave radiometry (Reinwater,

1978; Moffa et al, 2001; Goldsmith et al,1993;

Appleby and Lettington, 1991; Piechl, 2004;

Poradish and Habbe, 1982; Esepkina et al, 1973;

Skou, 1989; Shuchardt et al, 1981). With respect to

special application fields, it is interesting also to read

old publications and patents regarding military

designs and applications based on the deployment of

millimeter wave bands during the Cold War

(Shuchardt,1978; Moore et al., 1976; Parnell, 1988;

Seashore, Miley and Kearns, 1979; Corrado, 1988;

www.giws.de). After, due to the large potential

market, there has been a large diffusion of modern

microwave firms for the equipment of the airport

security systems and the detection of the concealed

objects.

Based on the growing problem of the terrorism,

especially after 11 September 2001, a large amount

of money has been devoted to special programs for

the design of special passive millimeter wave

imaging systems (PMMW) (Proc..of SPIE a lot of,

Huguenin,2006; Appleby,2007; Internet,Dill et al.

2009).

The focus of the present work is on the possibility

offered by PMMW radiometric systems in order to

detect remote aircraft with antiradar surfaces

coverage (Figure 1).

A relevant statement about the use of an anti-

radar surfaces adapted to the stealth ship is provided

just below. (wikipedia.org/wiki/Stealth ship). “In

designing a ship with reduced radar signature, the

main concerns are radar beams originating near or

slightly above the horizon (as seen from the ship)

coming from distant patrol aircraft, other ships or

sea-skimming anti-ship missiles with active radar

seekers. Therefore, the shape of the ship avoids

vertical surfaces, which would perfectly reflect any

such beams directly back to the emitter. Retro-

reflective right angles are eliminated to avoid causing

the cat’s eye effect. A stealthy ship shape can be

achieved by constructing the hull and superstructure

with a series of slightly protruding and detruding

surfaces”.

Anyway, it is sufficient to change the word from

“ship” to “aircraft”.

The stealth coating it is very suitable to avoid the

specular reflection for active systems, but at the

same time is practically useless for radiometric

systems, because nature produces radiations, which

re-reflect from this surfaces to antenna of radiometer

from the every directions (Figure 2 and 3)

Qiu J., Lan S., Liu H., Yin X., Denisov A., Qing X., Man Z. and Soldovieri F.

Is it Possible to Detect the Stealth Flying Objects by the.

DOI: 10.5220/0006227200610068

In Proceedings of the Fifth International Conference on Telecommunications and Remote Sensing (ICTRS 2016), pages 61-68

ISBN: 978-989-758-200-4

Figure 1: Photos of various modern famous special aircrafts with antiradar surfaces.

Objects reflect and emit radiation in the

millimeter wave range as they do it in the infrared

and visible ranges. The degree to which the object

reflects or emits is characterized by emissivity ε. A

perfect radiator (absorber) has ε = 1 and is known as

a blackbody (Esepkina, 1973). A perfect reflector

(non-absorber) has ε = 0. The earth and the sky can

be approximated as blackbody, whereas the metal

object is a reflector. Intermediate values of the

emissivity ε depends on several parameters as,

dielectric properties of the objects, angle of

observation (for example, for the water surface), the

polarization parameters, the surface roughness or

coatings, the wavelength and other factors.

8mm

0 10 20 30 40 50 60 70 80 90

100

200

300

Brightness temperature (K)

Incidence angle from zenith()

3mm

Figure 2: Brightness temperature of sky at the various

angles of relatively zenith.

The measurement of such a radiation is more

correct and understandable in terms of radio

brightness (simply brightness) temperature, which is

expressed in temperature T.

According to the usual definition (Esepkina,

1973), a radiometer (Figure 4) is a receiving device

designed for the measurements of the level of noise

radiation in an assigned band of the frequencies Δf.

The main functionality of the receiver in a

radiometer is to provide a measure of the input noise

power, expressed as an antenna temperature, in

equivalent black body temperature units. The

sensitivity of the radiometer ΔT

sens

is defined as the

minimum detectable signal and is determined by

amplitude of the fluctuations presented at the output

indicator in the absence of the signal.

Figure 3: Possible radio-brightness radiometric contrast

between metal and grass at the zenith angle for the case of

the observing an obects on the earth surface

Figure 4: Picture of the 8 mm radiometer (Gorishnyak et al,

2004).

Fifth International Conference on Telecommunications and Remote Sensing

Usually, the temperature sensitivity of radiometer

is evaluated for post-detection time 1 sec. More

details about various situations concerning ΔT

sens

(calculated and measured) can be found in (Esepkina,

1973; Skou 1989).

The bottom of the aircrafts will reflect the

radiation of the hot Earth and accordingly it will be

seen by the radiometer as an hot object with respect

to the background of the cold sky (space).

Even in case of the coating of all surfaces by

special absorption (small probability) material

(“painting”), such a “blackbody” will have surface

brightness temperature as from outside of the aircraft.

Remember standard information at the board during

the air flight: “Temperature of air overboard makes

minus 56 degrees.

In this paper, we evaluate the possibility of using

PMMW radiometric system for the remote control

and the finding of stealth aircrafts. The choice of the

real working frequency depends on the real size of

antenna and the microwave losses in atmosphere on

path radiometer – aircraft (Figure.5 and Table 1).

Figure 5: Absorption of the electromagnetic waves in the atmosphere

Table 1: Absorption in atmosphere for Three Window Frequencies (Seashore et al, 1979)

CHARACTERISTIC

FREQUENCY

35GHz

94GHz

140GHz

Wavelength

8.6mm

3.2mm

2.2mm

Clear Air Attenuation

0.12dB/km

0.4 dB/km

1.6 dB/km

Rain Attenuation

0.25mm/hr

0.07 dB/km

0.17 dB/km

0.2 dB/km

1.0 mm/hr

0.25

0.6

0.7

4.0 mm/hr

1.0

3.0

3.2

16.0 mm/hr

4.0

8.0

9.0

Fog Attenuation

Light 0.01g/m

0.006 dB/km

0.035 dB/km

0.07 dB/km

Thick 0.1 g/m

0.06

0.35

0.7

Dense 1.0 g/m

0.6

3.5

7.0

Apparent Sky Temperature

Clear

23°K

50°K

81°K

Moderate Overcast

120

200

Rain

110

220

250

Is it Possible to Detect the Stealth Flying Objects by the

Millimetre Wave Radiometer?

2 TECHNICAL DETAILS

2.1 Passive Millimeter Wave Imaging

A very increasing interest towards the design and

production various PMMW imaging system is due

to the real possibility to have fourth type of the

remote control system in addition to the existing

optical, radar and infrared (IR) systems. Attractive

feature of PMMW imaging systems is the capability

to operate under adverse weather conditions and to

be sensitive to non-metallic targets. In addition,

since PMMW sensor is passive, it can be operated in

all locations including friendly and hostile ports

where RF emissions may be disruptive to local

systems. (Moffa et al, 2001).

Figure 6 presents images produced by the 32

sensors 8 mm PMMW system with antenna diameter

90 cm (Gorishnyak et al, 2004; Denisov et al, 2009).

For this system, sensor sensitivity is 0,01 K for post

detecting time 1 sec and the sensor works without

any modulation-calibration at the entrance. This

system according to the theory works with full

power radiometers. In figure 6, it is worth noting

that the black spots on the image of city, at the top of

building, represent the mobile phone transmission

stations, producing harmonics in 8 mm band. Radio-

images in 90 GHz (3mm wavelength) frequency

band presented on Figure 7.

PMMW system are now gaining of deep cooling

based on superconductors for the space tasks

(Proc.of SPIE,2014) and the improvement of the

passive images, with the help of advanced data

processing, for the super resolution (Luxin et al.

2006). In the near future, it is expected the modern

application of the processed radio-images in the

various spectral bands with the help of a correlation

analysis.

2.2 Technical Peculiarities of the

Radiometric Discriminator

Good performance of radio-image systems is based

on the necessity to have identical sensors; this is not

challenging in case of a direct amplifier based on the

Monolithic Microwave Integrated Circuits (MMIC)

with enough high dynamic range. Instead, various

issues arises in the data processing, when we have to

turn from the measurements with a large number of

sensors in an image in a digital form or in optical up-

converting “looks”. Issuers regard also special

cooling (Moffa, 2001) and temperature stabilization,

which really increases the cost of PMMW system.

For example, prime cost of 8 mm sensor in Figure 4

has value around 700 $, and for modern European

direct amplifiers on 3 and 2,2 mm the cost is 5-20

times more expensive.

Figure 6: Radio-images in 8 mm wave band in comparing with the optical images of the same scenes.

Figure 7: Radio-images in 3 mm wave band

Fifth International Conference on Telecommunications and Remote Sensing

sensors, because the relevant angle of view is

enough small in this case.

Here, we focus on the differential modulation

radiometer, which is also named by discriminator.

Reciever

Voltmeter

Computer

Power

Source

reflector

Modulator

Demodulator

Intergrator

Modulator

Figure 8: Block-scheme of the simplest 8 mm microwave

discriminator.

The discriminator (see Figure 8) exploits two

feeders combined with the one antennas surface. The

special antenna system forms two space beams

directed on the two neighboring directions of the

space in the horizontal plane (the same device can be

done for the vertical plane). The angle between the

two beams depend on the technical specification.

The incoming signals from these two directions are

at the input of the switch and later to the radiometer

(Figure 8). Usually, it is convenient to deploy a

circulator in replacement of the switch where

radiometer is connected to the third gate of the

circulator. In this configuration, at the input of the

radiometer there are two microwave signals at the

modulation frequency (for example, 10 kHz). After

amplification (around 56 dB), the signal are detected

and are going as the meander with modulation

frequency. Afterward, the resulting signal can be

demodulated by the synchronous detector with time

constant around τ = 0.01sec. Finally, at the output of

the integrator (discriminator), there is a signal

proportional to the difference of the power in the

measured beams. This signal difference arises only

in case of observation of different observed scenes.

Therefore, as the result of the scanning, there is a

picture that resembles the two neighboring angles of

the space and accounts for the contour of the

observed object.

2.3 The Job of the Discriminator

For a simple explanation of the working principle of

the discriminator, we can refer to Figure 9.

Figure 9: Pictorial description of the working principle of a discriminator.

There the four channels PMMW discriminator

system produces the scanning of scene. The first

spot of the antenna beam S

b-left

, does not intercept

any part of the observing object whereas the object

Is it Possible to Detect the Stealth Flying Objects by the

Millimetre Wave Radiometer?

There the four channels PMMW discriminator

system produces the scanning of scene. The first

spot of the antenna beam S

b-left

, does not intercept

any part of the observing object whereas the object

is imaged in the second right spot S

b-right

For an evaluation of these two following

observing spots, the brightness temperatures T of the

object is used. The brightness temperature can be

evaluated according to the expression concerning the

microwave power P =

kTΔf, where k –Boltzmann’s

constant, T–brightness temperature, Δf- the band of

the receiving frequencies, which income to the

radiometer from the two antenna’s spots. Of course,

this task requires a critical technology

(Huguenin,2006, Dill et al,2009). In Eq.1 there is

back

= πA

, where A is the area of the aperture

antenna beam spot at the distance L till the aircraft.

Other main parameters are: the brightness

temperature of background (cold sky) is T

back

(concerned with S

-1, S

-2

on fig.9), this

value is dependent on the atmosphere conditions and

in particular on the absorption in atmosphere, which

increases the brightness temperature of sky

compared to the case of clear air. The diameter of

the antenna’s beam spot is A= 1,22 L sin λ/D. S

–

area of the aircraft surface inside of the beam spot

-2

with the brightness temperature of the aircraft

depending on the reflection from the earth or water

In the inset of Figure.9, we schematically present

two parts of the antenna beam spots S

b-left

and S

right

. Both S

b-left

and S

b-right

are two half of the full

surface of S

. Discriminator compares the received

signals from these two beam spots; the difference

between the received microwave power in terms of

the brightness temperatures of these two spots can

be evaluated in the simplest approximation as :

(1)

So, if the receiver of discriminator has a

sensitivity better that the result of eq. (1), it is

possible to detect the distinction inside of this

direction and the objects can be found by PMMW

discriminator system.

Every antenna has the efficiency η (for example,

in percents). On the track object – radiometer,

attenuation in atmosphere and losses from the

antenna to the input of the radiometer can be

accounted by the parameter α (expressed in dB or

times). Every measuring system must have a reserve

in the probability of detection of κ (signal/noise).

If we consider

Contrast

= T

back

sky

, it is

possible to recognize an aircraft, under a simple

approximation in case of :

Contrast

≥κ α Δ T

sens

b-right

/ S

η σ

(2)

Obviously, it is desirable to have a large contrast

Contrast,

which depends on the environmental

conditions, the brightness temperature of sky (Figure

2), the polarization effects, the working frequency,

the material and geometry of the reflecting surfaces

of the observed flying objects and position of

observing object of relatively horizon, zenith and

sun. Right part of Eq.2 depends on the sensitivity

ΔT

sens

, microwave losses in atmosphere α and (S

right

). If we consider the sensitivity of radiometer

for post-detection time 1 sec, real value is 0,01 K for

8 mm and in 3 mm practically too. There are no big

problems to do the radiometric system at the base of

modern MMIC with good thermal stabilization,

which is the key factor to filter out possible

amplification drift between the multitude of sensors.

In principle by using MMIC with more small noise

factor or special cooling, it is possible to reach best

sensitivity, as for example, for the space investigation

(Proc. of 22th ISSTT ), but this is not of interest in the

application considered in this paper. By turning to the

value S

b-right

which defines the percent of the

filling an aperture antenna ‘s beam spot by the

surface of aircraft, which according to aerodynamics

must have enough big figure in comparing with an

armor objects, for example, on the earth

(www.giws.de). Table 2 presents the diameter of the

antenna’s beam spot at the various distances from an

antenna as the function of its diameter. According to

the picture in fig.5 and Table 2, it is possible in good

approximation to evaluate the real microwave losses

on path between radiometric discriminator and the

aircraft to be observed.

b-right

back

b-right

back















)

(

Fifth International Conference on Telecommunications and Remote Sensing

Table 2: Diameter of antenna bean spot versus the distance from the antenna for the 8 mm wavelenght

Distance (km)

Diameter of antenna (m)

0.9

108

162

216

1.0

146

194

1.2

121

162

1.5

130

2.0

2.4 Simplest Calculation of the Possible

Distance

Let's try to use Eq.2 in two real cases, concerning

possible contrast levels

Contrast

as 250 K and

100 K (fig.9).

Range of radio brightness contrast ΔT

Contrast

for

the situation presented in Figures 2,3,9 and Table 1

can be from 23 till 270 K.

Real parameters of the discriminator:

- Radiometer sensitivity is 0.01 K for post

detection time 1 sec, at the real time for the analysis

(doing pixel) as τ = 10 msec, ΔT

sens

for this scanning

rate will be 0.1 K

- Wavelength λ = 8 mm

- The diameter antenna discriminator D, for

example - 200 cm (not so big problem to do it for 8

mm, if the accuracy of the surface must be worse

that λ/10). Diameter of an antenna spot at the

distance L is

/D)( 1.22L = A



and accordingly S

is π A

4//D)]( 1.22L [



. S

b-left

b-right

= 0,5πA

/4,

according to inset on fig.4.

- The antenna efficiency, for example, η = 0,8.

- The factor of the object position σ (σ = 0,8)

- The size of the appearing object inside of the

antenna beam spot S

is 5 x 10 m

(middle size ship).

- The probability of detection of κ (S/N) = 3-10

times. (Accept κ = 5)

- The attenuation or microwave losses between

discriminator and observed ship α (3….10 dB).

(Accept α =6 times).

In this case it will be Eq.3:

Δ T

sens <

η σ

Contrast

κ α 0,5 π (L 1,22 λ/D)

(3)

Arithmetical calculations will provide

0,1 K <

50 x 10

x 0,8 x 0,8 x ( 100…250 K) x (200 cm)

x 4 / 5 x 6 x 0,5 x 3,14 x L

x (1,22)

x 0,64 cm

(3.1)

< (100…250) x 50 x 0,64 x 16 x 10

/ 15 x 0,64 x 4,68 x 0,1

(3.2)

According to (3-1) the value L for the best case

(contrast equal to 250 K) will be 16,87 km. For

Contrast

of 100 K, the distance decrease at about

10,67 km. These evaluations have been made under

the assumptions of S

= 50 m

, but according to

Internet the real wing surface of the left aircraft on

the Figure 1 is 73 m

, and the right one has surface

478 m

It is worth noting that for the evaluations in the

case of a UAV, if we use the reflecting surface of

about 1 m

(really it is more smaller for the

observing UAV), the value L will be around 2,4 Km

for the same discriminator antenna size.

Is it Possible to Detect the Stealth Flying Objects by the

Millimetre Wave Radiometer?

3 CONCLUSIONS

For the cases where the PMMW kvazi image is not

so principle it can be used simple microwave

discriminator which is variety of the differential

modulation radiometer for the detection of an

objects. In this case, the receipted results can repeat

the contour of the observing objects.

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Fifth International Conference on Telecommunications and Remote Sensing