Download Key Technologies for Lidar Detecting Stealth Targets

PIERS Proceedings, Hangzhou, China, March 24-28, 2008
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Key Technologies for Lidar Detecting Stealth Targets
Bin Zhu1 , Jing Zhang2 , Yan Chen2 , Ke Deng2
Dagang Jiang2 , Peng Zhang2 , Zoushi Yao2 , and Wei Hu2
1
2
Chengdu University, Chengdu 610106, China
University of Electronic Science and Technology of China, Chengdu 610054, China
Abstract— Detecting and tracking stealth targets have drawn more and more attention. With
the advantages of high resolution and anti-jamming to mature radar technologies, the lidar has
become a new and unique radar mechanism. Key technologies of lidar detecting stealth target
have been proposed based on two methods called multi-band and multi-static anti-stealth. The
combination of the visible, infrared and laser technologies can improve the detection probability
of stealth targets.
1. INTRODUCTION
Stealth technology means that the launchers reduce their own detectable signals so that it is difficult
to detect them. It is a kind of disguising technique, and draws more and more attention. Stealth
targets will be very important in future high technology war and great threat in air-defense system.
Penetration abilities of aircrafts can be greatly improved by using stealth technologies. Meanwhile,
anti-stealth technologies is also promoted, which is mainly focusing on centimeter wave radars and
developing towards the whole-band detection. Lidars can be used in detecting stealth targets for
its higher angular resolution, strong ability of anti-jamming, good concealment, and small size and
light weight [1].
Traditional radars use microwave and centimeter wave as carriers, while the lidar uses laser,
which has much shorter wavelengths. The lidar uses amplitude, phase, frequency and polarization
carries information and does not have essential difference with traditional radars [1]. Several key
technologies need to be taken into consideration in detecting stealth targets by lidars.
2. MULTI-BAND ANTI-STEALTH
As the development of photoelectric detections, it has become an important method of obtaining
information in battlefields. If we combine photoelectric detection with traditional detection methods
to detect stealth targets, the ability of anti-stealth can be improved further.
The target designation radar needs not only discovering stealth targets but also tracking and
aiming so as to antagonize them. Extending radar wavelength is necessary. Laser radar can detect
stealth targets effectively because it has short wavelength, high beam quality, strong directionality,
high measuring accuracy and it has functions of target identifying, posture displaying and orbit
recording [2]. The normal operational wavelengths of laser radar include 0.532 µm, 1.064 µm,
10.6 µm, etc. Target and background optical properties on different wavelengths and atmospheric
effects of different wavelength need to be considered in lidar detection.
2.1. Target and Background Optical Properties
Targets act as a series of combined reflecting surfaces to lidars, and these reflection surfaces decide
the electric levels of echo signals. Both relative movement effects caused by targets movement and
vector speed of targets can lead to the variation in reflected signals of lidars. Observed echo signals
are called lidar characteristic signals which used to obtain target information. Reflection of several
typical targets on 1.064 µm laser is shown in Table 1 [3]. We can draw the conclusion from Table 1
that different target materials correspond to different reflection in the same wavelength.
The main background noise sources are sun light, moon light, atmospheric dispersion and its
own radiation, which cause background illegibility in the FOV (Field of View) of receiver. This can
be widely used in aircraft photoelectric stealth [4]. In order to improve abilities of anti-jamming
and stealth target detection, further research on target and background optical characteristic is
necessary.
2.2. Atmospheric Effects
There are three main atmospheric effects on lidar signal transmission. The first one is attenuation caused by atmosphere molecular absorption. H2 O, CO2 and O3 are the primary absorbing
Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008
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Table 1: Reflections of several typical targets on 1.064 µm laser.
Materials
Aluminum (weathered)
building cement
titanium alloy (new)
titanium alloy (weathered)
paint (saturated olive green)
Reflection (%)
55
50
47
48
8
sources. Another kind of attenuation arises from Mie scattering by floating particles. Atmospheric
turbulence leads to the random changes of refractive index of atmosphere and causes wave-front
aberration. Assuming that the original power of a lidar signal is P (λ), the power after transmission
of x can be calculated by
P (λ, x) = P (λ, 0) exp[−k(λ)x]
(1)
where k(λ) denotes the attenuation coefficient which contains absorption and diffraction. It can be
seen from formula (1) that atmospheric attenuation depends strongly on operational wavelengths
of lidars. So it is important to choose lasers with low atmospheric attenuation as the operational
wavelength, such as 10.6 µm and 1.064 µm.
Figure 1 shows the relationship between propagation range and atmospheric transmission on
the operational wavelength of 1.064 µm. It shows atmospheric transmission under conditions of
fine (with visibility of 25 km), clear (with visibility of 15 km), haze (with visibility of 5 km), mist
(with visibility of 1 km), light fog (with visibility of 0.7 km) respectively.
Figure 1: Relationship between propagation range and atmospheric transmission under different weather
conditions.
The atmospheric turbulence refers to the density fluctuations arising from atmospheric temperature fluctuations. It leads to the random changes of refractive index of atmosphere. Atmospheric
turbulence affects propagations of laser beams in different ways namely scintillation, beam wandering, beam broadening and fluctuations of the arriving angle [5]. Since turbulence effects depend
strongly on the operational wavelength, it is important to choose a proper operational wavelength
and to fully understand weather conditions of the working area so as to reduce turbulence effects.
As mentioned above, atmospheric attenuations relate strongly to wavelength, and different targets have different reflectivity under the same wavelength. We need to choose operational wavelength according to absorbing and reflective characteristics of different targets. At present, the
most familiar bands are 0.532 µm, 10.6 µm and 1.064 µm at present.
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PIERS Proceedings, Hangzhou, China, March 24-28, 2008
3. MULTI-STATIC LIDAR ANTI-STEALTH
Stealth targets degrade the performance of radars by reducing their RCS. The following will introduce the RCS and multi-static radar detections of lidar systems.
3.1. Laser Radar Cross Section — LRCS
The LRCS of target is the symbol of laser scattering ability of target. It refers to the ratio of incident
power in unit area to total scattering power when targets are isotropic scattering. This ratio has
a dimension of area, and it denotes how much power stealth targets have got from the incident
power. The LRCS is a complex function of targets’ dynamic and static features, propagation media
features and incident wave features. The LRCS can be calculated approximately by the method of
RCS in radar [6].
4πρAR
σ=
(2)
ΩR
where ρ denotes the reflectivity of target surface, AR denotes the projection area of target, ΩR
denotes the solid angle of scattering beam. Reflecting signal of diffusive reflection targets will be
scattered in a wide area, and the distribution of reflecting signals submit to the rule of Bidirectional
Reflecting Distribution Function (BRDF).
The detected power of lidars can be derived from lidar operating range formula [7].
µ
¶
µ
¶
AC
PT
PR =
(ρAR )
τ2
(3)
R 2 ΩT
R 2 ΩR
where PR denotes the receiving power, PT denotes the transmitting power, R denotes the operating
range, ΩT denotes the solid angle of transmitting beam, AC denotes the effective receiving area,
and τ denotes the transmission of unidirectional transmission.
The relationship between the LRCS and operating range can be derived from formula (2) and
(3) [6].
·µ
¶µ
¶ ¸1
4
PT
σAC
R=
τ2
(4)
PR ΩT
4π
When R is the maximum operating range, σM is called the Critical LRCS, and the target is
stealthy if inequality σ < σM is tenable. At this point, it is necessary to build a complex geometrical model and take account of the surface optical characters or material scattering characters to
calculate LRCS of a stealth target with complicated shape. The graphic EM calculating model of
a RF system can be used for reference of calculating the LRCS. In calculating RCS, the graphic
EM calculating model is the most efficient way [8].
3.2. Multi-static Lidars
Stealth planes are generally designed to restrain the backward scattering so as to reduce the echo
signals of radars reflected by the planes, and it is effective to the single-base radar. Stealth planes
do not have the same performance in every direction. They mainly restrain the backward scattering
of the angle range of ±45◦ in horizontal and ±30◦ in azimuth around the front of the plane and
they have obviously larger RCS on other directions [2]. Multi-static lidars detect the lateral or
forward scattering signals to detect stealth planes.
Theory and practice proved that the RCS of the targets increase obviously when the scattering
angle is larger than 130◦ [2]. LRCS will increase with the angles between bases, so the targets will
be easier to be detected.
Multi-static radar means that transmitters and receivers are placed on two or more stations
which are far from each other. Radar netting, selection of station and optimization of the detection
probability should be considered in multi-static radar detection. Stations can be on the ground,
air-platform or space-based platform (e.g., satellite). Transmitters are located in safe places which
are far away from battlefields [9]. According to experiments, the RCS obviously increases when
the targets are between the transmitter and the receiver and the received signals are forward or
lateral scattering waves. Therefore, the detection probability for targets with small RCS can be
considerably improved while the radar is installed on the aircraft or satellite. In addition, multistatic radar net composed by a transmitter on the satellite and several receivers on the ground is
a potential means for the stealth targets detection [9].
Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008
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3.3. Radar Net and Data Fusion
At present, stealth ability can not cover all the working bands. It is not omni-directionally stealthy
even at the stealthy band [10]. Setting up radar network by combining the predominance of
detection systems based on multi-band and multi-static can improve the anti-stealth ability at
frequency and spatial domains. Data fusing is necessary to radar netting. Target association and
tracking of general targets are relevant to PDA (Probabilistic Data Association) and JPDA (Joint
Probabilistic Data Association). Data fuzziness and discontinuous can not be effectively dealt with
when the RCS is small [11].
4. CONCLUSIONS
The lidar possesses higher resolution and anti-jamming ability due to its coherence property and
extremely high frequency [12]. This attribute indicates that lidar has huge potential in targets
detection, tracking and range. Further more, some transmitter/receiver components and digital
signal process technologies have become mature that in turn accelerates the development of the
lidar. Theory research proves that lidars can be very useful in stealth aircrafts detection with proper
coverage region when the range reaches 20–30 km, and the angle precision exceeds 0.3mrad [12].
This paper also proposes a multi-band and multi-static detection method which combines the
visible, infrared and lidar to improve the detection probability of stealth targets. It still needs more
efforts to carry out a solution to the system complexity problem.
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