The Advanced Simulation System for MMW Imaging Radar Seeker onboard

Air-to-air Missile Sun Yumeng Chen Jie Guo Caihong Sun Bing Zhou Yinqing

Beijing University of Aeronautics and Astronautics, Beijing 100083, China Email: sun_ryan1981@yahoo.com.cn

Abstract

Millimeter wave (MMW) radar is booming in application to target seeker onboard the air-to-air missile (AAM), which has the capability to obtain all-weather radar images for auto target recognition (ATR) and intelligent active homing guidance. An advanced simulation system for MMW imaging radar seekers of AAM was introduced in this paper. The system is composed of parameter initialization module, signal simulation module and image formation module. It is capable of investigation and demonstration for system performance of MMW radar seeker. The modeling of radar signal and the geometric model for missile-to-target were presented in particular. The echo signal for major radar operation modes could be generated by the system, including high-resolution range profile (HRRP) mode, doppler beam sharpening (DBS) mode as well as the high-resolution burst- SAR mode. The image formation algorithms for each radar operation mode were integrated in the system to generate highly accurate radar imagery. Computer simulation results for an actual aircraft were presented, which validate the correctness of the simulation system. It is an advanced tool for the investigation of imaging algorithms and the optimization of the system parameter selection, as well as the demonstration of the radar scenario.Keywords: Synthetic aperture radar, Air-to-air missile, Millimeter wave radar, Target seeker, Active homing guidance.

. Introduction As an all-weather, compact sensor for air-to-air

missile (AAM), millimeter waves (MMW) radar can perform homing function in various complex scenes with narrow beam-width. It is of special importance to investigate the technique of MMW radar seekers.

At present, the MMW radar seekers for AAM complete the homing guidance by fast matching classification for one dimensional high-resolution range profile (HRRP). The computation load for HRRP is suitable for missile-borne real-time processor. And the auto target recognition (ATR) algorithm for HRRP is relatively mature. But it is quite difficult to distinguish

false targets or multiple targets which are adjacent in range domain with HRRP. Compared to HRRP, 2-D radar images produced by synthetic aperture radar (SAR), which could offer more information, can enhance the capability of target acquisition and recognition of the seekers greatly but the imaging algorithm and implementation of air-to-air missile-borne SAR is much more complicated and need further investigation. In the absence of real data, an accurate raw data simulation becomes more significant for the investigation of imaging algorithm and the optimization of the system parameter selection, as well as the demonstration of the radar scenario.

A simulation system of MMW imaging radar seeker for AAM is presented in this paper. Echo signal of burst SAR mode, DBS mode and HRRP mode can be simulated through the system. And radar images of different modes can be generated by image formation.The system is very flexible for the users to input simulation parameters, in order to acquire raw data in different situations. Integrated with a full graphical interface, the simulation system provides an interactive and easy-to-use tool for performance analysis of the radar system and investigation of image formation algorithms and ATR techniques.

.Architecture of the Simulation System The system is mainly composed of parameter

initialization module, echo signal simulation module and image formation module.

The inputs of the system are provided in three parameter files. The first file contains movement parameters of the missile and the target, such as position, speed and attitude. The second file provides the parameters of SAR system, such as chirp duration, bandwidth, pulse repetition frequency, sampling rates and mode selection. Backscattering characteristic of the target, which is an important parameter for the simulation, is contained in the third file. The system can get the backscattering characteristic of the target from real SAR images also.

The output of the system consists of the echo signal and the imaging results. The echo signal of three modes possibly used for MMW seekers can be simulated in the system, including HRRP mode, Doppler beam sharpening (DBS) mode and burst SAR mode. Two kinds of waveform including frequency modulated continuous wave (FMCW) and linear FM pulse are

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employed in HRRP mode simulation. DBS mode is used to scan the scenario for target acquisition. Burst SAR mode is used to form 2-D radar images of high resolution. Imaging results of different modes are given by processing the raw data.

Flow chart of the system is presented in Fig.1.

Figure 1. The flow chart of the simulation system

. Simulation Model A. Modeling of the Radar Signal

The most targets of AAM are aircrafts, such as fighters, bombers and attackers that are noncooperative and highly maneuverable. The position, speed and acceleration of the missile and the target are changing enormously. During the homing guidance, the MMW radar mostly works in the forward-looking highly squint situation due to the geometric relation between the missiles and the targets. So it is quite difficult to complete the movement compensation and form SAR images of high quality. To solve the problem, burst SAR

mode is introduced and the method of forming SAR images with subaperture data was used in the simulation system. In a short time, the movement of the missile and the target can approximately be considered as the uniform motion. This approximation can simplify the echo signal model[1]. And using subaperture data for image formation can reduce the time spending for data processing, which is crucial for the homing guidance and the missile-borne real-time processor that has to continuously monitor the reflected target echoes and calculate the target’s angular coordinates and range.

The echo signal S(t) can be expressed by equation (1).

122 2

3 41 1

, 4exp ( )4 ( )

2 ( )

2 ( ) exp

N Mt m m

mn m m op

mprt

mprt

P GS t j R t

R t F

R trect t nTc

R tj t nT n tc

(1)

Where N is number of pulses to be simulated; M is the number of the scatters from the targets and the ground terrain; Pt is the maximum radiated power of the radar; m is the radar cross section (RCS) of each scatter; Gm( , ) is the antenna gain; is the elevation angle; isthe azimuth angle; is the wavelength; Rm(t) is the distance between the scatter and the phase center of radar antenna; Fop is the system losses; c is the velocity of light; Tprt is pulse repetition time; p is the chirp duration; (t)=- bt2, is the phase function of the chirp pulses, and b is the frequency rate; n(t) is the system noise, whose power can be calculated by kBnTn, where kis the Boltzmann constant; Bn is the noise bandwidth; Tnis the noise temperature.

The major parameters of radar system are taken into account to implement accurate echo signal simulation, including the radiated power of radar, the system noise of receiver, the antenna gain, the system losses as well as the attenuation of slant range. If the relative position vector R is obtained from the geometric model, the parameters of Rm(t), and are calculated and the echo signal can be easily simulated with equation (1). We can conclude that the critical point of the echo signal simulation is to provide pulse-by-pulse position of the missile and the target scatters according to their movement parameter.

B. Geometric Model of the Missile and the Target Differ from airborne and spaceborne radar

simulation[2], the platform and the target have more degrees of freedom in this model, then more sophisticated relative movement should be simulated. And the images of the target can fluctuate greatly with small attitude and squint angle changes. So the attitude

(roll, yaw, pitch) of the missile and the target should be taken into account.

In order to describe the geometric relation accurately, four orthogonal coordinates including antenna coordinate, ground coordinate, target coordinate and missile coordinate are set in the geometric model as presented in Fig.2. The position and speed vectors of the missile and the target can be converted from one coordinate into another conveniently by multiplying transfer matrix between each coordinate.

MXAX

AY

AZ

MY

MZ

TZ

TX TY

Figure 2. Coordinate setting

The relative position vector R between the antenna center and the scatter is

M A P TR = R + R + R + R (2) Where RM is the vector between the center of mass of the AAM and the origin of the ground coordinate; RA is the vector between the position of the antenna center and the center of mass of the missile; RP is the vector between the antenna center and the origin of the target coordinate; RT is the vector between the origin of the target coordinate and the scatter.

Then the distance between the antenna center and every scatter of the target and the range and azimuth off bore-sight angles can be determined by R, and echo data of burst mode can be calculated by Eq.3.

The position vector between the antenna center and the background scatter can be calculated in a similar way.

Once the position vector between the antenna center and the scatter is obtained, the echo data of HRRP mode and DBS mode can be simulated too. The detailed flow chart of the simulation model is presented in Fig.3.

Figure 3. Flow chart of the simulation model

. Image Formation Under the condition that the movement parameters

of the target are given, the image of HRRP mode can be formed by range compression and the image of DBS mode can be formed by range compression and FFT in azimuth domain.

The air-to-air missile-borne MMW radar mostly works in the forward-looking highly squint situation, so the imaging algorithm suitable for SAR mode of air-to-air missile-borne radar should meet the requirement of compensating the phase accurately and implementing the range cell migration correction (RCMC). There are several imaging algorithms suitable for subaperture data processing such as improved RD algorithm, SPECAN algorithm[3], and ECS algorithm[4].Compared to other algorithms, ECS algorithm has compact process and smaller computation load. Under the condition that the movement parameters of the target are known, slant range images can be formed using ECS algorithm, but slant range images can not show the real geometric position of the target. For ATR, geometric correction is applied and the imaging results are given in section V.

The movement parameter estimation of target by processing raw data and other imaging algorithm suitable for air-to-air missile-borne SAR need further investigation.

. Computer Simulation The RCS map of a B-52 bomber is obtained from

high-resolution airborne SAR image. Simulation parameters of burst SAR mode are listed in Tab.1. The simulated radar images are illustrated in Fig.4. The strong scatters of the aircraft are clearly reflected from the images, which demonstrate the capability and validation of the simulation system.

Tab.1. Simulation Parameters of Burst SAR mode Wavelength (m) 0.008 Pulse width (us) 5 Signal bandwidth (MHz) 120 Sampling frequency (MHz) 150 Pulse Repetition Frequency (Hz) 10000 Reference slant range (km) 10 Reference squint angle (degree) 60 Missile velocity (m/s) 1200 Target velocity (m/s) 400

(a)

(b)

(c)

(d)Figure 4. The imaging results of B-52: (a) HRRP mode image with resolution 1.25m; (b) DBS mode image with resolution 3.75 7.6m; (c) burst SAR mode image with resolution 1.25m 1.33m; (d) 3D vision of the burst SAR image compared with photo of B-52 bomber.

Conclusion An advanced simulation system of MMW imaging

radar seeker for AAM is carried out. Signal model and geometric model is established. The radar images of DBS mode, HRRP mode and burst SAR mode can be simulated by the system. ECS imaging algorithm is used to form SAR images of high quality. The imaging results of B-52 are presented for verification of the system. The simulation system satisfies the requirements for providing source data for air-to-air missile-borne radar imaging algorithm and ATR algorithm studies.

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