Non-Cooperative Classification of Helicopters using Millimetre Wave Radar and ISAR Processing

Transcription

1 Non-Cooperative Classification of Helicopters using Millimetre Wave adar and ISA Processing H. Essen*, Member, IEEE, M. Hägelen*, A. Wahlen*, K.-H. Bers**, M. Jäger** and M. Hebel** *GAN esearch Institute for High requency Physics and adar Techniques (H) Neuenahrer Str. 20, D-53343, Wachtberg, Germany **GAN esearch Institute for Optronics and Pattern ecognition (OM) Gutleuthausstr. 1, D Ettlingen, Germany phone: + (49) , fax: + (49) , essen@fgan.de Abstract Light weight and compact radars with high resolution capability can be built at millimeter wave frequencies. This has been demonstrated for a long period of time for missile seeker application for military use and as automotive radars in a civilian application. The technological advantages of this type of radar can be adapted to security applications in air traffic management at short and medium range as well as on the ground. The application discussed in this paper, focuses on the derivation of high resolution signatures of flying helicopters for non-cooperative classification schemes. Index Terms Millimetre Wave adar, ISA Imaging, High esolution I. INTODUCTION The control of the airspace and non cooperative classification of air vehicles under adverse weather conditions, during day and night is an increasing demand as well for the civil administration as for military observers. Especially during operations in the framework of peace keeping missions it has to be taken into account that under civil war conditions a mix-up of friendly and threat vehicles is appearing and the identification of aerial vehicles and especially the discrimination between different configurations of a certain type of helicopter is gaining more and more importance. Advanced radar sensors are able to deliver highly resolved images with considerable information content, as high resolution and polarimetry as well as robustness against changing environmental and operational conditions. Millimetre wave radars are able to operate over considerable ranges while delivering high resolution scattering center distributions independent on range. If a classification and discrimination between helicopters serving as weapon platforms or those being used for humanitarian purposes is done by the radar sensor, electrooptical sensor systems can deliver additional data at closer range for reliable target identification. Using the technique of ISA imaging it is possible to deliver high resolution scattering centre distributions of flying targets. These data can be compared with reference data of different possible target configurations. The process of signature comparison relies upon image based pattern recognition methods. II. BASIC IDEA O THE NON COOPEATIVE TAGET ECOGNITION DEMONSTATION The experimental millimetre wave radars MEMPHIS and KOBA have been used for the collection of high resolution scattering centre distributions of different helicopters in tower- /turntable configuration using the range-doppler (ISA) algorithm. To image flying helicopters also by means of the ISA technique, the MEMPHIS radar mounted on a tracking pedestal was used. The helicopters were measured flying along given straight and cycle trajectories at different velocities, heights and ranges. Scattering centre distributions were deduced in an offline process. These images were correlated with the reference data available for the 360 aspect angle range. At best orientation correlation the shapes were compared in an automatic correlation process to decide if a target state different from non hostile is probable. III. EXPEIMENTAL SET-UP A. The Experimental adar KOBA The experimental radar KOBA [1] is a modular system with four front-end modules at 10 GHz, 35 GHz, 94 GHz and 220 GHz respectively. The different front-ends are coupled to a common radar control and data acquisition electronics. The data processing is done in an off-line process. ig. 1 shows a simplified block diagram of the KOBA radar for the 94 GHz front-end. The basic M waveform is generated by an Arbitrary Waveform Generator, which is synchronized to an external reference oscillator at 100 MHz. The nominal frequency of the AWG is 1200 MHz with a

2 bandwidth of 500 MHz. This signal is amplified and doubled three times with appropriate filtering of the spurious responses thus resulting at a frequency of 9.6 GHz and a bandwidth of 4 GHz. A phase-lock oscillator, which is also synchronized by the 100 MHz crystal reference, generates an initial frequency of 14.1 GHz, which is multiplied by a factor of six to result in an output frequency of 84.6 GHz. After filtering, this signal is mixed with the 9.6 GHz signal carrying the chirp waveform. The resulting millimetre wave signal is amplified by a chain of HEMT amplifiers to result in a transmit signal with an output power of about 200 mw. A portion of the signal serves as local oscillator for the receive mixers. Low noise HEMT amplifiers in the input stages of the two channel, polarimetric receiver are employed to keep the noise floor sufficiently low. principally be operated with a closed loop for autonomous tracking, tracking was done manually using a TV camera during the experiments described here. This method was preferred to avoid glint effects during tracking and to maintain a stable aimpoint and a stable illumination of the complete target during flight manoeuvres. ig. 2 shows a photo of the MEMPHIS radar upon the tracking pedestal. igure 2 MEMPHIS with 35-GHz and 94-GHz ront-ends upon Pedestal Table 2 summarizes the performance data of MEMPHIS. Transmitter 35 GHz 94 GHz Output Power 500 W 700 W Waveform M Chirp + stepped frequency igure 1 94-GHz ront End of the KOBA adar Table 1 summarizes the performance data of KOBA. Transmitter Output Power Waveform Chirp Length Bandwidth ange esolution Polarization Dynamic ange 94 GHz 200 mw Linear Chirp M / CW 120 ms 4000 MHz 3.5 cm H-V and H-H 60 db Table 1 Performance Data of 94-GHz KOBA ront End B. The Tracking adar MEMPHIS The millimeter wave radar MEMPHIS is used for a variety of applications, ranging from airborne SA measurements [3] and land based signature measurements of ships [4] to 3-D- ISA measurements with fully illuminated targets on a turntable [5]. The MEMPHIS radar has already been described in detail [6]. or the investigations discussed here, it was used for polarimetric ISA measurements in the same way as the KOBA radar with reduced range resolution capability, In addition it was mounted on a VETEX [7] tracking pedestal to do signature measurements on flying helicopters. Although MEMPHIS is equipped with a monopulse antenna and can Pulse Length Bandwidth ange esolution Polarization Dynamic ange Data ecording 80 ns 2 ms 800 MHz 19 cm H-V and H-H 60 db 4 identical Channels for co- and cross-polarisation and Monopulse Deviations and Sum Table 2 Performance Data of 35/94-GHz MEMPHIS adar IV. TOWE-TUNTABLE ISA MEASUEMENTS To determine high resolution reference data of helicopters, tower-turntable measurements were conducted with different helicopters. or the measurements they were placed on a turntable for heavy loads. The measurement distance was about 200 m. The helicopters were fully illuminated by the 3- db beam of the antennas. ig. 3 shows a BO 105 helicopter on the turntable. igure 3 BO 105 Helicopter with Launcher and without upon Turntable

3 The method of ISA or ange-doppler (DI) imaging in tower-/turntable geometry to extract scattering centre distributions has been described in the literature [8], [9]. It is the standard method to determine two dimensional scattering centre distributions of a target under controlled conditions. In its simplest form it is applied to a target, which is continuously rotating on a turntable, fully illuminated by the antenna beam of the radar. Cross range resolution is achieved by the evaluation of the Doppler content of the echo signal. The Doppler shift of the received signal is associated to the distance r between scattering center and the center of rotation by D = 2 ω r c / λ (r c = component of r orthogonal to the radar beam). The Doppler frequency content is determined by a discrete ourier transformation (DT) over a certain number of pulse periods (N). ISA imaging is due to the same drawbacks as SA imaging, namely range walk and range offset. In the millimetre wave region these errors, however, can be neglected as long as the resolution is not too high and the radar frequency is sufficiently high and the target dimensions small enough. A detailed discussion of the limitations and how to overcome them is given in [10]. or the millimetre wave frequencies a resolution of about 20 cm, as delivered by the MEMPHIS measurements, allows linear DI, with increasing resolution as possible with the KOBA radar, a thorough reformatting procedure is necessary. The range/doppler map, which is yielded by the application of two consecutive ourier transformation, is a two dimensional representation of the geometric distribution of scattering centers on the target as far as they can be separated by the range resolution of the measurement radar. A method was developed to achieve a fast processing [11]. The algorithm takes advantage of the simple preconditions which are delivered by processing in the k-space and is done by successive two-dimensional ourier Transforms for discrete, equally spaced intervals and incoherent superposition of the images. ig. 4 gives examples for high resolution scattering centre distributions of the BO 105 helicopter derived with the KOBA radar. distributions. With two dimensional range/doppler imaging ambiguities arise in the assignment of scatterers to geometric features of the target in those cases, where different scattering centres are related to the same slant range at equal lateral position although they are located at different height. This drawback can not be solved by two dimensional imaging. To solve the ambiguity, three-dimensional processing has to be applied. A method to extract the third dimension is based upon the evaluation of the monopulse deviation in elevation. If in addition to the complex backscatter amplitudes also complex monopulse deviations are available, the same operations applied during the range/doppler extraction process can also be applied to the monopulse deviations. This operation yields high resolution monopulse deviations separated by range and cross-range. This means, that for each pixel of the range/doppler-map an elevation monopulse deviation is available. As the antenna beam width was chosen to maintain a full illumination of the target, and as there is a linear dependence between monopulse deviation and the position of the center of gravity of scatterers within the beam, the angular position within the beam is defined. ig. 5 demonstrates the principle of the algorithm. A detailed description has been given in [12]. Δ Σ t t igure 5 Schematics of 3-D- Monopulse ISA Imaging ig. 6 shows examples of representations for the main cuts derived from the 3-D scattering centre distributions for the BO 105 helicopter. Δ Σ igure 4 94-GHz ISA Images at H-H and H-V T/-Polarization, urther measurements were conducted using the MEPHIS radar equipped with monopulse antennas in a staring mode against the helicopter on the turntable. This measurement configuration allows the extraction of 3-D scattering centre igure 6 Scattering Centre Distributions derived by 3-D-ISA Imaging

4 V. ISA WITH LYING HELICOPTES In the configuration as shown in ig. 2 MEMPHIS was used for signature measurements of flying helicopters. Tab. 3 summarizes the geometrical and radar parameters effective during these measurements. ig.7 shows the helicopter in flight. requency simultaneous 35 GHz and 94 GHz Bandwidth 800 MHZ Antenna Tracking 35 GHz: Cassegrain D = 3 ft, 5 -Lens 94 GHz: Cassegrain D = 10 cm, 5 -Horn manual tracking using a tele-camera Calibration reference corner reflector light Path straight trajectories ange m Height m (AGL) Velocity 4 12 m/s Table 3 Measurement Parameters during Helicopter airborne Tests The algorithm to derive scattering centre distributions needs several processing steps, namely the range compression, the motion compensation and the cross range processing. ig. 8 demonstrates the range compression process, which is done by an inverse filtering method using calibration data from measurements again a reference reflector. raw measurement data range profiles igure 8 Demonstration of ange Compression The motion compensation process has to undergo the following steps: 1. Compensation of the zero range 2. Pre-alignment of range profiles by calculating the position of the CS centroid 3. Post-alignment by correlation of adjacent range profiles 4. Phase compensation using dominant scatterer or multiple scatterer algorithms (DSA, MSA) In order to achieve a continuous shifting of the range profiles, the measurement data must be resampled during each alignment step. ig. 9 shows the results for steps 1 and 3 applied on a series of rage profiles. igure 7 lying BO 105 during Measurements In principle the algorithm to determine scattering centre distributions of moving targets follows the same procedure as that described above for targets on a turntable. However is this measurement situation much easier to handle, as all parameters describing the movement of the target under consideration are very well controlled and registered simultaneously with the backscatter data. or a free flying helicopter the movement parameters are not readily fed into the data acquisition but have to be determined during flight. It is, however, obvious, that using a radar on a tracking pedestal the missing parameters can be determined. Generally the following parameters have to be determined: Target position Target motion state otation rate Projection plane. ange is delivered by the radar and the angular position of the antenna pedestal gives the pointing direction (azimuth and elevation), thus the trajectory of the helicopter can be determined from the measured data by a polynomial fit. The knowledge of the rotation rate is necessary for defining the integration time and a correct scaling in cross range. It can be calculated under the assumption that the drift angle of the target is negligible. To get stable information on the target motion, the observation time has to be sufficiently long.. igure 9 Series of ange Profiles for the first and third processing Step The resulting series of range profiles after the full motion compensation process is shown in ig. 10. igure 10 Series of ange Profiles after motion Compensation or the cross range processing care has to be taken regarding the limits of linear DI, which is: r 2δxδy /λ0 7.5 m This means, that imaging of objects with a diameter up to 15 m is possible. The range compression is done by application of an T to each range bin. To suppress side lobes a Hamming windowing function is used, which implies a reduction of the range resolution from 19 to about 25 cm, which was felt to be

5 tolerable. The resulting scattering distribution shows ig igure 13 Demonstration of Steps during the matching Process for Target and Outline from the Data Base igure 11 Scattering distribution for flying Helicopter VI. COMPAISON O GOUND BASED AND AIBONE SCATTEING CENTE DISTIBUTIONS The classification process for helicopters in the air is based upon an image based approach comparing the scattering centre distribution of the airborne target with a data base from ground based measurements. ig. 11 shows a respective comparison for the BO 105 Helicopter measured by ground based ISA and in the air. The matching process is done for each single ISA image. The resulting scattering centre distribution undergoes low pass filtering before the comparison with different outlines of targets is done. The automatic matching gives as well the momentary aspect angle of the helicopter as the type. This is done by calculation of the correlation coefficient. The situation with the optimum correlation is taken as the result. or the matching process the correct determination of the rotation rate of the target is most essential. A wrong rotation rate causes a wrong scaling of the target, which is inhibitive for a correct matching. ig. 14 shows four cases, where only for the first two aspects the scaling is correct und thus only two correct results have been produced in this case igure 12 Comparison of scattering Centre Distribution from ISA with airborne Target and from Turntable Measurements The comparison in this specific case refers to a BO 105 without missile launchers for the airborne measurement (ig. 11 left) and with launchers for the turntable measurement (ig. 11 right). This is obvious from the images. Inspection of different targets under different conditions showed, that the resolution of about 20 cm is sufficient for the purpose envisaged in this study. To be able to discriminate smaller alterations of the outer shape of the target, a higher resolution, as that demonstrated with the ground based KOBA measurements, would be of advantage. However it is felt, that it will be critical to achieve that high resolution for the airborne target configuration. VII. IST ESULTS O IMAGE BASED IDENTIICATION irst results could be achieved with a matching process of helicopter targets from a data base containing the characteristic outline of different helicopters based upon the ISA images of airborne targets. ig. 13 shows a series of images used during the matching process. igure 14 our Matching Approaches, the first two are successful, Case 3 and 4 suffer from a wrong scaling VIII. CONCLUSION During the study high resolution signatures of helicopters were determined by ground based (tower-turntable) ISA and using the same radar with different antennas, against airborne targets. It could be demonstrated, that it is possible to achieve data based upon which it is possible to apply image based comparison algorithms for non cooperative classification. or an application in the field a much bigger data base would be necessary and a real time ISA algorithm has to be implemented. Work towards such an algorithm is currently underway. EEENCES [1] H. Essen, A. Wahlen,. Sommer, G. Konrad, M. Schlechtweg, A. Tessmann, A very high Bandwidth Millimetre Wave adar, Electronic Letters, Oct. 2005, Vol. 41, No 23, pp 1247, 1248 [2] H. Schimpf, H. Essen, S. Boehmsdorff, T. Brehm, MEMPHIS a ully Polarimetric Experimental adar, Proc. IEEE IGASS 2002, Vol. III pp , Toronto 2002 [3] H. Essen, H. Schimpf, A. Wahlen: "emote Sensing with a 94- GHz Synthetic Aperture adar, Proc. EUSA 96, Königswinter, 1996 [4] S. Boehmsdorff, H. Essen, "MEMPHIS, an experimental platform for mm-wave radar," DGON Intl. Conf. On adar, 1998, Munich, pp

6 [5] H.Schimpf, H.Essen, S.Boehmsdorff, T.Brehm, MEMPHIS a ully Polarimetric Experimental adar, Proc.IGASS 2002, Toronto, Canada, June 2002 [6] H.Schimpf; A.Wahlen, H.Essen, High range resolution by means of synthetic bandwidth generated by frequency-stepped chirps, El.Letters, 39,18, pp , Sep.2003 [7] VETEX Antennentechnik, Baumstraße 50, D Duisburg, Germany [8] J. L. Walker, ange Doppler Imaging of otating Objects, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-16, No 1, Jan 1980, pp [9] D.L. Mensa, High esolution adar Imaging, Artech House, Dedham, MA, [10] Brehm, T., Wahlen, A., Sommer,., Wilcke, J., Essen, H., Tessmann, A., Schlechtweg, M., KOBA, a high resolution millimeterwave radar for the evaluation of targets and background signatures, DGON, IS- 2007, IS Proceedings, pp , Sep. 2007, Cologne, Germany, [11] H. Essen, M. Hägelen,. Brauns, G. Konrad,. Sommer, A. Wahlen, irst esults for Ultra-High esolution Tower-Turntable ISA and Tower-SA at 94 GHz, Oroc. EUSA-2006, Dreden, Germany, May 2006 [12] Biegel, G., Essen, H., Wahlen, A., Brauns,., Determination of Three- Dimensional Scattering Center Distributions by High esolution Monopulse, DGON, GS 2002, Bonn, pp , Sep

7 Year: 2008 Author(s): Essen, H.; Hägelen, M.; Wahlen, A.; Bers, K.-H.; Jäger, K.; Hebel, Marcus Title: Non-cooperative classification of helicopters using millimetre wave radar and ISA processing DOI: /TIWDC ( IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Details: Consorzio Nazionale Interuniversitario per le Telecomunicazioni -CNIT-; Institute of Electrical and Electronics Engineers -IEEE-: Tyrrhenian International Workshop on Digital Communications - Enhanced Surveillance of Aircraft and Vehicles, TIWDC/ESAV 2008 : Capri, Italy, 3-5 September 2008 Piscataway/NJ: IEEE, 2008 ISBN: pp.