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COLLISION AVOIDANCE RADAR ABLE TO DIFFERENTIATE OBJECTS / 27th EuMC’97 • 8-12 September 1997 • Jerusalem • Israel /

P.J.F. Swart and L.R. NieuwkerkIRCTR, Delft University of Technology, P.O. Box 5031, 2600 GA Delft, The Netherlands Tel.: +31 15 278 1023, Fax: +31 15 278 4046, e-mail: swart@irctr.et.tudelft.nl

ABSTRACT

The Collision Avoidance Radar Colarado (Collision Avoidance Radar Able to Discriminate Objects) is a multistatic frequency modulatedcontinuous wave (FMCW) radar system developed for use in the control system of an autonomous vehicle. The aim of the system is to detectthe presence of obstacles (targets) in the surrounding area and in addition to determine their positions and ultimately follow their movements.The advantage of using radar for this purpose is that unlike the sensors based on other physical principles (optical, ultrasound), its performanceis insensitive to the illumination level, atmospheric disturbances (like fog) or acoustic noise in the environment. In the next a laboratoryprototype system version, the demonstrator, is described and first results are presented.

1. INTRODUCTION

The Colarado project aims are the realisation and demonstration of a very flexible advanced anti-collision radar (Obstacle Warning Radar, OWR)with real-time FMCW radar signal processing. The Delft collision avoidance radar is designed for use on automatic guided vehicles, Jacobs (1)and Staalduinen (2). The radar is developed at the International Research Centre for Telecommunications-transmission and Radar, IRCTR. TheRF and LF parts are made by the Telecommunication and Teleobservation Technology group, the real-time radar signal processing developmentis carried out in co-operation with the laboratory of Computer Architecture and Digital Techniques, both at the department of electricalengineering of Delft University. Project funding is provided by the Netherlands Technology Foundation.

The realised demonstrator is one of the experimental radar facilities at IRCTR. The FMCW radar system has a rather high spatial resolutionthat is needed because of the multistatic antenna configuration on a relatively small base to achieve three-dimensional 'imaging' of the target area.The chosen strategy and original concept puts a large demand on real-time signal processing. The demonstrator validates the system conceptand forms the instrument to arrive at systems of practical use. The parallel multi-processor development system allows for reconfigurableprototyping.

Spin-off are mobile collision-avoidance systems (road, rail, shipping and aviation) based on initial use on automatically guided -container-vehicles for the European Combined Terminals company in the Rotterdam harbor area. The real-time signal processing algorithms are anotherimportant Colarado spin-off.

2. COLARADO SYSTEM

The radar is of the FMCW type and uses a multistatic antenna system consisting of two transmit and three receive antennas. The transmitsignal is produced by a voltage controlled oscillator that is sawtooth steered over a 1.5 GHz bandwidth around 9.75 GHz, resulting in 10 cmrange resolution, in 256 µs sweeptime followed by an idle time of 32 µs. The RF signal to the two transmit antennas is time-multiplexed on aper sweep basis. The three receiver channels work in parallel. Thus with 5 antennas 6 propagation paths of different length are obtained toenable the location of objects in 3-D space.

The antennas are located in the vertical plane on a support of 2.5 m width and 1 m height on the front of the automatically guided vehicle. Thebottom of the antenna frame is 0.5 m above the ground. The antennas are MLA (microstrip like waveguide antennas) small open-endeddielectric filled waveguides, Tian et al (3). The azimuth opening angle of the transmit antennas is 108°, the elevation 3 dB beamwidth is 80°.The antenna directivity is 7 dB. The effective isotropically radiated power is 10 mW. Specifications are listed in Table 1, the system blockdiagram is given in Fig.1.

The real-time signal processing tracking and tracing algorithms, scene display and collision avoidance signal are implemented on a parallelPowerPC’s computer. The processing aims at real-time reconstruction of 3-D scenes as sensed by the system. After sampling and FFTprocessing of the FMCW radar beatsignal into 128 range bins, i.e., a maximum range at 12.8 m, the beatfrequency spectra are scanned for peakscorresponding with individual objects (Peak Tracking and Tracing). The results from the three receiver channels are then used to reconstruct thespatial coordinates of each object as located in the scene (Object Tracking and Tracing). Switching between the two transmit antennas results intwo complete target maps that are finally matched to eliminate false targets resulting from combinations of spectral peaks belonging to differenttrue targets, Schier et al (4).

The signal source is one single VCO (Radian type 2829A) with 20 dBm output power that is within 0.5 dB constant from 9 to 10.5 GHz.Phase noise is specified as -85 dBc/Hz at 100 kHz. The VCO non-linearity amounts to only 0.5 percent which however was found still toresult in a spectral leakage, i.e., resolution degradation, that is too large with respect to the Colarado system requirements. After correctionusing a RAM table input to the sawtooth steering generator overall linearity is within 0.025 percent with a good long term stability. The1.5 GHz bandwidth gives a range resolution of 10 cm and depending on the placement of the antennas on the 2.5 by 1 m mounting frame thisresults in azimuth and elevation angle resolutions around 10 respectively 20 degrees. This may be improved by using the spectral phaseinformation.

The transmit antenna inputs and the receiver antenna external mixer LO inputs are connected with 3 m length SMA cables to the VCO outputsignal. The cable attenuation is around 1 dB/m at 10 GHz. The RF level at the transmit antenna input is 3 dBm. The mixer LO input is about6 dBm. The mixer down-converts the received RF signal to the beatsignal frequency band of 35 to 535 kHz. The minimum received signal basedon the RCS of a human at the maximum range distance of 12.8 m equals -101 dBm. Without the use of low-noise RF preamplifiers the noiselevel at the output of the receive antenna is -118 dBm. Since in practice a S/N ratio of 17 dB was not reached (mixer isolation, reflections, VCOphase noise, AM, LF-amplifier, 12 dB/oct dynamic range compression) RF pre-amplifiers are placed at the receive antenna outputs (20 dBgain, 4 dB noise figure).

The dynamic range in object RCS is set to 30 dB. An extra 20 dB is needed for maximally 10 objects in the scene. Accounting furthermore forthe antenna diagram and the deviation from 12 dB/oct spatial spreading loss due to the multistatic antenna configuration the total dynamic rangeis 60 dB. The minimum S/N ratio is 20 dB.

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3. CURRENT STATUS

The Colarado anti-collision radar demonstrator is operational since begin 1997, a photograph is shown in Fig. 2. The radar (X-band) RF-part,LF-part, analog to digital conversion, FFT, interfacing and processing hardware (Arcobel 4x transputer-PowerPC) are complete and functionwithin the imposed specifications (real-time). The RF-part and the (one of three identical, cyclic phase-locked printed circuit boards) receiverdigital part are shown in Figs. 3 and 4. The radar was linearised, system-parameters were measured followed by modifications mainly to reachthe required system sensitivity (detection of a human being). A full calibration of the radar (amplitude and phase over the complete range)remains to be done. Digitalisation, FFT spectrum determination, data transport to the processing part for the three parallel receiver channelswith optional windowing and complex or polair format output are in every detail working properly (6 Mbyte/s). For the demonstratorprocessing (parallel processor) and simulator (sequential, workstation(s)) a uniform software environment has been developed in such a waythat algorithms designed for and tested on the simulator can directly be used also in the demonstrator and vice versa so that radar data recordedwith the demonstrator can be played back on the simulator where results on the same data with various algorithms can be intercompared.

Thus, a very flexible advanced anti-collision radar (Obstacle Warning Radar, OWR) and data processing system is available. The system offersthree-dimensional multi-target scene information of which the geometric resolution can be improved by additional phase processing, andpossible velocity measurement.

4. MEASUREMENTS

To verify the system specifications and to provide data sets for further processing algorithm development, test measurements are performed.Extensive testing of the system is partly carried out in-door in a room of 9 x 6.5 x 3 m. The actual system coverage area of 12.8 x 10 x 5 mexceeds the available space and as a consequence many large reflections from the walls and ceiling as well as multi-path signals severely maskthe contributions of targets under test. To partly overcome this problem and yet to remain in a very well controllable laboratory situation atransponder is used in stead. The in-door measurement setup of the system and some early results are given in Fig. 5. From these in-doorexperiment results it is concluded that the detection algorithms based on only amplitude range data gives already reasonable results.

An initial out-door test showed that the system sensitivity is well above the RCS of a human being (0.4 m2), more exact quantification will bebased on a recently acquired large in-door (sports hall) data set for various multi-target scenes including reflectors, persons and transponder.First qualitative results are given in Fig. 6.

5. FUTURE PLANS

In the coming year the demonstrator is intended to be shown to work at the ECT (European Combined Terminals) terrain at the Rotterdamharbor area mounted on an AGV (automatic guided -container- vehicle).

As stated previously up to now only spectral amplitude data is used in the transformation of range to angular information. Although thesystem has a 10 cm range resolution the obtained angular resolutions of 10 and 20 degrees in azimuth and elevation respectively are rather poor.This is due to the relatively small distances between the antennas by the size limitation of the available mounting area of 2.5 by 1 m. Resolutionimprovement is therefore required and maybe obtained by additional processing of the spectral phase data that is available also. To this aim thephase change caused by distance changes between target and radar may be used in a first angular resolution refinement step, i.e.,SAR-processing, although this requires a priori knowledge of the velocity of the target with respect to the radar. Then a further resolutionimprovement is possible based on phase differences in the same way as using beatfrequency differences. The latter method suffers howeverfrom the 2π phase ambiguity and this is the reason for preceeding this by the mentioned first step to avoid resulting angle ambiguity. The angleambiguity may also be reduced by decreasing antenna spacing (wavefront reconstruction, inverse phased array) whereas the antenna spacingshould be large for the resolution based on beatfrequency. The use of a multistatic antenna configuration allows for both large and smallspacings.

The current X-band Colarado system was needed to show the principle of high resolution multistatic sparse array radar being able todifferentiate objects. Now that the soundness of the concept is proved, the used frequency has to be shifted to the bands allocated toautomotive application. Based on this and other considerations Colarado is going to be upgraded to 35 and 77 GHz. Two follow-upco-operation projects between IRCTR and the universities of Aachen and Leeds were started this year with a duration of three years. In theseprojects millimeter wave radar modules will be developed, radar system integration will be carried out at IRCTR. For the 35 GHz radar front-end monolithically integrated circuits, maybe single-chip, and for the 77 GHz modules hybrid technology will be used.

The real-time signal processing algorithms are in the demonstrator implemented in software. In a next project the large and costly paralleltransputer-PowerPC computer used for this aim is to be replaced by small and likely low-cost Application Specific Processing integratedcircuits.

6. CONCLUSIONS

Multistatic wideband radar used in the described way can provide three-dimensional position and velocity information of a multi-target scenewith a high repetition frequency. This approach relies on the real-time processing of large amounts of data. A demonstrator system version isoperational and first results are obtained. Future versions of the system include upgrades of the RF-part to Ka-band and W-band, anddevelopment of Application Specific Processing circuits to improve on real-time performance versus costs.

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REFERENCES

1. J.J. Jacobs, Feasibility study on bistatic anti-collision radar, Technical Report TVS A-318 TUD/ET, Delft, June 1990. (In Dutch)2. K.J. v.Staalduinen, System design of a multistatic FMCW anti-collision radar for unmanned vehicles, IRCTR Report S-006.95, Delft, Nov. 1995.(In Dutch)3. M. Tian, P.D. Tran, M. Hajian, L.P. Ligthart, “Air-gap Technique for Matching the Aperture of Miniature Waveguide Antennas”, in Proc. IEEEInstrumentation and Measurement Techn., California, IEEE Catalog No. 93CH3292-0, May 1993, pp. 197-201.4. J. Schier, H.J. Agterkamp, A.J.C. van Gemund and G.L. Reijns, “Object Tracking and Tracing - Incremental Approach”, in Proc. of the 2nd Eur.IEEE Workshop on Computer-Intensive Methods Control and Signal Processing, Prague, August 1996, pp. 151-154.

Table 1. Specifications

application anti-collision radar system on AGV, 3D-imagingfrequency 9.0 – 10.5 GHzmodulation FM-CW sawtooth PRF 3.47 kHzsignal source VCO Radian type 2829AEIRP 10 mWlocation Delft University, electrotechn. eng., in / out-doorantenna type MLA 2 step air-gap, directivity 7 dBantenna height multistatic, between 0.5 and 1.5 m above groundmaximum range 12.8 mradar cross section range 0.4 – 400 m2 (30 dB)maximum number of targets 10 (dynamic range max. +20 = 50 dB)maximum target velocity 10 m/sresolution (based on amplitude data only) range 0.1 m; azimuth 9°; elevation 22°

3x

3x

(3 m)

IF

RF LO

(3 m)

(3 m)

3x

f

H(f)

12dB/oct 500 kHz40 dB

4-waypower divider

RAM

linearizer

DAC

VCO

ADC

inter-face

inter-face

inter-face

Collis.Avoid.

Dig.SignalProc.

FFT

τ

scenedisplay

3x

20 dB

Figure 1. System block diagram Figure 2. Colarado demonstrator

Title: PADS Postscript Driver HeaderCreator: Andy Montalvo, 18 Lupine St., Lowell, MA 01851CreationDate: 06/08/90

Figure 3. RF-part of the Colaradosystem (X-band)

Figure 4. Analog to Digital Converter, FFT andcomputer interface printed circuit board

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1m

1m

T0 : 0, -1.25, 0.52T1 : 0, 1.25, 1.47R

0: 0, -1.25, 1.47

R 1 : 0, 0, 1.47R 2 : 0, 1.25, 0.52

5

4

3

2

1

T0

R0

T1

R2R1

x

yz

Actual transponder position (m):x y z

1: 4.39 -2.92 1.00

2: 1.84 -1.94 1.00

3: 3.32 -0.06 1.00

4: 2.65 2.36 1.00

5: 7.33 -0.12 1.00

Predicted target position (m):x y z

1: 4.42 -3.07 1.012: 2.06 -1.80 1.223: 3.46 -0.05 0.874: 3.07 1.96 1.315: 7.66 0.62 0.99

Position difference (m):∆x ∆y ∆z

1: 0.03 -0.15 0.012: 0.22 0.14 0.223: 0.14 0.01 -0.134: 0.42 -0.40 0.315: 0.33 0.74 -0.01

a) background b) 10 reflectors c) 10 reflectors + 5 persons

Figure 5. Colarado in-door test set up and object detection results Figure 6. Colarado large in-door (sports hall) test results