1st International Conference on Sensing Technology
November 21-23, 2005 Palmerston North, New Zealand
Millimetre Wave Radar for Tracking and Imaging
Applications
Graham M Brooker
Australian Centre for Field Robotics, University of Sydney, Sydney, Australia
gbrooker@acfr.usyd.edu.au
Abstract
This paper discusses the design and implementation of some millimetre wave radar systems for tracking and
imaging applications. It demonstrates the versatility of this technology and its application as the sensor of choice
for autonomous guidance and navigation applications.
Keywords: Millimetre wave radar, tracking, imaging, 94GHz, 77GHz, autonomous guidance, navigation
1
Introduction
Over the past two decades we have been building
millimetre wave radars for both military and
commercial applications. This paper discusses some
of the practical aspects of these developments with
illustrations of both the systems and their outputs.
2
Tracking Radar
A number of different 94GHz tracking radar systems
have been developed to investigate various aspects of
the technology. Probably the most simple is a single
stage non-coherent 5W pulsed IMPATT transmitter
with a conventional super heterodyne receiver shown
in Figure 1. Because of the cost and poor performance
of amplifiers at 94GHz it was decided to dispense
with RF amplification and feed a mixer directly. The
local oscillator (LO) is a conventional 2nd harmonic
Gunn device tuned to 93.7GHz to produce an IF
frequency of 300MHz.
locked oscillator chain driven by a fixed frequency
Gunn oscillator [3].
2.1
An analog split-gate range tracker performs this
function [4]. It comprises two fast sample-and-hold
boards each with a 50ns aperture time followed by
sum and difference circuits. The magnitude of the
sum signal is used for target detection, and the
difference signal drives a pair of cascaded integrators
to produce a second order tracker. The range voltage
controls a timing generator that produces two outputs
separated by 50ns to trigger the early and late gate
sample and hold circuits as shown in Figure 2.
Threshold
Sample
&
Hold
Successive
Detection
Log Amp
Horn-lens
Antenna
Mixer
Sum
+
+
Target
Detection
Range
Designation
Sample
&
Hold
94GHz
Amplifier
Matched
Filter
VE
Radar
Video
Pulsed IMPATT
Oscillator
Pulse
Generator
Range Tracking
+
VL
Difference
-
Range &
Velocity
Estimator
Velocity
Range
Early Gate
Late gate
Timing
Generator
300MHz
93.7GHz
Circulator
Figure 2: Analog implementation of a classical split
gate range tracking loop
Gunn
Oscillator
Figure 1: Pulsed millimetre wave radar configuration
One of the issues with this configuration is the large
intra-pulse chirp which is generated as the IMPATT
heats due to the extremely high current density [1]
which is typically greater than 100MHz for a 100ns
rectangular pulse. This can be reduced by shaping the
pulse current [2], but it does preclude using a filter
matched to the pulsewidth, and hence the overall
noise performance is not very good. It is however,
much less complex than implementing an injection
An external range designation input to the estimator
allows the range gates to be positioned manually
using a joystick, or scanned in range during the
automatic target detection sequence.
If the gates are swept through a point target, the
difference signal maps out the split gate error function
as shown in Figure 3. In general an AGC is required
to normalise this error function with respect to the
sum channel signal amplitude, but because a
successive detection log amplifier (SDLA) is used,
the error voltage is independent of the size of the
echo. It can be seen that there is a good (albeit slightly
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1st International Conference on Sensing Technology
November 21-23, 2005 Palmerston North, New Zealand
asymmetrical) linear region of +/-2m and a total
capture distance, in which the sense of the error is
correct, of +/-8m.
Over the past two decades, techniques to implement
this function for a wide range of W-band antenna
types including a horn-lens, a twist Cassegrain and a
standard Cassegrain antenna have been developed by
us for various applications [6].
For this application an antenna comprising a standard
rectangular horn illuminating a 110mm diameter
plano-convex lens was manufactured [7] as shown in
Figure 5. A disk of cross-linked polystyrene with a
relative dielectric constant εr = 2.6 was inserted at an
angle between the lens and the horn to displace the
beam off the boresight. The disk was spun to produce
the appropriate beam nutation for conical-scan
operation.
Figure 3: Measured range error function for the split
gate tracker
2.2
Reflective
Transducers
Angle Tracking
To determine the target offset from the antenna
boresight, conical-scan radars locate the centre of the
target image by nutation of a single feed that is
displaced from the axis as shown in Figure 4 [4, 5].
When the target is centred on the axis, the feed
receives equal amounts of power from all angles.
When the image is offset, more power is received by
the feed on the one side which results in amplitude
modulation of the received beam. This modulation is
compared to the in-phase and quadrature reference
signals generated by the nutation mechanism to
determine the sense of the error, while the amplitude
determines its magnitude.
It should be made clear that the received radar signal
that is demodulated by the reference signals will have
been gated in range to include only the target being
tracked. As with the range tracking case, the use of an
SDLA removes the requirement for an AGC loop.
Spinning
Polystyrene
Disc
Teflon
Lens
Horn
Figure 5: Schematic diagram of the horn-lens antenna
showing the position of a spinning polystyrene disc
designed to produce the beam nutation
For a 3dB beamwidth of 2.1° and a relative squint
angle of 0.3 [4], the actual squint angle is 0.6°. This
equates to a physical displacement of the beam focal
point of only 1mm for the focal length of 100mm
which was achieved using a 7.5mm thick disk tilted at
20°.
This radar uses an error demodulation technique
based on analog multiplier integrated circuits with
reference signals generated using a simple but elegant
technique shown in the following block diagram.
Reflective
Encoders
Signal
Conditioner
Range Gated
Radar Video
Index
Counter
(a)
count
index
Flip-Flop
State
Machine
I
Bandpass
Filter
Lowpass
Filter
Azim
Error
Q
Bandpass
Filter
Lowpass
Filter
Elev
Error
Demodulation
Tube housing motor
and modulator plate
Figure 6: Reflective encoders detect markers on the
spinning cylinder to produce the I and Q reference
signals used to demodulate the radar signal.
(b)
Figure 4: Conical-scan concept showing (a) the
pencil beam offset from the radar boresight by a small
squint angle and (b) the resultant amplitude
modulation of the received echo pulses
The spinning cylinder that houses the angled disc is
marked with four black bars equally spaced 90° apart
in a ring on its outer surface. These bars produce
pulses which then generate two quadrature square
waves that are filtered to produce the reference
signals. Four quadrant analog multiplier ICs perform
the angle error demodulation function.
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November 21-23, 2005 Palmerston North, New Zealand
The following photographs show the radar with its
associated antenna alongside a view of the conscan
mechanism obtained by removing the lens.
(a)
(b)
Figure 7: Photographs of (a) the pulsed radar seen
from the front and (b) the radar with lens removed
showing conscan mechanism
(a)
The angle error transfer function shown in Figure 8
was determined by scanning the beam across a distant
point reflector and plotting the amplitude of the error
signal as a function of angle.
(b)
Figure 9: Target tracking. (a) Measured and filtered
position estimates of the tracked vehicle (b)
Photograph of the unsealed road in a moderately
cluttered tracking environment
Figure 8: Measured angle error transfer function
obtained by scanning across a corner reflector
As with the range error function, the use of the SDLA
ensures that the angle error function is also
independent of the amplitude of the received signal.
This transfer function shows the classical shape with a
linear tracking region spanning +/-0.7° with a total
capture angle of +/-2°.
2.3
Ground Target Tracking
Some experimental work was undertaken to evaluate
the tracking performance of the pulsed radar in
cluttered environments. The radar was mounted along
with a boresight TV camera on a two axis pedestal
developed for the application. A joystick input or a
raster search pattern scanned the beam in azimuth and
elevation until a target was detected. After range
tracking was established the angle errors generated by
the conscan closed the angle tracking loops. The
system was capable of tracking ground vehicles in
clutter out to more than 1km as shown in Figure 9.
Figure 9a shows the measured position of a vehicle
driving slowly (≈10km/h) along a section of dirt road
in a moderately cluttered environment as shown in
(b). The target was lost for a short period as the
vehicle disappeared behind a spur, but was reacquired
about 10s later and tracked continuously for the
remainder of the 140s run.
The closed loop tracking function was performed
using analog processes discussed earlier, and the
range voltage and angle transducer data was digitised
at 20Hz and stored for processing and analysis. The
racking error variances for this run are σx2 = 1.9m2
and σy2 = 3.15m2 which is good considering the
clutter level.
3
Imaging
Two different modalities can be used to generate radar
images. Range gate limited imaging relies on the
ability of a radar to obtain returns from a large
number of contiguous range gates as the beam is
scanned in azimuth to produce a 2D backscatter
intensity plot of the surface. Beamwidth limited
imaging relies on the use of a pencil beam scanned in
two dimensions to build up a 3D image of the surface.
These mechanisms are illustrated in Figure 10 [8].
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1st International Conference on Sensing Technology
November 21-23, 2005 Palmerston North, New Zealand
continuous wave (FMCW), it is the method of choice
for range gate limited imaging [11].
The most basic open loop FMCW radar was
constructed around a Hughes broadband 10mW VCO,
an Alpha mixer and various circulators and couplers
as shown in Figure 12.
Hughes
VCO
Hughes
Isolator
Hughes
Coupler
Hughes
Circulator
B&K Ramp
Generator
Figure 10: The difference between beamwidth and
range gate limited imaging
3.1
Personal
Computer
Range Gate Limited Imaging
This imaging method is the most common for radar as
it can produce photographic quality images of the
earth’s surface from a low grazing angles as seen in
the strip map images produced by side looking
airborne radar and synthetic aperture radar systems [9,
10].
In real aperture scanned applications the images are
produced in polar space which is not ideal for further
processing, particularly if the frame of reference is
moving, and modern systems usually map the polar
image pixels into Cartesian space. This is most often
achieved by selecting a Cartesian grid with a
resolution equal to the cross-range resolution at the
smallest operational range and then over-sampling at
longer ranges.
The most simple method to allocate data to the
Cartesian image plane is to transform the coordinates
(R,θ) of the centroid of each polar pixel into Cartesian
coordinates (x,y), and use those to load the nearest
Cartesian pixel. However, this method often results in
unfilled pixels in the Cartesian plane which are
distracting, so the alternative, less efficient, method of
clocking through all of the Cartesian pixels in the
image and allocating to each the magnitude of the
nearest polar pixel is performed instead. Ideally,
partial pixel overlaps are integrated into their
respective bins in proportion to the percentage overlap
as shown in Figure 11.
HP
Spectrum
Analyzer
Hughes
Cassegrain
Antenna
Mini-Circuits
Amp
Alpha
Mixer
Figure 12: Block diagram of a prototype non
linearised FMCW radar
The unit was driven by the triangular output of a
standard function generator and the output spectrum
measured using an HP-8751 spectrum analyser.
Subsequent to this, more sophisticated high-resolution
closed-loop linearised FMCW radars were designed
and built by us for both ground based and airborne
imaging applications.
As part of the research into autonomous navigation
and landing, high resolution ground based images
produced using the HP spectrum analyser have been
analysed in minute detail with the specific goal of
determining the differences in reflectivity of various
forms of clutter at low grazing angles [12].
Magnified
View
Polar
Image
B
A
Figure 13: Image analysis of a section of runway
AB
D
E
E
Cartesian
Image
D
Figure 11: Polar to Cartesian image transform shows
an example where each Cartesian pixel becomes the
weighted average of the polar pixels with which it
intersects
Because of the ease with which the range gating
function is achieved using frequency modulated
Airborne measurements which followed could not use
the spectrum analyser because it was both too heavy
and too slow and so dedicated hardware based on the
Texas Instruments TMS320C40 was designed and
built. This hardware was capable of digitising the
radar video signal at 10MHz and running a 1024 point
FFT in near real time.
Major problems with EMI generated by the helicopter
avionics and power systems were encountered with
the result that the images obtained were rather noisy
as is shown in Figure 14.
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1st International Conference on Sensing Technology
November 21-23, 2005 Palmerston North, New Zealand
(b)
(a)
(a)
Figure 14: Airborne images of a section of runway
showing (a) composite millimetre wave radar and (b)
aerial photograph
The imaging range of a typical 94GHz FMCW radar
is limited to about 750m because of issues related to
reflected power and oscillator phase noise [13]. To
allow for longer range operation, a higher power
Interrupted FMCW radar was developed as shown in
Figure 15.
VCO
Isolator
Attenuator
Isolator PIN Switch
Circulator
Ramp
Gen
Loop
Filter
Phase
Det.
DRO
Harmonic
Mixer
Frequency
Discrim.
PILO
Mixer
(b)
LNA
Figure 16: Comparison between (a) an aerial
photograph and (b) a radar image of an airfield
Fref
(a)
In the last few years this imaging work has been
expanded with the development of light weight
stabilised mirror scanned radar that could be carried
by one of the UAVs developed at the ACFR [15].
3.2 Beamwidth Limited Imaging
Other high resolution 77GHz radars with mirror
scanners have also been developed to produce 3D
images of the terrain. As can be seen in Figure 17,
these radars were based on configurable modules
developed for the application.
(b)
RF
IN
-10
GU N N
V CO
Figure 15: FMICW radar details (a) schematic block
diagram and (b) photograph of integrated front-end
and antenna
RF
RF
O UT
IN
RF
B IA S
IF
B IAS , PR E- AM P
RAMP
GE N E R A T O R
D IS C R IM IN A T O R
IF
R F M O D UL E
(LOW DRIV E MIXER )
LO
(HIGH DRIV E MIXE R)
OUT
LO
R F M OD U L E
In this configuration, the transmitted signal is
interrupted during the receive period. This eliminates
the problems associated with reflected power and
allows the transmit signal to be increased to 250mW,
but it introduces a number of issues regarding the
spectrum analysis of the received radar video [14].
These problems notwithstanding, the system is
capable of producing excellent quality images out to a
range of more than 3km from extremely low grazing
angles as illustrated in Figure 16.
RF
-10
RF
B IA S, PR E- A MP
G UN N
LO
Figure 17: Configuration of a closed loop linearised
FMCW front end using two configurable modules
The use of mirror scanners allows the beam to be
directed both quickly and accurately as the servos
need only drive an extremely lightweight honeycomb
aluminium disc as can be seen in the photograph of
one of the larger systems shown in Figure 18.
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1st International Conference on Sensing Technology
November 21-23, 2005 Palmerston North, New Zealand
developed for high resolution millimetre wave radar
systems. It has demonstrated the versatility and
performance capability of these radar types for short
range navigation and guidance applications.
5
Figure 18: 3D Mirror Scanner with the 77GHz radar
Unlike laser scanners, these radars are capable of
producing high resolution images in the rain or
through dust [9, 16], and because they have some
foliage penetrating capability, even through trees [17]
as is shown in the following 3D perspective image of
the quadrangle outside the ACFR.
Figure 19: Point cloud image of the quadrangle
outside the ACFR
These images are being used to analyse terrain and
foliage characteristics for autonomous navigation
applications [18-20].
U. C. Ray and A. K. Gupta, "Intrapulse Frequency
Variation in a W-Band Pulsed IMPATT Diode," in
Microwave Journal, vol. April, 1994, pp. 238-244.
[2]
T. T. Fong and H. J. Kuno, "Millimeter-Wave Pulsed
IMPATT Sources," IEEE Transactions on Microwave
Theory and Techniques, vol. MTT-27, 1979.
[3]
H. J. Kuno, "Solid State Millimeter-Wave Power
Sources and Combiners," in Microwave Journal, June
1981, pp. 21-34.
[4]
D. Barton, Modern Radar Systems Analysis: Artech
House, 1988.
[5]
M. Skolnik, Radar Handbook: McGraw Hill, 1970.
[6]
G. Brooker, "Conical Scan Antennas for W-Band
Applications," presented at Radar 2003, Australian
International Conference on Radar, Adelaide, South
Australia, 2003.
[7]
A. Celliers, "The Design of a 94GHz High Resolution
Coherent Radar," University of Cape Town, 1987.
[8]
G. Brooker, S. Scheding, M. Bishop, and R. Hennessy,
"Millimetre Wave Radar Sensors for Mining
Applications," presented at Radar 2003, Australian
International Conference on Radar, Adelaide, South
Australia, 2003.
[9]
N. Currie and C. Brown, Principles and Applications of
Millimeter-Wave Radar: Artech House, 1987.
[10]
F. Henderson and J. Lewis, "Principles and Applications
of Imaging Radar," in Manual of Remote Sensing, vol. 2,
3 ed: John Wiley & Sons, 1998.
[11]
G. Brooker, "Long-Range Imaging Radar for
Autonomous Navigation," vol. Ph.D: University of
Sydney, 2005.
[12]
N. Currie, R. Hayes, and R. Trebits, Millimeter-Wave
Radar Clutter: Artech House, 1992.
[13]
G. Brooker, D. Birch, and J. Solms, "A W-Band
Airborne Interrupted Frequency Modulated Continuous
Wave Radar," IEEE Transactions on Aerospace and
Electronic Systems, vol. to be published.
[14]
G. Brooker, "A W-Band Interrupted FMCW Imaging
Radar," presented at SPIE Aerosense Conference,
Orlando Florida, 2003.
[15]
A. Goktogan, G. Brooker, and S. Sukkarieh, "A
Compact Millimeter Wave Radar Sensor for Unmanned
Air Vehicles," presented at 4th International Conference
on Field and Service Robotics, Yamanashi, Japan, 2003.
[16]
S. Ghobrial and S. Sharief, "Microwave Attenuation and
Cross Polarization in Dust Storms," IEEE Transactions
on Antennas and Propagation, vol. AP-35, pp. 418-425,
1987.
[17]
P. Bhartia and I. Bahl, Millimeter Wave Engineering
and Applications: John Wiley & Sons, 1984.
[18]
D. Langer, "An Integrated MMW Radar System for
Outdoor Navigation," Carnegie Mellon University,
1997.
[19]
A. Foessel, "Scene Modelling from Motion-Free Radar
Sensing," Carnegie Mellon University, 2002.
[20]
S. Clark and H. F. Durrant-Whyte, "The Design of a
High Performance MMW Radar System for
Autonomous Land Vehicle Navigation," presented at
International Conference on Field and Service Robotics,
Australia, 1997.
(a)
(b)
Figure 20: Comparison between (a) a volume
rendered perspective view using radar data and (b) a
photograph of the same row of trees
References
[1]
4
Conclusions
This paper has outlined extremely briefly some of the
tracking and imaging applications that we have
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