Professor Bill Mullarkey
Managing Director
dB Research Limited
and
Research Fellow
Denbridge Marine Limited
Perhaps the most important is
the appearance of FMCW
radars to compete with the
more traditional Pulse ones
Before going any further, there is an important
fact to bear in mind when considering the
differences between them.
In Physics, as in business,
THERE IS NO SUCH THING AS A FREE
LUNCH
Common signal processing
chain for all radars.
The first task is to illuminate the target scene with
energy and store the resulting echo returns on a B
Plane
Antenna
Radio Tx
and Rx
Signal
processing
B Plane
Scan Converter
r
Display
B Plane
Range
Bearing
Pulse Radar
amplitude
Tx
Pulse Repetition Period
Rx (not to scale)
treturn
treturn
Pulse Radar
time
FMCW Radar
frequency
Sweep Repetition Period (SRP)
Rx
The Rx frequency is different to the current Tx
one
treturn
treturn
FMCW (Broadband) Radar
time
Publicity image from the Navico
FMCW radar on a 1/16 mile range
Notice that
On the plus side
 There is no blank spot near to the centre
 Visibility close to own-ship’s bow is excellent
 The image is almost photographic over the
whole image
However
 It is on a very short range
 There are no publicity images for longer ranges
Radar performance
The quality of a radar is defined
by two metrics:
The ability to resolve as separate,
targets that are close together in
range and bearing; and
The ability to detect weak
targets.
The first is determined by receiver
bandwidth and pulse length for
range; and the antenna
characteristics for bearing
The second is by the ratio of the
echo’s energy to the receiver’s
inherent noise.
It is expensive to reduce receiver
noise so the only practical way to
improve target detection is to
illuminate the target scene with as
much energy as possible.
Energy Not Peak Power
Think in terms of Joules not
Watts
Some Numbers
A 2Watt FMCW radar will typically sweep the
frequency over a period of about 1ms and have a
PRF of 1kHz. It transmits all the time and radiates 2J
of energy every second.
A conventional 4kW pulse radar will typically use a
100nS pulse on the short ranges with a PRF of about
3kHz, which illuminates the scene with 1.2J per
second. On a longer range it might use a 1us pulse
that provides 4J per second
So What?
At very short ranges FMCW has a clear advantage
However, at ranges greater than 100 metres the
relative performance will be similar.
FMCW and Pulse radars use similar amounts of
energy so performance will depend upon the
quality of the engineering design
NOT ON THE TECHNOLOGY.
On longer ranges FMCW has its own
difficulties related to things such as
receiver bandwidth and phase noise.
Difficult for Leisure Marine.
After lunch, colleague Patrick
Beasley will talk about FMCW in
commercial and military radars
In summary
Inherent differences between the technologies
Broadband
(FMCW)
Pulse
Short range target detection
Better
Worse
Long range target detection
Worse
Better
Visibility of close in targets
Better
Worse
Target resolution in azimuth
Same
Same
Target resolution in range
Better
Worse
Sea clutter suppression
Better
Worse
Characteristic
Inherent differences between the technologies
Broadband
Characteristic
Pulse
(FMCW)
Power requirements
Similar
Similar
Power cabling
Thinner
Thicker
Requires standby period
No
No, once
switched on
Triggers Racon Beacons
No
Yes
Vulnerability to interference
from other radars
Difficult to solve
Easy to solve
Potentially a
problem
Not a problem
Only just begun
Mature
technology
Vulnerability to onboard
reflectors
Potential for future
development
A Related Radar Technology
There is a half way house,
beyond the scope of this lecture,
generally called “Pulse
Compression” that lies between
Pulse and FMCW.
Seahawk
A patented,
applied-mathematical
technology for improving
target detection and
resolution.
The Buoys are plastic
and it was a dry day, so
the only reflections
have to come from the
small holes the buoys
make in the water.
The next two slides show images
from a first generation SeaHawk
enabled Raymarine radar, which
used a 6ft open array antenna.
The first is with SH switched off .
The second with it on.
Seahawk doubles the effective
antenna size, to12ft .
So how does that work?
To understand how, we
need an intellectual
paradigm shift, so hold
on to your seats.
We need to think in the frequency
domain not the time one.
The polar diagram of an antenna is
the impulse response of a low pass
filter.
Importantly, whilst that filter
attenuates some frequencies
beyond its -3dB, so called “cut off”,
it does not eliminate them.
Imagine a HiFi system that
has a graphic equalizer.
It enhances some
frequencies to
compensate for room
acoustics. SeaHawk works
in a similar way.
It is that easy.
SeaHawk enhances the higher
azimuthal frequencies to give the
response of an antenna twice the
size of the original.
The next slide shows the frequency
response of a 6ft and what would be that
of a 12ft antenna, if a leisure–marine
vessel could carry such a thing.
That slide showed:
• the natural azimuthal bandwidth of a 6
ft antenna (Blue Trace);
• the natural azimuthal bandwidth of
a 12 ft antenna (Red Trace) ;
•
the SeaHawk filter (Green Trace) ;
and
• the overall SeaHawk-enhanced
frequency response (Black Trace) .
Notice how the SeaHawk
enhanced bandwidth
matches that of the 12 ft
antenna, with a little gain.
Target resolution of an antenna
that is twice the size
2.5
2
1.5
1
0.5
0
0
20
40
Input
60
1st iteration
5th iteration
80
12 ft Response
100
120
It gets better
• Target detection depends upon the
energy that illuminates the scene.
• The broad beamwidth antenna
illuminates every target with twice as
many pulses as would an antenna of
twice the size.
• That corresponds to twice the energy
less a 5% loss from the SeaHawk
algorithm.
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
6 ft Antenna
12 ft Antenna
15
Relative Peak Energy
20
So what next?
The first generation Seahawk was
designed against tight timescales with the
need to get the Raymarine SeaHawk
enabled Digital Radar to market as quickly
as possible.
Since then there has been the opportunity
to revisit the design and make some
significant improvements.
The next two slides are a taster.
The original presentation included two
images taken from the Second
Generation SeaHawk. For now they are
company confidential.
If you want to view them AND are either
and existing Collaborator of dB Research
OR Denbridge Marine OR have a
Confidentiality Agreement with one of
them, email bill.m@dbresearch.co.uk
with a request for a password to access it
and others.