Digital processing of today’s radar signals

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 Signal processing Radio Tx and Rx Scan Converter r B Plane Display

B Plane Bearing Range

Pulse Radar amplitude t return Pulse Repetition Period Tx Rx (not to scale) time Pulse Radar t return

FMCW Radar Rx The Rx frequency is different to the current Tx one t return Sweep Repetition Period (SRP) time FMCW (Broadband) Radar t return frequency

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 Characteristic 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

Inherent differences between the technologies Characteristic Broadband (FMCW) Pulse 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 Vulnerability to onboard reflectors Potentially a problem Not a problem Potential for future development Only just begun Mature technology

There is a half way house, beyond the scope of this lecture, generally called “Pulse Compression” that lies between Pulse and FMCW.
A Related Radar Technology

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

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.

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 [email_address] with a request for a password to access it and others.

Digital processing of today’s radar signals

by British Marine Electronic Association on Dec 15, 2010

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Digital processing of today’s radar signals — Presentation Transcript

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