Digital processing of today’s radar signals

Professor Bill MullarkeyProfessor Bill Mullarkey

Managing DirectordB Research Limited

andResearch Fellow

Denbridge Marine Limited

Perhaps the most important is Perhaps the most important is the appearance of FMCW the appearance of FMCW

radars to compete with the radars to compete with the more traditional Pulse onesmore 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

CommonCommon signal processing signal processing chain for all radars.chain for all radars.

The first task is to illuminate the target scene with energy and store the resulting echo returns on a B

PlaneAntenna

SignalprocessingRadio Tx

and Rx

Scan Converterr

B Plane Display

B PlaneB Plane

Bearing

Range

amplitude

treturn

Pulse Repetition Period

Tx

Rx (not to scale)

time

Pulse Radar

treturn

Pulse RadarPulse Radar

FMCW RadarFMCW Radar

Rx

The Rx frequency is different to the current Tx one

treturn

Sweep Repetition Period (SRP)

time

FMCW (Broadband) Radar

treturn

frequency

Publicity image from the Navico FMCW radar on a 1/16 mile range

Notice thatNotice thatOn the plus sideThere is no blank spot near to the centreVisibility close to own-ship’s bow is excellentThe image is almost photographic over the

whole image

HoweverIt is on a very short rangeThere are no publicity images for longer

ranges

Radar performanceRadar performanceThe 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 PowerThink in terms of Joules not

Watts

Some NumbersSome NumbersA 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?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 summaryIn summaryInherent differences between the technologies

CharacteristicCharacteristic Broadband Broadband (FMCW)(FMCW) PulsePulse

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

CharacteristicCharacteristic Broadband Broadband (FMCW)(FMCW) PulsePulse

Power requirements Similar Similar

Power cabling Thinner Thicker

Requires standby period NoNo, 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 begunMature

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 TechnologyA Related Radar Technology

SeahawkSeahawk

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?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.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: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 Target resolution of an antenna that is twice the sizethat is twice the size

It gets betterIt 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?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 protected] with a request for a password to access it and others.