Digital processing of today’s radar signals
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Transcript of 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.