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