Radar Revolution Page 2
Low clutter: pulsed radar shortens its pulse length at short ranges; the pulse length is what gives the unit the ability to discriminate between objects that are on more or less the same bearing but at slightly different ranges. The longer the pulse length, the bigger the gap between two objects must be before they show up as separate blobs on a radar screen. Usually this is about 50 feet at short range and about 500 feet at long range. Because FMCW radar doesn’t use pulses, it can distinguish between objects separated by a gap as small as about six to ten feet.
Another benefit is that FMCW radar is less affected by echo clutter from the sea surface or from rain. The clutter effect is more pronounced with rain and clouds because pulsed radar can often receive a return that is strong enough to register as a large mass on a radar screen. Navico says that its Broadband radar is at least five times less susceptible to rain and sea clutter than a conventional radar and that it can be removed altogether, using two automatic settings, harbor and offshore. There’s also a manual option for those who like to have something to tweak, or who like to use visible rain clutter to dodge squalls or showers.
To listen for echoes while still transmitting, FMCW radars need two antennas, one mounted above the other; one transmits and the other receives. This means that the radar has almost no minimum range. If you need to pick your way through piles or closely-moored boats you can do so without losing sight of the one you may be about to hit.
A 4kW-pulsed radar doesn’t use much power because it’s transmitting for such a small amount of time, but considerable power is consumed by heating the magnetron and lighting up the display. While FMCW units have no magnetron they do turn an antenna, run a processor, and illuminate a display. That is why power savings aren’t quite as much as you might like. But a power draw of 17 watts is still a useful saving compared to the 25 watts consumed by a conventional 2kW radar. For many users the biggest power savings will come from the instant on feature that allows a FMCW radar to be ready for use within about 20 seconds, rather than two or three minutes. This almost puts an end to the need to run the radar in “standby.”
A conventional pulsed radar can still do some things better than a FMCW unit. For instance, long-range performance is perhaps the most significant advantage of a conventional radar. No matter how clever FMCW technology may be, there’s a point at which a 100mW transmitter simply runs out of power. I can’t put a number on exactly what the maximum range might be as it depends entirely on the target. But Navico does say that the long-range performance of its FMCW radar is closer to their conventional 2kW units than it is to their 4kW models. Even so, I would say we’re seeing the start of a technological revolution that will forever change the way marine radar works.
I went out on the water with Navico’s new BR24 unit and, with some judicious tweaking of the controls, I could see large ships about twelve miles away and I could also get intermittent echoes from large targets on land over 20 miles away. The boat was not large and the antenna was less than fifteen feet above the surface of the water. Because it doesn’t transmit high-power pulses, FMCW radar will not trigger a response from radar transponders such as RACON’s (on navigation marks), SARTs (on liferafts), or any other active radar responder.
But most people will probably never see a SART used in earnest, and chart-overlay technologies and AIS are already making RACONS and radar responders less relevant. Unfortunately, the day I went out the seas were calm and the sky was clear. That meant I didn’t get a chance to see any rain clutter and not much in the way of sea clutter, either.