For the past 60 years or so the basic principles of marine radar have not changed. A radar transmits very short pulses of microwave energy and receives echoes reflected back from solid objects. The direction of an object is indicated by the direction from which the echo returns, and its range is determined by the time interval between when the pulse was transmitted and when the echo was received. The longer it takes for a pulse to complete its out-and-back trip, the farther away the object.
Because microwaves travel at the speed of light (162,000 nautical miles per second) it takes less than a tenth of a thousandth of a second for a pulse to go out-and-back from a target eight miles away. But even that is a long time compared with the length of an individual pulse. At short range, most marine radars have pulses that last about a tenth of a millionth of a second each (0.1 microseconds). But these short pulses don’t contain enough energy to produce reliable echoes at long distances, so for ranges of more than three miles or so, most small-craft radars automatically switch to long pulses lasting as much as a whole microsecond (one millionth of a second.)
Even using these long pulses, a marine radar needs a powerful transmitter—typically 2kW or 4kW—to achieve a useful range. Unfortunately, the explosive burst of power in each pulse is inherently dirty because it spreads energy over a wide range of frequencies, and getting too close to such a powerful transmitter won’t do your health any good.
For a user, the problems are simpler. The most obvious is that such powerful transmissions instantly overwhelm any receiver sensitive enough to receive the echoes that are produced. To overcome this, radar engineers incorporate a device that effectively disconnects the radar’s receiver when it’s transmitting; the result is that a conventional radar can’t see anything within about a hundred feet of its antenna.
The new technology
There is another radar technology, used in radio altimeters in passenger aircraft and in terrain–following radar in missiles and aboard fighter aircraft. Navico (navico.com) has successfully adapted it for use in marine radars. The technology is called Frequency Modulated Continuous Wave, or FMCW—though Navico calls it “Broadband” radar.
Continuous Wave refers to the fact that instead of transmitting pulses of energy, like conventional radar, FMCW radar transmits a sweep that lasts a thousandth of a second apiece. That’s a thousand times longer than conventional radar’s long pulse. So the difference between a pulse and a sweep is like the difference between a toot on a car horn and a burglar alarm blaring away for twenty minutes. The sweep is plenty long enough to qualify as continuous.
“Frequency Modulated” means that, unlike the pulse of conventional radar, the sweep is not made at a constant frequency. Rather, it rises steadily so that at the end of each sweep the frequency is 65 MHz higher that it was at the beginning. So when the echo returns to the FMCW antenna the radar is still transmitting, but at a higher frequency.
If the echo returns from something close at hand, the out–and–back trip might only take a few millionths of a second. Since the transmitter frequency won’t have changed much in that time, the difference between the received and transmitted frequencies will be small. If the echo has traveled a long distance, the transmitter frequency will have risen higher and the difference will be bigger. In other words, FMCW radar measures distance by comparing the received and transmitted frequencies rather than by measuring the time between transmission and reception.
Low power: One big advantage of this system is that transmitter power can be reduced dramatically. In pulsed radar, increasing the length of a pulse from 0.1 microsecond to 1.0 microsecond allows the range to be stepped up from a mile or so to 20 miles or more. On that basis, pulses (or sweeps) that are a thousand times longer might achieve an enormous range. In practice, marine radar’s range is limited by the curvature of the earth, which makes it more useful to accept a modest nominal range and reduce transmitter power.
The Navico transmitter produces just one-tenth of a watt, or about one twenty-thousandth the power of the smallest conventional-marine radar and only about one-twentieth the power of a mobile phone. Navico, in short, has swapped magnetron technology for a clean, controllable solid-state transmitter. Eliminating the magnetron also eliminates the need for a warm-up period. Navico’s FMCW radar can be switched off when not in use, and then switched on almost instantaneously if needed.
Low risk: Health risks from typical small marine pulsed-radar sets have never been considered very serious, unless you sit on top of the the radome or stare at it from a few feet away for hours. But you can sit on an FMCW radome while it is transmitting for as long as you like without suffering harm.