Radar Technologies

High Definition (HD) and Broadband Radar by Larry Brandt

The latest marine radar buzz words are “HD Radar” and “Broadband Radar,” now being advertised by reputable radar manufacturers as the latest and greatest technology. But how are they different from the radar we have come to know over the past decade? Here is a quick overview of these two technology advances.

The term HD rides along with the recent high definition television wave. HD TV has brought enhanced reality to the screen: favorite TV personalities are presented life-sized or larger, with amazing details plainly visible. This has been a remarkable step forward for television viewing. In marine radar, the step is not so large, but it is noticeable. HD processing in radar display is a manipulation of the video output of the received radar signal that can make the echo image appear sharper. Neither the transmitter nor receiver is radically changed to accomplish this, instead video processing after the receiver stage and the use of a color palette to display echo strength are used to make the target edges more defined. The result is a pleasing display appearance that lends confidence to the interpretation and may enhance target separation in some cases. In short, this is an improvement in the display and not a change in the target detection capabilities of the radar.

Broadband Radar is an arguable marketing term, intended to differentiate one manufacturer’s technology from others’. Marketing folks come up with these terms that sometimes convey the opposite of their intent. We suspect this is the case here. We could argue that this version of radar technology is not at all broadband, and that, in fact, it depends beneficially on an extremely narrow receiver selectivity (i.e., the opposite of broadband) to function. Nevertheless, this has been a good choice of terms as it has caught on and is widely used.

The technology is indeed radically different from that used previously in the marine industry, although it has been used in aviation applications for decades. The first change to note about this new marine application of a not-so-new technology is that it dispenses with the radar’s traditional high-power output vacuum tube–the magnetron. Instead, it uses a very low-power solid-state device as the transmitter’s main power generating element. A solid-state, low-power transmitter is a radical step forward in our industry.

The high-power magnetron is notoriously wasteful of energy: it splatters its output pulse across a wide band of the frequency spectrum. Talk about broadband! Adding disadvantage on top of disadvantage, the magnetron’s output pulse is not stable. Its transmitting frequency varies from pulse to pulse, and even within a pulse. This method of generating two thousand or more watts of pulsed power requires its paired radar receiver to have a wide selectivity that allows a lot of electrical noise to enter the receiver along with the faint target echoes.

A solid-state transmitter as in the new broadband radars generates energy that is more stable in frequency, and does not waste its energy in sidebands as much. This allows the radar engineers to design a receiver with very narrow selectivity that prevents much noise from entering the receiver where it competes with the very faint target echoes. If a magnetron radar can be called a blunderbuss, a solid-state radar is a rifle–that is, frequency-stable energy applied efficiently versus magnetron energy splattered inefficiently. What has kept magnetron radars alive so long is their low cost. Microwave ovens we cook with need high power to warm the soup, so a solid-state power device would not serve well in the galley. On deck, though, magnetron technology has about run its course.

The second change in broadband radar is that it doesn’t transmit a single output pulse. It transmits a continuous energy wave (called CW) that gradually sweeps across a frequency spectrum. If you could listen to the sound, it might be similar to a police siren’s whoop, whoop, sweeping in tone from low to high. Its paired receiver follows that frequency sweep with very narrow selectivity. In this it is very much like the function of an aviation product that we know of as the CW radio altimeter. As the transmitter sends out its constantly sweeping energy, its receiver keeps track of that output and compares it to the energy reflected from a target. When the incoming signal matches the outgoing, that identifies a target. Thus we come to the more generic name of this type of radar technology: Frequency Modulated Continuous Wave (FMCW).

One of the touted advantages of broadband radar is that it can see echoes virtually up to the rubrail of the boat. Much is made of this as a collision avoidance benefit; however, my own 2 kW radar mounted at the spreaders can readily see a small buoy barely 70 feet in front of the bow. My feeling is that that any situation within that distance that depends on radar to ameliorate is a collision that is unlikely to be avoided. Collision avoidance depends on proper watch at distances far beyond boat-length range–a virtue that some competitors still argue are lacking in the low-powered FMCW technology.

Today’s second generation broadband radars, however, are advertising much improved long-distance operation, which indicates that the first gen product probably wasn’t a great performer. There is no reason why a low-power solid-state radar cannot have long range performance equivalent to traditional multi-kW magnetron radars. The “No Substitute for Power” mentality in radar is based on myth. What matters is not transmitter power; what matters is Loop Gain, which is a function of transmitter effectiveness teamed with receiver sensitivity and selectivity, and modern video processing.

Eliminating the magnetron will reduce system power consumption because the filaments of the magnetron are a power-hog. Sailors will appreciate this power reduction, but the antenna drive motor and display back lighting, two other significant power consumers, remain unaffected. Radiation exposure due to radar is a topic that wins too little consideration, in my opinion. Solid-state radar would certainly reduce or eliminate any perceived hazard in this area, which has to be considered a virtue. Accepted values of radiation exposure seem to go down with each improved study.

The most serious disadvantage of broadband radar, in our opinion, is that its low transmitter output power is insufficient to trigger a RACON. This is a serious weakness, especially in challenging navigational environments where many vessels operate. Aside from this one major drawback, we believe that FMCW (“Broadband”) radar is worth consideration if one were contemplating a purchase.

Modern magnetron radars are darn reliable. On a delivery this writer has used an old monochrome CRT Decca to cross the Columbia River Bar at 0300 in pea-soup fog, a radar that has probably performed flawlessly for 30-plus years. And so long as the radiation hazard is recognized and respected, a magnetron radar will serve just fine.

But progress marches on. When the magnetron was removed from aviation weather avoidance radar, many “experts” stated that low power (e.g., 25 watts peak power) aviation radar would never work properly. Today, thousands of such radars perform exceptionally well, providing pilots and passengers with benefits they only dreamed of—such as Doppler turbulence detection—which were not available on high power magnetron radars.

Technological progress in the marine radar field will eventually herald the demise of the venerable “maggie.” And along with the frequency stability of solid-state transmitters may come a host of new advantages, such as Doppler analysis of squall winds, or even Doppler analysis of wind gusts based on radar reflection from the water surface. Stay tuned. There is a new radar world coming over the horizon.