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Radar revisited

Jun 24, 2010

Though AIS units now provide some of radar’s anti-collision capability, radar remains one of the mariner’s most effective electronic tools. While the technology of radar has advanced from heavy hooded displays to smaller LCD screens and now broadband signaling, a radar unit will make any voyager’s life easier — especially in the shipping lanes!

After World War II, radar was quickly embraced worldwide by commercial mariners, and its function proved valuable as both a navigation tool and an asset in collision avoidance. Operationally, the system remains a composite sender and receiver of RF energy, and displays are capable of accurately scaling distance and to a lesser extent the bearings of targets within the antenna’s line of sight. It is all based upon timing the return of transmitted microwave energy sent and received by the same rotating antenna. Wave theory laws regarding reflection, refraction and diffraction hold true for RF energy and targets in the beam’s path cause changes in the free space radiation pattern. In short, some of the radar energy bounces back as an “echo.”

Radar signal returns (the energy bounced back from targets) are used to modulate an electron beam scanning a phosphorous screen. This TV tube-like plan position indicator (PPI) reveals targets as bright spots or semi-differentiated blobs on the screen. The larger and more dense the target, the brighter and larger the image on the display. Signals attenuate with distance, and rain and sea state also degrade performance.

Steady improvements
Before long the marine electronics industry began standardizing features and providing filters to cope with rain and sea clutter, factors that had a negative influence on performance. Range resolution is a unit’s ability to discern multiple targets in close proximity. Bearing resolution is a reference to the ability of a unit to separate two targets at the same range, but differing in relative bearing angle. Obviously, in both of these cases the more resolution the better. It quickly became clear that a higher mounting position for the antenna helps a crew see further, and higher frequency operating units were better able to discern smaller targets. Conversely, lower frequency systems have longer range. This is why ships at sea rely on the superior range of S–band equipment, and in-shore boaters favor the improved signal detection of weaker signals from X-band radar.

The old days of analog PPI radar are well astern, and long gone are the hoods needed to see what’s on the screen in daylight conditions. Power hungry magnetrons have been upstaged by more energy-efficient designs, but the technology remains based upon the principle of transmitting and receiving RF pulses via a sweeping antenna. As in the past, larger, more metallic targets tend to yield stronger reflected signals.

Display technology changed for recreational users in about 1980 when more viewer-friendly raster units hit the marine market. Combined with smaller diameter domes, lighter weight and lower current demand solid state circuitry, these new X-band units had originated as weather radar in aircraft, and they became an instant hit among sailors and power cruisers alike. Their more compact size, lower current consumption and more modest price tag mainstreamed radar in the recreational boating market.

The LCD revolution took the technology to the next level of sophistication. Units by Furuno, Raymarine, Garmin and others can be networked with chart plotters and sounders allowing signals to be displayed on numerous monitors around the boat, and information to be overlaid from differing nav systems. The flat-screen LCD technology is compact, daylight viewable and even less current-consuming than the cathode ray tube displays of earlier raster scan radar. Smaller diameter 2-kW radomes have the smallest current draw of any magnetron-based system, and are a good choice for those on a tight dollar and energy budget. However there’s a noticeable improvement in sensitivity, selectivity and image resolution when a 4-kW unit is used in conjunction with a larger open array antenna.

A new twist
Navico’s line of Simrad, Northstar and Lowrance radars has turned the page on single-frequency, magnetron-based microwave radar. The new technology is based upon a broadband-modulated, FM, continuous wave system that uses a tiny fraction of the power and tallies up an impressive list of big plusses.

At the top of the upside column is exquisite detail on close ranges and no need to suffer auto-tune compromise or manual-tune complexity. Broadband radar returns are automatically digitally displayed as a sharp detailed image. The fact that the radiated energy is more than 1,000 times less than conventional radar lessens the concern about RF effects on human tissue and also lowers the power consumption linked to running radar aboard vessels under sail. Originally used in radar altimeters aboard aircraft, the sensitivity and selectivity of broadband radar gives an operator useful detection within the 1/16-mile scale ring. In fact, target discrimination remains crisp right down to 100 feet from the boat. Add to this the instant-on feature, and the ability to quickly flip through the always-tuned ranges and you get a big plus when it comes to collision avoidance and close quarters navigation.

As with most good things, there’s also a down side, and in the case of broadband radar, the negatives are few in number but important enough to pay heed to. The first is that target acquisition and presentation detail drops way off as the user scans beyond six miles. The second big deal downside linked to frequency modulated continuous wave (FMCW) radar is the blind eye it gives to all radar beacons. These are important aids to navigation in coastal waters and not having the ability to detect them is a shortfall. Lastly, FMCW radar has a heightened preference for hard targets and poor ability to discern large soft targets like rain-laden thunderstorm cells. These short comings should be considered carefully and played off against the upside. Poor performance in squall avoidance may be offset by the new system’s excellent in-tight maneuvering data displayed in overcast conditions. Some see broadband as an ideal way to augment a conventional X-band system, especially aboard a sailboat that has a need to be energy efficient while under sail. Switching to broadband radar and an LED tricolor light can cut electrical consumption considerably.

X-band units not eclipsed
The long story short is don’t count out the current reign of X-band small craft radar. It represents a very function amalgam of refinement and operational reliability. For example, Furuno’s dual band widths, narrow pulse length and change in antenna rotation speeds at differing ranges, optimizes resolution of nearby and distant targets. Raymarine’s simple plug-and-play networking makes it easy to add monitors on deck and below and their MARPA auto tracking makes monitoring contacts easier than ever. Garmin has brought aviation-proven technology to the boater and all of these systems remain X-band based.

Each system offers easy-to-use electronic bearing lines and variable range markers with network connectivity, even AIS contacts and vessel info can be placed on the screen. With all this relevant data and much more just a button push away, it may come as no surprise that sensory overload can become be an issue. And one of the radar user’s primary responsibilities is prioritizing what’s to be displayed on the screen. Each and every bit of eye grabbing information has the ability to eclipse or hide a weak intermittent target. It may be a seagull floating on the surface or kids in a skiff with a stealth-like radar footprint. Distracting the navigator with an avalanche of graphic detail and numerical readouts can hide such weaker targets. And that’s why many professional navigators put their radar display on a simplicity diet. In essence, they believe that if the information is not part of the radar detection process it doesn’t belong on the screen. There’s no scroll of engine data, way points or wind instrument readouts on their radar’s LCD screen, and split-screen operation is looked upon as detrimental to radar operation. Perhaps this approach is too draconian, but attempting to use a 6.5-inch screen as a source for plotter and radar information is just asking for trouble.

Turned on and in use
Another key aspect in radar’s bottom line is that it must be turned on to be an asset, and when it is operational a crewmember needs to put the displayed information to use. Some years back a cruising sailboat named Melinda Lee was approaching landfall in New Zealand in squally overcast conditions, and despite the fact that the crew was only 30 miles from the coastline, they chose not to turn on the radar in order to save battery bank energy. The crew of a Korean merchant ship failed to spot the yacht crossing its track as they made their way along the coast of New Zealand. In extremis, neither vessel made appropriate collision-avoidance maneuvers, and the yacht was run down with only the mother of a family of four surviving the ordeal. One of the lessons learned from this tragic episode is the need to use radar if you have it. And when it comes to picking the piece of gear that’s right for your vessel, care should be given to the unit’s energy appetite. Four or five amps may seem minimal up until it is multiplied by 12 hours of dusk to dawn operation.

Those voyaging with a limited ability to generate electricity may find broadband radar their best choice. Because, when it comes to having a system that’s compatible with the small amount of electrical energy, broadband wins hands down. A powerful X-band unit requires considerable current. The best answer to the problem is to have a hefty DC power system, and the budget and space to put broadband and conventional X-band technology to use.


Ralph Naranjo is a technical marine writer who is based in the Annapolis, Md., area.

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