Radio detecting and ranging was developed during World War II and was critically important to its outcome. The British were responsible for the greatest contributions to the technology and were also known for their self-deprecating humor even while in dire straits. The smart and vigilant chaps who worked the complex early radar sets trying to detect (and intercept) Luftwaffe bombers crossing the Channel called themselves ‘scope dopes.’
Some 60 years later, we mariners benefit hugely from the desperate radar work of WWII. Radar gives us a certain weird sight in darkness and fog, enabling us to navigate better, negotiate traffic and even spy out distant squalls. All sorts of advancements have made the acquisition and interpretation of radar imagery easier, yet this is still a complex technology awash in acronyms. You’ll get the most from your machine by becoming a bit of a scope dope yourself.
The following is a breakdown of radar features with emphasis on the newest ones. Warning: I use a lot of acronyms. I do so not out of cruelty, but because they are so often referenced in manuals and on radar displays. Besides, true scope dopes like to toss them around at marina parties.Target acquisition and discrimination
The gut technology of a radar set hasn’t really changed in recent years, in fact, not radically since Sir Robert Watson-Watt fathered the idea in the late 1930s. A magnetron pumps out high-frequency pulses from a spinning antenna that can also receive reflections of those pulses. The peak power of the set partially determines how far the pulse can reach and how weak a target sends back an echo. The length of the pulse can be varied with range; a shorter pulse is used at close range for better definition. The horizontal length of the antenna determines both the gain applied to the pulse and the narrowness of the pulse beam and hence, the side-to-side resolution of targets.
What have changed somewhat are the controls applied to the basic imaging. It used to be that a user carefully adjusted a tune control that matched the receiver frequency to the reflected pulses (and many skippers told their crew to never fiddle with the knob!). Tuning has now been entirely automated on most units. Likewise, other controls often have auto settings, though always with a manual alternative. Gain controls the general sensitivity of the receiver, much like volume, and is often adjusted at every range change. Sensitivity time control (STC) essentially reduces gain for nearby targets. Thus it’s useful for eliminating unwanted returns from nearby waves, and is alternately called sea clutter or just S on the display. Fast time constant (FTC) controls the receiver’s ability to discriminate against the clutter of weak rain reflections, also known as rain clutter or R. Note that some units, like Raymarine’s Pathfinder, have both a rain control, for close-in rain and snow, and FTC for farther away precipitation.
No doubt, some old timers are leery of automated controls, and certainly a user should evaluate their effectiveness and be able to apply manual adjustments as necessary. However, I recently saw a modern unit demonstrated at sea and was impressed. The operator put gain, sea and rain clutter in manual modes and turned them all to zero, then put them all in auto – ba-da-bing – up came what looked like the best possible image at every range. As marine electronics get more complex in general, it sure is nice when some functions just take care of themselves.
Another basic imaging function of note is interference rejection (IR), the common ability of a receiver to reject the distinctive, swirly jamming caused by another radar unit sweeping in its vicinity. Some users leave this off to help warn them of an active vessel. IR and other clutter filters can sometimes mask radar beacons, or racons, special aids to navigation that electronically respond to a radar echo.
While most radar manufacturers allow that basic transceiver circuitry and construction has changed little in recent years, they all like to point out certain subtle differences in their own hardware. Raymarine Product Manager Keith Wansley is proud of his Pathfinder’s low ‘noise’ specifications, supposedly resulting in more sensitive target acquisition. Eric Kunz, who holds a similar position at Furuno, talks about how his scanners use brushless motors and gear drives, supposedly more reliable than the alternatives. Radar build and image quality – not to mention the important business of technical support – are beyond the scope of this article, but the wise shopper will consult with experienced (and we hope, unbiased) experts, and remember that old reputations don’t necessarily reflect modern realities.A clean display
Acquiring clean, uncluttered targets is just the beginning of using radar. If you can’t see them well on your display screen, what good are they? Here’s where marine radar technology has made major moves in the last decade. The classic monochrome CRT with its black rubber hood for daylight viewing has become an anachronism. Some will argue that the CRT still offers more pixels and thus, subtler target discrimination, but LCD displays are winning at least the small to medium vessel market for their daylight viewability, compactness and capacity for weatherproofing. The recent introduction of rugged, sunlight-viewable color LCDs has caused a particular stir. Color permits a lot more information to be comprehensible on a given screen, and many navigators – particularly on shorthanded voyaging vessels – have always wanted their radar right out at an exposed helm.
The first generation of sunlight-viewable color LCDs mainly uses super-bright backlighting to accomplish the task. While this works well, it’s expensive, power hungry and susceptible to damaging heat build up. Simrad just introduced a ‘transflective’ display that overcomes these issues nicely, using reflected sunlight when available and backlighting when needed. The technology is not proprietary, and Simrad’s John Cavallero daringly predicts that most manufacturers will be using it within 18 months. Of course, given the purportedly coming world of omnipresent PDAs, automobile mapping systems, and video cell phones, efficient and bright displays are getting a lot of engineer attention. Accordingly, I hesitate to predict what exact LCD display type will become the marine standard, but I can say that there are beauties on the market now and no doubt better ones coming.
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The four-kW unit with an open array displays superior target discrimination.
It should be no surprise that radars have also gained substantial processing power, and software designers have used it to create various display features. For instance, navigators typically switch ranges frequently because a transceiver reconfigures itself for each range, in a sense refocusing its sight. The longer pulses and slower repetition rate of a larger range may blot out close targets. Modern displays can often be offset to look ahead or even in a variable direction (perhaps when operating along a shore) so as to make best use of a given range’s focus. Some can also zoom into a user-selectable area, displaying the results in a separate window. Most units have a Target Expand function, which overrides the pulse length for smaller ranges, thus amplifying the target size but at the expense of resolution.
New radars from Simrad and Si-Tex offer dual ranges in side-by-side windows, actually more a hardware feature, since it’s dependent on a double-speed scanner that can feed the display two sets of pulse lengths and repetition rates simultaneously. Several manufacturers also offer optional 3-D windows beneath the normal bird’s-eye display. Software interpolates target echoes into the image you might see if you were actually sighting ahead at about a 60° angle. Given how essentially crude marine radar vision really is, these images – which can look strikingly realistic – should be treated with caution. Simrad Product Manager Brian Vlad says their real usefulness is as an advanced form of sea-clutter filtering, looking for and emphasizing the peaks of targets.
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Screen shot of Raymarine’s recently introduced radar overlay. The unit is set for true motion, north up. There is much more that software and processor power is doing for the modern radar navigator, but a brief discussion of data integration is in order first.System architecture
Despite all of the above – and a few standard radar features I haven’t mentioned yet – the essential job of understanding radar imagery is tough. The operator is often stressed figuring out what’s land, what’s a buoy and ‘is that a big damn ship bearing down on me?’ As with many elements of the marine navigation puzzle, new and easier ways to use radar are built on data integration, which in turn is built on the complex and fast-changing business of system architecture.
The National Maritime Electronics Association (NMEA) 0183 standard for passing discrete chunks of numerical nav data from one machine to another has been around for a while. Even today’s least expensive little 1.5-kW, 6-inch LCD unit, the well-regarded RC 1000, will accept heading, position, course, speed and waypoint data from a boat’s electronic compass and/or GPS (this article is not about prices, but certainly value is another feature to consider). Unfortunately, 0183 works best for one-to-one communications, not whole systems of instruments. Raymarine modified 0183 into its ‘multi-talker, multi-listener’ SeaTalk protocol, but it has not become an industry standard. NMEA has introduced its more robust NMEA 2000 data exchange specification, but it has not yet shown up in the marketplace.
Meanwhile, Raymarine and Furuno have developed high-speed network connections, HSB2 and NavNet, respectively, that permit two or more devices to swap data, imagery, even complete control. Almost all the marine electronics companies have combination units, which typically integrate chart plotters and radars – and sometimes much more – into one box. Then there are the various MS Windows charting programs that are using the input capabilities of PCs to interface with all sorts of marine sensors, including radar transceivers. The choices for someone fitting out a new vessel are awesome and confusing, especially as so many different system architectures lead to similarly intriguing results.What’s land?
Traditionally, a navigator compared the radar image to the active chart – usually with a course track and DR penciled in – to determine which targets were fixed, often using the results to corroborate the DR. Range rings helped with the cross referencing, and electronic bearing lines (EBLs), variable range markers (VRMs), and/or a screen cursor could be used to plot identified land features or aids to navigation relative to the vessel or vice-versa.
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Another view of the Raymarine chart with radar overlay. The vessel will center itself when near the edge of the display. These techniques are not completely dead; in fact, a lot of modern machines have added ‘floating’ EBLs and VRMs, which facilitate exercises like checking that a supposed buoy echo actually fits the charted range and bearing from an identifiable shore. However, GPS receivers are ubiquitous, and a simple NMEA waypoint sentence can be automatically displayed on a radar as a ‘lollipop’ icon, both helping to orient the radar to the chart and perhaps identifying a target if your waypoint happens to be a buoy. Even simple GPS heading data – especially now with SA off and WAAS on – can be used to turn the radar image north up or course up. Either mode makes comparison to a chart easier while also eliminating the confusion of a head-up display on which everything swings relative to your steering.
Add SOG (speed over ground) data, and several of today’s high-end yacht radars can do all the navigational math to create a true-motion display. This is startling. Now your radar image sits there north or course up with your vessel moving across it, just like a DR walking across a paper chart. True motion display is particularly helpful for dealing with other vessels, whose own relative motion is deciphered in the process.
Data
integration
and
networking
also
encourage
the
happy
marriage
of
radar
with
chart
plotting.
Many
products
these
days
will
show
an
electronic
chart
side
by
side
with
radar,
either
on
split
display
or
on
synchronized
screens.
Heading
modes
and
ranges
can
often
be
matched
and
changed
with
a
single
control
or
not,
as
the
user
desires.
Like
a
waypoint,
the
radar
cursor
position
can
be
passed
to
the
plotter
for
target
identification,
even
with
just
a
slow
NMEA
0183
connection.
Ultimate
oneness
is achieved with radar overlay when the target images are semi-transparent and are placed over the same-scaled chart image. Given a decent display, the targets that correspond to fixed objects are immediately obvious. Overlay started in mega-yacht systems and then appeared as an accessory to PC charting programs; this year Furuno included it in the NavNet series and Raymarine just added it to their Pathfinder Plus upgrade (an upgrade that is neatly applicable to some older Pathfinders via a software patch delivered on a C-Map-style cartridge).
Si-Tex and Nobeltec co-developed the original RadarPC, essentially a two-kW black box radar transceiver that could feed targets to, and be controlled by, a PC. Si-Tex recently added four-kW domed- and open-array versions, which they market with their Ge’esis plotter/radar/fishfinder/video devices. Nobeltec sells the same units under their own brand for use with their Visual Navigation Suite (VNS).
So radar overlay has definitely arrived, despite the protests of some who think the resulting displays are just too darn complicated. I recently saw overlay in action on a Raymarine 10.4-inch, sunlight-viewable color display, and I thought it worked very well. I do question the utility of overlay on smaller, dimmer displays, and note that fast and accurate heading info is critical to solid chart/radar coordination. Most overlay schemes require data from at least a fluxgate compass and rate-of-turn gyro data is an added advantage. The Raymarine RL80 Plus was fed by their new T400G Course Computer, which includes a fairly inexpensive rate gyro developed by the automotive industry. When compared to the chart, the resulting radar overlay seemed bang-on, even during rapid maneuvers.
Aaron Bowman, a Nobeltec programmer who helped develop the RadarPC and its VNS implementation, has installed or tuned many systems. He says that, as a new product, the early sales went on large vessels as a backup, but typically became the favored unit. ‘A four-kW radar with chart overlay is more useful than a 12-kW without,’ he claims.Moving targets
Now, you might be asking yourself why you need to know which radar targets are charted if you have your boat constantly plotted correctly on a digital chart. The truth is that the value of radar for regular navigation is diminished on the modern integrated bridge (though a little independent position corroboration is a good thing). The bigger truth is that once you’ve identified which targets are fixed, then you’ve got a much better idea which ones are moving. The detection and plotting of possibly dangerous traffic – collision avoidance – has always been a prime use of radar, and several basic helper tools are available in most radar units.
One is ‘tracks’ or ‘wakes,’ which is simply the ability of the display to keep showing old target echoes, usually in a lighter shade or different color for a user-selectable time period. The result is that fixed targets show straight backtracks (actually plotting your relative motion), while moving targets show tracks that are the vector sum of your motion and theirs. Just as in eyeball navigation, constant bearing and closing range – implied by a track angled directly away from you – indicates a problem. Tracks are helpful unless you are maneuvering a lot, and then they make a mess (less so if you’re able to invoke one of the fixed heading modes).
EBLs, VRMs and screen cursors can also be used to plot moving targets. The traditional drill is to get bearings and ranges every six minutes while holding a steady course, and then plot the targets on a maneuvering sheet. After just two plots, you can determine how close the other vessel will get to you – the closest point of approach (CPA) – and when – the time of CPA (TCPA). With a little vector work on the sheet, you can calculate the other vessel’s true speed and course and even a new course to steer in order to achieve a certain desired CPA.
There is an easier way, though it goes by a confusing number of acronyms. Automatic radar plotting aid (ARPA) is an IMO standard that defines a big ship radar function dependent on the sort of integrated navigation data discussed above. An ARPA set can lock onto user-selected targets and soon show each one’s true or relative forward motion track and a data window with CPA, TCPA, speed and course. Apparently yacht radar manufacturers don’t meet the IMO’s display size requirements, nor can they agree on what to call their version of ARPA. Hence, you’ll find it called by many names – automatic tracking aid (ATA), automatic radar plotting (ARP), or mini or manual ARPA (MARPA), depending on which company you talk to.
A typical MARPA (that’s the dominant acronym) implementation can track 10 targets at once, which is handy, since multiple, moving target situations are when you might really appreciate some help. While most will only show one set of target data at a time, they can also change icons and/or sound alarms when a target’s CPA is dangerous. Most versions allow the user to specify the parameters of what makes a target ‘dangerous,’ and at least one can automatically lock onto targets in a selected zone. Like overlay, solid nav data is essential to smooth MARPA operation, as are strong and discrete targets. Rough seas, for instance, can cause some antenna scans to miss a target, and throw MARPA for a loop. It’s a terrific feature when it works, but a pad of maneuvering sheets (or just good radar sense) still has a place in the wheelhouse.Other features
There are various features that also make radar an effective tool. One is a guard zone, a selected area in which a new target will set off an alarm. It can be a simple circle around the vessel, a pie wedge selected with the cursor, or even a combination of the two. This feature is particularly useful on long, quiet passages, as is a timed-TX or watchman mode, whereby the scanner activates only at user selectable intervals (and thus conserves power).
Another important aspect of using radar is the ever-important man machine interface (MMI). I credit the engineers who design electronics or software interfaces with a very difficult job, but then I expect a lot. Many of us want to step on a boat and figure out the electronic tools without diving into a manual, and we often want to run them with one hand on the wheel. There are numerous MMI improvements in some of the newer radars – including on-screen help files and glossaries, user-programmable soft keys and hot pages, common control pads across product lines, and – with PC products – integration with already familiar Windows programs.
Nonetheless, you’ll find many folks in the marine electronics industry who subscribe to the 80/20 rule – the notion that 80 percent of operators use only 20 percent of any system’s features. It may not be the ideal situation, but it seems that the 20 percent of navigators who do make full use of today’s remarkable radar tools are true scope dopes, willing to put time and energy into their side of the MMI.
Perhaps some future genius will come up with a radically new user interface for the integrated bridge. Certainly, we’ll see bigger and richer displays and hopefully easier mixing of navigation sensors and controls. As to the far future of marine radar specifically, there’s talk in the industry of what might be done when broadband wireless really materializes, enabling connection from a boat’s radar set to powerful shoreside scanners. Imagine being able to look around a harbor before you get there. It’s enough to make a proper scope dope grin at the possibilities.
Contributing Editor Ben Ellison is a writer and ON seminar instructor living in Camden, Maine.
