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From DIY Boat Owner Magazine


Boat Radar Navigation

By David Anderson

Practice makes perfect when it comes to accurately interpreting the range and bearing of landmarks and boats on your radar screen. If you have radar onboard, follow these tips and do the math to maximize your navigational skills.

Learning to take full advantage of radar means you'll need to understand how it works, how to use its basic functions, and how to incorporate that knowledge into routine navigation. Radar "sees" by sending out microwave pulses, and detecting the pulses that are reflected back from objects, called "targets," around your boat. What it is not is a television camera. On the radar screen, the user sees only blips or echoes of the targets, not realistic representations. Consequently, it takes practice to read a radar screen and to interpret what's really out there. And, arguably, an accurate understanding of the images you see on-screen, especially the moving targets, is the most important aspect of using radar.

Broadband radar offers amazing close-range abilities;
the pilings seen onscreen are just a few feet from the boat.

Teach yourself to better read those blips by practicing in good visibility. Compare how nature, your charts, and/or chart plotter, and the radar image fit together when you can see what's around you with the naked eye. You'll find that there's usually quite a lot missing in the radar image, owing to the twodimensional illumination of the surroundings with your boat at the center of the display.

Just how far away will your radar be able to see those targets? Antenna and target heights are the key. Don't judge by the unit's maximum-range scale, because when a target is over the "radar horizon" you won't see it, no matter how much power you're broadcasting. Radar range is slightly farther than visual or geographic range due to the refraction of microwaves, but it still can't see over the radar horizon, which can be calculated as follows:

[1.2NM x √ antenna height (in feet)] + [1.2NM x √ target height (in feet)]

For example, if your antenna is mounted at a height of 12 feet above the water and you're looking for a vessel that is 25 feet high, the formula to determine radar range will be [1.2 x 3] + [1.2 x 5] = 3.6 + 6 = 9.6. Until it's within 9.6 nautical miles — even if you have a unit that ranges out to 24 or 36 miles — the target won't appear on-screen.

Naturally, if you mount the antenna higher by locating it on a spreader or mast, you can gain additional range. On a small boat at sea, however, an antenna located too high will be rocking so much that much of the advantage provided by elevation is wasted. For most small craft, pole-mounting an antenna at a height of nine to 12 feet is considered appropriate.

Far Sighted

Radar also has a minimum range, which is a bit more complex to determine as there are several variables: the pulse length and processing of the microwave signal, a geometric element that arises from the shadowed region that lies below the beam pulse, and the intentional squelching of excess close-proximity electrical noise.

The vertical width of a typical radar beam is about +/- 15 degrees from horizontal. That beam first strikes the water at distance of its height in feet, divided by tan (15 degrees). For a 30-foot antenna, this is 30/0.268, which is 112 feet from the antenna. With a 12-foot antenna, this distance is reduced to 44 feet. So on a typical small craft, even one with a high-mounted antenna, this is not a huge limitation.

The next limitation to add into the mix is electrical limitation, which is 164 yards for each microsecond of pulse length. Most radars switch to shorter pulse lengths at lower ranges, with something in the order of 0.12 microseconds being typical for ranges less than a mile. This translates to 0.12 x 164, or about 20 yards from the antenna — but signal processing usually doubles this electronic limitation. Thus the lowest range scale on many makes of radar is 0.25 miles or 0.125 miles. But often the last 50 yards or so is filled with so much noise that you'll see a solid blob on-screen; in some other cases the area within 50 or 100 yards of the antenna is intentionally filtered out with "bang suppression," which merely leaves a cleaner-looking blob or nothing at all at the center of your radar screen. So while the pulse length and height considerations are limiting factors, often they aren't the practical limitation to minimum radar range.

Radar resolution is dependent on many factore, including bearing and range.

There's one major exception to minimum-range limitations, which is provided by the cutting-edge technology found in broadband radar. Broadband doesn't emit strong microwave pulses, like traditional radar. Instead it emits a tiny fraction of the power and measures the frequency change between the emitted waves and those bounced back to the antenna. There's little excess electrical noise created, and bang suppression is completely unnecessary. As a result, it's possible to see objects that are just a few feet away from your boat — much less those 25 to 50 yards away, hidden by a pea-soup fog or inky darkness.

Radar Resolution

Resolution is a measure of how well two nearby objects are resolved, or separated, by radar. In the best-case scenario with outside factors such as sea conditions and target strength aside, it's determined by two separate factors: bearing resolution and range resolution. The typical horizontal width of a radar beam is about six degrees. This means that any two objects separated by less than six degrees can be smeared together into a single target. The tangent of six degrees is 1/10, so if two adjacent objects located a distance (D) away are to be resolved into separate targets on the radar screen, they must be separated by a distance of at least D/10.

For example, two boats five miles off must be 0.5 miles apart, or they'll appear as one. Similarly, if the entrance to a harbor is 0.2 miles across, it won't be seen as an opening (when headed straight toward it) until you're within two miles of it. It's a good idea to become familiar with bearing resolution and these relationships by making your own measurements with a chart in hand in broad daylight, to see how it works with your radar.

The pulse length of a radar signal also affects range resolution. A microwave travels at the speed of light, which is 186,000 miles per second, or 328 yards per microsecond. If two objects in line with each other are separated by less than one-half a pulse length, then the nearer target will still be reflecting signals from the end of the pulse when the farther one starts to reflect signals from the front of the pulse — and they will appear as one object. To be resolved, two objects on the same bearing must be separated by more than 164 yards per microsecond of pulse length. Typical pulse lengths vary from 0.1 to 1 microsecond. You can adjust pulse length in some units, but in most small craft units it adjusts automatically when you change ranges.

In one unit, for example, on a three-mile range, the pulse length is 0.3 microseconds, and on a four-mile range it's 0.8 microseconds. Consider the case of two close vessels (say a tug and the barge it's towing) separated by 100 yards at a distance of 2.8 miles. On the four-mile scale they'll appear as one vessel (resolution 131 yards), but on the three-mile scale they'll show as two distinct close vessels (resolution 49 yards).

"High definition" is a term you hear a lot today in the world of marine radar, and new high-def units do offer better resolution. But all of the above principals don't change; the main advantage of modern high-def systems is in the software. New algorithms and digitizing the signals allow for better target discrimination and detection of smaller, weaker targets. But remember, the same limiting factors still apply.

Land Ho

As with boats, how well a landmark shows on radar depends on the height of the land and the resolution of the radar. Isolated targets such as other vessels, buoys, small islands, or drilling rigs are easier to interpret than large, irregular landmasses. What one must also remember is that although the size of a large target increases on-screen as it grows closer, the size of the echo on-screen is not always a reflection of the actual size of the target. Especially at longer distances, isolated targets all appear as simple dots or small line segments.

The shape and material of the target also influences acquisition and resolution. Round and pointed bodies reflect only a small part of the incoming energy back to the scanner. The same applies to surfaces inclined towards the horizontal, such as the windscreens of some motor yachts. Remember, the most common material used to build pleasure boats today, fiberglass, provides a far poorer target than metals.

Bear in mind that when you're moving, the motion of any targets on the screen is relative motion, not true motion. If you're moving towards a stationary buoy at five knots, it appears on your radar screen as if the buoy is moving towards you at a speed of five knots. The only stationary target on a radar screen is one that is moving in exactly the same direction and at exactly the same speed as your boat.

Fixing the Problem

The two main uses of radar are preventing collisions and getting a position fix. When collision is a possibility, the first thing you must decide is whether or not a target poses a risk of collision; secondly, you'll have to determine what leads to this risk. For example, it's fairly easy to determine a target moving straight down your ship's heading line on a collision course — but is this a vessel you're going to run into from astern, or is it a target headed full-steam right for your bow?

The variable-range marker (VRM) and the electronic bearing line (EBL) are important tools for answering the questions raised by both collision-avoidance and position fixing. The EBL provides the bearing to a target, while the VRM indicates range to the target at that particular point in time. As time passes, it's easy to see if you are gaining or losing range to the target, and if the bearings remain the same. Ask yourself: Are you on a collision course with another vessel if you maintain the same bearing, and continue to close range? Is the current or tide sweeping you to the wrong side of a channel buoy, even though your compass heading implies that your heading is correct? Keep your eyes on the EBL and VRM to find out the answers.

Remember how your radar works and how to best use it, and you'll find that a good system gives you a new set of eyes — one that can pierce the night and fog — and makes all the difference in the world between safe navigation, and being scared of the dark.

This story originally appeared in Mad Mariner's DIY Boat Owner Magazine.





Contact Distances

Typical contact distances for a radar scanner mounted 12 feet (3.6m) above the waterline in nautical miles (nm):

SHIPS:
  • Tankers, bulk carriers, cruise liners 9–12 nm
  • Freighters 6–9 nm
  • Lightships, large Buoys
    w/radar reflectors 4–7 nm
  • Trawlers, coasters 3–6 nm
  • Metal-hulled boats 3–4 nm
  • Wood, fiberglass boats
    w/radar reflectors 2–4 nm
  • BUOYS:
  • Large w/reflector 3–5 nm
  • Large w/o reflector 2–3 nm
  • Medium-sized fairway buoys 1–2 nm

ICE:
Ice to windward is hard to pick up because the cooled air bends the radar beam upwards. Smooth ice doesn't produce an echo and neither do ice floes. With your radar antenna mounted at a height of 12 feet above the water you can expect to pick up icebergs and pack ice at a distance of two to nine nautical miles. Growlers are likely to be seen out to about two nautical miles.