Boat Radar NavigationBy David Anderson
Published: April/May 2011
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.
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 two-dimensional 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.
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.
No matter how powerful or expensive your radar is, its performance may be limited by how you mount the antenna
If you've ever been in a sudden fog or caught in a blinding rain storm, you'll appreciate radar's electronic vision
AIS has come of age for recreational boats allowing automatic communication of course and speed with other vessels
Typical contact distances for a radar scanner mounted 12 feet (3.6m) above the waterline in nautical miles (nm):
- 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
- Large w/reflector 3–5 nm
- Large w/o reflector 2–3 nm
- Medium-sized fairway buoys 1–2 nm
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.
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