2008 REED’s East Coast Edition - Navigation
Six-meter antennae produce 0.5° beams. may be used on both laterally significant
Antennae on merchant ships produce beams and non-laterally significant aids, the Racon
less than 2° wide. Because small craft can’t signal itself serves for identification purposes
carry large antennae, their average antenna only and carries no lateral significance. Ra-
size is about 0.5 meter and produces a 6° cons are also placed on bridges to mark the
wide beam. To allow for the motion at sea, optimal passage.
the vertical span of the beam is about 20°. All Racons operate in the marine radar
The transmitted pulse has to contain suffi- X-band from 9,300 to 9,500 MHz, though
cient energy to travel many miles and produce some also operate in the 2,900 to 3,000 mHz
a detectable echo return. The combined effect marine radar S-band.
of pulse length and horizontal beam width Racons have a typical output of 100 to 300
determines picture resolution, as well as the milliwatts and are thus short-range aids.
operator’s ability to discriminate between ob- Reception varies from a nominal range of six
jects of close proximity. The beam distortion to eight nautical miles when mounted on a
of echoes is evident in the fact that echoes buoy, to as much as 17 nautical miles for a
appear larger at longer ranges and get smaller Racon with a directional antenna mounted
as they near the center of the display. You may, at a height of 50 feet on a fixed structure.
for instance, not detect small river or harbor These nominal ranges are guides; actual
entrances. Multiple objects at the same range ranges vary with many factors.
within the beam will be depicted on the The beginning of the Racon presentation
screen, at that bearing, as a single echo. appears on your radar screen about 50 yards
The detectability of objects is also deter- beyond the Racon position and persists for a
mined by their shape, size, aspect and mate- number of revolutions of the radar antenna,
rial. Metal is a perfect reflector; while fiber- depending on its rotation rate.
glass is almost transparent to radar, which Distance to the Racon can be measured to
is why small fiberglass vessels should carry a the point at which the Racon flash begins,
radar reflector. Objects that present a sizable but the figure obtained is greater than your
vertical surface return stronger echoes. ship’s distance from the Racon due to the
Seawater is a very good re-radiator. Al- slight response delay in the Racon apparatus.
though most of the energy from surrounding Radar operators may notice some broaden-
seawater is reflected away, the scattered return ing of the Racon presentation when their
from nearby waves may give stronger echoes vessel approaches close to the source of
than close-by objects within the waves. the Racon. You can minimize this effect by
To remove this clutter around the center adjusting the IF gain or sweep gain of your
of the display, a special variable suppression radar. If desired, you can virtually eliminate
mode is provided that works outward from the Racon presentation by operating the “fast
the center, decreasing to nil at about three time constant” (FTC) controls of your radar.
miles. This is marked as the STC on the radar
Navigating Using Radar
display. Use this sea clutter control with cau-
tion; it’s easy to remove real echoes without
Even with its limitations, radar is a most
realizing the extent of the “hole” in your
useful navigational aid along the coast,
picture. This is one reason that small craft
presenting a view of surroundings that cor-
are sometimes not detected on a larger ship’s
respond to the chart. However, the radar
radar display.
picture will not look exactly the same. Small
insignificant objects may be enhanced, while
RACONs
small prominent objects may not give a de-
Some aids to navigation are enhanced by tectable return. Lower parts of a distant coast
the use of Racons (short for Radar beacons). may be below the radar horizon, in which
Racons, when triggered by pulses from your case the coast will appear on the radar to be
vessel’s radar, transmit a coded reply to your inland from the charted coast. Or, the slope
radar. This identifies the Racon station with of the shore may be so gradual that much of
a series of dots and dashes that emanate from the energy is reflected away.
the Racon and appear on your radar screen. Bearings are quite difficult to take with
This display indicates the approximate range small-craft radar. You must note your vessel’s
and bearing to the Racon. Although Racons heading at the instant you take the bearing and
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