Radio Detection and Ranging, is one of the most useful electronic
navigation aids carried aboard small vessels. This instrument provides
information which, if interpreted by a trained operator, can prevent
collision, determine position, and can be used to tell the course and
speed of other vessels in the area.
RADAR can do so much for the navigator, be aware that if your vessel is so
equipped, your liability to prevent collision may be altered and a greater
burden may be placed upon you. Generally, a vessel with a functioning
RADAR set may be obligated to have it operating while underway, and the
navigator is expected to be trained to use it correctly. It is NOT the
purpose of this module to provide that training, only to familiarize the
navigator with it's capabilities. It is recommended that professional,
hands-on training be obtained by every RADAR operator.
based on the fact that radio waves travel at a constant speed, namely the
speed of light. RADAR sets transmit brief pulses of radio waves which
travel outward in essentially straight lines. When these waves strike a
reflecting surface, like another vessel or the shoreline, some of the
energy is returned to the receiver. By measuring the time difference
between the transmission and receipt of these waves, the RADAR set can
calculate an object's distance. It also notes the relative direction of
the object and graphically plots this information on the screen.
signals travel along the "line of sight." The theoretical maximum range
of RADAR depends on the height of the RADAR scanner and the height of the
target. The higher the scanner and/or the target, the farther the RADAR
range would extend. Of course not every RADAR set has the power to reach
this maximum range. Be sure to read the manual. The RADAR horizon, or
the distance it's signal will travel before being blocked by the Earth's
surface, is actually slightly farther than the geographical or visual
horizon. This is because atmospheric conditions can bend the RADAR waves
over the horizon.
screen is set up as a Plan Position Indicator (PPI) where the RADAR
equipped vessel is at the centre of the screen and the ship's heading
In Fig. E2 you can see a
situation where the RADAR equipped patrol vessel is steaming towards a
larger ship. The patrol vessel's PPI shows it's own position as a cross
hair in the centre of the display. Straight up on the display is the
direction it's heading, whatever direction that might be. The rings are
Range Rings, circles of known diameter used for quickly measuring
distance. The large green blob at the top of the display is the target
echo. Note that an echo shown on the PPI may not correspond in size
with the relative size of the target, that is it may appear larger or
smaller than the actual object you're looking at. Consider the edge of
the echo that's closest to your vessel as the actual position of the
target. Finally, most modern RADAR sets will give you the Range
and Relative Direction of the target, in this case by placing the
cursor over the target with the trackball. Remember that you can convert
this relative bearing to an actual bearing by adding your ship's compass
heading to the relative bearing and adjusting that number for compass
error. If the resulting number is over 360°, just subtract 360 and the
remainder is the target's true bearing from your position.
Of course the RADAR
scanner is constantly rotating, so you're getting a 360° picture of what's
going on around you. The image is refreshed with each revolution of the
scanner. Assuming that your vessel is in motion on a constant heading,
stationary objects around you will look like they're moving down the PPI
at the same speed as your vessel, and on a course opposite yours. If
another vessel were travelling along side you and at the same speed, it's
blip would appear stationary on your PPI. Vessels ahead on courses that
would cross your vessel's bow would appear to travel across the PPI, but
remember that the courses and speeds of these vessels as they appear on
you PPI are not their actual course and speeds. It's all relative to what
your vessel is doing. Trained and experienced RADAR operators can quickly
calculate their course and speed by plotting right on the RADAR screen
with a grease pencil,
but remember that if a target appears to be closing
on you and remains on the same bearing, there is a danger of collision
with your vessel.
Not all targets reflect
RADAR waves efficiently. Surfaces that tend to deflect or absorb the
radio energy will not produce a good target on the PPI. Gradually sloping
beaches and small fiberglass boats are a couple of examples. Reflective
surfaces oriented perpendicular to the direction that the signal is coming
from produce the best targets. Large metal vessels, steep shorelines, and
anything with a RADAR reflector on it usually make excellent targets.
For more information on
Read the manual. This
document should be kept aboard the vessel. It explains the proper
adjustment and operation of your particular set.
Attend a professional,
USCG approved RADAR course. If you have a captain's license, you can
take the CG test and get the RADAR endorsement, required for high end
licenses and most un-inspected tow boat skippers.
Consider purchasing a
RADAR simulation program. There are some very good ones out there, and
they are great for learning how to interpret RADAR displays.
Read a book. Chapman's
has a good chapter, and of course Bowditch and Dutton's will tell you
everything there is about this and any other subject.