NAVIGATION TRAINING
Radar
Radar
Radar - How It Works
Radar
• Radar is an abbreviation for
Radio Detection and Ranging
• Note: the radar is best at
measuring range and that is
always where its strength lies
Radar
How does it Work
• The radar transmits out a short pulse of electromagnetic radiation.
Any object it encounters reflects that radiation, and a percentage of
that energy is reflected back towards the antenna
• The measurement of the time that that pulse takes to travel out
and return to the scanner is directly proportional to the range of the
object that reflected the pulse
Radar
How does it Work
The better the reflection back the
better the target
• Echo – returning pulse
• Targets – images resulting from
echoes
Radar and use it!
Radar Horizon
•Radar can only see as far as the radar horizon, very slightly further
than the visual horizon
•Objects (“targets”) beyond the curve of the earth of the radar sight
line remain out of view
Radar
Radar Horizon
• The radar horizon is very slightly further than the visual horizon
• Range improves with height of scanner
Radar
•Most small vessels use range of 3 nautical miles or less
•Produces “fuzzy” or granulated images, based on the
radar waves that get reflected back - no reflection - no
trace on the display
•Images on a radar screen do not precisely correspond
with the size and shape of scanned objects
•Origin of the radar trace is the radar scanner of your
vessel
Radar
Radar Range
The range of the display is the distance from the centre of the screen
to the top in nautical miles
• The ranges available include 0.75 nm, 1.5 nm, 3 nm, 6 nm, 12 nm
Radar
Radar Range
3 nm
Radar
Heads Up vs North Up
• Most RCM-SAR vessels do not have an accurate compass feed such
as a gyro compass.
• Therefore it is strongly recommended that the radar display is
always set on Head Up mode
• What is seen on the top of the radar is what is ahead
• Bearings seen are relative to own vessel’s bow
Heads Up v North Up
Radar
Heads Up vs North Up
Radar
Variable Range Marker (VRM)
• The Variable Range Marker is using radar’s greatest accuracy - the
measurement of range
• The VRM is moved in or out, but measures most accurately when
the outside of the VRM line touches the closest point of a target
Radar
Variable Range Marker (VRM)
Radar
Variable Range Marker (VRM)
• If the object is beyond the radar horizon, such as a
sloping foreshore there will be inaccuracies
• If you are travelling fast towards or directly away from
the object you are ranging off, inaccuracies will be present
• Ranges on the beam will be more accurate
Radar
Electronic Bearing Line (EBL)
•The electronic bearing line (EBL) is a radial line extending from the
origin (own ship) that can be set to any bearing
•It can be used for ascertaining the relative bearing of a target.
•When at speed, it has the greatest accuracy ahead and astern, and
prone to the greatest errors on the beam
Radar
Electronic Bearing Line (EBL)
Radar
Range Rings
• Range rings are equally spaced concentric circles
• The distance between rings will generally be indicated as part of
the range, e.g. 3 nautical miles range, 0.5 nautical miles between
rings
• They are used to quickly estimate the distance off of an echo
Radar
Range Rings
Radar
Collision Avoidance
Radar
Relative Motion
• When your vessel is moving, the image of the radar screen will be
constantly changing as you move relative to the land and other
vessels
• Other vessel’s direction and speed are relative to your own course
and speed
• Objects that stand still on the radar screen are other vessels that
match your speed and direction
Radar
Relative Motion
• Only when you are stopped will you be able to accurately
determine another vessel’s true course and speed
Radar
Risk of Collision
• Use the electronic bearing line (EBL) to place over the target on
the screen
• If your vessels keep its course and speed, and the target comes
down the EBL a risk of collision exists
• The quicker a target come down the EBL, the sooner a close
quarter situation will be
Radar
Risk of Collision
Radar
Risk of Collision
• Therefore with this in mind, focus only on target movement that
represents a risk of collision
Radar
Collision Avoidance
• Once a risk of collision is deemed to exist, the Collision Regulations
then require you to understand whether you are the stand on or
give way vessel, given the prevailing condition of visibility, AND
other factors
• These actions are not defined here.
Radar
Effect of Change of Own Vessel Course and Speed
•On a heads up display:
•When your vessel turns, all of the objects on the display will rotate
relative to you and will appear to move in the opposite direction
•When your vessel changes speed,other boats will acquire a new
motion relative to you.
•If another boat changes direction or speed, the display will not
change, only the relative motion of that boat
Radar
Guard Zone
• The VRM can also be used to set up
a guard zone as an area around one’s
own vessel into which we do not want
another vessel to come
Radar
Radar Navigation
• The biggest skill with radar is to match what you see on the radar
display with regards land and navigational aid targets, to that which
is to be found on a navigational chart
Radar
Radar Navigation
Radar
Radar Navigation
Radar
Radar Navigation
• Navigational aids such as navigation buoys, and marks are often
fitted with radar reflectors in order to make them more conspicuous
• In certain cases important navigation buoys or mark will be fitted
with Racons. These may encoded in morse code to identify them
Radar
Radar Navigation
Racon (B)
Radar
Radar Navigation
• Plotting by range alone can be used for sections of shoreline
where you can clearly measure the range and then draw an arc at
that radius
Radar
Radar Navigation
• An unstabilised radar is best used
when using three ranges to fix your
position.
Radar
Radar Navigation
• Radar is providing first hand information and does not
rely on outside electronics to work properly
Radar
Radar Navigation
•When plotting bearings from an
unstabilised radar, use the formula MB =
RB + MH
MB = magnetic bearing
RB = relative bearing
MB = magnetic heading
Radar
Radar Navigation
•Where no other fixing method is off a radar is possible,
plotting a range and bearing of a fixed isolated object can
be used.
•Navigation buoys are not considered fixed objects
Radar
Radar Navigation
• Radar can help you safely navigate along shorelines, and help stay
a standard distance off during a shoreline search
–Set the VRM at the minimum safe distance of approach
Radar
Radar Navigation
Radar
Radar Navigation with GPS
• Radar navigation is complimented by the appropriate use of the
Geographic Positioning System (GPS)
• Radar and a GPS can communicate using the NMEA 0183 protocol
• This is best achieved through the side by side displays on a chart
plotter, where the radar picture can be easily compared with the
electronic chart
Radar
Radar Navigation with GPS
Radar
Radar Reflectivity
•The strength of echoes are dependent on two features:
1. Material reflectivity to radar signals
Metal and sheer rock races are good
Wood and foliage are poor
Radar
Radar Reflectivity
2. Aspect angle to the radar
Verticals are generally good
The greater the angle from the vertical, the poorer the signal
are poor
Radar
Errors
• False echoes are possible with radar
• Almost all are connected with reflected signals
Radar
Errors
• Side Lobe effect
• When a large vessel is close the
side lobes of the radar
transmission can cause some
interference as shown
Radar
Errors
• Reflection from an overhead cable
• The result from this is the echo gets returned off the cable.
• As own vessel gets closer to the cable, the target appears to also
get closer, to be on a collision course with position of collision where
own vessel goes under the actual cable.
Radar
Weather & Radar
• The effect of rain can be reduced by the use of rain clutter, FTC or
Rain controls
• The effect of sea and swell waves can be reduced by use of sea
clutter or STC
• Excessive use of these controls can mask echoes
• Practice of the use of these controls is important to achieve the
appropriate level of masking for both STC and FTC
Radar
Weather & Radar
• Squall lines and rain can be seen on
radar
Radar
Weather & Radar
• Wind and swell waves can be
seen on radar, as can tide lines on
occasion