Transcript Chapter 15: Navigating with Radar

The Weekend Navigator
Part II
Copyright 2008 Coast Guard Auxiliary Association, Inc.
Chapter 15: Navigating with
Radar
Range Settings for Recreational
Boaters
• Radar is an invaluable tool for avoiding
collisions
• Radar can only see as far as the horizon
• Objects (“targets”) beyond the curve of the
earth remain out of view
Range Settings for Recreational
Boaters
• Range improves with height
• 90% of recreational boaters use radar at
ranges < 3 miles
Radar Display
• Produces “fuzzy” or granulated images
• Images on a radar screen do not precisely
correspond with the size and shape of
scanned objects
• Center of radar display is “you are here”
sign or the center of your boat
Radar Display
• A radar pulse encounters an object and the
pulse is reflected back toward the antenna
• Echo – returning pulse
• Targets – images resulting from echoes
Heading Up versus North Up
• Heading up display – radar reference point
aligns with the boat’s bow
– Top of the display represents the boat’s heading
– Most useful because it represents the world as
viewed from the boat
– Bearings on the display are relative to the boat
Heading Up versus North Up
Heading Up versus North Up
• North up display – radar reference point
aligns with north
– Radar screen better matches charts
– Targets stay put on the screen when you are
altering course
– Requires interfacing with an electronic
(fluxgate) compass
Range Rings and Bearing Marks
• Range rings – equally spaced concentric
circles for determining the range of an echo
• Bearing marks – short lines arrayed around
the perimeter of the display at 10o
increments
EBL and VRM
• Electronic bearing line (EBL) – radial line
that you can set to any relative direction
– Convenient tool for measuring a relative
bearing to a particular target
• Variable range marker (VRM) – circle of
constant range
– Can be set at ¼ nm
EBL and VRM
• EBL appears as dashed radial line from the
center of the display
• VRM appears as a dashed circle
EBL and VRM
Trails
• Radars scan at roughly 24 sweeps per
minute
• Target trails may be displayed from
previous sweeps
Directional Relationships and
Relative Motion
• When you are moving the radar screen is
constantly changing
• Objects that stand still are other boats that
match your speed and direction
• Other boats’ direction and speed are relative
to yours
Directional Relationships and
Relative Motion
• Focus only on movement patterns that
represent the risk of collision
• Watch out for any boat that closes directly
on the center of the display. That is a
collision course.
Collision Avoidance
• Collision avoidance is the most important
use of radar
• Targets that approach your location
represent a threat of collision
Collision Avoidance
• If a boat follows the EBL line toward you, a
risk of collision exists
• Rate of closure is the time it takes for the
other boat to reach you
Collision Avoidance
Effect of Boat Turn
or Change of Speed
• When you turn
– all of the objects on the display will rotate relative to
you and will appear to move in the opposite direction
• If you change speed
– other boats will acquire a new motion relative to you
• If another boat changes direction or speed, the
display will not change
Radar Navigation
• You must be able to recognize and identify
what you see on the display
• ATONs provide excellent point targets
Shorelines and Land-Based
Objects on Radar
• Echoes are dependent on two features:
– Reflectivity to radar signals
• Metal is good
• Wood is poor
– Aspect angle to the radar
• Verticals are quite visible
• Slants are less visible
– Ground is a decent reflector, but water is not
Navigating with Radar and GPS
• GPS receives and radar can communicate
using the NMEA 0183 protocol
• Radar can display GPS current position,
speed, bearing, and distance to waypoint
(“lollipops”)
Navigating with Radar and GPS
Navigating with Radar Alone
• Radar provides you relative position and is
more reliable than GPS
• Plotting by range and bearing use the
formula MB = RB + MH where,
– MB = magnetic bearing
– RB = relative bearing
– MH = magnetic heading
Navigating with Radar Alone
• By plotting by range and bearing, you can
improve your fix by combining your
recognition of the shoreline with the echo
from a nearby buoy
• 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 Avoidance Techniques
• Radar can help you safely navigate along
shorelines
– Set the VRM at the minimum safe distance of
approach
• Radar can help you set a danger circle
– Simply set your VRM at a safe radius
Tracking Weather
• Set the radar to reduce rain clutter
• Can help you spot severe squalls
• Helps you get a range to the storm
Tracking Weather
• Squall will appear as a large mass on screen
• Squall appears as a band of dense echoes
moving forward at a steady rate
• Rain generally returns as a speckled pattern
of random dots