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 improves with height 90% of recreational boaters use radar at ranges < 3 miles Range Settings for Recreational Boaters

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

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 10 o 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