Advanced Radar Topics Peter Hayden MV Tanglewood, N6062 Nord2AK Rendezvous Petersburg, AK July 2016

Quick Glossary COG – Course Over Ground. Your direction relative to the earth’s geography HDG – Heading. Direction the boat is pointing. Differs from COG based on current SOG – Speed Over Ground. Your speed relative to the earth’s geographic STW – Speed Through Water. Your movement through the water. Differs from SOG based on current CPA – Closed Point of Approach. Calculation of closest distance you will come to a passing boat TCPA – Time to Closest Point of Approach. Time until you reach your CPA. EBL – Electronic Bearing Line. A line that your radar can draw from your boat out in any direction. VRM – Variable Range Marker. A range ring that your radar can draw at any distance from your boat. AIS – Automatic Identification System ARPA – Automatic Radar Plotting Aid

Additional Resources “The Radar Book”, by Kevin Monahan, published by Fine Edge Publishing.

What’s covered and what’s not It’s assumed that you use your radar regularly and know the basics like: Interpreting a radar display, and correlation with what you see around you and on your chart Tuning for optimal returns Today’s discussion will focus on more advanced topics, primarily for collision avoidance.

Radar Tools to Assess Collision Risk Variable Range Markers (VRM) and Electronic Bearing Lines (EBL) Echo Trails ARPA Relative Motion Vectors

Assessing Collision Risk with VRM + EBL

Basics of collision detection Do you remember the basic rule? Constant bearing + closing distance = collision Observation of Relative Motion between the other boat and you.

Basic Features: VRM & EBL Variable Range Marker – a range ring that you can set to any distance Electronic Bearing Line – A line drawn from your boat in any direction VRM and EBL are you most basic radar tools

Target Tracking with EBL+VRM

Target movement over time Collision course

Target movement over time Target passes ahead of you

Target movement over time Target passes astern of you

Assess Collision Risk with Echo Trails

Basic Feature: Relative and True Motion Display Modes In Relative Motion Display Mode View is always centered on the ship, just like you are. Objects move by on the screen just as they move by while looking out your window. Motion of objects on the screen is relative to your boat’s position and movement.

Relative and True Motion Display Modes In TrueMotion Display Mode Screen view centered on a fixed geographic location. Land masses remain in fixed position on screen. Your boat and other moving objects move across the screen based on their geographic movement. Your boat will eventually move off the screen and the view needs to be moved to cover the new location

Echo Trails Old CRT radar displays used phosphorus with a long after glow. Successive sweeps of the radar would light up an objects current position, while past positions would slowly fade away. The result was a comet trail showing the movement of a target Modern radars have synthetic after glow which accomplishes the same thing, but can be turned on and off.

Echo Trail on a Modern Radar

Echo Trails for Collision Detection Echo trails show a target’s successive position over time, and can be used for collision detection much like an EBL Used in conjunction with different display modes provides different results with different tradeoffs.

Echo Trails in Relative Motion Display Mode Land masses and other objects standing still like buoys and anchored boats all develop comet trails because they are moving relative to you. Moving boats develop different comet trails depending on their movement. Same course, same speed – no trail develops Collision course – trail develops on a line that, if extended, intersects your boat. Positives: Trails leading towards you are collision threats Negatives: Screen quickly becomes obscured with trails and needs to be cleared, losing trails on unimportant and important targets alike

Echo Trails in Relative Motion Display

Echo Trails in True Motion Display Mode Moving boats develop comet trails. Land masses and other stationary objects like buoys and anchored boats do not develop comet trails Positives: Only moving objects develop trails, making them easy to spot Negatives: Screen quickly becomes obscured with trails and needs to be cleared, losing trails on unimportant and important targets alike

Echo Trails in True Motion Display

Assess Collision Risk with ARPA

AIS Automatic Identification System Provides definitive position, COG, SOG by sending encoded messages over a dedicated VHF channel Location and movement vector plotted on chart plotter, AIS, etc., and CPA/TCPA is calculated Obviously only works with AIS equipped boats Provides clear indication of target’s movement, but visually hard to assess collision risk. CPA helps, but even harder to assess whether target will pass ahead or astern of you, and how to turn to increase CPA

ARPA Automatic Radar Plotting Aid tracks and plots target over time to assess collision risk, just like you would do manually with VRM+EBL Shows target location and calculates COG, SOG, CPA/TCPA Provides all the same navigation info as AIS, except works with any target. Same challenges as AIS determining pass ahead or pass astern, and how to increase CPA What’s MARPA? Mini Automatic Radar Plotting Aid. Can’t acquire targets automatically, and typically tracks fewer targets (e.g. 10 vs 30 to 100)

ARPA Advantages Works with any target. Helpful to identify buoys, rocks awash, as well as other boats Targets can be displayed on many chart plotters placing them in context on your chart See AIS and ARPA targets together on same screen Eliminates the clutter of Echo Trails Allows you to pick the targets to focus on.

Combined View of AIS and ARPA Targets

Assess Collision Risk with True and Relative Motion Vectors

True Motion Vectors COG/SOG vectors from your boat, AIS, and ARPA are “true motion vectors”. They show geographic movement. Hard to visually judge how you will pass another boat CPA tells you how close, but doesn’t tell you whether you will pass ahead or astern of other vessel. Leaves you guessing Which way do you turn to increase CPA? It depends on whether you are crossing ahead or astern, and the wrong assessment will reduce CPA, not increase it.

Assessing Collision Risk with COG+SOG True Motion Vector

Relative Motion Vectors How do you tell how you will pass doing it the old fashion way? If the target’s bearing is moving towards your bow, he will pass ahead of you, and if it’s moving towards your stern, he will pass astern of you. With VRM+EBL, you monitor whether the target moves ahead of or astern of the original EBL Many radars can show this for you automatically using Relative Motion Vectors. It shows Relative COG and Relative SOG Similar to Echo Trails, except Relative Motion Vectors point ahead of the target and are predictive, where Echo Trails are historic and trail behind.

Target movement over time Collision course Relative Motion

Relative Motion Vector Showing Collision Course True Motion Vector Relative Motion Vector

Relative Motion Vector Showing Pass Ahead True Motion Vector Relative Motion Vector

Relative Motion Vector Showing Pass Astern True Motion Vector Relative Motion Vector

True vs Relative Motion Vectors

Advantages of Relative Motion Vectors Provides much cleared picture of how two boats will pass Differentiates ahead vs astern passing so you know how to increase CPA, if desired. Works with both AIS and ARPA targets

Locating Objects

Locating Objects

Locating Objects

Radar Features Feature Simrad TX10s, and 4G Raymarine Lighthouse Furuno NN3D Furuno TZTouch Furuno TZTouch2 Furuno FAR2xx7 Furuno 1835 Display Mode RM, TM RM ARPA/MARPA MARPA ARPA ARPA targets 10 30 100 ARPA Vectors R, T (1)(2) T R, T ARPA Track History none Echo trails R AIS Vectors IS Track History Notes 1 AIS and ARPA vector settings are linked. Changing one changes the other too. 2 Data box changes to report rCrs, rSpd, etc RM=Relative Motion TM=True Motion R=Relative T = True