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

2. 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

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

4. 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.

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

6. Assessing Collision Risk with VRM + EBL

7. 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.

8. 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

9. Target Tracking with EBL+VRM

10. Target movement over time Collision course

11. Target movement over time Target passes ahead of you

12. Target movement over time Target passes astern of you

13. Assess Collision Risk with Echo Trails

14. 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.

15. 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

16. 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.

17. Echo Trail on a Modern Radar

18. 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.

19. 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

20. Echo Trails in Relative Motion Display

21. 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

22. Echo Trails in True Motion Display

23. Assess Collision Risk with ARPA

24. 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

25. 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)

26. 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.

27. Combined View of AIS and ARPA Targets

28. Assess Collision Risk with True and Relative Motion Vectors

29. 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.

30. Assessing Collision Risk with COG+SOG
True Motion Vector

31. 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.

32. Target movement over time Collision course
Relative Motion

33. Relative Motion Vector Showing Collision Course
True Motion Vector
Relative Motion Vector

34. Relative Motion Vector Showing Pass Ahead
True Motion Vector
Relative Motion Vector

35. Relative Motion Vector Showing Pass Astern
True Motion Vector
Relative Motion Vector

36. True vs Relative Motion Vectors

37. 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

38. Locating Objects

39. Locating Objects

40. Locating Objects

41. 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