Question 1

What is parallel indexing on radar and why is it useful?

Accepted Answer

A technique using an index line parallel to the intended track to monitor the vessel's actual track relative to a radar-conspicuous fixed object.. Parallel indexing: draw a line on the radar display parallel to the intended ground track, offset by the planned CPA distance from a radar-conspicuous fixed object. As the vessel proceeds, the object's radar return should track along the index line. If the return moves off the line, the vessel is off track. Extremely useful in restricted visibility, ports approaches, and coastal navigation, effectively continuous position monitoring.

Question 2

You are using radar in relative motion (RM) display. Another vessel appears to be moving on the radar screen. What does this movement represent?

Accepted Answer

The other vessel's motion relative to your own vessel, combining the courses and speeds of both vessels.. In RELATIVE MOTION (RM) display, your own vessel is fixed at the centre. All other targets move on the display showing their motion RELATIVE to you, the vector combines both vessels' movements. In TRUE MOTION (TM) display, your own vessel moves across the screen at its true course and speed, other vessels show their true movement. For collision avoidance, true motion is more intuitive but RM is often used for close-quarters plotting.

Question 3

What do CPA and TCPA mean on a radar/ARPA display?

Accepted Answer

Closest Point of Approach and Time to Closest Point of Approach: minimum predicted separation between vessels and time until it occurs.. CPA (Closest Point of Approach): the minimum distance between the two vessels if both maintain current course and speed. TCPA (Time to CPA): time until that closest point is reached. ARPA/plotting displays calculate these automatically. A CPA of 0.0nm means collision course. Most navigators set a CPA alarm at 1-2nm as a warning. TCPA tells you how long you have to act. Together they quantify risk of collision from radar data.

Radar, questions and explanations

What is parallel indexing on radar and why is it useful?

You are using radar in relative motion (RM) display. Another vessel appears to be moving on the radar screen. What does this movement represent?

What do CPA and TCPA mean on a radar/ARPA display?

Practise this properly

Radar, questions and explanations

Questions

What is parallel indexing on radar and why is it useful?

You are using radar in relative motion (RM) display. Another vessel appears to be moving on the radar screen. What does this movement represent?

What do CPA and TCPA mean on a radar/ARPA display?

Practise this properly