RADAR part three: Answering plotting questions in your MCA exam
Exam technique
Relevant documents and M-notices: MGN 369 (Navigation practices relevant to restricted visibility), MGN 379 Navigation: Use of Electronic Navigation Aids
As of March 2024 and as per MIN 690 (Guidance on the MCA Oral Examination Booking, the Issue of Notices of Eligibility (NOEs) and the Online Oral Examination Processes) candidates are now sent, along with their NoE in email form, a radar plotting sheet.
Traditionally, in the days of in-person exams, radar plotting questions (and of course Rule 19) were assessed by the examiner first explaining the conditions of visibility then asking what you’d do on seeing a target on the radar.
Q - “A target appears on your radar here (places a counter on a plotting sheet). What are your actions?”
Now, with exams happening over Microsoft Teams, it is common for examiners to share their screen. They will use a digital whiteboard to do what they used to do using a plotting sheet and counters.
A - “I would commence plotting.”
With regards to plotting intervals, I recommend stating that you’d plot every three minutes. This is because plotting more frequently will likely be seen as impractical by the examiner, and that less frequently may mean that you find yourself in a dangerous situation by the time you realise it is developing.
From MGN 379 (Use of Electronic Navigational Aids):
A single observation of the range and bearing of an echo will give no indication of the track of a vessel in relation to own ship. To estimate this, a succession of observations must be made over a known time interval. The longer the period of observation, the more accurate the result will be. This also applies to ARPA/ATA which requires adequate time to produce accurate information suitable for assessing CPA / TCPA and determining appropriate manoeuvres.
You can find the ARPA performance standards in the article linked below.
Back to your exam. The examiner adds a second counter, giving you the time since the last plot.
Q - “What are your actions?”
A - “I could continue to plot.”
Don’t take any action until you have completed the plot; that means at least three plots, at known intervals. Then, and only then, may you complete the plot; that is, extrapolate the course and speed of the target relative to your vessel to find CPA and TCPA. Taking action based on anything less than this is acting on scanty RADAR information and is forbidden in Rule 7:
(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observations of detected objects.
(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.
Interpretation of this from MGN 369:
Take time to assess every situation properly, as it requires several minutes of systematic observation to produce useful information from a radar or ARPA set
Generally speaking the examiner will give you a situation where risk of collision exists. If in doubt (for example they have given you a smaller CPA than you are comfortable with) assume that such risk exists (this is as per Rule 7a).
We’re not going to go into Rule 19 here; that’s for a separate article, part of a series on the Rule of the Road, which will be linked here once published.
Q - “When is this estimation of the targets course and speed valid?”
A - “Only at the time of the last observation”
From MGN 379:
Estimation of the target’s true course is only valid up to the time of the last observation and the situation must be kept constantly under review. The other vessel, which may not be keeping a radar watch or plotting, may subsequently alter its course and/or speed. This will take time to become apparent to the observer. Electronic plotting will not detect any alteration of a target’s course or speed instantly and, therefore, should be checked by other means.
This makes sense, and it is why your answer, regardless of whether you alter course/speed or not, should always end with:
“…and I would continue to plot the target.”
This is as per Rule 8:
(d) Action taken to avoid collision with another vessel shall be such as to result in passing at a safe distance. The effectiveness of the action shall be carefully checked until the other vessel is finally past and clear.
So, is this what you do at sea? No, unless you are using some seriously outdated RADAR kit. We use ARPA now and let it calculate CPAs and TCPAs for us. Relative motion trails provide an immediate visual indication of whether risk of collision exists. Using an EBL is another approved technique; put the EBL on the side of the target that you think will pass closest to you (if it is crossing from the right hand side, put it on the left hand side of the target) and see if the target moves off the EBL. If it does, this shoes that the bearing is changing. Be ready to talk to the examiner about practical use of the RADAR as well as the somewhat outdated method discussed above, but remember that the method outlined above underpins everything we do with ARPA and that you need to understand it. It’s part of the syllabus, under ‘plotting techniques and relative and true-motion concepts’.
From MIN 653, at the operational level:
