11 lessonsabout 12 min reading5 visuals and tools10 scenario questions4 oral questions
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Lesson 1 of 11 · 1 min
How Radar Works — Basic Principles
Radar (Radio Detection And Ranging) transmits short pulses of microwave energy. When a pulse strikes a target it reflects back — the time for the echo to return is measured and converted to range. The direction of the antenna at the moment of transmission gives the bearing.
Parallel indexing uses a fixed radar-conspicuous object to continuously monitor the vessel's track without plotting a position fix. A line (the index line) is drawn on the radar display parallel to the planned track at the required distance off the reference object.
How to set up a parallel index
Select a radar-conspicuous fixed object (headland, isolated rock) — never a buoy
Determine the planned distance off the object when abeam
Draw the index line on the radar display parallel to the planned track at that distance
If the object's echo rides along the index line — the vessel is on track
If the echo moves inside the line — vessel is standing into danger
When to use it
Coastal passages with radar-conspicuous marks
Restricted visibility — replaces visual position fixing
Night passages in confined waters
Pre-plan in clear weather and use confidently in restricted vis
Use North-Up stabilised mode for consistency with the chart
Say it out loud. Talk “Parallel indexing” through with the live Tutor.
Reading the Screen — Relative Trails Are the Steady-Bearing Test
On a relative-motion display every echo moves along its RELATIVE track, own motion already subtracted. That gives you the fastest collision test on the bridge: a trail pointing at the display centre is a constant bearing with decreasing range — Rule 7's test, drawn for you, instantly, on every contact, before ARPA settles. What the trail does NOT give you is her true course or aspect: for that you plot, or read the settled true vector. Threat detection in relative; decision-making in true — and the first thing checked before reading anything is which mode and what vector time the display is actually in.
Say it out loud. Talk “Stabilisation — relative vs true, head-up vs north-up” through with the live Tutor.
Clutter Controls — A Clean Screen Is Not a Safe Screen
Sea (STC) and rain (FTC) suppression delete real echoes of the same character as the clutter — which is precisely the GRP yacht, the RIB, the buoy that was already marginal. Working discipline: minimum suppression that lets you work, a little speckle left alive so you know the receiver is listening, suspect areas interrogated at different ranges and gain settings, S-band brought into the argument in rain — and the honest conclusion drawn as a SPEED decision: Rule 6(b) lists radar limitations and the chance small craft are not detected at adequate range as safe-speed factors because the empty screen in clutter is a statement about the radar, not the water.
Say it out loud. Talk “Radar principles and controls” through with the live Tutor.
Parallel Indexing — Continuous Monitoring in Three Steps
At the planning stage: choose a FIXED, radar-conspicuous mark (a headland, never a buoy), draw the index line parallel to the planned track offset by the passing distance, note it in the plan. Underway: while the mark's echo rides the line, you are on track; the instant set or leeway moves you, the echo walks off the line and you see it NOW — not at the next fix. It is GNSS-independent, works in zero visibility, and remains the technique of choice for a controlled passing distance off a danger. Wheel-over points get the same treatment: a bearing or PI range as the objective cue for starting each turn.
Say it out loud. Talk “Parallel indexing” through with the live Tutor.
Scenario questions from the oral, with the reasoning after each one. The answers that look nearly right are there on purpose.
Module 09
Oral questions
Answer each one out loud before you open the model answer. If you can't say it, you don't know it yet.
Q1Your ARPA CPA limit is set at 1.0 nm and it alarms constantly in the Gibraltar Strait. What's the right response — and the wrong one?Model answer ▾
The wrong response is the common one: silencing or ignoring it until the alarm is dead as a defence — the MAIB's alarm-fatigue finding. The right response is a deliberate re-configuration for the waters: bring the CPA limit down to what the strait actually allows — with a compensating INCREASE in active watchkeeping, because a tighter limit means the machine warns later and the human must watch earlier. Alarm limits are standing-orders items: set to values the team will genuinely honour in these waters, recorded, and restored when sea room returns. An alarm the crew obeys at 0.5 nm beats one they ignore at 1.0.
Q2What are the limitations of radar?Model answer ▾
Blind sectors behind masts and superstructure. Sea and rain clutter can mask targets. Small vessels, buoys, and persons in the water may not appear. False echoes from indirect reflections and sidelobes. Second trace echoes at extreme range. Limited range and bearing discrimination means closely spaced targets may merge. Low-lying coastlines may not appear at expected range. The operator must always be aware that a clear radar screen does not mean a clear sea.
Q3What is CPA and TCPA and why do they matter?Model answer ▾
CPA is the Closest Point of Approach — the minimum distance the target vessel will pass if both vessels maintain course and speed. TCPA is the Time to Closest Point of Approach — how long until that minimum distance is reached. Together they tell you whether a risk of collision exists and how much time you have to take action. A small CPA with a short TCPA demands immediate action. Under COLREGS Rule 7, if there is any doubt — risk of collision shall be deemed to exist.
Q4What is parallel indexing and when would you use it?Model answer ▾
Parallel indexing is a technique using a fixed radar-conspicuous object to continuously monitor the vessel's track. An index line is drawn on the radar display parallel to the planned track at the distance the object should be abeam. If the object's echo rides along the line the vessel is on track — if it moves inside the line the vessel is standing into danger. I would use it in restricted visibility when visual position fixing is not possible, during night passages in coastal waters, and on any passage where there is a significant hazard to one side of the track.
Ready?
Sit a live oral on Radar & ARPA.
The live Examiner asks the questions out loud, coaches you when you miss, and gives you a debrief.