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.
- Frequency: S-band (3 GHz / 10cm) and X-band (9 GHz / 3cm)
- Pulse length: Short pulse = better close-range discrimination. Long pulse = better long-range detection
- PRF (Pulse Repetition Frequency): Number of pulses per second — affects maximum range
- Beam width: Narrower beam = better bearing discrimination
- X-band (3cm): Better discrimination, better for navigation — standard coastal use
- S-band (10cm): Better penetration through rain and clutter — better for long range and adverse weather
- Most vessels carry both — use X-band for close navigation, S-band for weather penetration
| Mode | Description | When to use |
|---|---|---|
| Head-Up (Unstabilised) | Own vessel's heading points to the top of the display. Display rotates as vessel alters course | Rarely — bearing distortion makes plotting difficult |
| North-Up (Stabilised) | North always at the top. Display does not rotate when course changes. Bearings are true | Passage monitoring — consistent with the chart |
| Course-Up (Stabilised) | Vessel's current course points to the top. Stabilised against gyro | Manoeuvring and collision avoidance — more intuitive ahead view |
| Relative Motion (RM) | Own vessel is stationary at the centre. All targets shown moving relative to own vessel | Collision avoidance — CPA and TCPA clearly visible |
| True Motion (TM) | Own vessel moves across the display. Targets show true course and speed | Traffic assessment — see actual movement of vessels |
| Limitation | Effect |
|---|---|
| Blind sectors | Masts, funnels, and superstructure create shadow sectors where radar cannot detect targets |
| Rain and sea clutter | Heavy rain and rough seas create returns that can mask targets — use clutter controls carefully; reducing clutter too much loses real targets |
| Small targets | Small vessels, buoys, debris, and persons in the water have poor radar cross-section — may not appear even at close range |
| Indirect echoes (false echoes) | Reflections from the vessel's own structure produce false returns at incorrect positions |
| Sidelobe echoes | Energy transmitted outside the main beam creates arc-shaped false returns near strong targets |
| Second trace echoes | At long range, echoes from the previous pulse return after the next pulse — target appears at wrong range |
| Range discrimination | Two targets close in range may merge into one return — depends on pulse length |
| Bearing discrimination | Two targets close in bearing merge — depends on horizontal beam width |
| Low-lying coastlines | May not appear at the range expected — sand dunes, beaches can be invisible until very close |
| ARPA term | Definition |
|---|---|
| CPA | Closest Point of Approach — the minimum distance the target will pass if both vessels maintain course and speed |
| TCPA | Time to Closest Point of Approach — time until CPA is reached |
| Acquisition | Manual or automatic — once acquired, ARPA tracks the target and begins building its vector |
| Vector | Line showing target's predicted future position — true vector shows actual course/speed; relative vector shows motion relative to own vessel |
| Trial manoeuvre | Allows OOW to test a proposed course/speed alteration and see the effect on CPA before committing |
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.
- Select a radar-conspicuous fixed object (headland, isolated rock, 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
- 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
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.
Own course 000°, speed 12 kn. Contact observed at 045° / 6.0 nm, and six minutes later at 045° / 4.0 nm:
| Step | Working | Result |
|---|---|---|
| CPA test | Bearing steady, range closing | Risk of collision exists |
| Relative speed | 2.0 nm in 6 min | 20 kn — about 12 minutes to collision: the decision clock |
| Own motion (O→A) | 12 kn × 6 min = 1.2 nm at 000°, laid from the first observation | The triangle's second side |
| Target true vector | Join A to the later observation W | Her true course and speed for the interval |
| X-band (3 cm) | S-band (10 cm) | |
|---|---|---|
| Strength | Sharp discrimination, small-target definition — and the band SARTs and most racons answer | Punches through rain and sea clutter; holds targets at range in weather |
| Weakness | Blinded by heavy precipitation | Coarser picture, weaker on small targets |
| Role | Primary for collision avoidance and pilotage | The all-weather cross-check running alongside |
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.
| Paint | What it is | Your response |
|---|---|---|
| Line of ~12 dots radiating OUTWARD along a bearing, ~0.6 nm apart — becoming arcs, then circles as you close | A SART: someone in distress at the innermost dot (X-band only) | Treat as distress: mark it, close it, inform the MRCC, SOLAS V/33 applies |
| A Morse character painting outward FROM a charted position | A racon identifying a major mark or bridge span | Navigation information — confirm against the chart |
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.
Your 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?
What are the limitations of radar?
What is CPA and TCPA and why do they matter?
What is parallel indexing and when would you use it?