Radar Principles: How Radar Works
Understanding the physics behind radar explains why its strengths and limitations exist — and helps you answer exam questions about what radar can and cannot do.
The radar transmitter emits short, intense bursts of microwave energy (pulses). Between pulses, the receiver listens for returning echoes. The time elapsed between transmission and echo return is converted to range: range equals (speed of light times time) divided by 2. The division by 2 accounts for the two-way travel.
Pulse repetition frequency (PRF) determines how many pulses are sent per second. Higher PRF improves close-range detection but limits maximum unambiguous range.
The radar antenna radiates energy in a beam with finite width in both the horizontal (azimuth) and vertical (elevation) planes. Horizontal beam width directly affects bearing accuracy — a narrow beam provides better angular resolution. Typical marine radar beams are 1 to 2 degrees wide horizontally.
Wide beam width causes bearing smear — a single target appears as an arc rather than a point. Two targets close together in bearing may merge into one echo.
Range resolution is the ability to distinguish two targets that are close together in range but on the same bearing. It is determined by pulse length — a shorter pulse gives better range resolution. Two targets separated by less than half the pulse length in range will merge into one echo.
Short pulse length gives better range resolution but lower average transmitted power. Long pulse gives more power and longer detection range but poorer resolution. Most radars switch pulse length automatically by range scale.
Bearing accuracy depends on horizontal beam width, scanner rotation accuracy, and display calibration. Marine radars typically achieve bearing accuracy of plus or minus 1 to 2 degrees. Bearing is measured from the center of the echo — but beam width causes smear, so the center of a large return may not be the geometric center of the object.
For collision avoidance, bearing is used to assess risk. A constant bearing with decreasing range means collision course regardless of bearing accuracy errors.
Radar Controls: Gain, Sea Clutter, Rain Clutter, Tuning, Brilliance
Misadjusted radar controls are a leading cause of navigational accidents in restricted visibility. The USCG exam tests your ability to select the correct control for each situation — and to understand what happens when a control is set incorrectly.
Gain
Controls overall receiver sensitivity — amplifies all received signals including targets and noise.
Too Low
Weak targets disappear; distant echoes drop out; dangerously clean display hides real hazards.
Too High
Display fills with receiver noise (speckle); small targets hidden in noise; difficult to read.
Correct Setting
Set so a small amount of random speckle is visible across the display — confirms receiver is near its sensitivity limit.
Exam focus: If distant targets are not showing, increase gain first. Gain is the master sensitivity control.
Sea Clutter (STC — Sensitivity Time Control)
Suppresses strong returns from nearby wave faces that would mask close targets.
Too Low
Wave returns fill the center of the display; close targets are completely obscured.
Too High
Suppresses genuine targets at close range; vessel could miss nearby vessels or buoys.
Correct Setting
Reduce until wave clutter is just suppressed in the center; targets at close range should still appear clearly.
Exam focus: Waves masking a close target: reduce sea clutter control. Critical: do not suppress completely or you will lose real targets inside the clutter zone.
Rain Clutter (FTC — Fast Time Constant)
Differentiates between sustained returns (land, vessels) and rapidly fluctuating returns (rain and precipitation).
Too Low
Precipitation fills large areas of the display; targets within rain undetectable.
Too High
Weakens genuine targets; can suppress buoys and small vessels.
Correct Setting
Adjust until precipitation return is reduced to a grainy texture; solid targets should stand out clearly.
Exam focus: Vessel inside a rain squall: reduce rain clutter (FTC). Rain also attenuates radar energy — targets may genuinely be masked beyond the squall.
Tuning
Aligns receiver frequency with transmitter frequency for maximum echo strength and sharpness.
Too Low
Returns are weak and blurry; display appears washed out even at normal gain.
Too High
Off-tune in the other direction produces the same degraded result.
Correct Setting
Maximize the strength and sharpness of a known land return or use the automatic tuning indicator.
Exam focus: If all returns are unusually weak and blurry with gain at normal setting, check tuning first.
Brilliance / Contrast
Adjusts display phosphor brightness and contrast between echoes and the background.
Too Low
Weak returns disappear into a dark background; difficult to see in daylight.
Too High
Strong returns bloom and spread, obscuring detail; range rings become thick and imprecise.
Correct Setting
Adjust to the ambient light conditions; reduce in darkness, increase in bright daylight.
Exam focus: Brilliance and gain interact — increasing brilliance can appear to increase sensitivity when it is actually just brightening existing returns.
Range and Bearing Measurement: VRM, EBL, and Range Rings
These three tools are the primary instruments for extracting position data from the radar display. Range is always preferred over bearing — understand why and you will answer most radar fix questions correctly.
Variable Range Marker (VRM)
An electronically generated circle centered on own ship that can be expanded or contracted to touch the inner edge of a target echo. The range displayed is the distance from own ship to the target.
Accuracy: Range accuracy is the primary strength of radar — typically better than plus or minus 1.5 percent of the range scale in use, or 70 meters, whichever is greater.
Primary Uses
- ►Measuring distance off a headland or charted hazard
- ►Checking distance to a destination buoy or waypoint
- ►Verifying off-track distance during parallel indexing
- ►Confirming CPA range to a traffic vessel
Always set VRM to the near (closest) edge of the echo — the center of a large echo adds target depth to the reading, giving a false (longer) range.
Electronic Bearing Line (EBL)
A straight line from the center of the display that can be rotated to align with a target. The bearing displayed is the direction from own ship to the target, in degrees true or relative depending on mode.
Accuracy: Bearing is less accurate than range — typically plus or minus 1 to 2 degrees. Beam width smear makes bearing measurement of large targets imprecise.
Primary Uses
- ►Monitoring bearing to a target for collision risk assessment
- ►Taking a bearing fix combined with VRM range measurement
- ►Checking whether a vessel is maintaining constant bearing
- ►Setting a reference bearing for transit alignment
On relative motion displays, EBL bearings are relative to own ship heading unless you switch to true bearing mode. Know which mode your radar is in before plotting.
Range Rings
Fixed concentric circles at equal range intervals centered on own ship. The interval depends on the range scale selected — for example, on a 6-mile scale, rings may appear at 1-mile intervals.
Accuracy: Same accuracy as VRM — range rings are derived from the same timing circuit.
Primary Uses
- ►Quick visual estimate of target range without using VRM
- ►Monitoring rate of closure between rings over time
- ►Maintaining a mental picture of traffic density within a given range
- ►Estimating distance to land masses during coastal pilotage
Count rings carefully — at small range scales the rings are close together and easy to miscount. Always verify against the scale indicator before reporting a range.
Parallel Indexing: Track Keeping in Any Visibility
Parallel indexing is the professional standard for radar-assisted pilotage. It requires no GPS and provides continuous, real-time cross-track error monitoring using the radar display alone.
Parallel indexing uses a fixed radar reference target to monitor cross-track error in real time. By drawing an index line parallel to the intended track on the radar display — offset by the planned distance from a radar-conspicuous object — the operator can see instantly whether the vessel is on track, set toward a hazard, or set away from the track.
How to Set Up and Use Parallel Indexing
- 1Select a radar-conspicuous object (buoy, headland, tower) that will appear on the display during the passage. The object must be positively identifiable on both chart and radar.
- 2On the radar display, measure the planned perpendicular distance from the vessel's intended track to the reference object. This is the index distance.
- 3Draw an index line on the display parallel to the vessel's intended track, offset by the index distance on the appropriate side.
- 4As the vessel proceeds, observe where the reference echo falls relative to the index line.
- 5If the echo is on the intended side and touching the index line, the vessel is exactly on track.
- 6If the echo has drifted toward the vessel's centerline, the vessel is set toward the hazard — alter course away immediately.
- 7If the echo has moved away from the centerline beyond the index line, the vessel has set away from the track — alter course toward the reference.
- 8Update the index whenever passing a reference point and selecting a new one ahead on the route.
Advantages of Parallel Indexing
- ✓Works in any visibility — day, night, fog, heavy rain
- ✓No GPS required — purely radar-based track monitoring
- ✓Provides real-time cross-track error without computation or calculation
- ✓Standard technique for restricted-visibility pilotage in narrow channels
- ✓Detectable even if set or leeway causes the vessel to drift off track gradually
Collision Avoidance: ARPA, CPA, TCPA, and COLREGS
Collision avoidance is the most operationally critical use of radar. The USCG exam tests both the technical concepts (CPA, TCPA, ARPA) and the regulatory requirements (COLREGS Rules 7, 8, 16, 17, and 19). These questions appear on nearly every exam.
Risk of Collision
Risk of collision exists when the compass bearing to an approaching vessel does not appreciably change as the range decreases. Even if bearing changes slightly, risk may still exist with large vessels at close range. Any doubt means risk exists — treat it as confirmed risk.
Required action: Take early and substantial avoiding action. Small, tentative changes are dangerous — the other vessel may not detect them on radar.
CPA — Closest Point of Approach
CPA is the minimum separation that will occur between own ship and the target if both maintain course and speed. Calculated by plotting relative motion or by ARPA. A CPA of zero means collision. Industry standard for acceptable CPA varies by traffic density and sea room — often 0.5 to 1.0 miles in open water.
Required action: If CPA is unacceptably small, maneuver early. Plot the new CPA after maneuvering to confirm it is safe before committing to the alteration.
TCPA — Time to Closest Point of Approach
TCPA tells you how much time remains until the minimum separation occurs. A short TCPA combined with a small CPA demands immediate large action. A long TCPA allows more time to monitor and plan, but early action is always better — it gives the other vessel time to observe your maneuver and confirm the situation is resolving.
Required action: Rule of thumb: act when TCPA is still large enough that both vessels have time to respond and verify the situation has resolved safely.
ARPA — Automatic Radar Plotting Aid
ARPA tracks selected radar targets automatically, computing CPA, TCPA, target course, and target speed continuously. It can also simulate the effect of a proposed own ship maneuver before execution. ARPA is required on vessels of 10,000 GRT and above by SOLAS. Smaller vessels may have MARPA (Mini-ARPA) integrated into chart plotters.
Required action: ARPA does not replace watchkeeping. It can lose tracks, acquire false targets, and react slowly to sudden course changes. Always verify ARPA data with visual observation.
Restricted Visibility — Rule 19 Action
In restricted visibility, when a target is detected forward of the beam and a close-quarters situation is developing: do not alter course to port toward a vessel forward of the beam. Do not alter course toward a vessel on the beam or abaft the beam. Reduce speed to bare steerage or stop if necessary.
Required action: Starboard alteration is generally preferred for targets forward of the beam. Sound the fog signal. Consider stopping engines to listen for the other vessel.
COLREGS Rules Tested Most Often with Radar
Rule 7 — Risk of Collision
Use all available means including radar. Constant bearing with decreasing range means risk exists. If in doubt, risk exists.
Rule 8 — Action to Avoid Collision
Action must be large, positive, made in ample time, and result in a safe passing distance. Small or hesitant alterations are ineffective.
Rule 16 — Give-way Vessel Action
The give-way vessel shall take early and substantial action to keep well clear. Do not make small, uncertain alterations.
Rule 19 — Restricted Visibility
If risk of collision exists with a target forward of beam: do not alter to port. Alter to starboard, reduce speed, or stop.
Radar Plotting: True vs Relative Motion and the OAW Triangle
Manual radar plotting was the original method for determining target course and speed before ARPA. The USCG exam still tests the OAW triangle because it teaches the geometry behind collision avoidance — the same geometry ARPA automates.
Relative Motion Display
Own ship is fixed at the center of the display. All other objects — targets, land, buoys — move relative to own ship. A stationary buoy appears to drift astern at own ship speed and opposite own ship course. Target echoes draw a relative motion trail across the display.
- ► Standard on most marine radars
- ► Relative motion vector of target is plotted directly
- ► OAW triangle applied to derive target true motion
- ► Stationary targets appear to move — can be confused with vessels
True Motion Display
Own ship moves across the display at its actual course and speed. Stationary objects (land, anchored buoys) remain fixed on the display. Moving vessels draw their true tracks. Easier to distinguish anchored from underway vessels at a glance.
- ► Requires reliable speed and heading input (GPS or log plus gyro)
- ► Own ship reaches edge of display — must be reset periodically
- ► Easier situational awareness in busy anchorages
- ► CPA and TCPA still require plotting or ARPA
The OAW Triangle
The vector representing own ship true course and speed, drawn from the origin (O) for a time interval equal to the plotting interval (e.g., 6 minutes). Length is proportional to own ship speed.
Always draw O first. Its direction is own ship true course; its length represents own ship speed over the ground for the plotting interval.
The vector of the target apparent motion on a relative motion display. Plotted from the first observed position (M1) to the second observed position (M2) over the plotting interval. This is the direction the echo moves across the radar screen.
A is the relative motion direction. In the OAW triangle, A equals W minus O (vector subtraction). Plot A parallel to the M1-M2 line from the tip of O.
The vector representing the target vessel true course and speed. Derived by adding own ship vector (O) to the relative motion vector (A) graphically. This is the solution — once W is known, target course and speed are determined.
W is the result. The direction of the W vector gives target true course; its length gives target speed. Extend M1-M2 line to find CPA distance to own ship.
Interpreting Radar Returns: Land, Buoys, Vessels, Precipitation
The radar display shows everything that reflects microwave energy. Skilled operators read the display like a chart — identifying each echo, distinguishing real targets from clutter, and knowing which targets may be missing entirely.
| Target Type | Appearance |
|---|---|
Land Masses | Strong, stable echoes; shape matches charted coastline when scaled correctly Sandy beaches, mudflats, and low lying coasts return weak echoes or may not appear at all. Elevation behind the shore is often displayed because the radar beam strikes it. Never assume the radar coastline matches the chart shoreline exactly. |
Buoys | Small, often intermittent returns; appear as single bright dots Buoys move with current and wind; their position may differ slightly from charted position. A RACON (radar transponder beacon) on a buoy emits a coded echo far stronger and easier to identify. Lighted buoys are not differentiated from unlighted buoys on radar. |
Vessels | Discrete bright echoes; size on display reflects radar cross-section, not physical size A large laden tanker returns a huge echo; a GRP sailing vessel may be nearly invisible. All small vessels should carry a radar reflector to enhance their return. Vessels at the same range but different bearings can be confused if beam width smear makes their echoes merge. |
Precipitation | Large amorphous patches of return that shift and change shape; grainy texture distinct from hard targets Precipitation attenuates the radar beam — targets on the far side of a heavy rain area may be significantly weakened or invisible. Reduce rain clutter (FTC) to distinguish hard targets within precipitation. Targets inside the squall are still attenuated regardless of control settings. |
Sea Return (Wave Clutter) | Dense, rapidly fluctuating returns in the center of the display, especially in high seas Sea return can completely mask targets at close range. Reduce sea clutter (STC) gradually to suppress wave returns while preserving target echoes. On a relative motion display, sea return remains centered because own ship is at the center. |
False Echoes (Multiple Reflections) | Ghost echoes at multiples of the true target range, on the same bearing as a strong nearby target Multiple reflection false echoes occur when radar energy bounces between own ship structure and a large nearby target before returning to the receiver. They move with the true target but at double or triple the range. Side lobe echoes appear as arcs at the same range as a strong target on different bearings. |
Radar Shadows and Blind Sectors
A radar shadow is a hidden danger — an area where targets exist but the radar cannot see them. Ignoring blind sectors is an operational hazard and a USCG exam topic.
What Creates Radar Shadows
- ►Masts and radar masts aboard own vessel blocking the antenna beam at certain bearings
- ►Funnels, davits, cranes, deck cargo, and superstructure creating permanent blind sectors
- ►Large vessels passing close aboard create a temporary shadow behind them
- ►Headlands, islands, and breakwaters create shadows extending to leeward of the obstruction
- ►The shadow angle widens with the height of the obstruction and narrows with distance
How to Compensate for Blind Sectors
- ►Learn the vessel blind sectors before operating in restricted visibility
- ►Alter course slightly to sweep a shadow area — the geometry changes with course change
- ►Post additional visual and auditory lookouts specifically covering blind sectors
- ►Reduce speed to increase time available to detect targets entering the blind area
- ►Use VHF to communicate with other traffic in an area where a large obstruction exists
- ►Treat the shadow area as potentially occupied — never assume it is empty
Blind Piloting with Radar: Fixing Position off Charted Objects
Blind piloting is radar-only navigation in restricted visibility — no visual bearings, no GPS confirmation. It requires systematic procedures and conservative seamanship.
Plan Before Departure
Identify all radar-conspicuous objects along the intended route on the chart before departure. Mark objects that will be on the radar display at each critical waypoint. Determine which objects are charted with sufficient accuracy for radar fixing. Note any radar shadows that may exist along the route.
Establish Radar Identification
On radar display, correlate observed echoes with charted objects by range, relative bearing, and position relative to other echoes. This positive identification is the foundation of every radar fix. If you cannot positively identify an echo, do not use it for navigation.
Take Range Fixes
Use VRM to measure the range to at least two positively identified objects. Plot these ranges as arcs on the chart. The intersection of two arcs is the fix. Three ranges provide a cocked hat — if small, use the center. Range fixes are preferred over bearing fixes because range accuracy exceeds bearing accuracy on radar.
Establish Parallel Index Lines
Draw index lines on the radar display for each leg of the route. Monitor target echoes against index lines continuously. Any deviation from the index indicates set, leeway, or helmsman error. Correct immediately and investigate the cause.
Fix Frequency
Fix more frequently in restricted waters. In open ocean radar passage-making, fixing every 30 minutes may be acceptable. In a narrow channel with hazards every half mile, fix every 2 to 3 minutes or use continuous parallel indexing. Speed and sea room determine fix interval.
Maintain Safety Margins
Never navigate in restricted visibility with the radar range scale so large that hazards appear only at the edge of the display. Use short range scales for pilotage, longer scales for traffic awareness. Maintain engine readiness to stop or maneuver. Sound fog signals continuously.
USCG Exam Focus Areas: What Gets Tested Most
These are the radar concepts with the highest exam frequency. Each is a common trap for test-takers who have general radar knowledge but miss the specific USCG exam angle.
Constant bearing, decreasing range = collision risk
This is the single most important collision avoidance concept on the USCG exam. If you observe a target whose compass bearing does not change while the range is decreasing, you are on a collision course. Take action: alter course significantly, reduce speed, or both. Do not wait.
Range is more accurate than bearing on radar
On every radar fix question, prefer range fixes over bearing fixes when both are available. Radar range accuracy is typically plus or minus 1 percent of range scale or 70 meters, whichever is greater. Bearing accuracy is plus or minus 1 to 2 degrees. At 3 miles, a 2-degree bearing error is about 0.1 miles of position error.
Sea clutter must not be fully suppressed
The USCG exam will present a scenario where waves are masking nearby targets. The correct answer is to reduce (not eliminate) sea clutter. Fully suppressing sea clutter also suppresses targets in that area. The correct setting leaves a slight grainy texture in the center of the display.
ARPA requires operator verification
ARPA calculates CPA and TCPA automatically, but it can acquire false targets, lose tracks on maneuvering vessels, and display incorrect data during initialization. The exam may present a situation where ARPA shows a safe situation but visual observation shows a vessel on collision course. Always trust visual observation over ARPA alone.
Rule 19 restricts port alterations in restricted visibility
COLREGS Rule 19 prohibits altering course to port for a vessel forward of the beam in restricted visibility, except when overtaking. This appears directly on USCG exam questions. The correct action for a radar target forward of the beam is to alter to starboard, reduce speed, or stop.
Buoys may be intermittent on radar
Buoy returns are small and often weak. In a seaway, buoys disappear behind wave crests periodically. Never rely on a single radar buoy return for a critical fix — verify range to a more stable charted object and use the buoy as secondary confirmation.
Radar shadows fall behind obstructions, not in front
If a large ship or headland is to starboard, the radar shadow extends behind it (on the far side from own ship). Vessels in that shadow zone are completely invisible. The USCG exam asks: where does the shadow fall? Behind the obstruction, away from own ship.
Short pulse on close range scales, long pulse on long range scales
Most marine radars automatically switch pulse length with range scale. Short pulse provides better range resolution for close-range pilotage. Long pulse provides stronger returns at distance. If two close targets are merging in range, switching to a shorter range scale improves resolution.
Practice Questions with Answers
Work through these questions in exam conditions — read carefully, commit to an answer, then check the explanation. Many are phrased the way the USCG exam phrases them.
You are observing a vessel on your radar at a bearing of 045 degrees relative, range 4 miles. After 6 minutes, the bearing is 046 degrees relative and the range is 3.2 miles. What is the risk of collision assessment?
Answer
Risk of collision exists. The bearing has changed only 1 degree while the range decreased by 0.8 miles. COLREGS Rule 7 states that a slight bearing change does not mean risk of collision does not exist, especially with a large vessel at close range. Take avoiding action now, while range and time allow a safe maneuver.
The radar display is showing a large area of bright returns covering the center of the screen, obscuring any close targets. What control should you adjust first?
Answer
Reduce the sea clutter control (STC). The bright returns near the center are sea return from nearby wave faces. Reduce gradually until the wave clutter diminishes, but do not suppress fully. After reducing sea clutter, check for any genuine targets that were hidden in the clutter.
You are in restricted visibility. Your ARPA shows a target forward of the beam with a CPA of 0.3 miles in 8 minutes. COLREGS Rule 19 applies. Which action is prohibited?
Answer
Altering course to port is prohibited under Rule 19(d) for a vessel forward of the beam in restricted visibility. The correct actions are: alter course to starboard, reduce speed to minimum steerage, or stop engines. A large, easily detectable starboard alteration is preferred.
In the OAW triangle, what does the vector from the origin to W represent?
Answer
The W vector represents the target vessel's true course and speed plotted over the plotting interval. It is the solution to the triangle. The direction of W gives the target's true course; the length of W represents the target's speed. From this, CPA and TCPA can be calculated by extending the target's relative motion line to own ship's position.
You need to take a radar fix. Two charted objects are visible as echoes at ranges of 2.1 miles and 3.4 miles. A third object is visible at a bearing of 320 degrees true, range 1.8 miles. How should you use this data?
Answer
Use all three for a cocked hat fix. Plot arcs at 2.1 and 3.4 miles from the two ranged objects, and plot an arc at 1.8 miles from the third object. The bearing is useful for identification but range is the primary measurement. The center of the triangle formed by the three arcs is your position. Range fixes are preferred over bearing-only fixes on radar.
During radar pilotage in a narrow channel, you notice that a charted headland echo has drifted from the index line toward your vessel centerline on the parallel index display. What does this indicate and what action is required?
Answer
The vessel has set toward the headland — cross-track error has developed and the vessel is too close to the hazard. Alter course away from the headland immediately to restore the echo to the index line. Investigate the cause: current set, helmsman error, or wind leeway. Once corrected, monitor closely and increase fix frequency.
A radar echo appears at a range of exactly twice the range of a large steel ship nearby and on the same bearing. What is the most likely explanation?
Answer
This is a multiple reflection (second trace) false echo. Radar energy is reflecting between own ship structure and the large target before returning, creating a ghost image at double the true range. This is not a real target. Verify by slightly altering course — a false echo will move with the true target; a real target would change bearing independently.
You are approaching a harbor in fog using radar. The chart shows a rocky headland to port. On the radar display, the area beyond the headland echo shows no returns at all. Should you navigate through that area?
Answer
No. The area of no returns beyond the headland is almost certainly a radar shadow — the headland is blocking the beam from illuminating targets behind it. Vessels, rocks, or other hazards in that shadow zone are completely invisible. Treat the shadow area as unknown. Navigate around it or use other means to verify safety before entering.
What is the difference in radar performance when switching from a 6-mile range scale to a 1.5-mile range scale, assuming the radar automatically adjusts pulse length?
Answer
On the 1.5-mile scale, the radar uses a shorter pulse length, which improves range resolution — two close targets that merged at 6 miles may be separated at 1.5 miles. Bearing resolution also improves visually because each target occupies a larger portion of the display. However, targets beyond 1.5 miles disappear from the display. Keep the longer range available on a second display or check it periodically for traffic.
An ARPA target is showing a CPA of 1.5 miles and a TCPA of 22 minutes. The duty officer proposes waiting 10 more minutes before taking action. Is this appropriate?
Answer
This requires careful judgment. With 22 minutes to CPA and 1.5 miles of separation, there is time to monitor — but close attention is required. If CPA is acceptable under vessel policy, continued monitoring is reasonable. However, if CPA tightens, act early. COLREGS require action in ample time. Waiting until TCPA is 5 minutes before acting is never appropriate. Monitor continuously and act the moment CPA becomes unacceptable.
Pro Tips: Operational Radar Technique
These are practical techniques used by professional mariners that go beyond the exam minimum — and help you understand the subject more deeply when questions involve operational judgment.
Use radar range to verify visual bearings
When taking a compass bearing to a charted object, simultaneously measure the radar range to the same object. Plot both — a bearing line and a range arc. If they intersect at the same point, your compass is accurate and the echo is correctly identified. If they do not agree, investigate before relying on either.
Know your vessel blind sectors before departing
Every vessel has radar blind sectors caused by masts, davits, stacks, or cargo. Learn them from the radar performance standards documentation or by rotating slowly in a marina and noting which bearings produce no returns. During watches in restricted visibility, mentally note when traffic is in a blind sector.
Increase fix frequency as hazard density increases
Open ocean: fix every 30 minutes is often acceptable. Coastal pilotage: fix every 5 to 10 minutes. Narrow channel approach: continuous parallel indexing plus fix every 2 to 3 minutes. The golden rule: your next fix should be complete before you reach a position where a navigation error would be irreversible.
Never trust a single radar identification without corroboration
Before using an echo for a fix, verify its identity using at least two independent criteria: the range matches the charted distance, the bearing is consistent, the shape corresponds to the charted feature, and its position relative to other echoes makes sense. One criterion alone is insufficient.
Plot the maneuver before you make it
When a collision risk exists, do not simply alter course and hope. Use ARPA trial maneuver function or manually plot the effect of your proposed course change. Confirm the new CPA is acceptable, then execute. After maneuvering, plot again to confirm the target is now passing safely.
Reduce speed in restricted visibility, not just course
COLREGS Rule 6 requires a safe speed at all times, and Rule 19 specifically requires reducing speed in restricted visibility when a close-quarters situation develops. Reducing speed gives more time to assess the situation, increases stopping distance available, and reduces damage potential in a collision.
Quick Reference Summary
Key facts in condensed form for final review before the exam.
Radar Accuracy
- ▪Range: better than +/- 1.5% of scale or 70 m
- ▪Bearing: +/- 1 to 2 degrees
- ▪Always prefer range over bearing for fixes
- ▪VRM reads inner (near) edge of echo
Control Rules
- ▪Gain: small speckle visible = correct setting
- ▪Sea clutter: suppress waves, preserve targets
- ▪Rain clutter: grainy texture OK, solid targets clear
- ▪Tuning: maximize sharpness of returns
Collision Avoidance
- ▪Constant bearing + decreasing range = collision
- ▪Rule 19: no port for target forward of beam
- ▪Act early, act large, act decisively
- ▪Verify CPA after every maneuver
OAW Triangle
- ▪O = own ship true vector
- ▪A = target relative motion vector
- ▪W = target true vector (the solution)
- ▪W = O + A (vector addition from origin)
Blind Sectors
- ▪Shadow falls BEHIND the obstruction
- ▪Not between own ship and obstruction
- ▪Alter course to sweep the shadow area
- ▪Post extra lookouts covering blind sectors
Parallel Indexing
- ▪Index line parallel to track, offset by planned distance
- ▪Echo on line = on track
- ▪Echo toward center = set toward hazard
- ▪Works in any visibility without GPS
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