Radar Observer Exam USCG: Full Prep Guide for 2026

The USCG Radar Observer endorsement is a mandatory add-on for any officer standing a navigation watch on vessels operating in restricted visibility. It's required under 46 CFR 10.480 for officers serving as watchstanders on inspected vessels, and it's an NMC-administered examination separate from your license exam modules.

If you're pursuing or renewing your Master 100 GT, Master 200 GT, or any inspected vessel credential, you need this endorsement — and it requires passing a specific radar examination at an approved testing center.

Here's a complete breakdown of what's tested, what the exam structure looks like, and how to prepare.

When You Need the Radar Observer Endorsement

The radar observer endorsement requirement applies under 46 CFR 10.480 to:

• Any officer standing a watchstanding role on an inspected vessel
• Officers on vessels required to carry radar (vessels over 300 GRT on international voyages, vessels over 1600 GRT domestically, and most inspected passenger vessels)
• Officers on towing vessels under Subchapter M

It does NOT apply to the OUPV (6-pack) in most cases, since OUPV vessels are uninspected. However, many 6-pack operators obtain it anyway because radar is standard equipment on serious charter vessels.

The endorsement is issued as an NMC endorsement on your MMC. It expires with your MMC (5-year cycle) and must be renewed by demonstrating continued use or retaking the radar course.

How to Obtain the Endorsement

There are two paths:

1. Approved radar course: Complete a USCG-approved radar observer course at an approved maritime training facility. These courses run 3-5 days and include practical simulator time, hands-on plotting exercises, and an end-of-course exam. The approved course satisfies the NMC application requirement without a separate exam.

2. Challenge exam at a Regional Exam Center: If you have documented radar watchstanding experience (typically 90 days of radar watchkeeping), you can challenge the exam directly at an NMC Regional Exam Center without taking the full course. This is the option most working mariners with existing sea service take.

The NMC will verify your documentation before issuing the endorsement regardless of which path you take.

What the Radar Observer Exam Tests

The challenge exam at an REC consists of written questions and radar plotting exercises. The written portion covers:

Radar Fundamentals (20-25 questions)

• Radar principles: How radar works — pulse transmission, the return echo, range measurement based on time-of-flight, azimuth from antenna rotation
• Range and bearing accuracy: Factors affecting accuracy — pulse length (affects range discrimination), beamwidth (affects azimuth discrimination), sea clutter, rain clutter, side lobes
• False echoes: Multiple echoes, side lobe echoes, indirect echoes, ghost targets — how to identify and discount them
• Performance monitoring: Daily checks — performance monitor, calibration marker, sensitivity time control (STC), fast time constant (FTC)
• Antenna height: Effect on radar horizon — higher antenna = more range on surface contacts, more sea clutter
• Frequency bands: X-band (3 cm, 9 GHz) vs. S-band (10 cm, 3 GHz) — X-band has better resolution, S-band better penetrates rain

ARPA (Automatic Radar Plotting Aid) Operation (20-25 questions)

• Target acquisition: Manual vs. automatic acquisition, acquisition zones
• Tracking targets: How ARPA calculates CPA and TCPA from multiple range/bearing fixes over time
• Vector display: True motion vs. relative motion vectors — what each shows, when each is preferred
• CPA/TCPA limits: Setting guard rings, understanding when an alarm triggers
• ARPA errors: Tracker errors in poor sea states, slow acquisition of new targets, errors from maneuvering too soon after a maneuver (the "wake up" period)
• Gyro input: The effect of incorrect gyro heading input on ARPA true motion calculations

Radar Plotting (Maneuvering Board or ARPA plot)

This is the practical section. Using a maneuvering board (Pub. 217 format) or plotted ARPA data, you must:

• Determine relative motion: Plot three or more range/bearing observations of a target to draw the relative motion (RM) line
• Find CPA and TCPA: The RM line extended to closest approach — measure the perpendicular distance from your origin to the RM line (CPA) and the time to reach it (TCPA)
• Determine true course and speed of a target: Add your vessel's true motion vector to the relative motion vector head to get the target's true vector
• Evaluate a proposed maneuver: Given a proposed course change or speed change, determine the new RM line and the resulting CPA
• Find the course to give a desired CPA: Back-solve the maneuver geometry to find what course change produces a specified new CPA

The plotting problems are the hardest part of this exam for candidates who haven't done systematic radar plotting practice. The geometry is logical but the construction takes time and precision.

Collision Regulations as Applied to Radar

The exam connects radar use directly to COLREGS:

• Rule 19 (Restricted visibility): No vessel shall proceed at a speed greater than that at which she can take proper and effective action to avoid collision before it is too late. Every vessel shall have particular regard to the prevailing circumstances when action in restricted visibility involves an alteration of course to port for a vessel forward of the beam.
• Rule 7 (Risk of collision): Risk shall be deemed to exist when the compass bearing of an approaching vessel is not appreciably changing. "Not appreciably changing" means what on the radar plot? A RM line pointing at the origin.
• Rule 8 (Action to avoid collision): Action must be large enough to be readily apparent to the other vessel — applies when determining the magnitude of a course or speed change as computed from radar plots.

Restricted Visibility Operations

Beyond COLREGS, the exam tests practical operations in fog:

• Fog signal schedules and requirements (Rule 35) — sounding signals at 2-minute intervals for power-driven vessels underway, single blast for vessels stopped, bell and gong for anchored and aground vessels
• Reducing speed in restricted visibility — what factors determine safe speed under Rules 5 and 6
• The requirement to stop engines and navigate with caution when a fog signal is heard forward of the beam (Rule 19)

Practical Plotting: Step by Step

For a maneuvering board problem (this is how it's tested on the challenge exam):

1. Plot your initial own-ship position at center (origin O)
2. For the target, plot range/bearing as follows: distance from center = range, angle = relative bearing. Call these points M1, M2, M3 (sequential observations at equal time intervals)
3. Draw the RM line through M1, M2, M3 (should be approximately linear for a constant-course target)
4. Extend the RM line — where it passes closest to the origin = CPA
5. Measure CPA distance on the range scale
6. Measure the distance from M3 to the CPA point along the RM line, divide by the time interval between plots × speed ratio = TCPA

The true vector of the target is found by adding your own velocity vector (from O in the direction of your course, magnitude = your speed) to the RM vector (from M1 to M3), and drawing the resulting vector from the tail of the RM vector.

Practice this at least 20-30 times before the exam. The construction is predictable but requires precision.

Renewal

The radar observer endorsement is tied to your MMC. At renewal (every 5 years), you must demonstrate:

• 1 year of sea service in the last 5 years on vessels equipped with radar, OR
• Completion of an approved refresher radar course

If your sea service lapses, the refresher course is required. Approved courses are listed on the NMC website.

Practice with Binnacle School

ARPA plotting and radar collision avoidance require understanding how to interpret radar data and apply it under COLREGS — not just memorizing radar specifications. [Binnacle School](/school) covers radar observer exam content including ARPA fundamentals, CPA/TCPA calculations, and the collision avoidance decision-making that the exam tests.

Build your radar observer exam confidence →

Binnacle AI is not affiliated with the U.S. Coast Guard or the NMC. Radar observer endorsement requirements are governed by 46 CFR Part 10 and applicable NMC policy letters — verify current requirements at uscg.mil/nmc. Not legal advice.