Radar Fundamentals and ARPA Basics

TL;DR — Marine radar provides range and bearing fixes from microwave echoes; ARPA automates target tracking and computes CPA/TCPA but does not replace visual watchkeeping. Federal carriage requirements for ARPA are triggered primarily at 10,000 GT for tank vessels and hazmat carriers, and at 10,000 GT for vessels built on or after 1 September 1984.

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What the Rule Says

Marine radar transmits microwave pulses and processes the returning echoes to display a plan-view picture of surrounding targets. Range and bearing to a target together yield a direct fix. Bowditch Ch. 13 §1301

The principal range scales used in piloting are 0.5, 1.5, 3, and 6 nautical miles. Short ranges give the best discrimination for collision avoidance; long ranges give the best landfall detection. Bowditch Ch. 13 §1302

Every commercial fishing vessel subject to 46 CFR Part 28 must be fitted with a general marine radar system for surface navigation, with the radar screen mounted at the operating station, and must also carry a suitable echo depth sounding device. 46 CFR §28.400

RACON Beacons

A RACON is a radar transponder beacon that responds to a vessel's radar pulse with a coded pattern — specifically a Morse character — painted on the radar display. The Morse code identifier for each RACON is given in the Light List. Light List — RACON

ARPA Carriage Requirements

An ARPA must comply with the IMO "Operational Standards for Automatic Radar Plotting Aids" and must have both audible and visual alarms. 33 CFR §164.38 The specific vessels required to carry ARPA are:

1. Each self-propelled vessel (except a public vessel) of 10,000 GT or more carrying oil or hazardous materials in bulk as cargo or in residue on the navigable waters of the United States, or which transfers oil or hazardous materials in any port or place subject to U.S. jurisdiction. 33 CFR §164.38

2. Each tank vessel of 10,000 GT or more operating on the navigable waters of the United States. 33 CFR §164.38

3. Each self-propelled vessel of 15,000 GT or more that is not a tank vessel and is not carrying oil or hazardous material in bulk, constructed before 1 September 1984, operating on the navigable waters of the United States — except when operating on the Great Lakes and their connecting and tributary waters. 33 CFR §164.38

4. Each vessel of 10,000 GT or more constructed on or after 1 September 1984, except when operating on the Great Lakes and their connecting and tributary waters. 33 CFR §164.38

Every required ARPA device must bear a permanently affixed label with the manufacturer's name and address, and a statement that the device was designed and manufactured to comply with the IMO Performance Standards for ARPA. 33 CFR §164.38

Speed and Distance Device Requirement

Every vessel required to carry ARPA under 33 CFR §164.38 must also be fitted with a device to indicate speed and distance, either through the water or over the ground. 33 CFR §164.40 That device must:

Have a display easily readable on the bridge by day or night. 33 CFR §164.40
Have speed errors not exceeding 5 percent of vessel speed or 0.5 knot, whichever is greater, when operating free from shallow water effect and from the effects of wind, current, and tide. 33 CFR §164.40
Have distance errors not exceeding 5 percent of distance run in one hour or 0.5 nautical mile per hour, whichever is greater, under the same free-from-effect conditions. 33 CFR §164.40

ARPA Performance Standards in Detail

The IMO performance standards incorporated into 33 CFR §164.38 establish the following key operational requirements:

Target acquisition may be manual or automatic, but a facility for manual acquisition and cancellation must always be available. When automatic acquisition is provided, the system must be capable of suppressing acquisition in operator-selected areas, and any suppressed area must be indicated on the display. 33 CFR §164.38

Tracking capacity:

With automatic acquisition: at least 20 targets simultaneously. 33 CFR §164.38
With manual acquisition only: at least 10 targets simultaneously. 33 CFR §164.38
The ARPA must continue to track an acquired target clearly distinguishable on the display for 5 out of 10 consecutive scans, provided the target is not subject to target swap. 33 CFR §164.38
On request, the ARPA must display at least four equally time-spaced past positions of any tracked target over a period of at least eight minutes. 33 CFR §164.38

Display requirements:

The effective display diameter must be at least 340 mm. 33 CFR §164.38
ARPA facilities must be available on at least two range scales: 12 or 16 miles, and 3 or 4 miles. 33 CFR §164.38
The display must be capable of relative motion with north-up and either head-up or course-up azimuth stabilization. True motion may also be provided; if so, the operator must be able to select either mode, and a positive indication of the mode in use must be shown. 33 CFR §164.38
Azimuth stabilization means own ship's compass information is fed to the display so that target echoes will not smear due to changes in own ship's heading. 33 CFR §164.38

Timing requirements:

Within one minute of acquisition: indication of the target's motion trend. 33 CFR §164.38
Within three minutes of acquisition: full predicted motion display. 33 CFR §164.38
After a maneuver by own ship or a tracked target: motion trend within one minute, full predicted motion within three minutes. 33 CFR §164.38
After changing range scales or resetting the display: full plotting information within four scans. 33 CFR §164.38

Alphanumeric data on demand for any tracked target must include: present range, present bearing, predicted CPA, predicted TCPA, calculated true course, and calculated true speed. 33 CFR §164.38

Trial maneuver: The ARPA must be capable of simulating the effect on all tracked targets of an own ship maneuver without interrupting the updating of target information. 33 CFR §164.38

Sensor input: Log and speed indicators providing inputs to ARPA must be capable of providing the ship's speed through the water. 33 CFR §164.38 The ARPA must not degrade the performance of any equipment providing sensor inputs, and connection of the ARPA to other equipment must not degrade that equipment's performance. 33 CFR §164.38

ARPA as a supplement, not a replacement: ARPA computes target tracks automatically and displays predicted CPA and TCPA, but it does not replace bridge watchkeeping — it supplements visual lookout. Bowditch Ch. 13 §1303

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Why It Matters on the Exam

Exam questions on this topic fall into three clusters:

1. Carriage thresholds. The exam will test whether you know the gross tonnage cutoffs and the construction-date dividing line (1 September 1984). Note that the 15,000 GT threshold applies only to non-tank, non-hazmat vessels built before 1984. Vessels built on or after 1984 face the lower 10,000 GT threshold regardless of cargo type. The Great Lakes exemption applies to the pre-1984 non-tank category and to the post-1984 general category, but not to tank vessels or hazmat carriers. 33 CFR §164.38

2. ARPA operational parameters. Questions frequently ask about tracking capacity (20 vs. 10 targets), timing (1 minute for trend, 3 minutes for full prediction), past-position history (4 positions over 8 minutes), and minimum display diameter (340 mm). 33 CFR §164.38

3. Speed log accuracy. The 5 percent / 0.5 knot tolerance for speed, and the 5 percent / 0.5 nm per hour tolerance for distance, are testable numbers. 33 CFR §164.40

4. RACON identification. Knowing that a RACON paints a Morse character on the radar display, and that the character is found in the Light List, is a standard exam item. Light List — RACON

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Common Pitfalls

Confusing relative and true vectors. A relative motion display keeps own ship fixed at the center; a true motion display moves own ship across the screen in accordance with its actual motion. Azimuth stabilization prevents smearing of echoes when own ship changes heading — it does not change whether the display is relative or true. 33 CFR §164.38

Misremembering the target-tracking minimums. With automatic acquisition the minimum is 20 targets; with manual acquisition only, it is 10. Candidates frequently invert these. 33 CFR §164.38

Assuming ARPA replaces lookout. ARPA supplements visual lookout; it does not replace it. This distinction is directly stated in the performance standards and is a frequent exam distractor. Bowditch Ch. 13 §1303

Ignoring the Great Lakes exemption. Tank vessels and hazmat carriers of 10,000 GT or more have no Great Lakes exemption. Only the non-tank categories carry that exemption. 33 CFR §164.38

Forgetting the companion speed log requirement. ARPA carriage automatically triggers the requirement for a speed and distance device. The two requirements travel together. 33 CFR §164.40

Short vs. long range scale selection. Short range scales give better discrimination for collision avoidance; long range scales are better for landfall detection. Selecting the wrong scale for the situation is a common operational error and an exam topic. Bowditch Ch. 13 §1302

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Quick Check

Q1 — At what gross tonnage threshold is a tank vessel required to carry ARPA on U.S. navigable waters?

10,000 GT or more. The Great Lakes exemption does not apply to tank vessels. 33 CFR §164.38

Q2 — How many targets must an ARPA with automatic acquisition be capable of tracking simultaneously?

At least 20 targets, whether automatically or manually acquired. An ARPA with manual acquisition only must track at least 10. 33 CFR §164.38

Q3 — Within what time must an ARPA display a target's full predicted motion after acquisition?

Within three minutes of acquisition. The motion trend must appear within one minute. 33 CFR §164.38

Q4 — What is the maximum allowable speed error for a speed and distance device fitted on an ARPA-equipped vessel?

5 percent of vessel speed or 0.5 knot, whichever is greater, when operating free from shallow water effect and from the effects of wind, current, and tide. 33 CFR §164.40

Q5 — What does a RACON display on a radar screen, and where do you find its identifier?

A RACON paints a Morse character on the radar display in response to the vessel's radar pulse. The Morse code identifier is given in the Light List. Light List — RACON

Q6 — Which radar range scale is preferred for collision avoidance, and which is preferred for landfall detection?

Short range scales give the best discrimination for collision avoidance; long range scales give the best landfall detection. Bowditch Ch. 13 §1302

Q7 — Does ARPA replace the requirement for a visual lookout on the bridge?

No. ARPA supplements visual lookout; it does not replace bridge watchkeeping. Bowditch Ch. 13 §1303

Q8 — A non-tank vessel of 12,000 GT was constructed in 1990 and operates on the Great Lakes. Is ARPA required?

No. Vessels of 10,000 GT or more constructed on or after 1 September 1984 are exempt from the ARPA requirement when operating on the Great Lakes and their connecting and tributary waters. 33 CFR §164.38