20 Automatic Radar Plotting Aid (ARPA) Functions You Must Know

by | Last updated Feb 28, 2024 | Navigation | 0 comments

An Automatic Radar Plotting Aid or ARPA is an essential tool for deck officers navigating the seas.

It’s a mariner’s best friend for enhancing situational awareness and collision avoidance by automatically tracking and displaying the movements of target vessels.

But this device can only be as good as the user. 

Since it has many functions due to its wide capabilities, Officers of the Watch must know and effectively use it to enhance the vessel’s navigation safety.

Key Takeaways

  • The primary purpose of an Automatic Radar Plotting Aid is to improve the standard of collision avoidance at sea.
  • ARPA reduces the workload of observers by enabling them automatically to obtain information about plotted targets.
  • Deck officers should familiarize themselves with essential ARPA functions, including display indications, display modes, target acquisition, etc.
  • While RADAR provides basic detection and location of targets, ARPA adds advanced functionalities such as automatic tracking.

What is an Automatic Radar Plotting Aid or ARPA

An ARPA or Automatic Radar Plotting Aid is a computerized addition to basic radar functionality that provides automated target tracking, vector analysis, and collision detection capability.

This tool integrates radar data with information about the ship’s speed and course to automatically track and plot the movements of other vessels in the vicinity.

The primary purpose of ARPA on board ships is to improve collision avoidance at sea. It does this by doing the following:

  • Automatically tracking other vessels – This allows officers to see the movements of other vessels and predict their future positions.
  • Reducing workload – Officers can handle multiple vessels as easily as one, thanks to automatic information gathering.
  • Providing real-time situation updates – ARPA helps officers make quick and accurate decisions based on the latest information about the target’s vectors.

20 ARPA Functions Every Deck Officer Must Know

You can fully appreciate the importance of an Automatic Radar Plotting Aid in high-traffic environments like passing the English Channel or the Gibraltar Strait.

In such congested waters where vessels of various sizes and types converge, ARPA’s capability to accurately monitor vessel movements in real-time is vital.

Here are 20 functions of an Automatic Radar Plotting Aid that you should be using.

1. Display Indications

Everything you see on the screen is the display indication. 

Your own position, bearing lines, range markers, cursors, display modes, range rings, range, heading line, etc., are all part of the display indication.

The screen can be overwhelming at first, but knowing what you are looking at is essential for the proper usage of ARPA.

2. Display Modes

Refer to the orientation on which your screen is set and your own ship’s movement in relation to other targets on the screen. It’s how your surroundings are represented on your radar screen.

There are two display modes and three display orientations.

Relative Motion

Means a display on which the position of own ship remains fixed and all targets move relative to own ship.

The three display orientations are:

  • North-up display – an azimuth stabilized presentation that uses the gyro input (or equivalent), and North is uppermost on the presentation.
  • Course-up display – an azimuth stabilized presentation that uses the input from the gyro compass or equivalent, and the ship’s course is uppermost on the presentation at the time of selection.
  • Head-up display – an unstabilized presentation in which own ship’s heading is uppermost on the presentation.

True Motion

A display in which own ship moves with its own true motion.

Head-up, Course-up, and North-up Display Orientation.

3. Range Scaling

Ability to adjust the radar’s range scale to focus on specific areas of interest. You must know which range scale to use depending on the traffic conditions and proximity of hazards.

4. Target Acquisition and Tracking

Knowledge of how to manually or automatically acquire and track targets on the radar display. The officer must also know how to stop the tracking and acquisition of such targets.

5. Vector Display

Vectors represent the speed and course of own ship and tracked targets. Depending on the timeframe used, it can also show the predicted position.

Relative vector and true vector can be confusing for the untrained eye. That’s why proper familiarization of this function is important.

6. Alarm Systems

An ARPA is equipped with audiovisual alarms. You can manually set parameters for the alarms to trigger, like the CPA and TCPA, including target alarms.

While at anchor, setting your vessel to anchor watch can also trigger the anchor watch alarm if your ship starts dragging.

7. Guard Zones

Guard zones are the predefined area in your radar that automatically acquires targets entering within that zone while setting off an alarm.

Understanding how to set up and monitor guard zones to receive alerts when vessels enter predefined areas can be useful, especially if the S-Band radar misses a specific target, but your X-Band doesn’t.

Guard Zones displayed on some areas of the ARPA.

8. Performance Monitor Test

Like any navigational tool in the bridge, an ARPA’s performance may degrade over time. One method to determine if this equipment is still displaying the correct range is by Performance Monitoring.

Different makes and models have various ways to do Performance Monitoring. On my previous ship, there was a button in the console that was labeled “PM”. Upon pressing, the display range automatically sets to 24 miles with your vessel at the center.

Rings or arcs appear on the screen, and we measure them using the VRM. 

When the range is not as specified in the manual, you may need to call service engineers or replace the magnetron.

Check your ARPA’s make and model before performing Performance Monitoring.

9. Trial Maneuver

This function simulates how specific changes in course, speed, or both will affect the relative movement of all tracked targets.

It’s one of ARPA’s most important features for both collision avoidance and navigation.

10. Anti-clutter Features

This is also called clutter suppression, swept gain control, or STC (sensitivity-time control). 

Sea waves and rain can create unwanted echoes that may “drown” targets on the radar screen.

Using features to minimize clutter on the radar display, such as sea clutter and rain clutter filters, is one of the most basic things to do when using your Automatic Radar Plotting Aid.

Anti-clutter of an ARPA in action.
The PPI screen of an ARPA before and after anti-clutter (sea and rain clutter) is applied.

11. AIS Interface

The Automatic Identification System extends the ARPA’s capability by providing enhanced tracking.

Some ships may be too small to be picked up by the radar. With an AIS, the target will still show on the PPI even if there is no echo detected.

The interface between the two makes collision avoidance and situational awareness very powerful.

12. Electronic Bearing Lines (EBL) and Variable Range Markers (VRM)

An Electronic Bearing line (EBL) is a continuous dashed line that radiates to the edge of the screen and measures the bearing of a target.

A Variable Range Marker (VRM) is a dashed ring that you can adjust to measure the range of a target.

Most ARPAs have two VRMs and EBLs. You can control them using a knob to adjust their range and bearing.

13. Waypoints Display

As part of your voyage planning, you can load the current route from your GPS to your radar or ARPA.

This makes it easier to navigate and alter courses as you already have the track to follow.

Though the second officer handles this part, all deck officers, including deck cadets, must know how to activate this function.

14. Ground and Sea Stabilisation

Ground stabilization is a mode of display whereby own ship and all targets are referenced to the ground, using ground track or set and drift inputs. Think of this mode as if your vessel has wheels and is moving on the ground.

Sea stabilization is a mode of display whereby own ship and all targets are referenced to the sea, using gyro heading and single axis log water speed inputs.

15. Off-Center Display

Off-center display allows you to move your ship’s position on any part of the PPI screen. It allows you to expand the view field without selecting a larger range scale.

Most navigators use this feature to move the ship a little backward from the center so that it can cover a bigger area in front.

16. Target Data Display

You must know how to interpret target data such as range, bearing, course, speed, CPA (Closest Point of Approach), and TCPA (Time to Closest Point of Approach).

Tracked targets with its information about their range, bearing, CPA, TCPA, course, and speed.
Target data is shown on the left side of the PPI screen.

17. Gain

Gain controls the amplification of all echoes on the screen.

If the gain is set very low, no targets would show up on the PPI. If set too high, clutters and noise would saturate the screen and drown the targets. 

The correct setting of gain is achieved by increasing it until a speckled background of noise (resembling fine sandpaper) is just visible on a medium or long-range scale. 

18. Brilliance

Brilliance is the “brightness” of your PPI screen. Technically, this function controls the strength of the electron stream in the CRT.

If set low, the picture would be dim and may not be visible. If it is too high, the screen will get saturated and will reduce the life of the equipment.

Most ARPA units nowadays have different brilliance controls for their various features.

19. Tracking History and Echo Trails

Tracking history enables an observer to check whether a particular target has maneuvered in the recent past.

The officer may be temporarily away from the display for other bridge duties, so having this feature may help him prepare for what the target may do in the future. 

Meanwhile, echo trails or trails are simulated afterglows of target echoes that represent their past movements.

Like the vectors, you can set the trails to either true or relative.


The STBY/ TX button activates the transmission of the radar pulses or puts the radar into standby mode.

When the vessel arrives in port, the radar is not used and put into standby mode. This is obligatory when you are on a tanker vessel.

After port operations and the vessel is ready to sail, pressing the STBY/TX button allows the scanner to rotate and transmit radar pulses.

Navigating Before Automatic Radar Plotting Aid

In your school, you probably saw an actual radar where you have to “peep” your face into a monitor called a “Plan Position Indicator (PPI)” to see the targets.

Then, you take a plotting sheet and plot the target’s range and bearing at 3, 6, or 12-minute intervals. After three observations, you should be able to manually compute the target’s vector, CPA, TCPA, etc.

Imagine doing this for five or ten targets in areas with heavy traffic!

Radar alone lacks the features of automatic plotting and continuous target monitoring that we have today with ARPA.

Even within congested waters, your situational awareness could be low, but your mental (and nervous) activity may be high!

However, with the integration of Automatic Radar Plotting Aid, a lot of manual activity from the radar was removed, and it was automated using this new technology.

A plotting sheet showing the manual tracking of target vessels before the ARPA's invention.
How seafarers track and analyze a target before the invention of Automatic Radar Plotting Aid.

Difference Between RADAR and ARPA

While RADAR and ARPA are synonymous nowadays, they do have differences that every seafarer, especially deck officers, should know.


  • Radar – Uses radio waves to detect the presence, range, and bearing of objects, such as ships, land masses, or weather systems. It provides a real-time display of targets relative to the radar-equipped vessel.
  • Automatic Radar Plotting Aid – Analyzes radar data to automatically track, process, and interpret target information.


  • Radar – shows a basic picture of contacts.
  • Automatic Radar Plotting Aid – provides processed data like vectors, motion trends, closest point of approach, collision warnings, and target histories.


  • Radar – requires manual plotting of targets to interpret data.
  • Automatic Radar Plotting Aid – incorporates automation to assist navigators in analyzing radar data, identifying potential collision threats, and providing alarms or alerts when necessary.

Target Tracking

  • Radar – provides raw data about detected targets but typically requires manual interpretation by the operator to track and predict the movements of these targets.
  • Automatic Radar Plotting Aid – automates the process of target tracking by continuously monitoring and updating the positions, speeds, and courses of detected targets.

In essence, a radar is a basic tool for detecting and locating targets. 

ARPA leverages marine radar by adding features like automatic tracking, collision avoidance, continuous monitoring, and other functionalities.

While a radar provides limited features, the Automatic Radar Plotting Aid adds more functionalities that enhance situational awareness to prevent collision.

But due to its complication, it’s up to the user, especially the officers, to know these functions to maximize the use of the shipboard ARPA.

May the winds be in your favor.



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