KUP 23: Ability to operate and to interpret and analyse information obtained from ARPA

KUP 23 Ability to operate and to interpret and analyse information obtained from ARPA, including

1) system performance and accuracy, tracking capabilities and limitations, and processing delays,

2) use of operational warnings and system tests,

3) methods of target acquisition and their limitations,

4) true and relative vectors, graphic representation of target information and danger areas,

5) deriving and analysing information, critical echoes, exclusion areas and trial manoeuvres

Learning objectives:

1. Understand system performance and accuracy, including ARPA’s tracking capabilities, limitations, and processing delays.
2. Gain the ability to use operational warnings and system tests to ensure proper ARPA functioning.
3. Learn the methods of target acquisition and recognize their limitations in various scenarios.
4. Interpret true and relative vectors, graphical target information, and danger areas for safe navigation.
5. Develop skills to derive and analyze critical information, including identifying critical echoes, exclusion areas, and conducting trial maneuvers for collision avoidance.

Content

Introduction

This learning content focuses on enhancing the skills and knowledge required to understand ARPA’s system performance, limitations, and practical application in real-world scenarios. Through this learning content, learners will explore key aspects such as target acquisition, the interpretation of vectors, and the analysis of critical information for collision avoidance. Mastery of ARPA systems will enable maritime professionals to make informed decisions and ensure navigational safety in complex and challenging environments.

1. System performance and accuracy

1.1 Understanding ARPA’s tracking capabilities, processing delays, and accuracy factors.

Tracking capabilities

A marine radar with automatic radar plotting aid (ARPA) capability can create tracks using radar contacts. The system can calculate the tracked object’s course, speed and closest point of approach (CPA), thereby knowing if there is a danger of collision with the other ship or landmass.

Processing delays

When operating the ARPA in Automatic Acquisition mode, the operator must be aware of the following tracking system limitations:

• a. Normally the sensitivity of the ARPA tracking system is reduced when operating in Automatic acquisition mode.

• b. When the ARPA tracking system uses the “guard ring” philosophy, echoes can escape acquisition because the radar at a range closer to the distance to the inner guard ring detects them or the echoes remain between guard rings.
• c. When the “search area” philosophy is used, echoes can escape acquisition because they are outside the specified area or too many echoes are picked up, resulting in system overload.

Accuracy factors

Factors affecting system performance and accuracy:

• Range resolution is the ability of radar system to distinguish between two or more targets on the same bearing but at different ranges.
• The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator.

1.2 Identifying factors that influence ARPA performance, such as radar resolution and sea state.

The performance of a radar system can be judged by the following [1]:

• (1) the maximum range at which it can see a target of a specified size,
• (2) the accuracy of its measurement of target location in range and angle,
• (3) its ability to distinguish one target from another,
• (4) its ability to detect the desired target echo when masked by large clutter echoes, unintentional interfering signals from other “friendly” transmitters, or intentional radiation from hostile jamming (if a military radar),
• (5) its ability to recognize the type of target, and (6) its availability (ability to operate when needed), reliability, and maintainability. Some of the major factors that affect performance are discussed in this section.

2. Operational Warnings and System Tests

2.1 Conduct operational system tests to ensure ARPA reliability.

The method of presentation should ensure that the ARPA data are clearly visible in general to more than one observer in the conditions of light normally experienced or the bridge of a ship by day and by night. Screening may be provided to shade the display from sunlight but not to the extent that it will impair the observers’ ability to maintain a proper lookout. Facilities to adjust the brightness should be provided [2].

2.2 Interpreting and responding to system warnings related to target tracking and accuracy.

Operational Warnings
There are six main situations which cause the Auto Plotter to trigger visual and aural alarms:

On the R-type, an inward or outward guard alarm can be selected on the RADAR 2 menu. On the IMO type, only the inward guard alarm is available. The inward guard alarm generates visual and audible warnings when a target enters the guard zone from any direction. The outward guard alarm is produced when a target leaves the guard zone.

When the predicted CPA of any target becomes smaller than a preset CPA alarm range and its predicted TCPA less than a preset TCPA alarm limit, the ARPA releases an aural alarm and displays the warning label COLLISION on the screen. In addition, the ARPA symbol changes to a triangle and flashes together with its vector.

Silencing CPA/TCPA aural alarm
Press the AUDIO OFF key to acknowledge and silence the CPA/TCPA aural alarm.
The warning label COLLISION and the flashing of the triangle plot symbol and vector remain on the screen until the dangerous situation is gone or you intentionally terminate tracking of the target by using the CANCEL key.

Setting a Guard Zone
When a target transits the operator-set guard zone, the buzzer sounds and the indication GUARD RING appears at the screen bottom. The target causing the warning is clearly indicated with an inverted flashing triangle [7].

3. Understanding target acquisition methods and their limitations

3.1 Automatic and manual target acquisition techniques.

Acquisition may be, manual, in which case the operator, indicates to the computer which targets are to be tracked, or may be ‘automatic’, when the computer is programmed to acquire targets which enter specified boundaries. 

When the ‘acquire’ button is pressed, an area centred on the screen marker is defined within the computer memory.  This area is termed the ‘tracking gate’ or tracking window’.  The gate is made to appear automatically on some ARPA displays; on others, the operator may display it if desired [3].

Example of Auto Acquisition Mode [4]:

In this mode, the acquisition areas are used to initiate tracking, but a target can leave the acquisition area and still be tracked. Restricted Mode (“Restrict ARPA to acquisition area” = ON): ARPA targets can only exist inside an acquisition area(s).

3.2 Limitations of ARPA in detecting and tracking multiple targets in complex traffic situations.

An ARPA assesses the risk of collision, and enables operator to see proposed maneuvers by own ship. While many different models of ARPAs are available on the market, the following functions are usually provided: 5. The ability to perform trial maneuvers, including course changes, speed changes, and combined course/speed changes. 6. Automatic ground stabilization for navigation purposes. ARPA processes radar information much more rapidly than conventional radar but is still subject to the same limitations. ARPA data is only as accurate as the data that comes from inputs such as the gyro and speed log.

Tracking System: Explains processing delay and other important limitations in the system. When operating the ARPA in Automatic Acquisition mode, the operator must be aware of the following tracking system limitations: a.Normally the sensitivity of the ARPA tracking system is reduced when operating in Automatic acquisition mode.

Problems may occur in using ARPA: 

The risks of over-reliance on ARPA: Appreciation that ARPA is only navigational aid and that’s its limitations, including those of its sensors, make over-reliance on the ARPA dangerous in particular for keeping a look-out, the heed to comply at all times with the basic principles and operational guidance for officers in-charged of a navigational watch.

1.  The risks of over-reliance on ARPA are including: impressive system, no system is better that the weakest part, the operator must be aware of the ARPA limitations. An ARPA system in the hand of unqualified personnel is not only dangerous, but can indirectly be the main reason for an accident.

2. Errors and Precautions. Errors in an ARPA system can be divided into groups:

• a. errors in sensors (radar, log, gyro, etc.)
• b. errors in ARPA software c. errors in ARPA hardware
• d. errors in interpretation of the actual display

When working with computerized systems, always remember “Rubbish-in-Rubbish-out” simple as that.

3. Errors in Interpretation of Display: Here are some possible treats:

• a. raster scan ARPA display “lock up”
• b. mixing trial and real time information
• c. wrong speed input or overlooking type of speed input to the ARPA
• d. no correction for gyro course error before input to ARPA
• e. misinterpretation of display symbols may cause severe problems.
• f. operating long periods in “impure presentation” may have serious consequences
• g. exclusive reliance of ARPA will sooner or later give you a problem.

Remember that ARPA is only a navigational aid and that its limitations including those of its sensors, make exclusive use of ARPA dangerous.

4. Automatic Acquisition Precaution: The majority of ARPA systems manufactured today provide and automatic acquisition feature. This feature may reduce the operator’s workload during busy periods and thus contributing possibility to safe sailing. However, the operator should be aware of the fact that most ARPA systems are less sensitive in auto- acquisition mode than in manual acquisition mode. This is one good reason not to rely on the new target warning only, but at regular intervals visually observe the ARPA screen to make sure that all targets are acquired.

5. Factors affecting system performance and accuracy:

• a. Knowledge of ARPA sensor input performance- radar, compass and speed inputs, effects of sensor malfunction on the accuracy of ARPA data.
• b. Effects of the limitations of radar range and bearing discrimination and accuracy, the limitations of compass and speed input accuracy on the accuracy of ARPA data.
• c. Knowledge of factors which influence vector accuracy.

6. Tracking capabilities and limitations

• a. Knowledge of the criteria for the selection of targets by automatic acquisition
• b. Factors leading to the correct choice of targets for manual acquisition
• c. Effects on tracking of “lost” targets and target fading
• d. Circumstances causing “target swoop” and its effects on displayed data

7. Processing delay: The delays inherent in the display of processed ARPA information, particularly on acquisition and re-acquisition or when target maneuvers.

8. When and how to use the operational warnings, their benefits and limitations: Appreciation of the uses, benefits and limitations of ARPA operational warnings, correct setting, where applicable, to avoid spurious interference.

9. System Operational test a. Methods of testing for malfunctions of ARPA systems, including functional self-testing b. Precautions to be taken after a malfunction occur.

10. Manual and automatic acquisition of targets and their respective limitations: Knowledge of the limits imposed on both types of acquisition in multi-target scenarios, effects on acquisition of target fading and target swoop.

11. When and how to use true and relative vectors and typical; graphic representation of target information and danger areas

• a. Thorough knowledge of true and relative vectors, derivation of targets true courses and speeds
• b. Threat assessment; derivation of predicted closest point of approach from forward extrapolation of vectors, the use of graphic representation of danger areas.
• c. Effects of alterations of courses and/or speeds of own ship and/or targets on predicted closest point of approach and predicted time to closest point of approach and danger areas
• d. Effects of incorrect vectors and danger areas
• e. Benefit of switching between true and relative vectors.

12. When and how to use information on past position of targets being tracked: Knowledge of derivation of past positions of targets being tracked, recognition of historic data as means of indicating recent maneuvering of targets and as a method of checking the validity of the ARPA’s tracking.

13. Setting up and maintaining display: Selection of the time scale of vectors/graphics

• a. Use of exclusion areas when automatic acquisition is employed by ARPA
• b. Performance checks of radar, compass, speed input sensors and ARPA

14. System Operational Test: System check and determining data accuracy of ARPA including the trial maneuver facility by checking against basic radar plot.

15. When and how to obtain information from ARPA display: Demonstrate ability to obtain information in both relative and true motion modes of displays including:

• a. Identification of critical echoes
• b. Used of exclusion areas in automatic acquisition mode
• c. Speed and direction of targets relative movement.
• d. Time and predicted range at targets closest point of approach
• e. Course and speed of the targets
• f. Detecting course and speed changes of targets and Limitations of such information.
• g. Effect of changes in own ship’s course or speed or both h. Operation of the trial maneuver.

4. True and relative vectors, graphical target information, and danger area representations

4.1 Understanding the difference between true and relative motion.

There are two basic displays used to portray target position and motion on the PPI’s of navigational radars. The relative motion display portrays the motion of a target relative to the motion of the observing ship. The true motion display portrays the actual or true motions of the target and the observing ship [5].

4.2 Thorough knowledge of true and relative vectors; derivation of targets’ true courses and speeds, including [6]:

• .1 threat assessment, derivation of predicted closest point of approach and predicted time to closest point of approach from forward extrapolation of vectors, the use of graphic representation of danger areas;
• .2 the effects of alterations of course and/or speed of own ship and/or targets on predicted closest point of approach and predicted time to closest point of approach and danger areas;
• .3 the effects of incorrect vectors and danger areas; and
• .4 the benefit of switching between true and relative vectors.

5. Deriving and analyzing critical echoes, exclusion areas, and conducting trial manoeuvres

Identifying critical echoes and analyzing the data for collision avoidance.

Using trial maneuvers to evaluate the effect of course and speed changes on navigational safety.

Understanding exclusion areas and applying them in congested waters for safe navigation.

Recommendation

References

1. Merrill I. Skolnik. “Factors affecting radar performance”. Fact-checked by The Editors of Encyclopaedia Britannica. https://www.britannica.com/technology/radar/Factors-affecting-radar-performance
2. 422(XI) Performance standards for Automatic Radar Plotting Aids (ARPA). https://puc.overheid.nl/nsi/doc/PUC_2474_14/1/
3. thenauticalsite.in Electronic Position Fixing (thenauticalsite.in) http://thenauticalsite.in/NauticalNotes/ARPA/MyARPA-Lesson02-Targets.htm
4. TimeZero User Guide. ARPA Auto Acquisition (mytimezero.com)
5. National Geospatial-Intelligence Agency. “Chapter 2 — radar operation – relative and true motion displays“
6. NAVIGATION VI Operational Use of Radar/ARPA. https://www.slideshare.net/slideshow/nav-6-semi-final-and-finalsppt/256285037
7. National Geospatial-Intelligence Agency. “CHAPTER 5 — AUTOMATIC RADAR PLOTTING AIDS (ARPA) – Radar Navigation and Maneuvering Board Manual“. Maritime Safety Information.