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9 STANDARDS FOR AUTOMATIC RADAR PLOTTING AIDS (ARPA)

9.1 INTRODUCTION 

9.1.1 Automatic Radar Plotting Aids (ARPA) required by the Navigating Appliances and Equipment Regulations shall, in order to improve the standard of collision avoidance at sea:

1. reduce the work-load of observers by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target;
2. provide continuous, accurate and rapid situation evaluation.

9.2 DEFINITIONS ^

9.2.1 Definitions of terms used in these standards are given in Appendix 1.

9.3 PERFORMANCE STANDARDS ^

9.3.1 Detection

9.3.1.1 Where a separate facility is provided for detection of targets, other than by the radar observer, it shall have a performance not inferior to that which could be obtained by the use of the radar display.

9.3.2 Acquisition

9.3.2.1 Target acquisition may be manual or automatic. However, there shall always be a facility to provide for manual acquisition and cancellation. ARPA with automatic acquisition shall have a facility to suppress acquisition in certain areas. On any range scale where acquisition is suppressed over a certain area, the area of acquisition shall be indicated on the display.

9.3.2.2 Automatic or manual acquisition shall have a performance not inferior to that which could be obtained by the user of the radar display.

9.3.3 Tracking

9.3.3.1 The ARPA shall be able to automatically track, process, simultaneously display and continuously update the information on at least:

1. 20 targets, if automatic acquisition is provided, whether automatically or manually acquired;
2. 10 targets, if only manual acquisition is provided.

9.3.3.2 If automatic acquisition is provided, description of the criteria of selection of targets for tracking shall be provided to the user. If the ARPA does not track all targets visible on the display, targets which are being tracked shall be clearly indicated on the display. The reliability of tracking shall not be less than that obtainable using manual recordings of successive target positions obtained from the radar display.

9.3.3.3 Provided the target is not subject to target swop, the ARPA shall continue to track an acquired target which is clearly distinguishable on the display for 5 out of 10 consecutive scans.

9.3.3.4 The possibility of tracking errors, including target swop, shall be minimized by ARPA design. A qualitative description of the effects of error sources on the automatic tracking and corresponding errors shall be provided to the user, including the effects of low signal-to-noise and low signal-to-clutter ratios caused by sea returns, rain, snow, low clouds and non-synchronous emissions.

9.3.3.5 The ARPA shall be able to display on request at least four equally time-spaced past positions of any targets being tracked over a period of at least eight minutes.

9.3.4 Display

9.3.4.1 The display may be a separate or integral part of the ship's radar. However, the ARPA display shall include all the data required to be provided by a radar display in accordance with the standards for radar.

9.3.4.2 The design shall be such that any malfunction of ARPA parts producing data additional to information to be produced by the radar as required by the standards for radar shall not affect the integrity of the basic radar presentation.

9.3.4.3 The display on which ARPA information is presented shall have an effective diameter of at least 340 mm.

9.3.4.4 The ARPA facilities shall be available on at least the following range scales:

1. 12 or 16 miles;
2. 3 or 4 miles.

9.3.4.5 There shall be a positive indication of the range scale in use.

9.3.4.6 The ARPA shall be capable of operating with a relative motion display with "north-up" and either "head-up" or "course-up" azimuth stabilization. In addition, the ARPA may also provide for a true motion display. If true motion is provided, the operator shall be able to select for his display either true or relative motion. There shall be a positive indication of the display mode and orientation in use.

9.3.4.7 The course and speed information generated by the ARPA for acquired targets shall be displayed in a vector or graphic form which clearly indicates the target’s predicted motion. In this regard:

1. ARPA presenting predicted information in vector form only shall have the option of both true and relative vectors;
2. an ARPA which is capable of presenting target course and speed information in graphic form shall also, on request, provide the target's true and/or relative vector;
3. vectors displayed shall either be time adjustable or have a fixed time-scale;
4. a positive indication of the time-scale of the vector in use shall be given.

9.3.4.8 The ARPA information shall not obscure radar information in such a manner as to degrade the process of detecting targets. The display of ARPA data shall be under the control of the radar observer. It shall be possible to cancel the display of unwanted ARPA data.

9.3.4.9 Means shall be provided to adjust independently the brilliance of the ARPA data and radar data, including complete elimination of the ARPA data.

9.3.4.10 The method of presentation shall ensure that the ARPA data are clearly visible in general to more than one observer in the conditions of light normally experienced on 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 shall be provided.

9.3.4.11 Provisions shall be made to obtain quickly the range and bearing of any object which appears on the ARPA display.

9.3.4.12 When a target appears on the radar display and, in the case of automatic acquisition, enters within the acquisition area chosen by the observer or, in the case of manual acquisition, has been acquired by the observer, the ARPA shall present in a period of not more than one minute an indication of the target's motion trend and display within three minutes the target's predicted motion in accordance with paragraphs 9.3.4.7, 9.3.6, 9.3.8.2 and 9.3.8.3.

9.3.4.13 After changing range scales on which the ARPA facilities are available or resetting the display, full plotting information shall be displayed within a period of time not exceeding four scans.

9.3.5 Operational warnings

9.3.5.1 The ARPA shall have the capability to warn the observer with a visual and/or audible signal of any distinguishable target which closes to a range or transits a zone chosen by the observer. The target causing the warning shall be clearly indicated on the display.

9.3.5.2 The ARPA shall have the capability to warn the observer with a visual and/or audible signal of any tracked target which is predicted to close to within a minimum range and time chosen by the observer. The target causing the warning shall be clearly indicated on the display.

9.3.5.3 The ARPA shall clearly indicate if a tracked target is lost, other than out of range, and the target's last tracked position shall be clearly indicated on the display.

9.3.5.4 It shall be possible to activate or de-activate the operational warnings.

9.3.6 Data requirements

9.3.6.1 At the request of the observer the following information shall be immediately available from the ARPA in alphanumeric form in regard to any tracked target:

1. present range to the target;
2. present bearing of the target;
3. predicted target range at the closest point of approach (CPA);
4. predicted time to CPA (TCPA);
5. calculated true course of target;
6. calculated true speed of target.

9.3.7 Trial Manoeuvre

9.3.7.1 The ARPA shall be capable of simulating the effect on all tracked targets of an own ship manoeuvre without interrupting the updating of target information. The simulation shall be initiated by the depression either of a spring-loaded switch, or of a function key, with a positive identification on the display.

9.3.8 Accuracy

9.3.8.1 The ARPA shall provide accuracies not less than those given in paragraphs 9.3.8.2 and 9.3.8.3 for the four scenarios defined in Appendix 2. With the sensor errors specified in Appendix 3, the values given relate to the best possible manual plotting performance under environmental conditions of plus and minus ten degrees of roll.

9.3.8.2 An ARPA shall present within one minute of stead state tracking the relative motion trend of a target with the following accuracy values (95 per cent probability values).

 

9.3.8.3 An ARPA shall present within three minutes of steady state tracking the motion of a target with the following accuracy values (95 per cent probability values).

9.3.8.4 When a tracked target, or own ship, has completed a manoeuvre, the system shall present in a period of not more than one minute an indication of the target's motion trend, and display within three minutes the target's predicted motion, in accordance with paragraphs 9.3.4.7, 9.3.6, 9.3.8.2 and 9.3.8.3.

9.3.8.5 The ARPA shall be designed in such a manner that under the most favourable conditions of own ship motion the error contribution from the ARPA shall remain insignificant compared to the errors associated with the input sensors, for the scenarios of Appendix 2.

9.3.9 Connexions with other equipment

9.3.9.1 The ARPA shall not degrade the performance of any equipment providing sensor inputs. The connexion of the ARPA to any other equipment shall not degrade the performance of that equipment.

9.3.10 Performance tests and warnings

9.3.10.1 The ARPA shall provide suitable warnings of ARPA malfunction to enable the observer to monitor the proper operation of the system. Additionally test programmes shall be available so that the overall performance of ARPA can be assessed periodically against a known solution.

9.3.11 Equipment used with ARPA

9.3.11.1 Log and speed indicators providing inputs to ARPA equipment shall be capable of providing the ship's speed through the water.

9.4 INTERNATIONAL STANDARD ^

9.4.1 The International Maritime Organization Resolution "A.422(XI) Performance Standards for Automatic Radar Plotting Aids (ARPA)" is the adopted standard.

9.5 EFFECTIVE DATE ^

9.5.1 This Standard comes into force on 1 September 1985.

Appendix 1 ^

DEFINITIONS OF TERMS TO BE USED ONLY IN CONNEXION WITH ARPA STANDARDS

Appendix 2 ^

OPERATIONAL SCENARIOS

For each of the following scenarios predictions are made at the target position defined after previously tracking for the appropriate time of one or three minutes:

Appendix 3 ^

SENSOR ERRORS

The accuracy figures quoted in paragraph 9.3.8 are based upon the following sensor errors and are appropriate to equipment complying with the Standards for Navigating Appliances and Equipment.

Note: s means "standard deviation".

RADAR ^

Target glint: (Scintillation) (for 200 m length target)

Along length of target s = 30 metres (normal distribution)

Across beam of target s = 1 metre (normal distribution)

Roll-pitch bearing: The bearing error will peak in each of the four quadrants around own ship for targets on relative bearings of 045° , 135° , 225° and 315° and will be zero at relative bearings of 0° , 90° , 180° and 270° . This error has a sinusoidal variation at twice the roll frequency.

For a 10° roll the mean error is

0.22° with a 0.220 peak sine wave superimposed.

Beam shape: - assumed normal distribution giving bearing error with s  = 0.05°.

Pulse shape: - assumed normal distribution giving range error with s  = 20 metres.

Antenna backlash: - assumed rectangular distribution giving bearing error ± 0.5° maximum.

Quantization

Bearing - rectangular distribution ± 0.01° maximum.

Range - rectangular distribution ± 0.01 nautical miles maximum.

Bearing encoder assumed to be running from a remote synchro giving bearing errors with a normal distribution s = 0.03°.

GYRO COMPASS ^

Calibration error 0.5° .

Normal distribution about this with s = 0.12° .

LOG ^

Calibration error 0.5 knots.

Normal distribution about this, 3s = 0.2 knots.

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