8.3 The Graphic Display of PPCs and PADs

The Predicted Point of Collision

• The Predicted point of Collision (PPC) is that the point towards which the observing ship should steer at her present speed (assuming that the target does not manoeuver) in order for a collision to occur.

Some Designers have recognized that the positions of these points can be quickly calculated and displayed for all tracked targets. The arguments made for displaying the PPCs. They do not give the indication of miss distance (other than zero CPA case) and any attempt to extrapolate the clearing distance either side of the point will be fraught with danger. A safe course is one which, among other things, results in passing at a safe distance, which implies knowledge of clearing distance. Safe and effective use of PPCs depends upon a thorough understanding of the factors which affect their location and movement. As is evident from the following treatment this is, in many cases, not a simple matter. Some systems make it possible to display these points as small circles when, but only when, true vectors are selected.

Exercise for Predicted Point of Collision

PAD (Predicted Area of Danger)

• The area within the figure is to be avoided to achieve at least the chosen passing distance and is referred to as predicted area of danger or PAD. The PAD approach is an extremely elegant solution to the problem of how best to present collision avoidance data. It is essential that the user has a thorough understanding of the principles underlying the presentation with particular user has a thorough understanding of the principles underlying the presentation with particular reference to the location, movement, shape of the PAD. As will be seen from the following explanation, this is not a simple subject.

Exercise for Predicted Area of Danger

8.4 The Threat of Collision by-forward Extrapolation of Vectors and by the Use of PADs

Assessing the Threat of Collision

• The ARPA should be capable of simulating the effect on all tracked targets of an own ship manoeuvre without interrupting the updating of target information. The simulation should be initiated by the depression either of a spring-loaded switch or of a function key with a positive identification on the display. With the availability of computer assistance, the problem of predicting the effect of a manoeuvre prior to its implementation by own ship is much simplified.

• While it is relatively easy to visualize mentally the outcome of a manoeuvre where two ships are involved, in dense traffic this becomes very difficult. In particular, with large ships and limited sea room, it is necessary to plan and update the whole collision avoidance strategy as quickly as possible in light of the continually changing radar scene.

While Planning, it is important to bear in mind the following points:

1. Own ship may temporarily need to be on a ‘collision course’ with more distant vessels, i.e. collisions may require sequential avoidance since it is unlikely that a single manoeuvre will resolve all the problems.

2. Extrapolation of the Present situation using the trial manoeuvre facility with current course and speed as inputs can provide valuable information on which of the ‘other’ vessels in the vicinity may have to manoeuvre in order to avoid collision between each other. Obvious avoiding manoeuvres may present themselves and should be watched for.

3.Constraints imposed by navigation may dictate the manoeuvre of ‘other’ vessels. This should be taken into account when planning strategy and watched for when carrying out the plan and assessing its effectiveness.

4. The ease with which this facility allows the navigator to establish the course to steer for a given passing distance may encourage the choice of a small alteration. This temptation must be avoided at all costs as it loses sight of the need to make a substantial alteration. The rules require the latter in recognition of the fact that the other vessels may be using much more rudimentary methods of data extraction and may not be able to detect a small manoeuvre.

8.5 How past positions of targets are displayed.

• ‘True history’ is without doubt the only meaningful way in which this data can be displayed since the nature of the maneuver is readily apparent. A curve in the trail indicates an alteration of course by the target, whereas a change in the spacing of the dots will occur when there has been a change in speed. Since changes in speed are much slower to take effect, they are consequently more difficult to detect.

• The ARPA should 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. This enables an observer to check whether a particular target has maneuvered in the recent past, possibly while the observer was temporarily away from the display on other bridge duties.

8.6 The results of trial maneuvers are approximations depending on the model of

“own ship” maneuvering characteristics

• The results of trial maneuvers are approximations depending on the model of “own ship” maneuvering characteristics and the speed and the course alteration input. Although there is a single requirement for all approved ARPAs to possess a facility for simulating a trial maneuver, different methods of providing this has been devised by the various manufacturers.

• It is important to select relative vectors when assessing the effect of maneuver as this will give an indication of how far the target will pass clear. It is also possible to vary the inputs while observing this display and note the effect on the CPA.

• This is a range of facilities available, with an increasing number of factors taken into account when presenting the trial data. In the simplest form, it is possible to feed in only the intended course and speed and observe their effect on the display.

• There is a provision in some equipment for the maneuver to be instantaneous, but at some later time, e.g. after a delay of some 6 minutes. In some ARPAs it is possible for the vessel’s handling characteristics to be included in the evaluation, but this will of necessity be restricted to one (possibly two) conditions of loading.

• In order that there should be no confusion between the trial data and the current situation, when trial is in operation the screen will display some distinctive indication such as the word SIM or TRIAL. The use of a ‘T’ to indicate trial is frequently mistaken for an indication that true vectors are being displayed. The letter ‘T’ as it stands is meaningless and has not been particularly helpful.

• There is the danger that one officer sets up display and another officer (or the master) observes it without realizing the special nature of display.

• The provision of the spring loaded switch, recommended in the specification, means that the observer has to make a positive decision to operate the switch and hold it over while he observes the display, few systems offer such a failsafe control.

• Note: 1.while trial maneuvers are being presented on the display, the computer continues its normal task of tracking all acquired targets,

The purpose of referring to the equipment manual is to provide a sample of the technical instruction that should be available to the officer. As a radar observer you should throughly read and understand the operating instructions for the radar units that you will be using. Operating instructions will of course differ not only between different radar manufacturers but also with different models for the same manufacturer.

As with all equipment, the operator should be completely familiar with safety instructions prior to turning on the radar.

8.7 The Need to Refer to the Equipment Manual for Description of the Manoeuvring Characteristics Model Used.

Collision Alert Visual and aural alarms are generated when the predicted CPA

and TCPAof any target become less than their preset limits. Press the AUDIO OFF key to acknowledge and silence the CPA/TCPA aural alarm.

Guard Ring/Zone alarm Visual and audible alarms are generated when a target transmits

the operator-set guard zone. Press the AUDIO OFF key to acknowledge and silence the guard zone audible alarm.

8.8 Setting and Acknowledgment of Operational Warning

Lost Target Alarm When the system detects a loss of a tracked target, the target

symbol becomes a flashing diamond. and the label “LOST” appears at the screen bottom. At the same time, an aural alarm is produced for one second.

Press the LOST TARGET key to acknowledge the lost target alarm. Then, the lost target mark disappears.

Target full alarm When the memory becomes full, the memory full status is

indicated and the relevant indication appears on the screen and a short beep sounds.

The anchor watch feature helps you monitor whether own ship is dragged by wind and/or tide while at anchor. This feature requires ship position data from a suitable radio navigational aid. Provided that own ship’s physical data has been entered, an own ship mark can be displayed when the anchor watch feature is activated. The message “ANCHOR WATCH ERR” appears in red when position data is not inputted.

Anchor Watch Alarm

Activating anchor watch1. On the ANCHOR WATCH menu, press the (2) key to select menu item 2ANCHOR WATCH OFF/ON.2. Further press the (2) key to select (or highlight) ON, followed by theENTER key to conclude your selection. The label WATCH appears at thelower left corner of the screen.3. Press the (3) key to select menu item 3 ALARM OFF/ON. Further pressthe (3) key to select (or highlight) ON or OFF, followed by the ENTERkey to conclude your selection. (This operation determines whether toactivate the anchor watch audible alarm).

Manually acquired targets

The indication “MAN TARGET FULL” appears at the screen bottom and

a short beep tone sounds when the number of manually acquired targets

reaches 20 or 40 depending on whether auto acquisition is activated or not.

Automatically acquired targets

The indication “AUTO TARGET FULL” appears at the screen bottom and

a short beep tone sounds when the number of automatically acquired targets

reaches 20.

System failure alarm

When the ARP board receives no signal input from the radar or external equipment, the screen shows both “SYSTEM FAIL” associated with an indication denoting offending equipment, also releasing an aural alarm.

8.9 The Benefits and Limitations of Operational Warnings

ARPA Alarms/Warnings

There are several visual and aural alarms that an ARPA unit can provide the operator to alert him from any danger, or when an immediate operator’s attention is needed in order to avoid any danger. Usually the operator himself presets these alarms, thus, these alarms may be different whenever different operators are using the equipment.

Again, erroneous inputs will produce, .of course, erroneous results. Extreme care should be considered when altering these settings.

The following alerts/alarms are just some of the available nowadays. There are probably more, as newer and more advanced features are being installed on modern ARPA units.

8.10 Using Area Rejection Boundaries where Appropriate to Avoid Spurious Interference

Guard Rings and Area Rejection Boundaries (ARBs)

With this method of acquisition, the usual provision is for up to two ‘rings’ (of predetermined depth) plus up two area rejection boundaries (ARBs). When a target is automatically acquired in a guard zone/guard area, it is usual for an alarm to be activated to attract the operator’s attention. The target activating the alarm will be indicated on the screen by, for example, a flashing symbol.

Setting of the zones/ areas and in general, automatic acquisition has not been as successful as was at first predicted. There is a tendency to acquire sea clutter, rain clutter, noise and interference, while disassociated elements of land echoes will very quickly fill up the available tracking channels. Land echoes can be excluded by careful ARBs, but spurious targets, after having been acquired, are quickly lost and the ‘lost target’ alarm can sound continually.

While it is argued that automatic will reduce the operator’s workload, in practice there is a tendency for it to acquire spurious targets, also to ‘over acquire’ and so clutter the screen with unnecessary and unwanted vectors. This has led to auto-acquisition falling out of favor. Enquiries have indicated that it is rarely used in areas of high-density traffic, but can be useful on long ocean passages where the number of targets is small and there is the danger of loss of concentration by the officer of the watch due to boredom.

Manual acquisition can be very quick and also selective and hence the perceived need for automatic acquisition has not really materialized. Guard zones/areas should be regarded as an additional, rather than an alternative means of keeping a proper lookout.