2. 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.
4. 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.
6. 8.4 The Threat of Collision by-forward
Extrapolation of Vectors and by the Use of
PADs
7. 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.
8. 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.
10. • ‘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.
11. • 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.
12. 8.6 The results of trial maneuvers are
approximations depending on the model of
“own ship” maneuvering characteristics
13. • 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.
14. • 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.
15. • 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.
16. • 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,
17. 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.
18. Collision Alert
Visual and aural alarms are generated when the predicted CPA
and TCPA
of 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.
19. 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.
20. 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.
21. Activating anchor watch
1. On the ANCHOR WATCH menu, press the (2) key to select menu item
2
ANCHOR WATCH OFF/ON.
2. Further press the (2) key to select (or highlight) ON, followed by the
ENTER key to conclude your selection. The label WATCH appears at the
lower left corner of the screen.
3. Press the (3) key to select menu item 3 ALARM OFF/ON. Further press
the (3) key to select (or highlight) ON or OFF, followed by the ENTER
key to conclude your selection. (This operation determines whether to
activate the anchor watch audible alarm).
22. 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.
23. 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.
25. 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.
26. 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.
27. 8.10 Using Area Rejection
Boundaries where Appropriate to
Avoid Spurious Interference
28. 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.
29. 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.
30. 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.
31. 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.
