ARPA
Trial Manouevre
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.
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 visualise 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.
Own ship may temporarily need to
be on a ‘collision course’ with more distant vessels, while evading nearer
targets.
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 collisions between each
other.
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.
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.
It is important to select
relative vectors when assessing the effect of a manoeuvre 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.
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 or T.
The ARPA display
The continued availability of radar data in the event
of an ARPA malfunction is mandatory.
The size of
the display
The size of the display on which
ARPA information is presented should have an effective display diameter of at
least 340-mm.
This is equivalent to the normal
16-inch radius radial CRT whereas a raster-scan display requires a 27-inch
(690-mm) tube.
The range scales on which ARPA facilities should be
available
The ARPA facilities should be
available on at least the following range scales:
12 or 16 miles
3 or 4 miles
ARPA facilities are provided on
all range scales from 1.5 n miles to 24 n miles inclusive.
The ARPA
data brilliance control
Means should be provided to
adjust independently the brilliance of the ARPA data, including complete
elimination of the ARPA data.
Unfortunately, many a mariner
has been caught out by this control and has spent some frustrating minutes
trying to find the screen marker, only to realise that
the ARPA data brilliance control was turned down.
The effect
of changing range scales
After changing range scales on
which the ARPA facilities are available or re-setting the display, full
plotting information should be displayed within a period of time not exceeding
four scans.
It should be appreciated that,
in order to fulfil this requirement, the ARPA needs to track and plot the
acquired targets continually out to some 16 miles, irrespective of the range
scale selected by the operator. Because
of this, if the shorter range scales are selected and accompanied by a short
pulse, targets at a longer range returning a poor response may be lost.
The display
of alphanumeric data
At the request of the observer
the following information should be immediately available from the ARPA in
alphanumeric form in regard to any tracked target.
Present range to the target.
Present bearing of the target.
Predicted target range at the
closest point of approach (CPA).
Predicted
time to CPA (TCPA).
Calculated
true course of target.
Calculated
true speed of target.
Although vectors are suppressed
during the first minute of tracking, the observer can normally select a target
during that period and read out the alphanumeric data.
This is acceptable as a means of
quickly obtaining the range and bearing of the target, but it must be
appreciated that other alphanumeric values will at that stage be based on only
a few observations and hence can be dangerously misleading.
When trial manoeuvre is selected, some systems continue to provide the
real alphanumeric data while others produce the trial values. In the case
of any given ARPA, it is essential to establish exactly which data are being
made available.
Alarms and
warnings
It should be possible to
activate or de-activate the operational warnings.
Guard zone
violation
The ARPA should 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
should be clearly indicated on the display.
It is possible to specify an
area in the vicinity of own ship, which, if entered by a target, would activate
an alarm.
It is usual to have two zones,
one, which may be at some pre-set range and the other at a range, which may be
varied according to, circumstances.
The target, which has activated
the alarm, may be made to ‘flash’ or alternatively be acquired.
It is important to remember that a target which is
detected for the first time at a lesser range than the guard ring will not
activate the alarm.
This warning system should not
be regarded as an alternative to keeping a proper lookout, but rather as an
additional means of ensuring the safety of the vessel.
In the above, target D will be acquired by ARPA
and will sound the alarm as it crosses the outer zone.
Target A, if detected at its
present position will be acquired once it crosses the inner zone and the alarm
will be activated.
However if target B is detected
at its current position will not be acquired by the ARPA and neither will there
be any alarm.
Similar is the case with target
C.
Predicted CPA/TCPA violation
The ARPA should 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 should be clearly indicated on the display.
It is possible to specify a CPA
and TCPA (sometimes referred to as safe limits), which will activate an alarm
if both, are violated
Where own ship’s heading marker
intersects a predicted area of danger (PAD), a warning will be activated and
will continue until such time as own ship’s course is altered to clear the PAD.
Lost target
The ARPA should clearly indicate
if a tracked target is lost, other than out of range. Also the target’s last
tracked position should be clearly indicated on the display.
Consider a target, which is
being tracked but, for one of a number of reasons does not return a detectable
response on one scan: the tracker will open up the gate and, if it finds a
response, will continue to track. If it fails to find a response, it is
required that the tracker should continue to search for the echo in an area
where it might be expected for up to five successive scans. If, after this searching, the target is still
not detected, the ‘target lost’ warning is activated and the last observed
position of the echo is marked on the screen.
It is also normal to activate an audible alarm.
A double effect of target ‘Lost’
with a target ‘swop’ also may take place when a
target is lost. The gate having widened to search for the earlier target comes
into contact with another separate target either acquired earlier or acquired
new. The target specification then would be of the new target and not of the
original ‘lost’ target.
Performance
tests and warnings
The ARPA should provide suitable
warnings of ARPA malfunction to enable the observer to monitor the proper
operation of the system. Additionally,
test programmes should be available so that the
overall performance of the ARPA can be assessed periodically against a known
solution.
Connections
with other equipment
The ARPA should not degrade the
performance of any equipment providing sensor inputs. The connection of the ARPA to any other
equipment should not degrade the performance of that equipment.