ARPA
Trial Manouevre
The ARPA should be capable ofsimulating the effect on all tracked targets of an own ship manoeuvre withoutinterrupting the updating of target information.
With the availability ofcomputer assistance, the problem of predicting the effect of a manoeuvre priorto its implementation by own ship is much simplified.
While it is relatively easy to visualise mentally the outcome of a manoeuvre where twoships are involved, in dense traffic this becomes very difficult. In particular, with large ships and limitedsea room, it is necessary to plan and update the whole collision avoidancestrategy as quickly as possible in light of the continually changing radarscene.
While planning, it is importantto bear in mind the following points.
Own ship may temporarily need tobe on a ‘collision course’ with more distant vessels, while evading nearertargets.
Extrapolation of the presentsituation using the trial manoeuvre facility with current course and speed asinputs can provide valuable information on which of the ‘other’ vessels in thevicinity may have to manoeuvre in order to avoid collisions between eachother.
Constraints imposed bynavigation may dictate the manoeuvre of ‘other’ vessels. This should be taken into account whenplanning strategy and watched for when carrying out the plan and assessing itseffectiveness.
The ease with which thisfacility allows the navigator to establish the course to steer for a givenpassing distance may encourage the choice of a small alteration. This temptation must be avoided at all costsas it loses sight of the need to make a substantial alteration.
It is important to selectrelative vectors when assessing the effect of a manoeuvre as this will give anindication of how far the target will pass clear. It is also possible to varythe inputs while observing this display and note the effect on the CPA.
In order that there should be noconfusion between the ‘trial’ data and the current situation, when trial is inoperation the screen will display some distinctive indication such as the wordSIM or TRIAL or T.
The ARPA display
The continued availability of radar data in the eventof an ARPA malfunction is mandatory.
The size ofthe display
The size of the display on whichARPA information is presented should have an effective display diameter of atleast 340-mm.
This is equivalent to the normal16-inch radius radial CRT whereas a raster-scan display requires a 27-inch(690-mm) tube.
The range scales on which ARPA facilities should beavailable
The ARPA facilities should beavailable on at least the following range scales:
12 or 16 miles
3 or 4 miles
ARPA facilities are provided onall range scales from 1.5 n miles to 24 n miles inclusive.
The ARPAdata brilliance control
Means should be provided toadjust independently the brilliance of the ARPA data, including completeelimination of the ARPA data.
Unfortunately, many a marinerhas been caught out by this control and has spent some frustrating minutestrying to find the screen marker, only to realise thatthe ARPA data brilliance control was turned down.
The effectof changing range scales
After changing range scales onwhich the ARPA facilities are available or re-setting the display, fullplotting information should be displayed within a period of time not exceedingfour scans.
It should be appreciated that,in order to fulfil this requirement, the ARPA needs to track and plot theacquired targets continually out to some 16 miles, irrespective of the rangescale selected by the operator. Becauseof this, if the shorter range scales are selected and accompanied by a shortpulse, targets at a longer range returning a poor response may be lost.
The displayof alphanumeric data
At the request of the observerthe following information should be immediately available from the ARPA inalphanumeric form in regard to any tracked target.
Present range to the target.
Present bearing of the target.
Predicted target range at theclosest point of approach (CPA).
Predictedtime to CPA (TCPA).
Calculatedtrue course of target.
Calculatedtrue speed of target.
Although vectors are suppressedduring the first minute of tracking, the observer can normally select a targetduring that period and read out the alphanumeric data.
This is acceptable as a means ofquickly obtaining the range and bearing of the target, but it must beappreciated that other alphanumeric values will at that stage be based on onlya few observations and hence can be dangerously misleading.
When trial manoeuvre is selected, some systems continue to provide thereal alphanumeric data while others produce the trial values. In the caseof any given ARPA, it is essential to establish exactly which data are beingmade available.
Alarms andwarnings
It should be possible toactivate or de-activate the operational warnings.
Guard zoneviolation
The ARPA should have thecapability to warn the observer with a visual and/or audible signal of anydistinguishable target, which closes to a range or transits a zone chosen bythe observer.
The target causing the warningshould be clearly indicated on the display.
It is possible to specify anarea in the vicinity of own ship, which, if entered by a target, would activatean 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 bevaried according to, circumstances.
The target, which has activatedthe alarm, may be made to ‘flash’ or alternatively be acquired.
It is important to remember that a target which isdetected for the first time at a lesser range than the guard ring will notactivate the alarm.
This warning system should notbe regarded as an alternative to keeping a proper lookout, but rather as anadditional means of ensuring the safety of the vessel.
In the above, target D will be acquired by ARPAand will sound the alarm as it crosses the outer zone.
Target A, if detected at itspresent position will be acquired once it crosses the inner zone and the alarmwill be activated.
However if target B is detectedat its current position will not be acquired by the ARPA and neither will therebe any alarm.
Similar is the case with targetC.
Predicted CPA/TCPA violation
The ARPA should have thecapability to warn the observer with a visual and/or audible signal of anytracked target, which is predicted, to close to within a minimum range and timechosen by the observer. The targetcausing the warning should be clearly indicated on the display.
It is possible to specify a CPAand TCPA (sometimes referred to as safe limits), which will activate an alarmif both, are violated
Where own ship’s heading markerintersects a predicted area of danger (PAD), a warning will be activated andwill continue until such time as own ship’s course is altered to clear the PAD.
Lost target
The ARPA should clearly indicateif a tracked target is lost, other than out of range. Also the target’s lasttracked position should be clearly indicated on the display.
Consider a target, which isbeing tracked but, for one of a number of reasons does not return a detectableresponse on one scan: the tracker will open up the gate and, if it finds aresponse, will continue to track. If it fails to find a response, it isrequired that the tracker should continue to search for the echo in an areawhere it might be expected for up to five successive scans. If, after this searching, the target is stillnot detected, the ‘target lost’ warning is activated and the last observedposition 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 atarget is lost. The gate having widened to search for the earlier target comesinto contact with another separate target either acquired earlier or acquirednew. The target specification then would be of the new target and not of theoriginal ‘lost’ target.
Performancetests and warnings
The ARPA should provide suitablewarnings of ARPA malfunction to enable the observer to monitor the properoperation of the system. Additionally,test programmes should be available so that theoverall performance of the ARPA can be assessed periodically against a knownsolution.
Connectionswith other equipment
The ARPA should not degrade theperformance of any equipment providing sensor inputs. The connection of the ARPA to any otherequipment should not degrade the performance of that equipment.
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