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.
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.