The acquisition of targets

Acquisition may be, manual, in which case the operator, indicates to the computer which targets are to be tracked, or may be ‘automatic’, when the computer is programmed to acquire targets which enter specified boundaries. 

When the ‘acquire’ button is pressed, an area centred on the screen marker is defined within the computer memory.  This area is termed the ‘tracking gate’ or tracking window’.  The gate is made to appear automatically on some ARPA displays; on others, the operator may display it if desired.

The acquisition specification

There should always be a facility to provide for manual acquisition and cancellation.  ARPAs with automatic acquisition should have a facility to suppress acquisition in certain areas.  On any range scale where acquisition is suppressed over a certain area, the area of acquisition should be indicated on the display.

If automatic acquisition is provided, a description of the criteria of selection of targets for tracking should be provided to the user.

The main problem with automatic acquisition is that the ‘sensitivity’ of the detection circuitry, if set too high, will acquire thermal noise and clutter, leading to false alarms, while if its sensitivity is reduced, poor response targets can evade detection.

Guard rings

With this method of acquisition, the usual provision is for up to two ‘rings’ (of predetermined depth).  The operator may position the rings.

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.

In general, automatic acquisition has not been successful.  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 setting of the zones, but spurious targets (e.g. clutter), after having been acquired, are quickly lost and the ‘lost target’ alarm can sound continually.

While it is argued that automatic acquisition 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 favour. 

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.

Guard zones should be regarded as an additional, rather than an alternative means of keeping a proper lookout.

The tracking of targets

The tracking specification

In many cases it may be obvious that a target is being tracked by virtue of the fact that a vector will indicate its predicted movement.

However, the need for tracked targets to be clearly indicated on the display is important because in the early stages (up to about one minute) of tracking a fresh target, in most systems the vector is suppressed because the available data is unlikely to be sufficiently accurate or stable. 

Furthermore, in certain cases, even when the vector is present it may have zero length (e.g. the true vector of a stationary target or the relative vector of a target on the same course and speed as the observing vessel).

The number of targets to be tracked

The ARPA should be able to automatically track, process, simultaneously display and continuously update the information on at least:

Twenty targets, if automatic acquisition is provided, whether automatically or manually acquired.

Ten targets, if only manual acquisition is provided.

It has been found that an excess of vectors can produce ‘ARPA clutter’ and be counter productive.

It should be noted that a higher tracking capability is required by the Performance Standard where the manufacturer has elected to provide automatic acquisition.

Target loss

Provided the target is not subject to target swop, the ARPA should continue to track an acquired target, which is clearly distinguishable on the display for 5 out of 10 consecutive scans.

The term scan tends to be used rather loosely in radar terminology.  Sometimes it is used to describe one line, as in the term ‘interscan period’; while on other occasions it refers to one aerial rotation.  In the above context it refers to the latter.)

It should be noted here that if, for some reason, a response from a tracked target is not received on a particular scan, the ARPA must not immediately declare the target lost. 

Also it is implied that some form of  ‘search’ for it must take place, e.g. by opening the tracking gate.

Target swop

Target swop is likely when two targets respond within the tracking gate at the same time.  When this happens, the tracker can become confused and the vector(s) may transfer to the wrong target. 

To minimize this problem, the gate should be made as small as possible, the movement of the target should be predicted and the gate moved on at each scan as described under ‘rate aiding’.

The two requirements that target swop be minimised by the ARPA design and that tracking be continued even if no response is received for a period of time are thus to some extent achieved by the common solution of rate aiding.

For the observer, since the size of the gate is beyond his control, the only way left out to him is to be prepared for a ‘swop’ by monitoring visually as two targets close in. If this were left to the ARPA, then in the advent of a swop, the observer would take the readings of a wrong target and complacency can set in.


The course and speed information generated by the ARPA for acquired targets should be displayed in a vector or graphic form, which clearly indicates the target’s predicted motion.  In this regard:

ARPA presenting predicted (extrapolated) information in vector form only should have the option of true and relative vectors.

An ARPA which is capable of presenting target course and speed information in graphic form should also, on request, provide the target’s true and/or relative vector.

Vectors displayed should either be time adjustable or have a fixed time-scale.

A positive indication of the time-scale of the vector in use should be given.

Vectors must be capable of indicating the rate and direction of the target’s relative motion (relative vectors), or indicating the rate and direction of the target’s proper motion (true vectors). 

In all cases, the displayed vector length is time related.

The fixed physical length generally remains the same irrespective of the range scale, e.g. 3 minutes on the 6 n mile range scale, 6 minutes on the 12 n mile range scale, etc.

Note: True vectors can be selected to appear on a relative motion presentation and vice versa. 

Relative vectors

The ARPA must track the target(s) for a period of time, after which a vector can be displayed. Using the vector length control, the vectors can be extended to determine the CPA by observation against the background of the range rings and the TCPA can be read off from the vector length control.

True vectors

As an alternative, the observer may request that the true vector(s) be displayed. In this case, own ship will also have a vector, which will increase in length as the time control is increased. 

The likelihood of a close quarter’s situation developing can be ascertained by running out the true vectors progressively to show the predicted development of the encounter. 

The dynamic nature of this technique appeals to many users but it must be borne in mind that any evaluation of CPA/ TCPA is a matter of trial and error and thus better avoided.  It is essential to appreciate that the CPA is not represented by the point at which the target’s true vector intersects own ship’s true vector, except in the case of zero CPA.