
APRA-SPECIFIED FACILITIES
209
The history dots can be likened to a vessel dropping
buoys (with radar reflectors) at
equal intervals
of time, thus
a change in the spacing is indicative of a change in the
target's speed but it is important to appreciate the sig-
nificance of the distance between the echo and the last
dot. For example, in Figure 4.10, target A has not just
executed a crash stop but has just dropped a buoy and is
now steaming away from it
When history is requested, only that of tracked tar-
gets can be displayed and even then the number of dots
which are displayed will depend upon the period for
which a particular target has bee