User's Manual Part 2

Operator Manual
3−18
3748DOC020102 Edition: 14.JAN.2005
3.4.2 Collision Assessment (Surveillance)
Once the true velocities of own ship and targets are established (within one
minute of acquisition), it is valid to interpret displayed information for collision
assessment. TRUE and RELATIVE VECTORS are available to present target
velocities.
The NSC provides two additional surveillance features to aid in rapid
assessment of collisions potential in congested situations. When displaying
TRUE VECTORS, the operator may choose to display Potential Collision Point
(PCPs). The appearance of one or more of these symbols, on or near own ship’s
heading line, immediately signals collision potential. When displaying RELATIVE
VECTORS, the operator may choose to display all CPAs within the set CPA limit,
plus one mile of own ship whose TCPA is less than 30 minutes. The appearance
of one or more of these symbols near own ship’s current position immediately
signals collisions potential.
3.4.2.1 Accuracy of Collision Assessment
The position of PCP and CPA are the key parameters in collision assessment
and of vital importance. Both are calculated from knowledge of present position
and velocities of targets and own ship.
3.4.2.2 Displaying of CPAs
CPA is the Closest Point of Approach for any target vessel if both the target
vessel and own ship maintain course and speed. The calculated CPA will match
the actual CPA only if the present course and speed of the target vessel has
been maintained long enough for the target velocity filters to settle. The ARPA
specification allows up to three minutes for this purpose. The NSC accomplishes
this in one minute .
Furthermore, if own ship’s speed−log has no smoothing or delay in its readings,
the NSC will produce an accurate CPA even as own ship comes to a new course
and/or speed (target not maneuvering). CPA and TCPA are calculated for all
targets; once for each revolution of the radar antenna (approximately two
seconds for Raytheon radars).