Small Boat RADAR Exercise - The Natale Method of Determining CPA and TCPA
How to use a Floating EBL (FEBL) and Floating VRM (FVRM) to determine a contact's Closest
Point of Approach (CPA) and Time of Closest Point of Approach (TCPA).
Rule 7 of the Navigation Rules states that proper use shall be made of RADAR equipment if
fitted and operational. Rule 7 also requires RADAR plotting or equivalent systematic observation
of detected objects. The exercise below does not suggest that contacts always be analyzed in
this manner but it simply promotes a method of introducing the next generation of coxswains to
the concept of relative motion and associated terminology. Understanding this concept may
lead them to develop a systematic method of analyzing contacts of their own. Preventing
collisions at sea is what this is all about.
Contact and own ship to maintain course and speed for this exercise.
1. In open water, select a contact with a decreasing range that is likely to maintain course and a
speed of 15 knots or less.
2. Place the origin of the FEBL/FVRM at the contact and note the time (start your stopwatch).
3. As the 06 minute mark approaches, rotate the FEBL around to connect the contact's original
position and its current position at minute 06. The FEBL now represents what is known as the
Relative Motion Line (RML). The contact will now move along this line as long as both the
contact and own ship maintain course and speed. The FEBL now indicates whether the contact
will pass ahead or astern and it also gives you a fairly accurate idea of CPA that will be refined in
steps 4 and 6.
4. Using a VRM originating at own ship, expand it out to the FEBL (this is the RML created in
step 3). This distance is the contact's CPA (step 6 may provide a more accurate CPA but for now
this will do).
5. Expand the FVRM out from the contact's original position at minute 00 to its position at
minute 06. This distance now represents a distance traveled in 6 minutes which can easily be
converted into the Speed of Relative Motion (SRM) using the 6 Minute Rule. Note: This is not
the contact's speed but the algebraic sum of own ship’s course and speed + contact's course and
speed (more on vectors some other day).
6. Using an EBL originating at own ship, rotate it around until it is perpendicular to the FEBL. The
intersection created and the VRM from step 4, are now used to determine a more precise CPA.
It will also be used to calculate the Time of CPA, or TCPA. Here's how: Using the FVRM, measure
the distance from contact's original position at minute 00 to CPA. Knowing this distance and the
SRM calculated in step 5 will allow you to now solve for TCPA (multiply the total distance by 60
then divide by SRM = total time in minutes to CPA. Now add total time in minutes to the time
this exercise commenced and your answer is the actual Time of CPA, or TCPA).