Q: Would you explain what is meant by running CPA?

A: Called the Radar Dead Reckoning /Running CPA method, it satisfies the need both to navigate safely and to assess the risk of collision effectively in a narrow channel or inland waterway. This method has some merit in open waters as well. For example, Running CPA enhances the rapid interpretation of the ARPA display in congested lakes, bays, or sounds.

Most passing approaches between vessels on the U.S. Inland Waterways or Western Rivers involve meeting or overtaking situations with parallel banks on each side of the channel line or defined narrow channels.  Rarely do pilots have the time to complete a conventional radar plot.  In the Radar Dead Reckoning/Running CPA technique, radar is used as a tool of collision avoidance to aid the pilot in determining visually, by quick-eye reference, changing bearings and ranges to approaching vessels.  It is a method by which the pilot can “set and forget” while using the accuracy of the radar without further adjustment.

When a mariner is using the Radar Dead Reckoning/Running CPA technique, time-consuming recognition of the distortion effects of relative motion on radars operated in the heads-up, unstabilized, relative-motion can be overcome by a quick look at the radar image rather than a formal plotting analysis.  In addition, the technique can give the pilot feedback on the effects of current or wind set in a manner similar to the feedback obtained from parallel indexing.

Radar Dead Reckoning/Running CPA serves the pilot by identifying when a risk threshold is crossed in a vessel-to-vessel approach situation.  The pilot, by employing this technique, can determine when a risk-of-collision situation transitions into a close-quarters situation with two possible outcomes.  This is accomplished in spite of course changes or the distortion effects of relative motion. 

Obviously, the desired outcome would be a stabilized approach that is monitored by the lookout or lookouts until both vessels are finally “past and clear” (Rule 8(d)).  The undesirable outcome would be both vessels In Extremis with the possible outcome of collision or near miss.  In either case, Radar Dead Reckoning / Running CPA would provide an equivalent systematic observation by providing feedback of bearing drift, or the lack of it, range change and a clear indicator of when assessment should turn into action.

Rule 7 requires “long range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects.”  The author maintains that this method of observation and measurement meets the requirements of a systematic observation equivalent to a radar-collision-avoidance plot in circumstances found in narrow channels.  Bearings and ranges to targets are thus visually observed and measured by quick-eye references on radar during vessel-to-vessel approach situations in narrow channels. This assertion takes into consideration the following factors:

• To qualify Radar Dead Reckoning/Running CPA further as an equivalent systematic observation, it should be noted that the basic elements of a radar plot are bearing and range measurements coupled with the establishment of a minimum CPA (safe passing distance).

• Radar Dead Reckoning/Running CPA replicates and measures all of the basic elements of a relative motion plot without working out the space-limiting and time-consuming mechanics. 

• The Running CPA established ahead or astern of own ship predetermines a safe passing distance as required by NavRule 8(d).

NavRule 19 (d) requires that “a vessel which detects by radar alone the presence of another vessel shall determine if a close quarters situation is developing or risk of collision exists.” Radar Dead Reckoning/Running CPA sets up a system of bearing and range observations by quick-eye references, including a determination by the pilot on watch when risk of collision transitions into  close quarters.

Six minutes is 1/10th of an hour. Using this six-minute rule of navigation, the variable range marker (VRM) is extended to a distance based upon own vessel’s speed over ground.

For example, if the speed is 5 mph, extend out the VRM for 0.5 miles.  This creates a radial approach CPA (ACPA) and a DR position in six minutes at the intersection of the VRM and header flash (marker) giving the pilot the ability to anticipate vessel’s position on the water in six minutes. 

For short-range scales, three minutes might be better, and in the case of fast or highly maneuverable vessels, an established minimum approach CPA (ACPA) could be based upon a pre-determined distance appropriate to prevailing operating circumstances, conditions of visibility, and established operating procedures.

The required sectors are arranged by streaming the electronic bearing lines (EBL) No. 1 and No. 2  (danger bearings) off the port and starboard head of the vessel’s radar image.  Keep in mind that the distortion effects of beam width and pulse length cause the vessel image to bloom on the indicator. A vessel’s radar image will look wider than it is in real-world scale.  For smaller vessels, EBL No. 1 and No. 2 settings of 3Â¥ to 5Â¥ on either side of the header flash have worked effectively to set up this technique.

The intersection of the EBLs and VRM creates four sectors ahead of own ship on the radar indicator. Two sectors, port and starboard, immediately ahead of the vessel are named the “extremis sectors.” Two sectors ahead of the VRM are named the “guard sectors.” The intersection of the EBLs and VRM are identified as the minimum port/starboard running CPAs. The VRM also establishes a minimum approach CPA for 360Â¥ around own ship to monitor vessels approaching with a crossing or overtaking aspect.

As a target enters the “guard sectors”, movement toward the running CPAs is easy to monitor closely.  Ensuring that an approaching vessel passes outside of either the port or starboard running CPA (predetermined safe passing distance) stabilizes the approach and allows the pilot to monitor the target’s movement closely until it is past and clear (NavRule 8 (d)). If the target enters the “extremis sector” an extreme close-quarters situation is developing and urgent action to avoid collision must be determined and initiated.

  The ratio of the relative speed to response time between two vessels is at its greatest value when the approach target enters the “extremis sector”.  It is important that the target vessel not remain inside of the extremis sector as the approach continues unless avoidance maneuvering makes this condition necessary, for example, when holding up in a cross wind or current. 

Targets approaching with crossing aspects have slower speeds of relative motion.  For this reason electronic bearing lines can be set on the target as it approaches minimum approach CPA (ACPA) to reveal bearing drift, or the lack of bearing drift, and pending approach threat. Targets approaching from astern can be assessed effectively by inverting the Radar Dead Reckoning/Running CPA collision-avoidance technique.

Once this easily learned technique has been mastered, the pilot will find that using an active/passive equivalent systematic observation for defined narrow channels enables an efficient mental recognition of the determination of risk of collision and/or the development of a close quarters situation. 

 Even in open waters, the method has merit, because a vessel approaching from outside the fore-and-aft direction should ideally indicate a line of relative motion that crosses outside the six-minute circle, if this is the chosen minimum CPA. The line of relative motion is easy to predict by using a parallel index line and placing it along the target’s trail.

The Running CPA is a simple and effective method of  collision avoidance and navigation.  Many who have become accustomed to using this method have reported that when the lines and circles of the Radar Dead Reckoning/Running CPA method are removed, the radar presentation does not provide a clear indication of the situation six minutes ahead.   When the lines are displayed, the pilot’s attention is drawn to that point. This factor alone is an eloquent argument in favor of Radar Dead Reckoning/Running CPA.   This simple and convenient method provides a significant enhancement of the margin of safety on the inland waterways.

          John Moyle is a licensed master who has served as captain aboard vessels in near coastal waters, the Great Lakes, and the Mississippi River system. His experience includes teaching as a U.S. Coast Guard approved instructor for numerous programs and currently serves as a maritime licensing specialist.

By Professional Mariner Staff