Disappearing Targets: Radar Blind Spots

Disappearing Targets

“Safety Moments, presented at CCA Stations and Posts”

By Ted Parish, Chesapeake Station

Marine radars often have "blind sectors" created by the antenna location. At the right is an example of a placard that informs the user of these "blind spots”; relative heading angles where a target may not be displayed. Awareness should be that these are fixed known discrepancies, while temporary blocks like a wet sail, masts, close proximity flags (particularly wet), bimini and awnings, and even painting a name on the radar dome may create additional "blind spots". The solution is to operate the radar in a known fixed (docked) location and visually compare for horizontal discrepancies. The vertical angle (placard 2) should include the vessel at various pitch and heel (hull angle).

Before GPS, I was hired to deliver a 53' Oceans motoryacht to Florida in December. The vessel was built with a huge flying bridge/upper deck with an arch crossing the after section. The radar, VHF, and Loran-C antennas were mounted to this arch. 

At a later date, the owner installed a fixed bimini that ran from the arch to the helm area, shading the entire upper deck, a logical deterrence to the Florida sun. 

We departed the Chesapeake in December and, in an effort to get the boat south, ran sunrise-sunset. Offshore weather required staying inland, adding additional limitations to making miles. 

Visibility was decreasing with afternoon rain and 20 knot following wind, so the arrival on a lee shore harbor entrance was required. (Murphy) The entrance had a red ATON fixed pole with a light. The Loran was simple in nature (before plotters) and the Loran lines needed pencil plotting while crossing the NC sound. As I approached the entrance with premature darkness, rain and fair wind, I was required to operate in a semi-planing pitch bow up, perhaps 10 knots, to keep steerage. 

During the day operating at full speed, I had no concerns for the radar, including being tuned, and I was ready for the anticipated afternoon reduced visibility.

So out of the gloom and rain I simultaneously see surf to port and perceive a red flash 1 point starboard bow but nothing on the radar. Meanwhile, I had identified targets abeam and behind me. The immediate full reverse did prevent a full grounding, but the running gear took a beating. 

The next day (we did safely dock that night) enroute very slowly to get hauled out, I began examining the antenna configuration/install and belatedly realized that at idle and full speed the vessel pitch was level, but at semi-planing the pitch was bow up The installed awning blocked the radar down-angle significantly and so blocked all critical targets forward to 2 points either side. The future solution for me was to periodically do "Crazy Ivans": 30 degree turns to port and starboard to clear the heading blind sector. The permanent solution for that vessel would be to raise the radar antenna or move it to the bow, but both suggestions had serious conflicts and never happened. 

I will add that sailboats have serious sail conflicts, particularly when wet. Personal experience in the high latitudes where fog and ice congregate, the backstay gimballed antenna blind spot was the mast and the mainsail when reaching. This made it challenging to see the bergy bits to leeward and find the leads in the ice pack. 

I have experienced mast-mounted radar doing similar when the spinnaker or reacher was being used. 

Radar is not mysterious; it just takes a bit more understanding and is unique to every vessel.

The Cruising Club of America is a collection of accomplished ocean sailors having extensive boat handling, seamanship, and command experience honed over many years. “Safety Moments” are written by the Club’s Safety Officers from CCA Stations across North America and Bermuda, as well as CCA members at large. They are published by the CCA Safety and Seamanship Committee and are intended to advance seamanship and safety by highlighting new technologies, suggestions for safe operation and reports of maritime disasters around the world.