Tuesday, June 15, 2010

Altered Reality: Navigating with radar

Navigating with radar is an exercise in perception in an altered reality.

Instead of seeing with light, you are seeing with radio waves.  Things are different (on purpose!) - there are some consequences:
  • Things reflect radio waves differently than light.  Some things which are opaque to light are essentially transparent to radio waves (eg. wood, fiberglass).
  • Surface roughness matters.  Rough surfaces (with feature size down to say 3-4 inches) will appear brighter than smooth surfaces.
  • Size on the radar screen will be related to reflection efficiency more than physical size.  A canoe with a good radar reflector will appear bigger than a 30' wood cabin cruiser with the engine below the waterline.  In fact, that cabin cruiser may be invisible.
  • The radar beam does not continuously illuminate the surroundings like the sun.  Instead, it is a narrow beam which is swept around the horizon - imagine trying to understand your surroundings by swinging a flashlight around your head in a circle.  This means that you are very obviously viewing a series of snapshots rather than continuous video.
  • In most cases, the radar antenna will be mounted near a mast.  This arrangement will cause a blind sector - a wedge-shaped area on the radar screen in which nothing can ever appear.  Think of it as a shadow.  But there will be no evidence on the screen of this.  
For these and other reasons, it is very disorienting for the helmsman to switch back and forth between the radar display and looking out over the bow.  A far, far better arrangement is to have a radar officer (on Eolian this is Jane) who lives continuously in the altered reality of the radar world, and a helmsman who remains in the real world.  To complete the link, a communication protocol needs to be worked out between the radar officer and the helmsman.  Not surprisingly, we fell into the one where angles are identified as a clock position on the screen, and distance is in miles or fractions of a mile.  For example, Jane might say, "You have something which appears to be stationary, out about 1/2 mile at 2 o'clock."  And then I squint in that direction.

Try it in broad daylight first.  In this way, you will be able to link up the altered reality of the screen with the one out the window.  And do not be surprised if several of the boats in your vicinity are completely invisible on radar.  (Do you have a radar reflector?).  Try out all your controls to learn how they affect what you see (or don't see).

Do not spend all of your observation time at short distance scales.  Switch to longer range scales frequently - big ships move quickly (and you don't). 

Illustrating some of these points, here are two stores from our logs:
Cruising north in thick fog just off of Apple Tree Cove, we knew we were in the ferry lanes.  I was having no luck seeing, but I could hear a ferry out there somewhere.  I kept asking Jane where it was, and she kept telling me that she could not see it at any range.  Seemingly very quickly, he sounded like he was right behind us.  And suddenly Jane shouted - There he is - on both sides of us! 
The ferry had been approaching us from astern, and was in the shadow wedge created by our mizzen mast.  When he got close enough that his ends extended outside the shadow, indeed, he did appear to be on both sides.  He also was turning into Apple Tree Cove to the Kingston ferry terminal, so the story had a happy ending.  Im pretty sure he had us on his radar.
And another:
Cruising again in thick fog, we were entering Port Ludlow.  Now, this is is a little scary in good weather. Having the fog did nothing to make it easier.  Jane began to report that the area beyond the ledge was filled with boats, all apparently stationary.  "Hmmm...," I thought, "a bunch of fishing boats?"  I asked her for a safe course, and she said there were too many blips to pick one.   I ducked down the companionway to see for myself, and try to get a feel for things.  Yikes!  They were everywhere!

I throttled way back, hoping to slow to the point where I'd be able to see boats and be able to avoid them.  As I approached the first one, Jane kept telling me how close I was, and kept asking me if I could see it.  Then why I couldn't see it, as we were right on top of it.  And about then, we passed a crab pot buoy with a little flag on it.  And then a whole lot more of them.  It seems that there was a wire in the flag, to hold it out - a wire that made an excellent radar reflector.

This story too has a happy ending. As we rounded the little sandy hook and entered the inner bay, we drove out of the fog into bright sunshine.


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