Hello Augustin!
Thursday November 14 1996, Augustin Henriques writes to Ralf Zehender:
 AH>> v)      Oc (2) 20s
 RZ>> Light, group of 2 occultings, period 20s
 RZ>> Found at INT1 Chart, International Hydrographic Organization, Monaco.
 AH>    Hmmm, you got the description spot on, but I must confess I don't 
now
 AH> anything else about it's name, location, nor useage.
Sometimes used for landfall lights or leading lights on channels.
 AH>                         a)  Iso 2s
Isophase (on for as long as it's off) with a 2 second period (on one second, 
off one second).
 AH>                         b)  Al
Now, I must confess you've got me.
Bye for now,
          Jackson
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