The definition is Early sub detector — ASDIC being that old sonar system for spotting submarines. Now, regularly seen is doing the heavy lifting: take every third letter of near us in Dominica — nearuSinDomIniCa, positions 3, 6, 9, 12, 15 — and out come the letters R, S, D, I, C... wait, that first one's actually an A if you count from the N (the A in "near" at position 3 if you consider "nea" — yes, it's the A at position 3), giving you A, S, D, I, C in order.