-- Last Updated: Jun-29-08 10:27 PM EST --
Aircraft pilots are required to know these according to the Aeronautical Information Manual, Chapter 6-2-7, Search and Rescue, Fig. 6.2.1
MESSAGE SYMBOL
V Require assistance
X Require medical assistance
N No or Negative
Y Yes or Affirmative
(arrow) Proceeding in this direction
SOS International Distress Symbol
From my experience as a SAR pilot, X is the easiest and least ambiguous to spot.