cone of visibility

cone of visibility

[′kōn əv ‚viz·ə′bil·əd·ē]
(aerospace engineering)
Generally, the right conical space which has its apex at some ground target and within which an aircraft must be located if the pilot is to be able to discern the target while flying at a specified altitude.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.