Constant bearing, decreasing range (CBDR) is a term in navigation which means that some object, usually another ship viewed from the deck or bridge of one's own ship, is getting closer but maintaining the same relative bearing. If this continues, the objects will collide.
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Absence of visual clues
Sailors, especially deck hands and those who stand watch on the bridge, are trained to watch for this situation and make it known when they detect it happening. The reasoning is subtle: non-sailors who are used to driving automobiles normally are able to detect the possible risk of a collision with implicit reference to the background (e.g., the street, the scenery, the landscape, etc.) At sea, when vessels are borne in water, the sea surface removes this vital visual clue. Unaware individuals tend to gauge the risk of two vessels colliding based on which direction each vessel is heading. In other words, a novice will think that two vessels which appear to be heading apart from each other cannot collide. This is false, as when a faster vessel is overtaking a slower one, they can in fact have a collision even though the vessels are on substantially different headings.
Presence of visual clues
When a non-moving background (typically the shore line) is present, CBDR situations can be recognized by the fact that some part of the moving target vessel is not visually moving against the background. On the other hand, if the target vessel visually moves forward against the background, with relative bearing decreasing, then the target vessel will pass in front of the observing vessel and there is no risk of collision, provided both vessels maintain constant speed and course. Similarly, if the target vessel visually moves backwards against the background, with relative bearing increasing, the target vessel will pass behind the observing vessel. However, this method of recognizing non-CBDR conditions is only true for relatively small observing vessels. With larger vessels, only an observer positioned near the bow can reliably make the first conclusion, whereas the second conclusion can only be left to an observer positioned near the stern.
Relative bearings and true bearings
Another source of confusion arises from the distinction between relative bearing and absolute bearing. One might think that another vessel which seems to be progressing from, perhaps, dead ahead to apparent movement down one's starboard side cannot result in a collision. This is not quite true. Because a vessel afloat can change its true heading (relative to north) without obvious detection (unless there are landmarks in sight) two vessels may collide even if one presents a substantially changing relative bearing.
Aircraft and spacecraft
As humans have begun to maneuver other forms of vehicle independent of the visual clues of the earth's surface, the phrase Constant Bearing, Decreasing Range has come into use elsewise. It can be applied to aircraft or spacecraft, or any other objects which are maneuvering within proximity of each other.
Colloquial use
Colloquially, the phrase can also be used to speak of situations not involving physical objects that might come into contact with each other. People familiar with the phrase (such as sailors) might observe a situation with danger or obstacles which are not obvious to another individual and give a warning of the condition using the term "CBDR".