Critical Separation

Critical Separation is the spacing in a grid search where, if the search objective is halfway between two searchers, the object will be at the limit of the visible range of both searchers.  This can be very useful when estimating a Probability of Detection (POD).  As with the discussion of anything related to POD, it is important to define what it is your are searching for.  You would have a different POD when searching for a responsive subject versus an unresponsive subject, or for searching during a sunny day versus a rainy night, so you would also have different critical separations for those situations. 

Another way of saying critical separation is to say 100% coverage.  This is to say that every portion of the area in question was searched exactly once, with no overlap, during the search effort.  You can see from the picture below that the two searcher's visual ranges do not overlap as they search with critical separation.  


The Probability of Detection associated with critical separation is defined as \( 1-e^{-1} \), which is numerically about 63%.  Many people round this down to 50% in an attempt to be conservative while reporting the POD from an assignment, although this can be unnecessarily conservative.