A blind area occurs when the background in the field of view varying from above-the-horizon to below-the-horizon in
air to air detection of the moving target. Factors that effect on the detection blind area are emphatically analyzed in terms
of the relative distance between sensors and targets and the velocities and altitudes of the targets. A range capability of
IR sensors is modeled and verified by using experiment data, a reasonable result is obtained. At last, some suggestions
that would overcome the problem are given.