To overcome the problems in passive ranging of infrared small targets used in photoelectric countermeasure
systems, a passive ranging scheme is proposed based on the distance-ratio between two adjacent
sampling times and the direction information about the azimuth and pitch angles of the target. A model of
motion is build to describe the relative position between the observer and the target in a 3D space. With
this model, a quadratic equation is derived, whose parameters can be calculated by using the distance-ratio,
the 3D coordinates of the observer, and the azimuth and pitch angles of the target. The proposed method
has a simple form, and can avoid the occurrence of near-zero values in the denominator. The performance is
demonstrated by simulation. It is applicable to the estimation of the target distance with a dual-band infrared
detection system installed on an airborne or missile-borne platform.