Traditionally, a Doppler radar only measures a relative velocity at line-of-sight (LOS). In FIG. 3, the relative velocity of a stationary scene point is calculated in terms of Doppler frequency byfD=Kνr cos(θr),  (1)the relative velocity of an approaching moving object is calculated as (FIG. 4)fD=K[νr cos(θr)+νt cos(φt)]  (2)and the relative velocity of a leaving moving object is (FIG. 5)fD=K[νr cos(θr)−νt cos(θt)]  (3)where K is a Doppler frequency conversion constant. No range and other 3D structure information is available from single Doppler radar.
There are a number of methods of deriving 3D structure information from either a single image or a sequence (or sequences) of images, collectively called the “structure from X methods”. These methods include: structure from texture, structure from shadow, structure from motion, structure from stereo, structure from focusing/defocusing, etc. However, none of these methods can reliably provide 3D information without strict conditions.
Since a Doppler radar and a camera measure different properties of a 3D scene, fusion of radar and camera signals is desired to obtain reliable information about the scene. Correct registration of a Doppler radar with a camera is crucial for information fusion. So far, there is no reliable way of registering a Doppler radar and a camera, such that the measurements from the Doppler radar and camera all refer to the same physical point in the scene.
FIG. 4 illustrates the relative velocity measurement of an approaching moving object.
FIG. 5 illustrates the relative velocity measurement of a leaving moving object.