1. Field of the Invention
The present invention relates to an FM-CW radar apparatus using a transmitted signal resulting from frequency modulation (FM) of continuous wave (CW).
2. Related Background Art
The FM-CW radar apparatus is suitable for detection of an object in a relatively near range, as compared with pulse radar apparatus. In recent years, research and development is being carried out in the field of the FM-CW radar apparatus as means for detecting the position and relative speed of a preceding car or the like while being mounted on a car.
In the FM-CW radar apparatus, the distance and velocity of a target existing in a beam direction are calculated from a beat frequency in a modulation frequency increasing interval (hereinafter referred to simply as an up interval) and a beat frequency in a modulation frequency decreasing interval (hereinafter referred to simply as a down interval) in the same beam. Here, the beat frequency means a frequency difference between a transmitted wave and a reflected wave from the target or the like exposed to the transmitted wave. This operation is carried out for all beams in the scanning range to obtain information of targets existing in the scanning range.
When there is only one target, a single level peak (maximum) based on the reflected wave from the target is obtained at a beat frequency in each of the up interval and the down interval. Therefore, the distance and speed of the target can be calculated using the beat frequencies of the level peaks in the respective intervals.
If various objects are mixed in the detecting range there will exist a plurality of targets in one beam, however. In this case, there exist a plurality of level peaks of beat frequencies in each of the up interval and down interval. It was thus not easy to determine which beat frequency level peak in the down interval should be paired with an arbitrary level peak of beat frequency in the up interval, and it was also easy to make pairing errors.
Particularly, when the FM-CW radar apparatus is used as an on-vehicle radar apparatus for detecting a vehicle around it, reflection originates not only from the vehicle as a primary target, but also from various objects such as fixed objects on the road side except for the primary target. It was thus easy to make the pairing errors as described above.
For example, supposing reflection takes place from two objects, a moving object and a fixed object, the relation of magnitude of the beat frequencies corresponding to the respective objects can be reverse between in the up interval and in the down interval; in that case, if pairing is done simply in the order of magnitude of the beat frequencies, the apparatus will fail to perform correct recognition of the targets.