Field of the Invention
The invention relates to a signal process that is used to derive a position of a target.
Description of the Background Art
Conventionally, a radar apparatus mounted on a vehicle sends a transmission wave from a transmitting antenna and a receiving antenna of the radar apparatus receives a reflection wave that is the transmission wave reflected at a point on an object (hereinafter referred to as “target”). Accordingly, the radar apparatus derives information of the target (hereinafter referred to as “target information”), including a position of the target relative to the vehicle.
Concretely, the radar apparatus generates a beat signal by mixing a reception signal and a transmission signal of which a frequency changes in a predetermined cycle. Then the radar apparatus processes the beat signal by using fast Fourier transform (FFT) to generate a frequency spectrum. A signal exceeding a predetermined threshold (hereinafter referred to as “frequency peak signal”) is extracted in each of an up period and a down period in the frequency spectrum. The up period is a period in which a cyclically-changing frequency of a transmission signal increases, and a down period is a period in which the cyclically-changing frequency of the transmission signal decreases. Then the radar apparatus performs pairing that pairs the frequency peak signal in the up period and the frequency peak signal in the down period based on a predetermined condition and finalizes a pair data set. The radar apparatus derives a filtered data set obtained by filtering the pair data set finalized in the current process and the pair data set in a previous process, at a predetermined percentage. The two pair data sets to be filtered have the highest possibility of being associated with a same reflection point.
Next, the radar apparatus derives a prediction peak signal based on the derived filtered data set. The prediction peak signal is a prediction of the frequency peak signal in each of the up period and the down period to be processed in a next process. The prediction peak signal includes parameters such as a prediction frequency and a prediction angle. In other words, the prediction peak signal represents a prediction position of the target that will be derived in the next process.
Based on the prediction frequency of the prediction peak signal, the radar apparatus extracts the frequency peak signal, in each of the up period and the down period, that has time continuity with the filtered data set (hereinafter referred to as “history peak signal”).
Moreover, the radar apparatus performs a process of calculating azimuth directions of the history peak signals in the up period and in the down period. As a result of the process of calculating the azimuth direction, an angle spectrum is derived. The radar apparatus derives a peak signal exceeding a predetermined threshold (hereinafter referred to as “angle peak signal”) in each of the up period and the down period in the angle spectrum. For example, three or less angle peak signals are derived from one history peak signal.
The radar apparatus performs a process of pairing the angle peak signal in the up period and the angle peak signal in the down period based on a prediction angle of the prediction peak signal. Concretely, the radar apparatus selects an angle peak signal that has an angle closest to the prediction angle in each of the up period and the down period, as a candidate for a combination. Then, in a case where a signal level difference between the candidate angle peak signals for the combination is equal to or less than a predetermined value, the radar apparatus finalizes the combination of those angle peak signals as a pair of the angle peak signals associated with a same reflection point with which a target data set derived in a past process is associated (hereinafter referred to as “history pair data set”).
Based on the filtered data obtained by filtering, at the predetermined percentage, the history pair data set and the target data set derived in the past process that is associated with the same reflection point as the history pair data set, the radar apparatus derives a distance from the vehicle to the target, an angle of the target to the vehicle and a relative speed of the target to the vehicle. The radar apparatus outputs to a vehicle controller a position and the relative speed of the target. Accordingly, the vehicle controller performs necessary vehicle control depending on the position and the relative speed of the target.
However, in the process of pairing the angle peak signals in the up period and the down period, in a case where the signal level difference between the angle peak signals exceeds the predetermined value, the radar apparatus does not finalize the combination of the peak signals as the history pair data set because in the case where the signal level difference between the angle peak signals in the up period and the down period in the pair exceeds the predetermined value, there is a high possibility that the angle peak signals in the combination are associated with different reflection points.
Herein, although the angle peak signals closest to the prediction angle in the periods are paired, a reason why the signal level difference exceeds the predetermined value is because the reflection wave of the angle peak signal in the up period is reflected at a different reflection point from a reflection point at which the reflection wave of the angle peak in the down period.
There is a time difference of some msec. between when the radar apparatus receives the reflection wave corresponding to the angle peak signal in the up period and when the radar apparatus receives the reflection wave corresponding to the angle peak signal in the down period. For the some msec., at least one of the vehicle and the target travels so that a position of the target to the vehicle is changed. Therefore, the reflection waves corresponding to the angle peak signals in the up period and in the down period are reflected at different reflection points. Moreover, the radar apparatus receives the reflection waves reflected at plural reflection points on a vehicle traveling in front of a host vehicle in a traffic lane (hereinafter referred to as “front vehicle”) in which the host vehicle is traveling. In a case where number of the plural reflection points exceeds a maximum number that the radar apparatus can calculate the azimuth directions of the angle peak signals, a slight difference is caused between angles of the angle peak signals derived by the radar apparatus and an actual angles of the targets.
In a case where the radar apparatus cannot derive the history pair data set, the radar apparatus derives a prediction data set that is a history pair data set predicted based on the target data set derived in the previous process. Then the radar apparatus performs a process of replacing the prediction data set with the history pair data set (hereinafter referred to as extrapolation process”). However, the extrapolation process is performed based on the prediction data set, not based on an actually derived data set. Therefore, a derived position of the target may be different from an actual position so that the vehicle controller that controls the vehicle may not perform proper control in a case where the vehicle controller performs the vehicle control by obtaining position information of the target output from the radar apparatus.