There are various types of radar apparatus for detecting a distance and a direction of an object with respect to the radar apparatus. A frequency modulated continuous wave (FMCW) radar apparatus is a radar apparatus that continuously transmits a frequency-modulated radar signal to the object and detects the distance or a relative velocity of the object based on the transmitted radar signal reflected from the object.
In one method for detecting the direction of the object, a transmitting means for transmitting a radar signal mechanically turns and scans the transmitted radar signal reflected from the object. In another method that uses a digital beam forming (DBF) algorithm, the transmitting means is fixed and an antenna having multiple elements arranged in an array receives the transmitted radar signal. The received radar signal is digitally processed and the direction of the object is detected based on the digital signal. Specifically, in the DBF algorithm, an angular spectrum is generated based on the received radar signal on each of the elements and a peak of the angular spectrum is detected. The direction of the object is estimated based on the peak of the angular spectrum.
In a beamformer algorithm as the DBF algorithm, the angular spectrum is generated such that amplitudes of the received signal at a given time are connected as shown in FIGS. 10A and 10B. A multiple signal classification (MUSIC) algorithm is known as a high-resolution direction of arrival (DOA) estimation algorithm.
In the DOA estimation algorithm, correlation matrices are calculated, eigenvalue expansions are performed on each of the correlation matrices, the angular spectrum is calculated from eigenvectors of the correlation matrices, and the direction of the object is calculated based on the angular spectrum.
A FMCW radar apparatus disclosed in U.S. Pat. No. 6,121,917 corresponding to JP-A-H11-133142 detects the direction of the object by using the beamformer algorithm. In the FMCW radar apparatus, a fast Fourier transform (FFT) is applied to the received wave signal to obtain a peak frequency of a distance power spectrum. Then, the beamformer algorithm is applied to only the peak frequency component of the received signal so that the amount of calculation required to detect the direction of the object is reduced.
However, when the beamformer algorithm is used in the radar apparatus, resolution of the radar apparatus depends on the number of elements arranged in the array. Therefore, the radar apparatus using the beamformer algorithm needs to be increased in size to obtain high resolution.
The high-resolution DOA estimation algorithm such as the MUSIC algorithm achieves the high resolution without an increase in the number of the elements. In the DOA estimation algorithm, the resolution may be increased by reducing noise with summation of the received signal with respect to time. The summation is performed such that a present angular spectrum calculated in a present process and a previous angular spectrum calculated in a previous process are summed up.
However, when the object moves, a frequency corresponding to the distance changes between in the previous process and in the present process. Therefore, the DOA estimation algorithm needs to be applied to all the frequency components of the received signal on each process to calculate the angular spectrum, and the calculated angular spectrum needs to be stored in a memory. The calculation of the angular spectrum requires an eigenvalue expansion that requires a lot of calculation. Therefore, when the angular spectrum is calculated on each frequency component, the amount of calculation is significantly increased. In the DOA estimation algorithm, the high resolution results in a significant increase in the amount of calculation.
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