Radar apparatus and direction-of-arrival estimation device

1. A radar apparatus comprising:

a transmitter that transmits a radar signal through a transmitting array antenna;

a receiver that receives, through a receiving array antenna, a reflected-wave signal produced by the radar signal being reflected by a target; and

a direction estimator that estimates an angle of arrival of the reflected-wave signal, and outputs an estimation result indicating the angle of arrival of the reflected-wave signal,

wherein the direction estimator includes

a first estimator that, by performing a maximum likelihood estimation process on a first direction, calculates, using signals received by a first virtual linear array constituted by virtual receiving antennas, linearly arranged in the first direction, that belong to a plurality of virtual receiving antennas included in a virtual receiving array constituted on the basis of an arrangement of the transmitting array antennas and the receiving array antennas, first maximum likelihood values corresponding to NW (where NW is an integer of 1 or larger) angles in the first direction and extracts first candidate angles of arrival of incoming waves in the first direction including at least the first maximum likelihood values,

a second estimator that, by performing a maximum likelihood estimation process on a second direction that is orthogonal to the first direction, calculates, using signals received by a second virtual linear array constituted by virtual receiving antennas, linearly arranged in the second direction, that belong to the plurality of virtual receiving antennas, second maximum likelihood values corresponding to the NW angles in the second direction and extracts second candidate angles of arrival of incoming waves in the second direction including at least the second maximum likelihood values, and

a third estimator that estimates, using the first and second candidate angles of arrival, angles of arrival of the NW incoming waves in a two-dimensional plane extending in the first and second directions, and

wherein the radar apparatus outputs an estimation result indicating a presence of a target and a direction of the target.

2. The radar apparatus according to claim 1 , wherein the third estimator determines the angles of arrival of the NW incoming waves in the two-dimensional plane from among combinations of the NW angles corresponding to the first maximum likelihood values and the NW angles corresponding to the second maximum likelihood values.

3. The radar apparatus according to claim 1 , wherein the first estimator extracts the first candidate angles of arrival including the NW angles corresponding to the first maximum likelihood values and the NW angles corresponding to at least one extreme value other than the first maximum likelihood values in a first evaluation function that is used in the maximum likelihood estimation process,

the second estimator extracts the second candidate angles of arrival including the NW angles corresponding to the second maximum likelihood values and the NW angles corresponding to at least one extreme value other than the second maximum likelihood values in a second evaluation function that is used in the maximum likelihood estimation process, and

the third estimator determines the angles of arrival of the NW incoming waves in the two-dimensional plane from among combinations of the first and second candidate angles of arrival.

4. The radar apparatus according to claim 1 , wherein the third estimator estimates the angles of arrival of the NW incoming waves in the two-dimensional plane by performing a maximum likelihood estimation process on the two-dimensional plane with respect to combinations of the first and second candidate angles of arrival.

5. The radar apparatus according to claim 1 , wherein the third estimator estimates the angles of arrival of the NW incoming waves in the two-dimensional plane by a maximum grade method using the first and second candidate angles of arrival as initial values.

6. The radar apparatus according to claim 1 , wherein the maximum likelihood estimation process involves use of an evaluation function including signals received by the virtual receiving antennas and directional vectors of the virtual receiving antennas, and

the evaluation function uses parameters including prestored parameters pertaining to the directional vectors.

7. A direction-of-arrival estimation device for estimating angles of arrival of signals received through a plurality of receiving antennas, comprising:

a first estimator that, by performing a maximum likelihood estimation process on a first direction, calculates, using signals received by a first linear array constituted by receiving antennas, linearly arranged in the first direction, that belong to the plurality of receiving antennas, first maximum likelihood values corresponding to NW (where NW is an integer of 1 or larger) angles in the first direction and extracts first candidate angles of arrival of incoming waves in the first direction including at least the first maximum likelihood values;

a second estimator that, by performing a maximum likelihood estimation process on a second direction that is orthogonal to the first direction, calculates, using signals received by a second linear array constituted by receiving antennas, linearly arranged in the second direction, that belong to the plurality of receiving antennas, second maximum likelihood values corresponding to the NW angles in the second direction and extracts second candidate angles of arrival of incoming waves in the second direction including at least the second maximum likelihood values; and

a third estimator that estimates, using the first and second candidate angles of arrival, angles of arrival of the NW incoming waves in a two-dimensional plane extending in the first and second directions, and outputs an estimation result indicating the angles of arrival of the NW incoming waves,

wherein the direction-of-arrival estimation device outputs an estimation result indicating a presence of a target and a direction of the target.