An apparatus for detecting a direction of a target is provided. The apparatus transmits and receives radio waves through a plurality of transmission/reception channels causing a phase difference in signals to be received through the transmission/reception channels and calculates the direction based on the phase difference. The apparatus comprises a direction calculating device, range determining device, and direction correcting device. The direction calculating device calculates the direction of the target based on the phase difference in the received signals on the assumption that the phase difference is within a range of -.pi. to +.pi. [rad]. The range determining device determines that the target exists in any of azimuthal angle ranges each corresponding to ranges defined by (2m-1) .pi. to (2m+1) .pi. [rad] (m is an integer). The direction correcting device corrects the direction according to a range-determined result.

To realize a monopulse radar system wherein the velocity of a mobile body, distance between an obstacle and the mobile body and relative velocity can be detected and simultaneously, the direction of the obstacle can be detected, in a monopulse radar system wherein an azimuth is detected depending upon amplitude difference or phase difference between signals respectively received by plural receiving antennas, an array antenna composed of plural antenna elements is used for each transmitting antenna and each receiving antenna, at least one of the transmitting antenna and the receiving antenna is provided with an antenna switch for switching an antenna beam shape to a short angle/long distance or a wide angle/short distance and a switch control device that controls the switching of the antenna switch is provided.

The present invention relates to a monitoring device (1) with a transmitter unit (2) and a receiver unit (3) for monitoring an area. Slotted cables serve as antennas. According to the invention, a first slotted cable is arranged as a transmitting an antenna (4) for transmitting a pulsed high-frequency signal and a second slotted cable is arranged as a receiving antenna (9) for receiving the reflected signal. The slotted cables are arranged in association with each other and essentially parallel to each other, and the transmitter unit (2) is connected to the first slotted cable at one end of the antenna arrangement and the receiver unit (3) is connected to the second slotted cable at the other end of the antenna arrangement.

A monopulse radar system for detecting an azimuth depending upon amplitude difference or phase difference between signals respectively received by plural receiving antennas, includes: a transmitting array antenna and a receiving array antenna each including antenna elements forming an antenna train, at least one antenna switch disposed among the elements of the transmitting array antenna and/or the receiving array antenna, and a switch controller for turning ON/OFF the antenna switch to change an aperture length of the transmitting array antenna and/or the receiving array antenna thereby changing a beam shape thereof. The antenna elements of the transmitting array antenna and the receiving array antenna are formed on a dielectric substrate. A dielectric length between the antenna train formed by the antenna elements and the antenna switch is 1/2 dielectric wavelength.

An apparatus for estimating direction of arrival of signal is provided that has excellent performance in terms of angular resolution and the number of signals that can be identified. In an array sensor comprising a plurality of sensor elements, the two outermost sensor elements are alternately selected by a switch for use as a transmitting sensor, and the other sensor elements are selected in time division fashion as receiving sensors. With this arrangement, the effective aperture is increased to about twice the physical aperture, to improve angular resolution in a direction-of-arrival estimator.