1. Field of the Invention
The present invention relates to an interferometer-type radar which observes a flying target.
2. Related Art
A conventional radar uses a very narrow beam antenna so as to know the direction of the target. In order to observe a wide area with the narrow beam width, it is required to scan the antenna at a high speed. In a case of searching a potential target, a certain time period is required from the time a specific direction is scanned to the time the same direction is scanned again, so a target which appeared suddenly may be overlooked.
In the conventional radar, a distance is measured with a delay time until a transmitting signal is reflected from a target and received. In order to improve the distance resolution, the transmitting pulse width must be reduced, which causes a decrease in the transmitting average power, an expansion of the receiving bandwidth, and a drop of S/N. Therefore, there is a case where a target, in which the radar cross section area for reflecting the transmitting signal is small, cannot be found.
In the conventional pulse radar, a Doppler frequency shift of a reflection signal is generally used for measuring the velocity of the target. In order to measure the velocity of a high speed moving target, the pulse repetition frequency should be twice as large as the Doppler frequency. In a case that the target is a flying object moving at an extremely high speed such as a missile, the receiving time window becomes extremely small, whereby ambiguity is caused in the distance measurement. In order to prevent this problem, it has been required to calculate the distance to the target by performing analytical processing using plural pulse repetition frequencies.
A conventional phased array radar forms a multi-beams, and by using these beams, tracks a plurality of targets independently. However, since there is a limit in the number of beams capable to be divided, the number of target which can be tracked is limited.
As a method of obtaining the distance to the target and the velocity of the target by using a means other than a pulse radar, there is one using an FMCW radar. In a conventional FMCW radar, the velocity of the target is detected from the Doppler shift component of the baseband frequency in the frequency ascending time and the frequency descending time of a FMCW signal, and the distance to the target is obtained by correcting the Doppler component. (see, for example, Japanese Patent Application Laid-open No. 2000-46941 and Japanese Patent Application Laid-open No. 2003-177175).
However, when the target is moving, there is a problem that a baseband signal, which is a frequency-converted receiving signal by a transmitting signal, is affected by a Doppler shift due to the velocity of the target, and at the same time, the frequency changes according to the movement of the target, whereby it is impossible to measure the distance accurately. Further, if the velocity and a change in the velocity of the target are large, or if there are a number of targets of different velocities at the same time, it is difficult to apply the above-mentioned method.
Further, in the conventional FMCW radar, an influence of delay is disregarded by taking a repetition cycle which is much larger than a delay due to the distance. Therefore, when measuring a long distance, there is a problem that a fast repetition cycle cannot be adopted, whereby it is difficult to measure the distance to a target moving at a high speed with higher accuracy.