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Basic Principle

Vergrößerung

In order to explain the SIREV principle it is reasonable to have first a look at the synthetic aperture radar (SAR) which forms the basis of the SIREV technique. The main objective of every imaging radar is to obtain a high spatial resolution in both image dimensions, i.e., range and azimuth. In range, the pulse compression technique allows us to achieve a high spatial resolution while minimizing the requirements on the peak transmit power. In azimuth, a high resolution can only be obtained by means of a narrow antenna diagram. Since beam width and antenna length follow an inverse relationship, improved resolution in azimuth requires an extension of the aperture size. In SAR systems such extended apertures are generated by a spatio-temporal sampling process which takes advantage of the relative movement between an airborne platform and the illuminated targets on the ground. The basic idea behind this approach is to synthesise an extended virtual antenna, thereby realizing an effective aperture perpendicular to the flight path which by far exceeds the dimension of the physical antenna. The major drawback of this technique is, however, that it can only be operated in a side looking imaging mode.

In contrast to SAR, the SIREV antenna consists of a physically existent linear array of single elements, which is oriented horizontally and perpendicular to the flight direction as shown in the Figure. The antenna elements may now be switched in sequential order to transmit and receive the radar signals. This monostatic mode with equally located transmit and receive antennas still corresponds to a SAR system, yet we don't have to move a single antenna along a straight trajectory. The disadvantage of the monostatic mode is, that it requires a T/R-module for each antenna element, thereby increasing the complexity and costs of the necessary hardware. A more cost-effective realization may be achieved by using only one transmit antenna, which leads to a bistatic configuration where the receive antennas are spatially separated from the transmit antenna. This configuration can be operated in either a simultaneous or a sequential receive mode. The simultaneous mode makes optimum use of the transmitted signal power by a large effective antenna area whereas the sequential receive mode minimises the hardware requirements. The major disadvantage of the bistatic configuration is, that it reaches only half of the image resolution as compared to the monostatic case. In principle, it is also possible to combine the advantages of the monostatic and bistatic configurations using a small number of transmit antennas.