ISAR
Figure 1: Comparison of SAR methodology with ISAR
ISAR
Inverse Synthetic Aperture Radar (ISAR) is an inversion of the SAR because not the flight path of the airborne radar is taken as the basis for the required relative motion but the motion vector of the located target. The relative motion between the radar system and the target must be sufficiently known during the entire measurement period to allow coherent processing of the measurement data. This relative movement changes the targets orientation to the radar system resulting in different aspect angles of viewing the target. As a result, the echo signal of each reflecting point of the target receives a phase change. As with SAR, coherent data processing thus yields high spatial resolution even in the azimuthal direction.
Three methods are available for ISAR processing of radar raw data:
- Doppler frequency analysis
The achievable azimuth resolution is determined by the possible frequency resolution in the Doppler spectrum, which is determined by the Fourier relation through the measurement duration. - Correlation method
The image generation or focusing of each image point is performed by the coherent summation of the radar raw data for each individual image point. - Back projection method
The back projection method is an approximation to the correlation method by variably adjusting the computational effort for ISAR image generation from the measured raw data.
The ISAR method became very important on board of naval patrol aircraft in order to obtain radar images of ships with such a quality that target identification is possible.