Basics of SAR and ISAR Radar – Radar Operation – Part 4

Synthetic aperture radar (SAR) and inverse SAR (ISAR) are methods of using radar to map a stationary object, more usually terrain. SAR/ISAR systems are deployed on aircraft or satellites that move at high velocities in respect to the surface of the earth or potentially any stationary object or terrain. SAR/ISAR benefits from the phenomenon that an object, and/or, antenna in a horizontal direction in relation to the terrain, or antenna if the antenna is stationary, enables the antenna to appear as if it has a much larger aperture than it actually does. ISAR is predominately used in surveillance (maritime surveillance and ship classification) and during astronomical observation, where SAR is more commonly used with terrain mapping.

In the case of ISAR, the object is moving in respect to the antenna’s azimuth and an image is generated by employing a 2D or 3D Fourier Transform as a function of the Doppler shift and the target’s aspect ratio. The Doppler frequency of an echo is what is used to determine the position of the object/terrain in respect to the radar. If an object/terrain is ahead in the direction of the “flight path” then the Doppler frequency will have a positive offset, while anything behind the radar in the “flight path” will have a negative Doppler frequency offset. The exact Doppler frequency shift can be calculated to determine a target’s azimuthal position in respect to the radar antenna.

With SAR/ISAR the antenna aperture is synthetically enhanced as a function of the perpendicular velocity of the radar platform in respect to the object being mapped or imaged. A larger aperture antenna enables much more precise radar range resolution, which works in conjunction with the bandwidth of a pulse radar. Typically, SAR/ISAR radar are both moving in high velocity in respect to the object or terrain and use high bandwidth pulses in order to achieve imaging resolutions below 10 centimeters, and even down to a few millimeters for the latest high resolution satellites.

As with any imaging system, the sensitivity of the system is limited by the noise, phase noise, distortion, harmonics, spurious behavior, and other non-linearities and signal degrading aspects of both the channel and the transceiver circuitry. Therefore, with SAR/ISAR systems, like with other radar systems, high precision components and devices are typically used in the design and assembly of these radars. This includes the antenna, as misalignment of the antenna phase center position results in aberrations in the geometry of the resulting SAR/ISAR imagery. Corrections, or calibration, can be applied during processing to mitigate the geometric aberrations, or in cases of frequency dispersive antenna, a phase correction can be applied to the receive signal.

Given that SAR/ISAR are typically used to image objects or terrain at great range, extremely sensitive receivers and powerful transmitters with highly linear performance are used. In some cases, namely ISAR, geometrically simple and narrowband antenna are used to mitigate aberrations and errors.