In this letter, pioneering experimental results of passive synthetic aperture radar (SAR) imaging are presented. The classical active SAR radar operates in monostatic geometry. The SAR sensor presented in this letter is a passive radar utilizing commercial Digital Video Broadcasting-Terrestrial transmitters as illuminators of opportunity. It works in a bistatic configuration, where the receiver is placed on a moving platform and the transmitter is placed on the ground and is stationary. The imaged scenes are stationary surfaces on Earth such as agriculture or urban areas, buildings, etc. In this letter, pioneering results of signal processing verified by a measurement campaign are presented. In the experiment, two synchronized passive radar receivers were mounted on a small airborne platform. The main goal of the presented experiment was to verify the possibility of ground imaging using passive SAR technology and validate previously presented theoretical results.

This article presents a new approach to the estimation of shift and rotation between two images from different kinds of imaging sensors. The first of the image is an orthophotomap that is created using optical sensors with georeference information. The second one is created utilizing a Synthetic Aperture Radar (SAR) sensor.The proposed solution can be mounted on a flying platform, and, during the flight, the obtained SAR images are compared with the reference optical images, and thus it is possible to calculate the shift and rotation between these two images and then the direct georeferencing error. Since both images have georeference information, it is possible to calculate the navigation correction in cases when the drift of the calculated trajectory is expected. The method can be used in platforms where there is no satellite navigation signal and the trajectory is calculated on the basis of an inertial navigation system, which is characterized by a significant error. The proposed method of estimating the navigation error utilizing Affine Scale-Invariant Feature Transform (ASIFT) and Structure from Motion (SfM) is described, and techniques for improving the quality of SAR imaging using despeckling filters are presented. The methodology was tested and verified using real-life SAR images. Differences between the results obtained for a few selected despeckling methods were compared and commented on. Deep investigation of the nature of the SAR imaging technique and noise creation character allows new algorithms to be developed, which can be implemented on flying platforms to support existing navigation systems in which trajectory error occurs.

The article presents a novel fast barycentric-based back projection (BP) algorithm for radar imaging using the Synthetic Aperture Radar (SAR) technique. The innovation in the proposed method is in the determination of the reverse virtual synthetic aperture beam for each point of the observed radar scene, and in classifying which portion of raw radar data must be considered to complete imaging processing. This approach allows the classical well-known BP algorithm to be significantly accelerated, which is one of the most computationally demanding methods in SAR imaging. The proposed barycentric-based BP method gives the opportunity to control the resolution of the processed image by changing the width of the calculated beam. Moreover, the described solution is still highly parallelizable, like the classical BP algorithm. The solution can be adapted to a real-time SAR system with modern high-end computational units, such as graphical processor units (GPUs) that support parallel computing. In the paper, the BP algorithm is clearly described with its barycentric-based modification. The proposed method is compared with a classical BP algorithm based on the measurement of simulated and real data.

In this paper a system for radar measurements using a low terahertz frequency band is presented. The architecture of the system is briefly described and some preliminary results obtained with the developed system are attached. A concept to rebuild this system for outdoor radar experiments is presented and the design process described, with an emphasis on the design challenges. Design choices regarding signal generators and recorders, analog intermediary circuitry, and the navigation system are discussed in this paper.