Low-weight full-polarimetric SAR in Drone
Airborne Synthetic Aperture Radar (SAR) sensors have been commonly used during the last decades to monitor different phenomena in medium-scale areas of observation, such as object detection and characterization or topographic mapping. The use of Unmanned Aerial Vehicles (UAVs) is a cost-effective solution that offers higher operational flexibility than airborne systems to monitor these type of scenarios. The Universitat Politècnica de Catalunya (UPC) has developed the first fully polarimetric SAR system at X-band integrated into a small UAV Multicopter Platform (UAV MP). The sensor, called AiRBased REmote Sensing (ARBRES), has been integrated into the platform overcoming restrictions of weight, space, robustness and power consumption. The ARBRES-X SAR system represents an evolution of a previous version developed by the Remote Sensing Laboratory of the UPC  . It is a SAR sensor operating at 9.65 GHz that works with a Stepped Linear Frequency Modulated Continuous Wave (SLFM-CW) signal. The complete system has been designed to be fitted in small UAVs, as is the case of the UAV MP. This has imposed strong constraints in its design in terms of weight, power consumption, compactness and robustness.
The capability of the UAV MP to execute different types of trajectories are exploited to evaluate the system performance. It has been observed that the defocusing problem in the retrieved images is due to the deviation of the platform from the nominal trajectory and the flight instabilities. To overcome this undesired effect, it has been concluded that the use of a cost-effective, small-size IMU is a non-viable option. This is because of the lack of accuracy of these systems, which has forced us to discard the use of a Motion Compensation technique to focus the image. Alternatively, an autofocus algorithm has been applied to refocus the SLC images, improving its quality for large apertures.
We use two different kind of radar calibrators. On the left, a corner reflector with relative low cross-pol response. On the right, a Polarimetric Active Radar Calibrator (PARC)
The inherent instability of the platform and the insufficient positioning accuracy of the GPS system make it extremely difficult to obtain interferometric results. The autofocus algorithm is well suited to compensate amplitude images, but the problem is not yet efficiently resolved with the phase in long apertures. Because of that, the interferometric measurements have been processed using small apertures, where the flight deviations are less important.