Synthetic Aperture Radar (SAR) is an established remote sensing technique that can robustly provide high-resolution imagery of the Earth’s surface. However, current space-borne SAR systems are limited, as a matter of principle, in achieving high azimuth resolution and a large swath width at the same time. Digital beamforming (DBF) has been identified as a key technology for resolving this limitation and provides various other advantages, such as an improved signal-to-noise ratio (SNR) or the adaptive suppression of radio interference (RFI). Airborne SAR sensors with digital beamforming capabilities are essential tools to research and validate this important technology for later implementation on a satellite. Currently, the Microwaves and Radar Institute of the German Aerospace Center (DLR) is developing a new advanced high-resolution airborne SAR system with digital beamforming capabilities, the so-called DBFSAR, which is planned to supplement its operational F-SAR system in near future. It is operating at X-band and features 12 simultaneous receive and 4 sequential transmit channels with 1.8 GHz bandwidth each, flexible DBF antenna setups and is equipped with a high-precision navigation and positioning unit. This paper aims to present the DBFSAR sensor development, including its radar front-end, its digital back-end, the foreseen DBF antenna configuration and the intended calibration strategy. To analyse the status, performance, and calibration quality of the DBFSAR system, this paper also includes some first in-flight results in interferometric and multi-channel marine configurations. They demonstrate the excellent performance of the DBFSAR system during its first flight campaigns.

This paper presents a design of a horizontally polarized antenna array for a new DLR airborne Ka band synthetic aperture radar system with the maximum gain of 24 dB, low cross-pol level of 40 dB and a shaped radiation pattern with low sidelobe level in both azimuth and elevation planes below -18 dB. It has been designed using the slotted waveguide antenna (SWA) technology for the center frequency equal to 35.5 GHz. The transverse slots have been cut into the narrow waveguide wall and the use of iris pairs placed inside enable the radiation. A suitable feeding network consisting of a conventionally H-plane T junction power divider and a directional waveguide coupler has been also described. Based on theoretical principles, the antenna array as well as the corresponding power splitter have been developed and fabricated while meeting mechanical and electrical system demands. The measurement results of the manufactured prototype have been presented and a good agreement with requirements has been achieved.

An extremely compact design of a power divider in Substrate Integrated Waveguide (SIW) and fed by means of a Coplanar Waveguide (CPW) line is presented in this paper. The proposed solution presents a relative low insertion loss and good adaptation (measurements show an S11 better than -20 dB for the centre frequency and below -15 dB for a 30% bandwidth) in X-band, despite the use of a substrate with a high dielectric permittivity (εr = 9.8). In order to validate the design, various topologies of the power divider are manufactured, measured and compared with the simulated data. These power splitters will be implemented in the feed network of a phased array for an airborne radar system with Digital Beamforming capabilities operated by the German Aerospace Center.

Phased array antennas are typically designed from a set of largely similar radiating elements that are fed by a beam forming network (BFN). The ambient structure thereby impacts the installed performance and needs to be considered in the excitation coefficients determination. However, obtaining reliable results with purely numerical tools for aerospace composite material structures is questionable without electromagnetic parameter investigation. This work describes a method based on individual far-field measurements of the radiating sub-arrays in a Compact Antenna Test Range (CATR). The full array pattern is synthesized by planar near -field (PNF) techniques. Thereby, the excitation coefficients are optimized to mitigate the actual structure influence without the need for additional electromagnetic material properties investigation. The procedure is exemplified with a linear polarized Ka-band phased array for an airborne synthetic aperture radar application. Individual on structure slotted waveguide antenna (SWA) element measurements, excitation coefficients determination and the synthesized radiation pattern are shown. In a proof of concept with structures that are assumed perfectly conductive, measurementbased processing results are compared with simulations.