In-Orbit Relative Amplitude and Phase Antenna Pattern Calibration for Tandem-L Gerhard Krieger, Sigurd Huber, Marwan Younis, Alberto Moreira, Jens Reimann, Patrick Klenk, Manfred Zink, Michelangelo Villano, Felipe Queiroz de Almeida Microwaves and Radar Institute, German Aerospace Center (DLR), Germany Abstract Precise knowledge of the far-field antenna patterns associated with individual receive channels of a multi- channel SAR system is of fundamental importance to operate the radar instrument in advanced imaging modes. A prominent example is Tandem-L which uses a large deployable parabolic reflector that is illuminated by a digi- tal feed array with multiple receive channels. This architecture enables, in combination with an appropriate on- board signal processing, the acquisition of a wide image swath by simultaneously recording multiple scattered radar echoes with a set of narrow elevation beams that are steered in real time towards the angles of the arriving wavefronts. As such a multiple elevation beam technique is prone to range ambiguities, optimized real-time beamformers are considered that maximize their gain in the direction of the desired radar echo and suppress, at the same time, the range ambiguous radar echoes arriving from different angles. The implementation of these advanced real-time beamforming techniques requires, however, precise knowledge of the amplitude and phase of the secondary far-field antenna patterns associated with the excitation of single feed elements. As it is impossible to measure these complex antenna patterns on ground with sufficient accuracy, we propose here a novel tech- nique that enables a highly accurate in-orbit measurement of the relative amplitude and phase of the far-field secondary antenna patterns associated with individual feed elements. The key idea is to collect SAR data in space by a set of dedicated calibration flights, where the signals from all feed elements are simultaneously rec- orded in a transparency mode, i.e., without any real-time beamforming on board the satellite. The multichannel data are then transferred to the ground, where the relative antenna pattern information is extracted and calibrated beamforming coefficients are derived, as required for the implementation of advanced SAR modes. This paper provides an overview of the proposed multichannel antenna calibration technique and demonstrates the superior SAR imaging performance that can be achieved by employing this technique in conjunction with a series of es- pecially designed calibration flights over natural terrain with known topography. 1 Introduction Tandem-L is a proposal for a highly innovative SAR mission to monitor the Earth system with unprecedented spatial and temporal resolution [1]. To meet the de- manding mission requirements, a new SAR instrument architecture and imaging mode has been developed [2], [3]. The system architecture is based on a large parabol- ic reflector antenna that is illuminated by a digital feed with multiple elevation channels (cf. Figure 1) [4]. As each feed element is associated with a different second- ary beam, it becomes possible to image a wide swath with high Rx gain by a time-variant combination of the feed signals in synchrony with the expected direction of arrival of the desired radar echo [5], [6]. The imaging capacity is further increased by using not only one but multiple elevation beams that map multiple swaths at the same time [7]. As these swaths are separated by blind ranges, several strategies and modes have been proposed to avoid such gaps in the SAR image [8]. Out of these modes, Tandem-L will employ a technique where the pulse repetition interval is rapidly changed from pulse to pulse [9]. This technique, now denoted as staggered SAR, has been further analyzed and elaborat- ed in detail in [10], [11]. In combination with an opti- mized Tandem-L reflector and feed system it becomes then possible to map a 350 km wide swath with an azi- muth resolution of 7 m, thereby significantly improving the imaging capacity if compared to state-of-the-art L-band SAR systems like ALOS-2 or even the C-band satellite constellation Sentinel-1A and 1B. While staggered SAR enables the acquisition of an ultra-wide image swath with high resolution, it requires also a notable oversampling in azimuth. Such an in- crease of the average pulse repetition frequency (PRF) is mandatory to avoid a rise of azimuth ambiguities caused by missing samples along the synthetic aperture [12]. The high PRF will, however, also increase the sus- ceptibility to range ambiguities. Range ambiguity sup- pression is further challenged by the required wide swath illumination, which causes multiple mutually ambiguous radar echoes to arrive at the same time from different elevation angles but with comparable magni- tudes (cf. Figure 1). This poses high demands on the multichannel receiver system which has to steer multi- ple elevation beams in real time towards the radar ech- oes’ expected directions of arrival. The shape of each of these receiver beams must be adjusted to maximize for each instant of time the antenna gain in the direction of the desired radar echo, while minimizing the gain to- wards the arrival angles of the interfering range ambig- uous radar echoes. 421 EUSAR 2018 ISBN 978-3-8007-4636-1 / ISSN 2197-4403 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach