Data Volume Reduction in High-Resolution Wide-Swath SAR Systems Michelangelo Villano, Gerhard Krieger, Alberto Moreira Microwaves and Radar Institute German Aerospace Center (DLR) Wessling, Germany michelangelo.villano@dlr.de Abstract— High-resolution wide-swath (HRWS) synthetic aperture radar (SAR) systems are very attractive for the observation of dynamic processes on the Earth’s surface, but they are also associated with a huge data volume. In order to comply with azimuth ambiguity requirements, in fact, a pulse repetition frequency (PRF) much higher than the required processed Doppler bandwidth (PBW) is often desirable. The data volume can be drastically reduced, if on-board Doppler filtering and decimation are performed prior to downlink. A finite impulse response (FIR) filter with a relatively small number of taps suffices to suppress the additional ambiguous components and recover the original impulse response. This strategy is also applicable and especially relevant to staggered SAR systems, where on-board Doppler filtering and resampling can be jointly implemented. Index Terms—Synthetic aperture radar (SAR), high-resolution wide-swath (HRWS) imaging, staggered SAR, data volume reduction, finite impulse response (FIR) filter, on-board processing. I. INTRODUCTION Synthetic aperture radar (SAR) is a remote sensing technique, capable of providing high-resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earth’s surface [1]. However, conventional SAR systems are limited, in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by high-resolution wide-swath (HRWS) systems based on digital beamforming (DFB) on receive, where multiple swaths can be simultaneously imaged using multiple elevation beams [2]. Moreover, if the system is operated in staggered SAR mode, i.e., if the pulse repetition interval (PRI) is continuously varied, it is also possible to get rid of the “blind ranges”, present between adjacent swaths, as the radar cannot receive while it is transmitting [3], [4]. Due to their resolution and coverage requirements, HRWS systems are inherently associated with a huge data volume, thereby increasing the demands for internal data storage, downlink, ground processing and archiving. Recent studies related to Tandem-L, a proposal for a polarimetric and interferometric satellite mission to monitor dynamic processes over the Earth’s surface with unprecedented accuracy and resolution, quantify the volume of the acquired data as 8 TB/day [5]. Moreover, in order to comply with azimuth ambiguity requirements, a pulse repetition frequency (PRF) much higher than the required processed Doppler bandwidth (PBW) is often desirable. For a HRWS SAR system with constant PRI and multiple elevation beams, in order to achieve a good azimuth ambiguity-to-signal ratio (AASR), the required PRF is usually even larger than twice the PBW. As an example, in a SAR system with PRF = 1800 Hz and PBW B p = 780 Hz, the data volume to be downlinked increases by more than 130% due to the azimuth oversampling. The system, in fact, downlinks data included in the Doppler frequency interval [-PRF/2, PRF/2], while only data in the Doppler frequency interval [-B p /2, B p /2] are needed to achieve the desired azimuth resolution. The information contained in the Doppler frequency intervals [- PRF/2, -B p /2] and [-B p /2, PRF/2] is useless and discarded in the SAR processing. If the system is operated in staggered SAR mode, the ratio of the mean PRF on transmit to the PBW can be even larger than 3. This determines a further increase of the data volume to be downlinked with a direct impact on the cost of the mission. As an example, for a staggered SAR system with a mean PRF on transmit PRF meanTX equal to 2700 Hz and a PBW B p = 780 Hz, the data volume to be downlinked increases by almost 250%. II. DATA VOLUME REDUCTION CONCEPT Let us first consider the case of a SAR system with constant PRI. If data were just decimated prior to downlink (e.g., by a factor of 2 in the latter example where PRF = 1800 Hz and B p = 780 Hz), a considerable degradation of the AASR would occur. Fig. 1 (a) shows the power spectral density (PSD) of the azimuth SAR signal at near range for an L-band reflector antenna with a diameter of 15 m. The PSD is the joint transmit- receive antenna pattern, displayed as a function of Doppler frequency. The unambiguous energy, the ambiguous energy, and the additional ambiguous energy due to the decimation are highlighted in green, red, and blue, respectively. As is apparent, the additional ambiguous energy due to decimation is significant, i.e., the total ambiguous energy is the same obtained for PRF = 1800 Hz / 2 = 900 Hz. However, if Doppler filtering is performed before decimation, the additional 119 978-1-4673-7297-8/15/$31.00 c 2015 IEEE