PRF Selection in Formation-Flying SAR: Experimental Verification on Sentinel-1 Monostatic Repeat-Pass Data

Abstract

Formation-flying synthetic aperture radar (FF-SAR) enables new working modes and can achieve very high performance through a series of very compact, low-weight, satellite platforms thanks to passive operations of conveniently distributed formation-flying receivers. System timing is a crucial aspect of FF-SAR. The manuscript presents a novel approach to pulse repetition frequency (PRF) selection in order to obtain a uniform distribution of samples at given platform positions. A digital beamforming algorithm is applied on a stack of monostatic repeat-pass images collected by the Sentinel-1 system to test the validity of the PRF selection method. Processed images were thus properly selected to achieve the best merit index measuring the quality of samples distribution. The results show that: (a) the image resulting from beamforming features better azimuth ambiguity-to-signal ratio and (b) the proposed approach for PRF selection allows one to individuate a subset of the available images leading to uniform distribution of samples which can be used to support FF-SAR processing.

Time-Domain and Monostatic-like Frequency-Domain Methods for Bistatic SAR Simulation

“…A very good agreement is obtained: differences are only related to the use of the phase stationary method for the evaluation of the system TF, so that small amplitude oscillations of the signal spectrum are lost by the frequency domain simulator, but the spectrum mean amplitude and the bandwidth are perfectly reconstructed. In the second example, we consider system parameters of a receiver belonging to an along-track FF-SAR configuration, conceived for wide-swath imaging and SNR enhancement [8][9][10]. More specifically, we refer to the system described in [10], in which the receivers' formation follows the transmitter, used as an illuminator of opportunity, at large distance (several tens km), to ensure that the functionalities of the transmitting platform (e.g., telemetry, tracking, and command) remain unaffected by the presence of the receiving formation.…”

“…However, in recent years, an increasing interest has been devoted to bistatic and multistatic SAR acquisition geometries [ 1 , 2 , 3 , 4 , 5 , 6 ]. Indeed, the presence of a spatial separation between receiver and transmitter platforms can be effectively exploited to improve system performance and to widen the range of retrievable physical information from the acquired data [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. For instance, in the framework of Formation Flying SAR (FF-SAR), specific acquisition configurations can be considered in order to improve the signal-to-noise ratio (SNR), the spatial resolution, and the range coverage [ 7 , 8 , 9 , 10 ].…”

“…Indeed, the presence of a spatial separation between receiver and transmitter platforms can be effectively exploited to improve system performance and to widen the range of retrievable physical information from the acquired data [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. For instance, in the framework of Formation Flying SAR (FF-SAR), specific acquisition configurations can be considered in order to improve the signal-to-noise ratio (SNR), the spatial resolution, and the range coverage [ 7 , 8 , 9 , 10 ]. Moreover, more general bistatic/multistatic configurations grant an increased number of degrees of freedom with respect to the monostatic one in terms of acquisition geometry (no longer restricted to the backscattering case) that could be used to increase the retrievable information about the observed scene, also thanks to the availability of appropriate bistatic scattering models [ 11 , 12 , 13 , 14 ].…”

“…As a matter of fact, these acquisition geometries are of special interest in practical scenarios. The TI configuration well describes the basic configuration of an FF-SAR, where the flying paths of two (or more) platforms are jointly planned to guarantee the best working conditions for attaining specific tasks (e.g., SNR enhancement, interferometry applications, wide-swath imaging) [ 7 , 8 , 9 , 10 ]. As for the one-stationary configuration, it can be fruitfully used to exploit the signal transmitted by either orbiting satellites (space-surface configuration) or ad hoc planned airborne platforms [ 24 ].…”

Formation Flying SAR: Analysis of Imaging Performance by Array Theory

“…small and micro satellites, or when an illuminator of opportunity is used, is a rough estimation of the offset from the GPS time which is then refined by suitable post-processing of the collected raw bistatic data [45]. As for positioning errors, it is important to remark that tight orbit control is not required for FF-SAR operation [46], which sets instead tight requirements on the knowledge of the relative positions. Specifically, different requirements are posed by bistatic SAR processing and FF-SAR beamforming.…”