Using GNSS signals as a proxy for SAR signals: Correcting ionospheric defocusing

Abstract

Ultrahigh frequency space-based synthetic aperture radar (SAR) can suffer from the degrading effects of a scintillating ionosphere which modulates both the phase and the amplitude of the radar signal. In this paper, we use Global Navigation Satellite System (GNSS) signals to synthesize an L-band SAR point spread function (PSF). The process of transforming the GNSS signal to the equivalent SAR PSF is described. The synthesized PSF is used to explore the possibility of using a phase correction determined from a point target in a SAR image to correct the ionospheric degradation. GNSS data recorded on equatorial Ascension Island during scintillation events are used to test the feasibility of this approach by applying a phase correction to one GNSS receiver from another located along a magnetic east-west baseline. The peak-to-sidelobe ratios of the synthesized L-band SAR point spread functions before and after the correction are compared, and it is shown that this approach improves the L-band PSF over distances of~3000 m in the ionosphere, corresponding to 6000 m on the ground.

Measurement of Ionospheric Scintillation Parameters From SAR Images Using Corner Reflectors

“…The raw GPS data were processed in the manner described by [7]. Slant total electron content (TEC) values were calculated by differencing the raw L1 and L2 phase, and these were converted to an equivalent phase (advance) time series.…”

“…That this is a very real problem was demonstrated by [3], and a series of papers [4]- [6] has sought to relate the SAR image statistics and point spread function (PSF) to the strength of ionospheric turbulence. A recent paper [7] has also explored how L-band Global Navigation Satellite System (GNSS) signals can be used as a proxy for L-band SAR signals and explored how smallscale irregularities affect the signal correlation distances. The latter paper builds on earlier work [8] using V/UHF signals to evaluate SAR performance.…”

Minimum-Entropy Autofocusing Based on Re-PSO for Ionospheric Scintillation Mitigation in P-Band SAR Imaging

“…We define that χ 1 is selected as the premier parent which has the worst fitness among the selected chromosomes. The chromosome χ 1 is updated by (10). This operator will increase the diversity of the swarm and help some of the particles escape from the local optimum.…”

“…The spaceborne SAR signal propagating through the ionosphere electron density irregularities suffers the ionospheric deterioration, including the background ionosphere effect and the scintillation [5]- [9]. The background ionospheric effect can be effectively compensated by the prior knowledge of the total electron content [10], [11] or by other technologies [12], [13]. The ionospheric scintillation which is introduced by small scale irregularities (less than 10 km), typically occurs at equator and polar regions, from sunset until midnight [14], [15].…”

Performance Analysis of Ionospheric Scintillation Effect on P-Band Sliding Spotlight SAR System

“…Varying from the scale of spatial distribution, the ionosphere is typically categorized into the background ionosphere (larger than 10 km) and ionospheric irregularities (less than 10 km) [8,14]. The background ionospheric effect can be mitigated using the split-spectrum method or using the ionospheric prior knowledge acquired from the global navigation satellite system (GNSS)/BeiDou system [15,16,17,18]. In recent papers, the multi-squint (MS) interferometry methodology is proposed [5], which provides a new ionospheric mitigation approach for SAR system with limited bandwidth.…”