An Extended Model of Ionospheric Dispersion Effects for Nonlinear Frequency Modulation Signal and Correction Method

Nonlinear frequency modulation (NLFM) signal can construct the signal’s power spectral density (PSD) to reduce sidelobes without loss of signal-to-noise ratio. LuTan-1 (LT-1) is an L-band spaceborne synthetic aperture radar (SAR) mission which is launched in the beginning of 2022, and a high-precision NLFM signal generator is developed in LT-1. However, the existing model, i.e., the traditional frozen ionosphere model, cannot accurately describe ionospheric dispersion effects faced by the NLFM signal due to the nonlinear characteristic of the instantaneous frequency. Thus, an extended model is established in this letter to describe ionospheric dispersion effects of the NLFM signal. Then, the differences of ionospheric dispersion effects on the NLFM and linear frequency modulation (LFM) signals are compared. Afterward, a method that embedded into the focusing procedure is proposed, which aims to eliminate ionospheric dispersion effects for the NLFM signal. Finally, the hardware-in-the-loop simulations of point targets and distributed targets are performed to verify the proposed method. The method proposed in this letter is used in the ground processing system of LT-1.

I. Introduction

Ionospheric disturbances are not negligible for the low-frequency (for example, L-band and P-band) spaceborne synthetic aperture radar (SAR). The phase dispersion effect is one of the main interference factors [1]–[4]. It focuses on the time delay and phase change experienced by the signal when the SAR signal passes through the ionosphere. At the echo domain, the phase advance caused by the dispersion effect will lead to the problems such as image defocusing and image shift. Furthermore, at the image domain, the phase error introduced by the dispersion effect will affect the accuracy of the DEM reconstruction.