An improved comtrast method with variable window limitation for dispersion compensation of MARSIS echoes and TEC inversion

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on onboard the European Space Agency Mars Express spacecraft is a low-frequency nadir-looking radar sounder. The operation frequency is close to the maximum plasma frequency of Martian ionosphere. Accurate estimation of dispersion phase is beneficial for focusing imaging of echo signals, and accurate inversion of Total Electron Content (TEC). This paper proposes an improved Contrast Method (CM) to estimate the dispersion phase with Gamma model and local window constraint. Firstly, the local maxima detection is used to determine accurate group delay value based on the horizontal continuity of ionosphere. Then, the Gamma model and window function are used to optimize the higher-order terms of dispersion phase. Finally, all terms of dispersion phase are synthesized to compute the compensation phase for echo correction and to precisely estimate TEC along track. Compared to the conventional CM, the new method not only corrects group delay correctly, but also achieves stable high-order expansion terms, providing a better correction approach for ionospheric compensation of Mars subsurface radar.