MARSIS is a low frequency, nadir-looking pulse limited radar sounder
and altimeter with ground penetration capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections.

    The operation altitudes for MARSIS are up to 800 km above the Martian surface for subsurface sounding and up to 1200 km for ionospheric sounding. 
    In its standard operating mode, the instrument is capable of making measurements in 1 MHz wide bands centred at 1.8, 3.0, 4.0 and 5.0 MHz
    
    This data set contains Total Electron Count data from the MARSIS subsurface sounder on Mars Express.  Useful information can  be found in the peer-reviewed article:
    Correction of the ionospheric distortion on the MARSIS surface
    sounding echoes, by Mouginot et al., Planetary and Space Science, Volume 56, Issue 7, p. 917-926, 2008