This paper describes the operating principles and the design of the Mars Shallow Radar Sounder (SHARAD), an HF sounding radar devoted to the map of sub-surface features of Mars and embarked on board the Mars Reconnaissance Orbiter spacecraft. SHARAD is characterised by its low carrier frequency (20 MHz), its relatively high range resolution (and, therefore, a large fractional bandwidth) and the need to achieve very low chirp sidelobes in order to allow discrimination of sub-surface echoes in presence of much stronger surface returns. Several processing results of SHARAD data are presented to demonstrate that the instrument is properly working around Mars and it is able to achieve primary scientific objectives: map the distribution of liquid and solid water in the upper portions of the
