A GPR field experiment in the Republic of Djibouti provides evidence for very low radar penetration in arid volcanic materials, in the range 100--500 MHz. This phenomenon is attributed to the high iron oxide and evaporite concentration in soils, which significantly increases the conductivity, thus leading to poor subsurface imaging performances. The geologic context in Djibouti is shown to provide a good terrestrial analogue to Mars geology. Results of this study show that the future sounding radar missions to Mars may not reach the penetration depths previously anticipated. ¿ 2001 American Geophysical Union