Mars Express instrument production. Technician assembling an oscillator for recording the frequency of transmission and reception signals from the MARSIS instrument on board the European Space Agency's Mars Express orbiter spacecraft. MARSIS is a sub-surface sounding radar altimeter. It emits low frequency radio waves from a 40 metre long antenna towards the Martian surface. It can map sub-surface structures to a depth of a few kilometres. Its primary objective is to map the distribution of water (both liquid and solid). Mars Express was launched in June 2003 and entered Martian orbit on 25 December 2003.