The ESA (European Space Agency) ERS (Earth Resources Satellite) programme was composed of two missions, ERS-1 and ERS-2, which were launched into the same orbit in 1991 and 1995 respectively. The two spacecraft were designed as identical twins with one important difference – ERS-2 included an extra instrument (GOME) designed to monitor ozone levels in the atmosphere. Due to the satellites' shared orbit, a tandem mission was implemented following the launch of ERS-2, whereby ERS-2 passed the same point on the ground one day later than ERS-1.