Artist's impression of a Defense Satellite Communications System Phase III (DSCS III) satellite. A network of six DSCS III satellites are planned, two being in-orbit spares. From their geosynchronous orbit, the satellites in the network can carry 1,300 duplex voice links for tactical and strategic users in the US armed forces. The satellites have a degree of nuclear hardening, enabling them to operate in the intense electromagnetic field generated by nuclear weapons blasts. The first DSCS III pair was launched in October 1985 on Shuttle Atlantis, by early 1991 a total of four were believed to be in service.