2.1 POSITIONING SYSTEM FOR PA
Positioning system works by the help of differentconstellations of satellites. Developments of
these different positioning systems are the main technological milestone, which made PA
concept a reality. Global positioning system (GPS), developed by US Department of Defense
(DOD). Similar positioning system is Russian GLONASS positioning system. A European global
navigation satellite system (GNSS), "Galileo" isunder discussion (Spiller et al., 1998) and for
that a 'definition study' funded by European Union (EU) and the European Space Agency
(Stafford, 2000) is going on.