Control Segment (Monitor Stations) The GPS-System is controlled
by the United States Air Force. The “master control station”
(Schriever AFB) and four additional monitoring stations (on
Hawaii, Ascension Islands, Diego Garcia and Kawajalein) were set
up for monitoring the satellites. During August and September
2005, six more monitor stations of the NGA (National Geospatial-
Intelligence Agency) were added to the grid. Now, every
satellite can be seen from at least two monitor stations. This
allows to calculate more precise orbits and ephemeris data. For
the end user, a better position precision can be expected from
this. In the near future, five more NGA stations will be added
so that every satellite can be seen by at least three monitor
stations. This improves integrity monitoring of the satellites
and thus the whole system. The passive monitor stations are GPS
receivers which track all satellites in their range and collect
data of the satellite signals. The raw data are then sent to the
mater control station