Geoscience Reference
In-Depth Information
are tracking of the satellites for the orbit and clock determination and predic-
tion, time synchronization of the satellites, and upload of the data message
to the satellites.
Master control station
The master control station is located at the Consolidated Space Opera-
tions Center (CSOC) at Shriver Air Force Base, Colorado Springs, Colorado.
CSOC collects the tracking data from the monitor stations and calculates
the satellite orbit and clock parameters by a Kalman estimator. These re-
sults are then passed to one of the three ground control stations for eventual
upload to the satellites. The satellite control and system operation is also
the responsibility of the master control station.
Monitor stations
There are five monitor stations located at Hawaii, Colorado Springs, As-
cension Island in the South Atlantic Ocean, Diego Garcia in the Indian
Ocean, and Kwajalein in the North Pacific Ocean. Each of these stations
is equipped with a precise atomic time standard and receivers which con-
tinuously measure pseudoranges to all satellites in view. Pseudoranges are
measured every 1.5 seconds and, using ionospheric and meteorological data,
they are smoothed to produce 15-minute interval data which are transmitted
to the master control station.
Ground control stations
These stations collocated with the monitor stations at Ascension, Diego Gar-
cia, and Kwajalein are the communication links to the satellites and mainly
consist of the ground antennas. The satellite ephemerides and clock informa-
tion, calculated at the master control station and received via communication
links, are uploaded to each GPS satellite via S-band radio links.
User segment
The diversity of the military and civilian users is matched by the type of
receivers available today.
On the basis of the type of observables (i.e., code pseudoranges or phase
pseudoranges) and of the availability of codes (i.e., C/A-code, P-code, or
Y-code), GPS receivers can be classified. For the majority of navigation ap-
plications, C/A-code pseudorange receivers will suce. With this type of
receiver, only code pseudoranges using the C/A-code on L1 are measured.
Typical devices output the three-dimensional position either in latitude, lon-
gitude, and height or in some map projection systems, e.g., universal trans-
verse Mercator (UTM) coordinates and height.
