R&S SMW-K44 User Manual page 18

Satellite Navigation
Figure 4-2: GNSS system components (simplified)
1 = Space segment or satellites
2 = Ground segment or ground stations
3 = User segment or receivers
4 = Ephemeris (broadcasted satellites orbit and clock)
5 = Broadcasted navigation message
Space segment
The space segment consists of the satellites that orbit the earth on their individual
orbits. Satellites broadcast signals at specific frequency in the L band and spread by
predefined codes. For the GPS satellites using L1 frequency band, for instance, these
are the coarse/acquisition (C/A) or the precision (P) codes.
The transmitted signal carries the navigation message, on which each satellite broad-
casts its major characteristics, its clock offsets and precise orbit description, where the
latter is called ephemeris. The navigation message contains also satellites status
information, ionospheric and time-related parameters, UTC information and orbit data
with reduced accuracy for all other satellites, commonly referred as almanac.
Ground segment
The ground segment is a network of ground stations whose primary goal is to mea-
sure constantly the satellites' location, altitude and velocity, and the satellites signals.
The ground stations also estimate the influence of the ionosphere. They calculate the
precise orbit (and orbit perturbation) parameters and clock drifts parameters of
each satellite. This corrected highly accurate information is regularly broadcasted back
to the satellites so that their navigation messages can be updated.
User Manual 1178.6379.02 ─ 01
About GNSS
GNSS Components Overview
18