GNSS, such as GPS,
GLONASS, and the
planned GALILEO and
COMPASS systems, can
also be used to monitor
changes in the atmosphere.
Due to the fact that
the GNSS signals travel
through the atmosphere,
they can be used to derive
some very important parameters
such as:
the total electron content
of the ionosphere,
and
the precipitable water
vapour in the troposphere.