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Navigation
Almanac data takes a long time to download for a GPS receiver.
Ephemeris data is very precise data from each individual satellite regarding its
position and the exact time of transmission. This data is transmitted every 30
seconds.
You could say that Ephemeris data enables the GPS receiver to calculate position,
while almanac data helps the GPS receiver know where to look for other
satellites.
If you start a GPS that has not been started recently or has been moved and does
not know its position, it takes about 12.5 minutes to transfer almanac data and
easily 15 minutes before you have a 3D position.
The Control Segment
The Control Segment is located in the USA. It sends information to each satellite
via Ground Antennas. This information, as well as information from the satellite's
atomic clock, is transmitted in the form of two sideband signals at 1575.42 MHz
and 1227 MHz.
Using this information, a GPS receiver can calculate the aircraft's position.
The User Segment
The User Segment consists of the
receivers we have in our airplanes.
They can have different numbers of
channels.
Often 5, 8 or more and often they can
receive both GPS and Glonass
satellites.
Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 898
