Honda 2008 Civic Sedan Owner's Manual page 146

Audio System (Models with navigation system)
Signal may be blocked by
mountains or large obstacles to
the south.
SATELLITE
140
Signal weaker in
these areas.
GROUND REPEATER
The XM satellites are in orbit over
the equator; therefore, objects south
of the vehicle may cause satellite
reception interruptions. To help
compensate for this, ground-based
repeaters are placed in major
metropolitan areas.
Satellite signals are more likely to be
blocked by tall buildings and
mountains the farther north you
travel from the equator. Carrying
large items on a roof rack can also
block the signal.