Acura 2008 MDX Owner's Manual page 222

Playing the XM Satellite Radio (Models with navigation system)
Signal may be blocked by
mountains or large obstacles to
the south.
SATELLITE
216
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.