If you're going through tunnels, under or beside large trees, through underpasses, or in the canyons of a big city with skyscrapers, this will be normal. The antenna needs line of sight to the satellite. In some large cities, SIRIUS broadcasts locally.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles