Blaupunkt TravelPilot 54 Guide De Mise En Route Rapide page 21

GPS satellite signal can not pass through solid materials (except glass). When you
are inside a tunnel or building, GPS positioning is not available.
A minimum of 4 GPS satellite signal is needed to determine current GPS position.
Signal reception can be affected by situations such as bad weather or dense
overhead obstacles (e.g. trees and tall buildings).
Other wireless devices in the vehicle might interface with the reception of satellite
signals and cause the reception to be unstable.
A car holder id needed when using the device in a car. Be sure to place the device
in an appropriate place, so as not to obstruct the driver's view or deployment of air
bag.
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