The XM weather data system relies on the XM radio system for accessing the data inflight.

The XM system uses two satellites (called interestingly enough "Rock" and "Roll").  These two satellites are in syncronous orbit and are fixed in the south eastern and south western sky.  Each satellite covers the entire US with its powerful signal, however since the data comes from two directions, signal coverage on the ground where trees and other obstacles might exist is improved greatly.  The XM signal, being a microwave frequency is quite directional and is very line of sight. The satellites are approximately 40 degrees above the southern horizon in the central US.

In flight, the only obstructions to be concerned about are the actual structure of the aircraft and accessories such as heated windshield plates.  

If you are experiencing XM TIMEOUT messages or data dropouts and you are flying northbound, your problem is most likely antenna placement.  Since the satellites are in the South Eastern and South western skies, antenna plcement is most critical when the aircraft is heading northbound. 

The ideal place in any aircraft for the antenna is near the top center of the windshield.  This location assures that the antenna can see the satellites when the aircraft is heading in any direction. There is minimal possibility of aircraft structure interfernig with the antenna reception.

On many aircraft, the antenna can simply be placed on the glare shield with good performance.  If the glare shield is small, performance in this location may be inadequate when the aircraft is northbound.    On some aircraft, it may be necessary to position the antenna on the southmost side of the glareshield when heading nearly north to insure adequate reception.