That's interesting and not unexpected.

With GPS signals the many birds aloft send weak signals to Earth and its up to the receiver to decode the identities and match them to the ephemeris. This can get confusing to GPS if there are radio-reflecting surfaces very near the antenna. Then the reflected signals reach teh antenna with slight time delays and if the surfaces are sufficiently reflective it may be the the receiver will fail to lock onto the signal. This is multipath reflection.

I wonder if XM or Serius have a similar problem with multipath? I mean, if your antenna is close to some reflective surface, will the antenna capture multiple signals and the receiver perhaps fail to discriminate the direct signal from the multipath time-delayed signals?

Is there an EE in the house?

Botnst