I mean that to get a good signal, the southern sky should be as unobstructed as possible. Satellites use microwaves to transmit their signals and they are almost purely line-of-site. It doesn't matter if you have an omnidirectional antenna, if there is a mountain or large building to your immediate south, it may block the satellite signal. If you are in a city with earth repeaters, then this isn't an issue.
Think of this example. The moon is a satellite. Our eyes can be turned any direction, so essentially they are an omnidirectional antenna/receiver. If the moon is below the horizon or is obstructed from view by a building or mountain, then our eyes don't receive the signal (reflected light) from the moon.
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XM Coverage Map