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hippy-
When using both GPS and GLONASS, the receiver has the ability to lock on to 24 more satellites than using GPS alone. This is particularly helpful for users who need reliable satellite navigation in challenging environments such as deep canyons or urban environments where a large portion of the sky is blocked by solid objects. The disadvantage to using both systems simultaneously is a reduction in battery life.
I can only assumed why, but the gist is that using both GPS/GLONASS take a little more power for a lots more precise lock on.
edscoble
The lower battery is likely down to the addition of using GLONASS as well.