Does glide ratio change with altitude?

During a recent discussion about calculating glide ratio, I began to wonder
what the effects of pressure altitude were on the glide ratio of an
aircraft. Since air is 50% as dense at FL180 as at sea level, would the
glide ratio increase (glide further) as altitude decreases? AND if there is
a difference in glide ratio as altitude changes, then what values do most
manufacturers use when they publish their numbers (if they do)?

Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the
wags can skip that part of the reply...

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Jim Carter
Rogers, Arkansas