After hijacking Kur's blog (sorry Kur), I tried to post my logs, but it appears if you post in a blog post, you can't post files. Anyways, here is my logs using ALT_Hold with a sonar.
Note: I don't think anything is wrong with SONAR, but check out GPS Alt. It goes lower and lower and lower. I swear I'm just hovering it around the front yard.
My APM2 is in the hard case, so it's not a barometer problem.
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I forgot to summarize for those who have not read Kur's blog. The problem is that when you use Alt_Hold with Sonar, the copter just climbs and climbs.