Mission Planner Ground Control Station

In a Nutshell, Mission Planner Ground Control Station...

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  has had 7,003 commits made by 172 contributors
  representing 1,697,870 lines of code
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  is mostly written in C#
  with a well-commented source code
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  has a well established, mature codebase
  maintained by a large development team
  with decreasing Y-O-Y commits
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  took an estimated 478 years of effort (COCOMO model)
  starting with its first commit in February, 2013
  ending with its most recent commit 3 months ago