My best guess (about 90% certainty) is that it is a Helipad.

http://www.themilton.co.uk/images/pic-helipad.jpg

http://www.airfields-freeman.com/CA/WarEagleHelo_CA_05_n.jpg

~It's located about where I would expect one.
~It's about the right size.
~I don't see a Helipad anywhere else on the field.
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* I have lived long and seen much - But GE has allowed me to see new things and revisit old *