What is an open array scanner?

I'm familiar with electronically steered phased array antennas and the
traditional fixed phase array antennas which use mechanical rotation to

sweep the radar beam but I have never heard of an open array scanner

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Could be an example of a commonly used, commonly understood term that
is not technically correct.

An "open array" radar is a term often used to describe a radar where
the antenna rotates in the open air, rather than under a dome.