SearchResearch Challenge (9/6/17): What's that thing on the ground?

Every time I fly into San Francisco airport... 

... I see this very strange antenna on the left side of the plane as we fly west to land.  (I figured out the lat/long for you so you don't have to extract it:  37.5469956,-122.2338807)    I've seen it for years, and yet, I don't have any idea what it is or why it's there.  It's all very mysterious.  (What ARE those red letters on the ground around it?) 

It's hard to see in this aerial image, but it's an odd antenna, with a large plate-like thing at the top of the mast. I was so surprised by this oddity that after my last flight, I drove over and took a closer look after landing.  It took a while to get there, as the roads around it aren't very straight, but I managed.  

Can you see the hexagonal frame at the very top?  (Click on the image to zoom in. Or if you want to, download the image and you can zoom as much as you'd like.)  

At the fence I also found this, in case it helps: 

This is one of those everyday mysteries--things you see all the time without understanding what it is. But that's NOT what SRS is about!  When we see mysteries, we try to understand what's happening.  So.. .

Can you figure out the story here? In particular: 

1.  What IS the story behind this antenna?  (A bit of history, please.)   

2.  Why does it have that funny plate plate at the top?

3.  Who uses this antenna now, and for what purpose? 

Next week, we'll talk about what we've found (and most importantly, HOW you can use your SRS skills to answer your own questions like this).  

Leave your notes in the comments--and please tell us all HOW you found out!  Did you use any special search methods?  Did you need to use any special databases?  

Search on!