What is a large grey A map symbol?

I spotted a large A symbol at 62,000 ft squawking 7777 (military interception) at a speed of 5 kts. What is it, an ADSB testing facility?

It’s a ADSB groundstation indeed.
Here’s an example of 3 stations around EBBL (Liege Airport Belgium)

It’s a tower, not an A.

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