And just to be clear, the POTUS/super-special VIP transport is run by the USMC out of Quantico[1], a bit further to the south along the Potomac. Belvoir is US Army, and does have VIP heelicopters (obviously), but it's not the same group that lands at the White House.

1 - https://en.wikipedia.org/wiki/HMX-1 They have some Ospreys and other things as well, but HMX-1 is the most famous and recognizable.