Robert,

tests and/or usage of ground-based jamming equipment.

As the people at Schriever AFB have explained to me, uploads of data
(each satellite has to be "touched" by the ground station at least once
per day - talk about maintenance intensive) do NOT disrupt service.
Also, it is not really possible to deny service to well defined
geographic areas by switching off satellites.

--
Thomas Borchert (EDDH)