The FISA is basically used by America to spy on its own citizens. This could be constitution but it needs more regulation and amendments added to it. It is easy to say that someone is suspected of spying or may commit a terrorist act, in order to make it acceptable to spy on someone. There needs to be more protocols in place.
No, FISA is not constitutional, because it goes far beyond the bounds of constitutional authority. The federal government has used FISA as a mechanism to start warrantless wiretaps, without even informing the public that it is happening. The United States should only intervene and do such activities when there are specific threats to American security.