Factual error: The film's plot violates the third of the Ten Immutable Laws of Security by Scott Culp. The premise of the law is: When one has physical access to a computer system, given enough time, he can take over the system. In the film, the reverse happens: Dane takes over an ATAC site remotely while people inside (who have complete physical access) cannot do anything to wrest the control back. In the real world, it is usually possible to simply cut the connection cable or antenna and take control back. (Even Windows XP and later have such simple lockdown provisions as part of Windows Firewall.) After the lockdown, the passcodes can be changed and other measures taken.
Plot hole: Dane's motivation for hijacking the satellite is supposedly because ATAC fired him. But he would have already had the satellite programmed to respond if he used it remotely like he does on the train except he wouldn't have known he'd be fired at the time, so he'd have no reason to have the satellite ready to hijack.