Less than meets the eye
Since the Shuttle will be flying to the ISS, it's actually pretty easy to determine the launch window. The security measures really aren't accomplishing anything. According to this CNN article, there is speculation that the security measures are actually to protect Columbia's next launch because it'll be carrying Israel's first astronaut.
"Oberg theorizes that the chief intent is not really to protect Atlantis' flight to the international space station or Endeavour's launch on May 31, but to safeguard Columbia's liftoff in July with the first Israeli astronaut."