"Scalia created a "firm but also bright" line drawn by the Fourth Amendment at the "'entrance to the house'".[1] This line is meant to protect the home from all types of warrantless surveillance and is an interpretation of what he called "the long view" of the Fourth Amendment."
Mobile apps that use GPS to track people=brain freeze.
Given his standing on Clapper v. Amnesty, I think this falls under his more general view of privacy rights...which is not great, as Scalia doesn't really acknowledge Griswold v. Connecticut when the right to privacy was first established, and he has a strong reluctance to question national security agencies on issues he doesn't seem to know very much about...
“The consequence of that is that whether the NSA can do the stuff it’s been doing ... which used to be a question for the people ... will now be resolved by the branch of government that knows the least about the issues in question, the branch that knows the least about the extent of the threat against which the wiretapping is directed,” Scalia said.