The Constitutional questions here are very interesting and rapidly evolving. For example, is requesting cell site location data like tracking a beeper in a can of ether on a public highway or like a pen register? Or is it more like a wiretap of a search of a building? What happens when GPS is turned on?

There isn't a lot of case law here. In general though courts have expressed concern about the sorts of surveillance that are becoming possible. See, for example the cell site location arguments at the 3rd circuit appeals court or the opinions of Sotomayor and Alito in Jones v. United States last year (Alito's opinion concurring in judgement was joined by Kagan, Breyer, and Ginsberg. Sotomayor joined the majority of Roberts, Scalia, Thomas, and Kennedy as a narrow way to resolve the case, but she also went beyond that and endorsed Alito's logic as well, which complicates counting votes).