The Berkley Technology Law Journal posted a short article last week on this issue, Privacy Expectations in the Use of GPS Tracking Devices: United States v. Maynard.
In this article, Musetta Durkee observes that GPS tracking is different in kind, not just degree, from discrete observations:
As with most aggregations of information where the individual pieces of information, on their own, would be meaningless, when taken together these pieces reveal patterns of information that convey more than the individual pieces. As such with the aggregation of an individual's movements over a prolonged period: previously private information, in which our society recognizes an expectation of privacy, becomes public. In the case of GPS tracking devices, this private-turned-public information thereby is made available to authorities without a warrant.
Durkee addresses a key criticism to this analysis, mainly "how would law enforcement know, before actually reaching the threshold where an individual's privacy has been breached, when patterns of behavior, instead of discrete acts, are being revealed?" His conclusion that a legislative solution to the question of GPS tracking is premature, I believe. This is because the law is too unsettled on this matter. As long as law enforcement has a fairly strong argument that the warrantless use of GPS tracking devices raises no Fourth Amendment issues, then law enforcement and homeland security interests can push hard to be exempted from any legislation on this issue.
My view remains that GPOS devices implicate the Fourth Amendment. GPS devices permit law enforcement to conduct surveillance beyond a targeted investigation into a certain crime and could permit law enforcement to undertake surveillance of a particular individual over an extended period of time in the hope of piecing together evidence of illegal conduct that was not suspected prior to the surveillance. Especially where the cases involve sustained and long-term surveillance of a targeted individual unrelated to any particular criminal action, no reasonable person would expect to be the target of such a massive police surveillance operation. Accordingly, because the use of these devices infringes on a legitimate expectation of privacy, the use of these devices constitutes a search which, absent the present of another exception, requires a warrant.
For more details, see here for a draft of an article I am publishing on the topic this winter.