Last night we filed an amicus brief in United States v. Pineda-Moreno, a Ninth Circuit case that could play a significant role in determining how broadly the Supreme Court’s recent GPS tracking decision, United States v. Jones, is applied to protect Americans’ privacy.

To back up a step, in January the Supreme Court issued its decision in Jones, holding that attaching a GPS device to a car and tracking its movements is a search under the Fourth Amendment. Although this conclusion was unanimous, the court issued three separate decisions that are something of a puzzle to fit together.

1) Writing for five justices, Justice Scalia relied on common-law trespass doctrine to hold that intruding onto a person’s property (in this case his vehicle) and gathering information (in this case a record of his movements) is a search under the Fourth Amendment.

2) Writing for four justices, Justice Alito agreed that the government had conducted a search, but held that it was the prolonged nature of the tracking itself, rather than any trespass, that implicated the Fourth Amendment.

3) Justice Sotomayor both joined Justice Scalia’s opinion and filed a separate concurrence explaining that she agreed with Justice Alito that at least long-term tracking is a search under the Fourth Amendment. (She also had some interesting things to say about the viability of the court’s older Fourth Amendment doctrines in the face of the dramatic technological changes of the past 20 or so years, but they’re not relevant here.)

While all the justices agreed that GPS tracking is a search, albeit for different reasons, none of them addressed whether it’s the sort of search that requires a warrant. While the default rule is that a warrant is necessary whenever a search is carried out—indeed, the Supreme Court has remarked that “warrantless searches are per se unreasonable under the Fourth Amendment…”—there are exceptions to the rule.

Pineda-Moreno, the case we filed in last night, may provide the Ninth Circuit with an early opportunity to weigh in on this question of whether a warrant is required.

Back in 2010, the Ninth Circuit concluded that tracking someone’s movements by attaching a GPS device to their car does not implicate the Fourth Amendment at all. In other words, it concluded that law enforcement agents were free to attach GPS devices to anyone’s car for any reason whatsoever, and that the Constitution has nothing to say about that. But this interpretation of the Fourth Amendment is not tenable in light of Jones, and the Supreme Court recently ordered the Ninth Circuit to reconsider its decision taking the Jones decision into account.

We haven’t seen the government’s brief yet in Pineda-Moreno, but presumably it will make the same argument here that it has made elsewhere: far from probable cause and a warrant being necessary, it is sufficient that an officer have “reasonable suspicion to believe that the defendant was involved in criminal activity and that evidence bearing on that offense would be found through the search.”

Our brief explains why this low standard provides inadequate protection for individuals’ reasonable privacy interests. We contend that, under Fourth Amendment jurisprudence, warrantless searches are presumptively unlawful. The exceptions to this rule are few and do not apply here: this case does not involve exigent circumstances or a stop and frisk, nor did it occur in an “exempt area,” such as a border or an airport, where special needs make obtaining a warrant impractical. Nor does this case implicate the automobile search exception the Court has created. To uphold the police conduct in this case would require the creation of an entirely new exception to the warrant clause, and no such exception can be justified.

As a majority of the Court recognized in Jones, government tracking of the whereabouts of a citizen for months on end is a massive intrusion on the expectation of privacy. Only the neutral evaluation of probable cause by a magistrate can justify a search of such magnitude.