The breakdown in this case is very odd. All nine justices agree that the search was unconstitutional.

Scalia, Roberts, Kennedy, Thomas, and Sotomayor (kinda) rely on Nino’s originalist common-law trespass view of the Fourth Amendment and need not reach Harlan’s reasonable expectation of privacy test. The opinion is short and sweet–12 pages.

Sotomayor concurs, by herself, writing that since there was a physical intrusion, she joins the narrower basis. Soto could have totally split a 4-1-4. Curious.

Alito concurs in judgment, joined by Ginsburg, Breyer, and Kagan, in a distinctly unoriginalist manner, focusing on whether the reasonable expectation of privacy was violated  by the long-term monitoring of the vehicle.

(I’ll have more shortly).

Scalia majority opinion

Scalia goes all-Olmstead on the Fourth Amendment, and demands a physical search.

It is important to be clear about what occurred in thiscase: The Government physically occupied private property for the purpose of obtaining information. We have no doubt that such a physical intrusion would have beenconsidered a “search” within the meaning of the FourthAmendment when it was adopted. Entick v. Carrington, 95 Eng. Rep. 807 (C. P. 1765), is a “case we have described as a ‘monument of English freedom’ ‘undoubtedly familiar’to ‘every American statesman’ at the time the Constitutionwas adopted, and considered to be ‘the true and ultimate expression of constitutional law’” with regard to searchand seizure. . . .

The text of the Fourth Amendment reflects its close connection to property, since otherwise it would have referredsimply to “the right of the people to be secure against unreasonable searches and seizures”; the phrase “in their persons, houses, papers, and effects” would have been superfluous

And lots of cites to Kyllo:

Consistent with this understanding, our Fourth Amendment jurisprudence was tied to common-law trespass, at least until the latter half of the 20th century. Kyllo v. United States, 533 U. S. 27, 31 (2001); Kerr, The Fourth Amendment and New Technologies: Constitutional Myths and the Case for Caution, 102 Mich. L. Rev. 801, 816 (2004). Thus, in Olmstead v. United States, 277 U. S. 438 (1928), we held that wiretaps attached to telephonewires on the public streets did not constitute a FourthAmendment search because “[t]here was no entry of the houses or offices of the defendants,” id., at 464.

But then, drats, Katz!

Our later cases, of course, have deviated from that exclusively property-based approach. In Katz v. United States, 389 U. S. 347, 351 (1967), we said that “the Fourth Amendment protects people, not places,” and found a violation in attachment of an eavesdropping device to a public telephone booth. Our later cases have applied theanalysis of Justice Harlan’s concurrence in that case,which said that a violation occurs when government officers violate a person’s “reasonable expectation of privacy,” id., at 360.

But Scalia effectively says Katz need not apply, relying on his baby Kyllo, which is not the end-all-be-all of the Fourth Amendment:

But we need not address the Government’s contentions, because Jones’s Fourth Amendment rights do not rise or fall with the Katz formulation. At bottom, we must “assur[e] preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted.” Kyllo, supra, at 34. As explained, for most of our history the Fourth Amendment was understood to embody a particular concern for government trespass upon the areas (“persons, houses, papers, and effects”) it enumerates.3 Katz did not repudiate that understanding. Less than two years later the Court upheld defendants’ contention that the Government could not introduce against them conversations between other people obtained by warrantless placement of electronicsurveillance devices in their homes. The opinion rejectedthe dissent’s contention that there was no Fourth Amendment violation “unless the conversational privacy ofthe homeowner himself is invaded.”4 Alderman v. United States, 394 U. S. 165, 176 (1969). “[W]e [do not] believethat Katz, by holding that the Fourth Amendment protectspersons and their private conversations, was intended to withdraw any of the protection which the Amendment extends to the home . . . .” Id., at 180.

Nino replies to Alito in a footnote to try to save originalism:

JUSTICE ALITO’s concurrence (hereinafter concurrence) doubts the wisdom of our approach because “it is almost impossible to think oflate-18th-century situations that are analogous to what took place inthis case.” Post, at 3 (opinion concurring in judgment). But in fact it posits a situation that is not far afield—a constable’s concealing himself in the target’s coach in order to track its movements. Ibid. There is no doubt that the information gained by that trespassory activity would bethe product of an unlawful search—whether that information consisted of the conversations occurring in the coach, or of the destinations to which the coach traveled. In any case, it is quite irrelevant whether there was an 18th-centuryanalog. Whatever new methods of investigation may be devised, our task, at a minimum, is to decide whether the action in question would have constituted a “search” within the original meaning of the Fourth Amendment. [JB: Original expected application originalism or something like that] Where, as here, the Government obtains information byphysically intruding on a constitutionally protected area, such a search has undoubtedly occurred.

Alito has a funny rejoinder to this silly constable example:

The Court suggests that something like this might have occurred in 1791, but this would have required either a gigantic coach, a very tinyconstable, or both—not to mention a constable with incredible fortitude and patience.

And this:

The concurrence begins by accusing us of applying“18th-century tort law.” Post, at 1. That is a distortion. What we apply is an 18th-century guarantee against un- reasonable searches, which we believe must provide at a minimum the degree of protection it afforded when it was adopted. The concurrence does not share that belief. It would apply exclusively Katz’s reasonable-expectationof-privacy test, even when that eliminates rights thatpreviously existed.

And Scalia stresses that Katz could remain viable for cases involving *just* the electronic transmisison of signals (this screams of something added by another Justice, as it departs so heavily from what Nino has written elsewhere):

For unlike the concurrence, which would make Katz the exclusive test, we do not make trespass the exclusive test. Situations involving merely the transmission of electronic signals without trespass would remain subject to Katz analysis.

Here Scalia suggests that monitoring a car 24/7 with humans would be constitutional, but the same approach through electronic means would be unconstitutional.

Thus, even assuming that the concurrence is correct tosay that “[t]raditional surveillance” of Jones for a 4-weekperiod “would have required a large team of agents, multiple vehicles, and perhaps aerial assistance,” post, at 12, our cases suggest that such visual observation is constitutionally permissible. It may be that achieving the sameresult through electronic means, without an accompanying trespass, is an unconstitutional invasion of privacy,but the present case does not require us to answer that question.

Scalia has another interesting footnote in response to Alito exploring the interplay between a search and a trespass:

5The concurrence notes that post-Katz we have explained that “ ‘an actual trespass is neither necessary nor sufficient to establish a constitutional violation.’ ” Post, at 6 (quoting United States v. Karo, 468 U. S. 705, 713 (1984)). That is undoubtedly true, and undoubtedly irrelevant. Karo was considering whether a seizure occurred, and as the concurrence explains, a seizure of property occurs, not when there is a trespass, but “when there is some meaningful interference with an individual’s possessory interests in that property.” Post, at 2 (internal quotation marks omitted). Likewise with a search. Trespass alonedoes not qualify, but there must be conjoined with that what was present here: an attempt to find something or to obtain information. Related to this, and similarly irrelevant, is the concurrence’s point that, if analyzed separately, neither the installation of the device nor its use would constitute a Fourth Amendment search. See ibid. Of course not. A trespass on “houses” or “effects,” or a Katz invasion of privacy, is not alone a search unless it is done to obtain information;and the obtaining of information is not alone a search unless it isachieved by such a trespass or invasion of privacy.

Scalia reconciles this opinion with Knotts:

The first case, Knotts, upheld against Fourth Amendment challenge the use of a“beeper” that had been placed in a container of chloroform,allowing law enforcement to monitor the location of thecontainer. 460 U. S., at 278. We said that there had been no infringement of Knotts’ reasonable expectation of privacy since the information obtained—the location of the automobile carrying the container on public roads, andthe location of the off-loaded container in open fields nearKnotts’ cabin—had been voluntarily conveyed to the public.
6 Id., at 281–282. But as we have discussed, the Katz reasonable-expectation-of-privacy test has been added to, not substituted for, the common-law trespassory test. The holding in Knotts addressed only the former, since thelatter was not at issue. The beeper had been placed inthe container before it came into Knotts’ possession, withthe consent of the then-owner. 460 U. S., at 278. Knotts did not challenge that installation, and we specifically de- clined to consider its effect on the Fourth Amendment analysis. Id., at 279, n. Knotts would be relevant, perhaps, if the Government were making the argument thatwhat would otherwise be an unconstitutional search is not such where it produces only public information. The Government does not make that argument, and we knowof no case that would support it.

Oh boy that is a narrow distinction.

In any event, resolving this case based on trespass obviates the need to drawn lines. When is a search too long to be unreasonable?

And answering it affirmatively leads us needlessly into additional thorny problems. The concurrence posits that “relatively short-term monitoring of a person’s movements on public streets” is okay, but that “the use of longer term GPS monitoring in investigations of most offenses” is no good. Post, at 13 (emphasis added). That introduces yetanother novelty into our jurisprudence. There is no precedent for the proposition that whether a search has occurred depends on the nature of the crime being investigated. And even accepting that novelty, it remainsunexplained why a 4-week investigation is “surely” toolong and why a drug-trafficking conspiracy involving sub- stantial amounts of cash and narcotics is not an “extra- ordinary offens[e]” which may permit longer observation. See post, at 13–14. What of a 2-day monitoring of a suspected purveyor of stolen electronics? Or of a 6-month monitoring of a suspected terrorist? We may have tograpple with these “vexing problems” in some future case where a classic trespassory search is not involved andresort must be had to Katz analysis; but there is no reason for rushing forward to resolve them here.

Sotomayor Concurring

Sotomayor stresses that a trespass on property is not the only way to violate the Fourth Amendment:

Of course, the Fourth Amendment is not concerned onlywith trespassory intrusions on property. See, e.g., Kyllo v. United States, 533 U. S. 27, 31–33 (2001). Rather, even in the absence of a trespass, “a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable.” Id., at 33; see also Smith v. Maryland, 442 U. S. 735, 740–741 (1979); Katz v. United States, 389 U. S. 347, 361 (1967) (Harlan, J., concurring). In Katz, this Court enlarged itsthen-prevailing focus on property rights by announcing that the reach of the Fourth Amendment does not “turn upon the presence or absence of a physical intrusion.” Id., at 353. As the majority’s opinion makes clear, however, Katz’s reasonable-expectation-of-privacy test augmented,but did not displace or diminish, the common-law trespassory test that preceded it.

Soto strikes back at Alito’s rejection of the trespass-test.

JUSTICE ALITO’s approach, which discounts altogether the constitutional relevance of the Government’s physical intrusion on Jones’ Jeep, erodes that longstanding protection for privacy expectations inherent in items of property that people possess or control. See post, at 5–7 (opinion concurring in judgment). By contrast, the trespassory test applied inthe majority’s opinion reflects an irreducible constitutionalminimum: When the Government physically invadespersonal property to gather information, a search occurs.The reaffirmation of that principle suffices to decide this case.

Soto also addresses Alito’s point that many searches can occur in the absence of a physical search, noting the shortfalls of what seems to be the majority’s compromise-position:

Nonetheless, as JUSTICE ALITO notes, physical intrusionis now unnecessary to many forms of surveillance. Post, at 9–12. With increasing regularity, the Government will becapable of duplicating the monitoring undertaken in thiscase by enlisting factory- or owner-installed vehicle tracking devices or GPS-enabled smartphones. See United States v. Pineda-Moreno, 617 F. 3d 1120, 1125 (CA9 2010) (Kozinski, C. J., dissenting from denial of rehearing enbanc). In cases of electronic or other novel modes of surveillance that do not depend upon a physical invasionon property, the majority opinion’s trespassory test mayprovide little guidance. But “[s]ituations involving merely the transmission of electronic signals without trespass would remain subject to Katz analysis.” Ante, at 11. As JUSTICE ALITO incisively observes, the same technologicaladvances that have made possible nontrespassory surveillance techniques will also affect the Katz test by shaping the evolution of societal privacy expectations. Post, at 10–11. Under that rubric, I agree with JUSTICE ALITO that, at the very least, “longer term GPS monitoring in inves- tigations of most offenses impinges on expectations of privacy.” Post, at 13.

Sotomayor culls from circuit opinion to focus on the threats of unbounded-GPS-tracking.

Awareness that the Government may be watching chillsassociational and expressive freedoms. And the Government’s unrestrained power to assemble data that revealprivate aspects of identity is susceptible to abuse. The net result is that GPS monitoring—by making available at a relatively low cost such a substantial quantum of intimate information about any person whom the Government, in its unfettered discretion, chooses to track—may “alter the relationship between citizen and government in a waythat is inimical to democratic society.” United States v. Cuevas-Perez, 640 F. 3d 272, 285 (CA7 2011) (Flaum, J., concurring).

And here she puts forth her own test!

I would take these attributes of GPS monitoring into account when considering the existence of a reasonable societal expectation of privacy in the sum of one’s public movements. I would ask whether people reasonably expect that their movements will be recorded and aggregated in a manner that enables the Government to ascertain, more or less at will, their political and religious beliefs, sexual habits, and so on. I do not regard as dispositivethe fact that the Government might obtain the fruits of GPS monitoring through lawful conventional surveillancetechniques. See Kyllo, 533 U. S., at 35, n. 2; ante, at 11 (leaving open the possibility that duplicating traditional surveillance “through electronic means, without an accompanying trespass, is an unconstitutional invasion ofprivacy”). I would also consider the appropriateness ofentrusting to the Executive, in the absence of any oversight from a coordinate branch, a tool so amenable tomisuse, especially in light of the Fourth Amendment’s goalto curb arbitrary exercises of police power to and prevent“a too permeating police surveillance,” United States v. Di Re, 332 U. S. 581, 595 (1948)

Justice Harlan in Katz, it is not.

The admonition to reconsider Smith is apt though.

More fundamentally, it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties. E.g., Smith, 442 U. S., at 742; United States v. Miller, 425 U. S. 435, 443 (1976). This approach is ill suited to the digital age, in which people reveal a greatdeal of information about themselves to third parties in the course of carrying out mundane tasks. People disclose the phone numbers that they dial or text to their cellu- lar providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medi- cations they purchase to online retailers. Perhaps, asJUSTICE ALITO notes, some people may find the “tradeoff” of privacy for convenience “worthwhile,” or come to acceptthis “diminution of privacy” as “inevitable,” post, at 10, and perhaps not. I for one doubt that people would accept without complaint the warrantless disclosure to the Government of a list of every Web site they had visited in the last week, or month, or year. But whatever the societal expectations, they can attain constitutionally protectedstatus only if our Fourth Amendment jurisprudence ceases to treat secrecy as a prerequisite for privacy. I would not assume that all information voluntarily disclosed to some member of the public for a limited purpose is, for that reason alone, disentitled to Fourth Amendment protection.

Alito Concurring in Judgment

Justice Alito takes a distinctly un-originalist view of this case, focusing on the “current” case law rather than what Scalia views as the original meaning of the Constitution.

This case requires us to apply the Fourth Amendment’s prohibition of unreasonable searches and seizures to a21st-century surveillance technique, the use of a Global Positioning System (GPS) device to monitor a vehicle’s movements for an extended period of time. Ironically, the Court has chosen to decide this case based on 18th-century tort law. By attaching a small GPS device1 to the underside of the vehicle that respondent drove, the law enforcement officers in this case engaged in conduct that might have provided grounds in 1791 for a suit for trespassto chattels.2 And for this reason, the Court concludes, the installation and use of the GPS device constituted a search. Ante, at 3–4.

This holding, in my judgment, is unwise. It strains the language of the Fourth Amendment; it has little if any support in current Fourth Amendment case law; and it is highly artificial.
I would analyze the question presented in this case byasking whether respondent’s reasonable expectations of privacy were violated by the long-term monitoring of themovements of the vehicle he drove.

This really reads more like a dissent (ignoring that both agree the search was unconstitutional).

Alito disagrees with the Court’s grounding of the trespass-theory in the case law (this is probably right) and proceeds to identify other issues:

In sum, the majority is hard pressed to find support in post-Katz cases for its trespass-based theory. Disharmony with a substantial body of existing case lawis only one of the problems with the Court’s approach in this case. I will briefly note four others. First, the Court’s reasoning largely disregards what is really important (the use of a GPS for the purpose of long-term tracking) and instead attaches great significance to something that most wouldview as relatively minor (attaching to the bottom of a car a small, light object that does not interfere in any way with the car’s operation). . . . By contrast, if longterm monitoring can be accomplished without committinga technical trespass—suppose, for example, that the Federal Government required or persuaded auto manufacturers to include a GPS tracking device in every car—the Court’s theory would provide no protection.

Yes, this is indeed the 800-pound GPS-gorilla the majority largely glosses over.

Fourth, the Court’s reliance on the law of trespass will present particularly vexing problems in cases involving surveillance that is carried out by making electronic, as opposed to physical, contact with the item to be tracked.For example, suppose that the officers in the present case had followed respondent by surreptitiously activating astolen vehicle detection system that came with the carwhen it was purchased. Would the sending of a radiosignal to activate this system constitute a trespass to chattels?

Alito also discounts the Katz test, in light of the way that evolving technologies can change privacy expectations (similar to Sotomayor’s point)

The Katz expectation-of-privacy test avoids the problems and complications noted above, but it is not without itsown difficulties. It involves a degree of circularity, see Kyllo, 533 U. S., at 34, and judges are apt to confuse their own expectations of privacy with those of the hypothetical reasonable person to which the Katz test looks. See Minnesota v. Carter, 525 U. S. 83, 97 (1998) (SCALIA, J., concurring). In addition, the Katz test rests on the assumption that this hypothetical reasonable person has awell-developed and stable set of privacy expectations. But technology can change those expectations. Dramatic technological change may lead to periods in which popular expectations are in flux and may ultimately produce significant changes in popular attitudes. New technologymay provide increased convenience or security at the expense of privacy, and many people may find the tradeoff worthwhile. And even if the public does not welcome the diminution of privacy that new technology entails,they may eventually reconcile themselves to this development as inevitable On the other hand, concern about new intrusions on privacy may spur the enactment of legislation to protect against these intrusions. This is what ultimately happened with respect to wiretapping. After Katz, Congress did not leave it to the courts to develop a body of Fourth Amendment case law governing that complex subject.Instead, Congress promptly enacted a comprehensivestatute, see 18 U. S. C. §§2510–2522 (2006 ed. and Supp.IV), and since that time, the regulation of wiretapping hasbeen governed primarily by statute and not by case law.7 In an ironic sense, although Katz overruled Olmstead, Chief Justice Taft’s suggestion in the latter case that theregulation of wiretapping was a matter better left forCongress, see 277 U. S., at 465–466, has been borne out.

In other words, let Congress–and not the courts–deal with this evolving technologies.

And this is the passage that will be cited ad infinitum in law reviews

Recent years have seen the emergence of many newdevices that permit the monitoring of a person’s movements. In some locales, closed-circuit television video monitoring is becoming ubiquitous. On toll roads, automatic toll collection systems create a precise record of the movements of motorists who choose to make use of that convenience. Many motorists purchase cars that are equipped with devices that permit a central station to ascertain the car’s location at any time so that roadsideassistance may be provided if needed and the car may be found if it is stolen. Perhaps most significant, cell phones and other wirelessdevices now permit wireless carriers to track and recordthe location of users—and as of June 2011, it has been reported, there were more than 322 million wireless devices in use in the United States.8 For older phones, the accuracy of the location information depends on the density of the tower network, but new “smart phones,” which are equipped with a GPS device, permit more precise tracking. For example, when a user activates the GPS onsuch a phone, a provider is able to monitor the phone’s location and speed of movement and can then report back real-time traffic conditions after combining (“crowdsourcing”) the speed of all such phones on any particular road.9 Similarly, phone-location-tracking services are offered as “social” tools, allowing consumers to find (or to avoid)others who enroll in these services. The availability and use of these and other new devices will continue to shape the average person’s expectations about the privacy of his or her daily movements.

Alito is right-on to note that in the pre-computer age, the biggest constraint on surveillance was cost. With technology, those costs are no longer a factor:

In the pre-computer age, the greatest protections of privacy were neither constitutional nor statutory, but practical. Traditional surveillance for any extended period of time was difficult and costly and therefore rarely undertaken. The surveillance at issue in this case—constant monitoring of the location of a vehicle for four weeks—would have required a large team of agents, multiplevehicles, and perhaps aerial assistance.10 Only an investigation of unusual importance could have justified such an expenditure of law enforcement resources. Devices like the one used in the present case, however, make long-term monitoring relatively easy and cheap. In circumstances involving dramatic technological change, the best solutionto privacy concerns may be legislative. See, e.g., Kerr, 102 Mich. L. Rev., at 805–806. A legislative body is well situated to gauge changing public attitudes, to draw detailed lines, and to balance privacy and public safety in a comprehensive way. . . .

But the use of longer term GPS monitoring in investigations ofmost offenses impinges on expectations of privacy. For such offenses, society’s expectation has been that lawenforcement agents and others would not—and indeed, inthe main, simply could not—secretly monitor and catalogue every single movement of an individual’s car for a very long period. In this case, for four weeks, law enforcement agents tracked every movement that respondent made in the vehicle he was driving. We need not identify with precision the point at which the tracking of this vehicle became a search, for the line was surelycrossed before the 4-week mark. Other cases may present more difficult questions. But where uncertainty exists with respect to whether a certain period of GPS surveillance is long enough to constitute a Fourth Amendmentsearch, the police may always seek a warrant.11 We also need not consider whether prolonged GPS monitoring inthe context of investigations involving extraordinaryoffenses would similarly intrude on a constitutionallyprotected sphere of privacy. In such cases, long-termtracking might have been mounted using previously available techniques.

Oh this was somewhat unsatisfying. If only Alito’s opinion became the majority. Sotomayor’s vote is so, so odd.

But the use of longer term GPS monitoring in investigations ofmost offenses impinges on expectations of privacy. For such offenses, society’s expectation has been that lawenforcement agents and others would not—and indeed, inthe main, simply could not—secretly monitor and catalogue every single movement of an individual’s car for a very long period. In this case, for four weeks, law enforcement agents tracked every movement that respondent made in the vehicle he was driving. We need not identify with precision the point at which the tracking of this vehicle became a search, for the line was surelycrossed before the 4-week mark. Other cases may present more difficult questions. But where uncertainty exists with respect to whether a certain period of GPS surveil