Concluding that Fourth Amendment precedents already define GPS surveillance in a public place is this thoughtful piece: Does the Fourth Amendment Prohibit Warrantless GPS Surveillance? by Orin Kerr on the Volokh Conspiracy:

Does the Fourth Amendment require a warrant to conduct surveillance of a government-installed GPS device, such as a device installed on a suspect’s car to monitor the car’s location? This issue comes up occasionally, and the DC Circuit has a case pending on the issue. I don’t think I have ever blogged about it, so I want to offer my thoughts. This post will explain why I think the doctrine here was settled by a pair of Supreme court cases from the 1980s, and why those cases draw a pretty reasonable Fourth Amendment line.

My personal view is not far off, but with a caveat: United States v. Knotts, 460 U.S. 276 (1983), and United States v. Karo, 468 U.S. 705 (1984), two beeper tracking cases, are hard to overcome. The only real difference is that the beeper cases had to be surveilled up close. GPS doesn't require the observer to leave the office. But that is the Fourth Amendment. Somehow, the upheaval in technology since Knotts and Karo (see, e.g., Kyllo, the thermal imaging case) has to be accounted for under the "reasonable expectation of privacy."

The New York Court of Appeals decision in People v. Weaver, 12 N.Y.3d 433, 882 N.Y.S.2d 357, 909 N.E.2d 1195 (May 12, 2009), posted here, is hard to disagree with, too. And it distinguishes Knotts and Karo under the state constitution:

Here, we are not presented with the use of a mere beeper to facilitate visual surveillance during a single trip. GPS is a vastly different and exponentially more sophisticated and powerful technology that is easily and cheaply deployed and has virtually unlimited and remarkably precise tracking capability. With the addition of new GPS satellites, the technology is rapidly improving so that any person or object, such as a car, may be tracked with uncanny accuracy to virtually any interior or exterior location, at any time and regardless of atmospheric conditions. Constant, relentless tracking of anything is now not merely possible but entirely practicable, indeed much more practicable than the surveillance conducted in Knotts. GPS is not a mere enhancement of human sensory capacity, it facilitates a new technological perception of the world in which the situation of any object may be followed and exhaustively recorded over, in most cases, a practically unlimited period. The potential for a similar capture of information or "seeing" by law enforcement would require, at a minimum, millions of additional police officers and cameras on every street lamp.

That such a surrogate technological deployment is not -- particularly when placed at the unsupervised discretion of agents of the state "engaged in the often competitive enterprise of ferreting out crime" (Johnson v United States, 333 US 10, 14 [1948]) -- compatible with any reasonable notion of personal privacy or ordered liberty would appear to us obvious. One need only consider what the police may learn, practically effortlessly, from planting a single device. The whole of a person's progress through the world, into both public and private spatial spheres, can be charted and recorded over lengthy periods possibly limited only by the need to change the transmitting unit's batteries. Disclosed in the data retrieved from the transmitting unit, nearly instantaneously with the press of a button on the highly portable receiving unit, will be trips the indisputably private nature of which takes little imagination to conjure: trips to the psychiatrist, the plastic surgeon, the abortion clinic, the AIDS treatment center, the strip club, the criminal defense attorney, the by-the-hour motel, the union meeting, the mosque, synagogue or church, the gay bar and on and on. What the technology yields and records with breathtaking quality and quantity, is highly detailed profile, not simply of where we go, but by easy inference, of our associations -- political, religious, amicable and amorous, to name only a few -- and of the pattern of our professional and avocational pursuits. When multiple GPS devices are utilized, even more precisely resolved inferences about our activities are possible. And, with GPS becoming an increasingly routine feature in cars and cell phones, it will be possible to tell from the technology with ever increasing precision who we are and are not with, when we are and are not with them, and what we do and do not carry on our persons -- to mention just a few of the highly feasible empirical configurations.

Ever the originalist, Justice Scalia, wrote the opinion in Kyllo v. United States, 529 U.S. 334 (2001), but Kyllo involved surveillance of the home, a significant distinction.

Where does Kyllo and the reasonable expectation of privacy go from here? It is not a violation of the Fourth Amendment to use basic police work and tail a suspect. That's what the police did before beepers. With beepers, their target didn't have to be in sight, but they at least had to be nearby. Now, with GPS, tracking can occur for weeks or months or years without even being in the same city, state, or country. The battery has to be replaced periodically on a transmitter on a vehicle (wiring it in probably would require a warrant, but that's not even clear). A cellphone, however, has its own power. As to police cellphone tracking with the complicity of cellphone providers see posts here, here, and here, referring to 8 million law enforcement requests for cellphone GPS tracking, without a warrant.

Also, how does Katz's "reasonable expectation of privacy" standard fit in? Katz had a reasonable expectation of privacy in his phone calls even though he could be seen in the phone booth:

For the Fourth Amendment protects people, not places. What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. See Lewis v. United States, 385 U.S. 206, 210; United States v. Lee, 274 U.S. 559, 563. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected. See Rios v. United States, 364 U.S. 253; Ex parte Jackson, 96 U.S. 727, 733.

The Government stresses the fact that the telephone booth from which the petitioner made his calls was constructed partly of glass, so that he was as visible after he entered it as he would have been if he had remained outside. But what he sought to exclude when he entered the booth was not the intruding eye - it was the uninvited ear. He did not shed his right to do so simply because he made his calls from a place where he might be seen. No less than an individual in a business office, in a friend's apartment, or in a taxicab, a person in a telephone booth may rely upon the protection of the Fourth Amendment. One who occupies it, shuts the door behind him, and pays the toll that permits him to place a call is surely entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world. To read the Constitution more narrowly is to ignore the vital role that the public telephone has come to play in private communication.

Then there is Bond v. United States, 529 U.S. 334 (2000), which seemingly offers some support. There, a bag in the overhead of a bus was subject to manipulation, and the Court rejected that the bag is open to being felt up by the police as a matter of course because the expectation of privacy still protected it:

Here, petitioner concedes that, by placing his bag in the overhead compartment, he could expect that it would be exposed to certain kinds of touching and handling. But petitioner argues that Agent Cantu's physical manipulation of his luggage "far exceeded the casual contact [petitioner] could have expected from other passengers." Brief for Petitioner 18-19. The Government counters that it did not.

Our Fourth Amendment analysis embraces two questions. First, we ask whether the individual, by his conduct, has exhibited an actual expectation of privacy; that is, whether he has shown that "he [sought] to preserve [something] as private." Smith v. Maryland, 442 U. S. 735, 740 (1979) (internal quotation marks omitted). Here, petitioner sought to preserve privacy by using an opaque bag and placing that bag directly above his seat. Second, we inquire whether the individual's expectation of privacy is "one that society is prepared to recognize as reasonable." Ibid. (internal quotation marks omitted). When a bus passenger places a bag in an overhead bin, he expects that other passengers or bus employees may move it for one reason or another. Thus, a bus passenger clearly expects that his bag may be handled. He does not expect that other passengers or bus employees will, as a matter of course, feel the bag in an exploratory manner. But this is exactly what the agent did here. We therefore hold that the agent's physical manipulation of petitioner's bag violated the Fourth Amendment.

By Katz, the phone had long before become "vital." Now GPS.

As Kerr recognizes, technology now advances almost monthly, not over decades. And the Fourth Amendment jurisprudence of the Court has not kept up because it hasn't taken a case that resolves any of these issues.

Thirty-five years ago, Anthony G. Amsterdam, Perspectives on the Fourth Amendment, 58 Minn. L. Rev, 349, 384 (1974), essentially predicted what GPS has done to our expectation of privacy:

An actual, subjective expectation of privacy ... can neither add to, nor can its absence detract from, an individual's claim to fourth amendment protection. If it could, the government could diminish each person's subjective expectation of privacy merely by announcing half-hourly on television that ... we were all forthwith being placed under comprehensive electronic surveillance.

What about the "right to be let alone"? See Olmstead v. United States, 277 U.S. 438 (1928) (Brandeis, J., dissenting) (discussed in the Treatise § 1.26):

The protection guaranteed by the [fourth and fifth] amendments is much broader in scope. The makers of our Constitution undertook to secure conditions favorable to the pursuit of happiness. They recognized the significance of man's spiritual nature, of his feelings and of his intellect. They knew that only a part of the pain, pleasure and satisfactions of life are to be found in material things. They sought to protect Americans in their beliefs, their thoughts, their emotions and their sensations. They conferred, as against the government, the right to be let alone-the most comprehensive of rights and the right most valued by civilized men. To protect, that right, every unjustifiable intrusion by the government upon the privacy of the individual, whatever the means employed, must be deemed a violation of the Fourth Amendment.

What will SCOTUS do? We already know, just from our experience, that there are at least four votes in favor of broad GPS surveillance. The other five are all, to me, wild cards on this issue.