The majority (Scalia, Roberts, Thomas, Kennedy, Sotomayor) distinguished
Knotts on the basis that in that case there was no trespass onto an owner's property to install the device:
"The beeper had been placed in the container before it came into Knotts’ possession, with the consent of the then-owner. Knotts did not challenge that installation, and we specifically declined to consider its effect on the Fourth Amendment analysis."The four remaining justices (Alito, Breyer, Ginsburg, Kagan) would've made a stronger holding, instituting a new general principle that pervasive, long-term monitoring requires a warrant, regardless of the method: they summarized Knotts as "relatively short-term monitoring of a person’s movements on public streets" as opposed to this case's "use of longer term GPS monitoring". (The're a need to come up with some kind of dividing line, because short-term tailing of people on public streets has long been considered something police can do on mere suspicion, without a warrant.)
Sotomayor's concurrence indicated a potential willingness to join those latter four in an appropriate case, but she didn't feel it was necessary to deciding this case, since it was enough for this case to just hold that the physical installation required a warrant.