Jones argues the use of the GPS device violated his ‘reasonable expectation of privacy,’ Katz, . . . and was therefore a search subject to the reasonableness requirement of the Fourth Amendment. Of course, the Government agrees the Katz test applies here, but it argues we need not consider whether Jones’s expectation of privacy wasreasonable because that question was answered in United States v. Knotts, 460 U.S. 276 (1983), in which the Supreme Court held the use of a beeper device to aid in tracking a suspect to his drug lab was not a search. As explained below, we hold Knotts does not govern this case and the police action was a search because it defeated Jones’s reasonable expectation of privacy.” Id. at 16 (Katz full cite omitted).
This decision is in conflict with the Ninth Circuit's opinion in United States v. Pinedo-Moreno, 591 F.3d 1212 (9th Cir, 2010). For more detailed discussion on how to use as a practitioner, see the Ninth Circuit Blog. The Ninth Circuit Blog article also contains an interesting discussion on the use of cell phone towers for pinging that is easier than ever before to use as a "tracking device". It is becoming routine that federal law enforcement are using cell-phone location data through sealed applications. (Further reading here).
When working this type of case as an Investigator, it is important to get discovery that includes the type of device used, how the device stores the information, whether there was a warrant application that included a time limit, and how the device works (e.g. whether the device can be remotely accessed or whether it has to be retrieved and manually downloaded) so that the investigator is armed with all information possible to assist in a motion to suppress.