Judge John Rogers, in his ruling, says multiple times that the phone was emanating information that the authorities tracked, therefore no invasion of privacy. He compares it to the scent a dog uses to follow someone, or the color of a car, or the numbers on a license plate, or the location of a car when it is on a public road.
"The government used data emanating from Melvin Skinner’s pay-as-you-go cell phone to determine its real-time location." (http://www.ca6.uscourts.gov/opinions.pdf/12a0262p-06.pdf, p2, par1)
Rogers says the information the phone was emanating was just a "proxy" to this other publicly visible information. It's not clear what this "proxy" status is for the phone, but more importantly, the GPS in the phone could not have been emanating position information because that's not how client stations work in GPS. The satellites in the sky emanate position information, and the ground units just receive this and triangulate position from it. Otherwise, it would be a terrible technology for the military to use, as they would be easily located!
The judgement acknowledges that "ping" data had to be accessed from phone company, and that "pinging" the phone is an activity engaged in by the DEA agents on the suspect's phone, but somehow sticks with the emanating logic: "a federal magistrate judge, on July 12, 2006, authoriz[ed] the phone company to release subscriber information, cell site information, GPS real-time location, and “ping” data for the 6447 phone in order to learn Big Foot’s location while he was en route to deliver the drugs." (p4, par1)
(By pining the first phone and realizing it was at Big Foot's home, and not on the road with him, the DEA agents got another "authorization" for the release of the same data for a second "6820" phone.)
"By continuously “pinging” the 6820 phone, authorities learned that Big Foot left Tucson, Arizona on Friday, July 14, 2006, and was traveling on Interstate 40 across Texas. At no point did agents follow the vehicle or conduct any type of visual surveillance. At around 2:00 a.m. on Sunday, July 16, 2006, the GPS indicated that the 6820 phone had stopped somewhere near Abilene, Texas." (p4, par3)
That's where they moved in, did a K-9 walk-around of the RV, and then arrested him.
In the judgement about the 4th Amendment violation:
"If a tool used to transport contraband gives off a signal that can be tracked for location, certainly the police can track the signal."
Of course, GPS clients like the suspect's phone do not actually give of a position signal, which is why they had to ping it. Rogers' logic is most carefully stated in the following, and so perhaps this could be the grounds for a new appeal:
"This case is different from the recent Supreme Court decision in United States
v. Jones, 132 S. Ct. 945 (2012). That case involved the secret placement of a tracking
device on the defendant’s car, id. at 948, and the Court’s opinion explicitly relied on the
trespassory nature of the police action. Id. at 949. Although Fourth Amendment
jurisprudence includes an assessment of the defendant’s reasonable expectation of
privacy, that “d[oes] not erode the principle ‘that, when the Government does engage in
physical intrusion of a constitutionally protected area in order to obtain information, that
intrusion may constitute a violation of the Fourth Amendment.’” Id. at 951 (quoting
Knotts, 460 U.S. at 286 (Brennan, J., concurring)). No such physical intrusion occurred
in Skinner’s case. Skinner himself obtained the cell phone for the purpose of
communication, and that phone included the GPS technology used to track the phone’s
whereabouts. The majority in Jones based its decision on the fact that the police had to
“physically occup[y] private property for the purpose of obtaining information.”
132 S. Ct. at 949. That did not occur in this case." (p10, par2)
This decision relies on a faulty understanding that GPS emanates a signal that the agents passively received, while paradoxically also admitting earlier that the unit was "pinged" to determine its location. Somehow, the use of the term "ping" elides what should have been a more thorough analysis, which could only have shown that pinging the device is physically interacting with (or physically occupying?) the suspect's private property to obtain information from it that would not otherwise be available to the public.