Do the Police Need a Warrant to Track You With Your Cellphone?

Technology and laws are constantly evolving. The U.S. Supreme Court recognizes some privacy protections when it comes to tracking cellphones, but the law lags far behind new technology capabilities and law enforcement surveillance techniques.

By , Attorney · Mitchell Hamline School of Law

Electronic devices track and store an amazing amount of data regarding our personal lives, including location data. Our cellphones, smart devices (phones, watches), and even cars know our almost every move. What privacy protections, if any, does the Fourth Amendment provide when it comes to the government accessing this data? Does law enforcement need a search warrant? As the typical legal answer goes, it depends.

The law requires—in most situations—that the police get a warrant in order to gather historical cellphone location information kept by cellphone and wireless network providers. The U.S. Supreme Court established this privacy rule for all the country in the 2018 case Carpenter v. United States. (138 S. Ct. 2206.) But, this ruling addressed technology used in 2011. So where does the law stand on newer technology?

Cellphone and Smart Device Location Data: Where Does It Come From?

Cellphone and wireless service providers and tech companies (like Google) store certain historical and real-time location data gleaned from cellphone connections with cell towers, GPS satellite tracking, and smart device applications and operating systems. Law enforcement can request this data using a court order or warrant. Some examples of this data include:

Historical CSLI. A typical cellphone produces a time-stamped record every time it connects to a cell site. This record is referred to as "cell-site location information" (CSLI). With CSLI, police can get a really good idea of where someone's phone has been, allowing them to piece together past events—for instance, connecting the dots as to a suspect's location at the scene of a crime.

Real-time CSLI. To obtain real-time data, a service provider can either contemporaneously monitor a phone's CSLI connections or "ping" a user's cellphone and force it to reveal its location. This real-time data can provides police with information on a suspect's (phone's) current whereabouts.

GPS movements. A phone or car's GPS system tracks its movements using satellite signals. This information might be stored on the phone or vehicle itself or transmitted to a repository. It can also be relayed to police in real time.

Tower dumps: reverse location search. Instead of providing CSLI records on a certain phone number, a tower dump refers to a request for all the CSLI data connected to certain cell towers at a specific time—so every and any phone that connected during that time. Law enforcement can use the data to work backward in identifying or tracking a suspect.

Geofence: reverse location search. A geofence refers to a search of a location history database to identify all users whose devices were in a certain location at a certain time. This data is often available through location tracking applications. Police use the information to increasingly narrow their search parameters (the "geofence") and try to pinpoint devices that might be connected to the crime—again, working backward to identify a suspect but from a database of user movements that far exceeds a tower dump.

Do Police Need a Warrant for Location Data?

For a typical search warrant, police need to convince a judge that they have probable cause to believe that evidence of a crime will be found in the place or item to be searched. To establish probable cause, an officer must typically submit an affidavit based on reliable sources that identify objectively suspicious activities.

In recent investigations, however, law enforcement has relied on a few different statutory authorities that generally require a lower standard of proof than probable cause. For instance, one statute allows law enforcement to obtain a court order directing access to a company's stored communications when the officer shows a "reasonable basis" to believe the information is "relevant and material to an ongoing criminal investigation." (18 U.S.C. § 2703(d).)

Carpenter v. United States

In the 2018 case Carpenter v. United States, the Supreme Court found that this lower standard ("reasonable basis" listed above) did not suffice for a request of a month's worth of historical CSLI data on a defendant's phone. The Court held the acquisition of these records constituted a Fourth Amendment search and the police needed to get a probable cause warrant. (138 S. Ct. 2206.)

This decision was consistent with the trend suggested by recent decisions dealing with the relationship between technology and privacy. The Court had held in 2012 that the police can't arbitrarily slap a GPS on a car and track the vehicle's movements (United States v. Jones, 565 U.S. 400), and in 2014, that officers normally need a warrant to search the cellphone of someone they've arrested (Riley v. California, 134 S. Ct. 2473).

Carpenter: Guidance for New Surveillance Technologies?

We don't yet know what Carpenter means for searches involving newer forms of tracking technology. When the Carpenter ruling was issued, it already lagged behind several advances in tracking and surveillance technologies used by law enforcement. As the case was being decided, police had moved on to obtaining real-time CSLI and reverse location tracking data. On top of that, the Supreme Court was very careful to limit the Carpenter ruling to only historical CSLI data and requests to obtain more than six days' worth of data. (The Court even noted that emergency situations will likely justify warrantless searches at times.)

Basically, until Congress acts or a case comes before the Supreme Court, lower courts are left to grapple with competing interests of the government and privacy rights of citizens. Several lower courts—both before and after the Carpenter decision—have held that law enforcement must usually obtain probable cause warrants to obtain real-time CSLI. Some state legislatures have also stepped in and enacted laws that require a probable cause search warrant to obtain certain real-time or reverse location data. But without clear guidance, judges must navigate a patchwork of laws and court rulings when issuing search orders or warrants.

What's Next for Privacy Rights in the Digital Tracking Age?

Some commentators are confident that lower courts will rely on Carpenter to extend the probable cause warrant requirement to newer data searches that arguably infringe on greater privacy rights. Whereas historical and real-time CSLI searches are often particularized to an identified individual, reverse location searches may look at the whereabouts of hundreds or thousands of individuals—most of whom are not suspected of any wrongdoing. And some question the effectiveness of reverse location searches in general—arguing that such searches are highly prone to errors and shouldn't be used at all.

This area of law is evolving. As courts and legislatures try to catch up to the technology, more cases and statutes will likely emerge. If you have questions regarding your privacy rights, contact a lawyer who specializes in civil rights or criminal defense.

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