In our era of expanded tracking technologies that rapidly outpace our laws, there's a lot of work to be done to get back in line with the Fourth Amendment if we want to provide reasonable protections against warrantless surveillance.

The mere fact that the police are capable of tracking someone doesn’t mean they’re entitled to do so without first getting a warrant, any more than the mere fact that it’s easy to break down someone’s front door or open their postal mail gives the police the right to take these steps without a warrant.

Allie Bohm, on the ACLU Blog of Rights, writes that some police departments in Colorado have established clear policies for getting probable cause warrants before tracking cell phone information:

The Denver Police Department requires a warrant for location tracking as a general rule, and provides an exception for missing people or true emergency situations where law enforcement must then go to the court and show probable cause after the fact.

The fact that the police in Denver follow privacy-protective principles when tracking cell phone location belies claims by some that such procedures would impose unworkable burdens on law enforcement.

They’re reasonable in Denver, and they would be reasonable nationwide.

Not surprisingly, a warrant requirement works for law enforcement.  More importantly, a warrant means we don't sacrifice our privacy rights by using a cell phone.