GPS navigation and cellular-signal triangulation can help us to find our way around — or to help someone else find us. If you happen to be a lost pet, an Alzheimer's patient or a small child, that's a good thing.
But what about the rest of us? Where do we stand legally in terms of being tracked by cellular carriers, smartphone app makers or the government?
Government tracking
You might assume that there's a federal law preventing government tracking of you, at least without a warrant. But you'd be wrong.
Several court decisions in recent months have sent mixed messages about the legality of GPS and cellphone tracking by the government, and the issue has just landed in the U.S. Supreme Court.
In August, a federal judge in New York ruled that police would need a warrant to track an individual using cellular-tower triangulation. In early October, a different federal judge, this one in Washington, D.C., ruled that police did not need a warrant to use same method to track the cellphone of an armed-robbery suspect in an ongoing case. And just last week, a third federal judge, this one in Houston, ruled that a warrant was necessary.
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Earlier this month, the U.S. Supreme Court heard arguments in U.S. vs. Antoine Jones, a case involving a suspected drug dealer whose wife's Jeep was tracked in 2005 using an attached GPS locator.
District of Columbia authorities obtained a warrant to attach the locator to the Jeep, but continued tracking it for a month, long after the 10-day warrant had expired. Prosecutors used the vehicle's movements in Maryland, outside the warrant's jurisdiction, to convict Jones of conspiracy in 2007.
A federal appeals court overturned Jones' life sentence in 2010, ruling that the use of the warrant outside of its boundaries violated the Fourth Amendment. The Justice Department appealed the lower court's ruling.
The Supreme Court's eventual ruling in the case, due by next summer, could potentially shed some light on the issue. But even a Supreme Court ruling may conflict with sections of the USA PATRIOT Act, which many federal agencies cite to claim that they never need a warrant to track potential suspects. (The time limits on those sections were recently extended.)
"Right now, there is conflict in the circuit courts. We don't have a national policy," said Robert Ellis Smith, a Rhode Island lawyer who is the publisher of Privacy Journal, a monthly newsletter, and who helped write an amicus briefing to the Supreme Court on behalf of Jones.
Smith explained that requiring warrants would not prevent GPS tracking, but would leave a legal paper trail if the tracking went on for months.
"When you are talking about long-term surveillance, there is no reason not to get a warrant," Smith said.
The real question asked by U.S. vs. Antoine Jones is: In the digital age, how will the current Supreme Court interpret the Fourth Amendment's prohibition on unreasonable searches and seizure?
Is tracking of individuals by the government bad? Well, that depends on the situation.
Most of us are already familiar with the E911 system, which helps 911 call operators to locate emergency callers by using cell-tower triangulation. A recent Federal Communications Commission ruling requires all cellphones enabled with E911 to be accurate to at least 50 feet by 2018.
Non- government tracking
But if the government can track individuals using their cellphones, so can a lot of other people. Tracking software can be installed by anyone with access to a smartphone. And sometimes a resourceful person doesn't even need access to the phone.
"Your phone type, your carrier name and your cell phone number" are often all that's needed," said Angela Daffron, the founder of Jodi's Voice, an online anti-stalking advocacy group based in New Mexico.
Some companies even market tracking apps that allow a spouse or parent to track both the location and the communications of a smartphone. It's not illegal. Earlier this year, a New Jersey appellate court ruled that it was not an invasion of privacy for one spouse to install a tracking device on a jointly owned vehicle driven by the other spouse.
Are you your own worst enemy?
As it turns out you, may be your own problem — if you don't know how to make the right choices with your smartphone apps. Many apps, especially on Android phones, ask you to allow them to track your location even if they have no need for it.
Of course, some applications use your location for fun. "Check-in" applications, such as Foursquare or Facebook's "Places" feature, allow a user to let all his friends or followers know that he is at a specific location.
But this feature, known as "checking in," has the potential to make an unprecedented amount of public.
So how worried should you be about the security of "checking in?" Experts are conflicted.
"[There's] no reason to think they're not secure. Both [Facebook and Foursquare] take data security very seriously," said Christopher Wolf, the co-chair of the Future of Privacy Forum, a Washington, D.C., public-policy advocacy group.
Daffron countered that "checking in" is fine, but only in moderation.
"Be careful not to check in at home, or at specific locations," she said. "It's fine to check in at the mall, but don't list specific stores."
Your safety
If you want to minimize the ability of others to track you, you have a few options.
First and foremost, say no to smartphone apps that want to track you, and turn off the GPS chip on the phone. Look for "location services" or similar settings on your smartphone or tablet.
Google maps Wi-Fi hotspots around the world to aid in its location data; if you don't want your home wireless network included, you'll need to add "_nomap" to the end of your network's name.
To prevent tracking software being installed on your phone, Daffron suggests keeping your phone with you at all times, and also limiting who knows your mobile phone number.
You can also install a good piece of anti-virus software, such as Norton, on your phone to keep away spyware.
If you really want to make sure that your cellphone is not being tracked, even for E911 purposes, then you need to set it to airplane mode or shut it off.Of course, you won't be able to receive calls during those times.