The specter of an Orwellian world in which Americans’ every movement is monitored is no longer a chimera. It is upon us.

This term the Supreme Court will decide whether the warrantless GPS monitoring of an individual’s car violates the Fourth Amendment. The court’s answer must be a resounding yes.

The Fourth Amendment generally requires officers to obtain a search warrant from a judge prior to performing an invasive search or seizure. The threshold for a warrant is minimal: Officers must show probable cause, defined as a reasonable belief, that a crime was committed. This mandate protects individuals from over-zealous officers and provides officers with an objective perspective on the efficacy of their investigations.

A GPS tracking device should only be employed after probable cause is established and a warrant secured. The government contends that Americans forfeit their privacy whenever they enter the public domain and that GPS technology can be used before probable cause is established. However, this would allow individuals to be targeted without any established factual cause, obliterating longstanding notions of privacy that Americans regard as sacrosanct.

Absent the requirement of a warrant, a single officer could surreptitiously affix a GPS device to an individual’s car and observe its movements for weeks or months on end. Activities once thought private – health choices, religious affiliations, political associations, romantic liaisons – would become a matter of public concern, traceable by police.

Worse, unchecked GPS surveillance may chill such activities. Freedom sometimes presupposes anonymity. Indeed an individual contemplating a trip to the abortion clinic, treatment at a drug rehabilitation center, or membership in an unpopular political or religious group, may reconsider if that person believes the government could be watching.

That people use public roads, or otherwise enter the public realm, to participate in the political and cultural fabric of society does not mean that they surrender their expectation of privacy. People having to search their vehicles for a tracking device every time they get behind the wheel is antithetical to a democracy committed to protecting individual rights. As Justice Louis Brandeis once famously noted, “the right to be let alone [is] the most comprehensive of rights and the right most valued by civilized men.”

Requiring a warrant for GPS technology is also a necessary prophylactic against government abuse. History is replete with examples of the government conducting dragnet type surveillance against perceived dissenters. Victims have included civil rights groups such as the NAACP and supporters of the women’s liberation movement after World War II, university and church groups opposed to the Vietnam War, and America’s Muslim population following the September 11, 2001 terrorist attack.

The Associated Press revealed earlier this fall that undercover NYPD officers have clandestinely surveilled over 250 mosques and Muslim student groups in New York sometimes without evidence of wrongdoing. Permitting officers to deploy GPS technology without judicial oversight would facilitate and legitimate such practices.

Ask Yasir Afifi, a Muslim-American college student. In October 2010, an auto mechanic found a GPS tracker on his car. The FBI later retrieved the device and told Mr. Afifi that he needn’t worry because he was “boring.” To this day, Afifi has never been charged with a crime.

The sophistication and facility of GPS devices only exacerbate their potential for abuse. Small and durable, they require little to no human intervention apart from installation, and can yield data that would otherwise necessitate hundreds of hours of surveillance.

Law enforcement has argued that a warrant requirement would delay investigations, jeopardizing public safety. However, GPS evidence is most useful if gathered over a lengthy period of time, and it takes only a short period to secure a warrant, sometimes just minutes.

Officers pursuing fleeing suspects need not even seek a warrant because they can invoke the “hot pursuit” doctrine, which permits warrantless searches of suspects fleeing a crime scene. Some police cars in fact are already equipped with launchers that fire GPS projectiles.

The story doesn’t end here. The march of science and technology will soon yield GPS devices small enough to attach to satchels and clothing. Officers may even opt for “stingrays,” cell phone tower technology that forces phones to reveal their location. Unless the Supreme Court imposes a warrant requirement on the GPS tracking of cars, the dark cloud cast over our nation by the threat of ubiquitous surveillance will only loom larger.

Arjun Sethi is an attorney.