In Florida v. Riley, 488 U.S. 445 (1989), the Supreme Court held 5–4 that police officers who spotted marijuana plants inside a backyard greenhouse by flying a helicopter over the property at 400 feet did not conduct a “search” under the Fourth Amendment, and therefore did not need a warrant. The decision produced a four-justice plurality and a separate concurrence by Justice O’Connor that supplied the fifth vote on different reasoning, leaving lower courts to sort out which test controls. Riley remains the Court’s most direct statement on low-altitude aerial surveillance by police.
What Happened on Michael Riley’s Property
Michael Riley kept a greenhouse on rural property in Pasco County, Florida. Trees, shrubs, and his mobile home blocked any view of the interior from the ground, and corrugated panels covered the roof. Two of those roof panels were missing, leaving roughly 10 percent of the greenhouse open to the sky.1Justia U.S. Supreme Court Center. Florida v. Riley, 488 U.S. 445 (1989)
After a tip about illegal cultivation, an investigating officer confirmed he could see nothing from ground level. He then went up in a helicopter, circled the property twice at 400 feet, and identified what he believed to be marijuana plants through the gaps in the roof using his naked eye. That observation supported a search warrant. Officers executed the warrant, seized the plants, and charged Riley, who moved to suppress the evidence on the ground that the warrantless flyover was itself an unconstitutional search.
The Plurality’s Rule: Legal Altitude, No Warrant
Justice White, joined by Chief Justice Rehnquist and Justices Scalia and Kennedy, reversed the Florida Supreme Court. The plurality accepted that the greenhouse sat within the curtilage of Riley’s home, the area where Fourth Amendment protections are strongest. But the missing roof panels left the interior exposed from above, and the helicopter was in airspace where the public was legally free to fly.1Justia U.S. Supreme Court Center. Florida v. Riley, 488 U.S. 445 (1989)
The altitude mattered because of how federal aviation rules are structured. Fixed-wing aircraft generally must stay at least 500 feet up over non-congested areas, but helicopters may operate below that floor as long as they do not create a hazard to people or property on the ground.2eCFR. 14 CFR 91.119 – Minimum Safe Altitudes: General Because the helicopter’s 400-foot altitude was legal, the plurality reasoned, “any member of the public or the police could legally have observed respondent’s greenhouse from that altitude.” An officer seeing what a private citizen could lawfully have seen did not invade any expectation of privacy the Fourth Amendment protects.
The plurality built on California v. Ciraolo, 476 U.S. 207 (1986), which had allowed police to fly a fixed-wing plane at 1,000 feet over a fenced backyard and spot marijuana without a warrant.3Justia U.S. Supreme Court Center. California v. Ciraolo, 476 U.S. 207 (1986) Riley extended that principle to a lower altitude and a more maneuverable aircraft.
Justice O’Connor’s Concurrence and Why It Complicates the Rule
Justice O’Connor supplied the fifth vote to reverse but rejected the plurality’s reliance on FAA compliance. In her view, aviation regulations exist to prevent midair collisions, not to define constitutional privacy. What should matter, she wrote, is whether the public actually flies over residential property at that altitude “with sufficient regularity” that a homeowner cannot reasonably expect to be shielded from view.4Supreme Court of the United States. Florida v. Riley, 488 U.S. 445 (1989)
She concurred in the result only because Riley had not proved that helicopter traffic at 400 feet over his area was rare. Under her framework, a homeowner who could show that virtually no one flies that low in a given area might still win a Fourth Amendment challenge.
The two approaches point in different directions. The plurality’s test is bright-line: legal altitude means no search. O’Connor’s test is fact-specific and turns on how common public overflights actually are. Because neither approach commanded a majority, courts handling later aerial surveillance cases have not settled on a single standard.
The Dissent
Justice Brennan, joined by Justices Marshall and Stevens, argued that the plurality had drained the Katz reasonable-expectation test of meaning. A helicopter hovering at 400 feet, he wrote, depended on “a very expensive and sophisticated piece of machinery to which few ordinary citizens have access,” and equating that vantage point with a pedestrian on a public street ignored the practical reality of what people expect above their fenced yards.4Supreme Court of the United States. Florida v. Riley, 488 U.S. 445 (1989)
Brennan also called it “a curious notion that the reach of the Fourth Amendment can be so largely defined by administrative regulations issued for purposes of flight safety” and said the government, not the homeowner, should bear the burden of showing that overflights at a given altitude are common. Justice Blackmun filed a separate dissent along similar lines. Together the four dissenters warned that the ruling removed “virtually all constitutional barriers to police surveillance from the vantage point of helicopters.”
Where Riley Stops: Naked Eye Only
Riley turned on the fact that the officer used no equipment beyond his own vision. Twelve years later, in Kyllo v. United States, 533 U.S. 27 (2001), the Court drew the technology line. Agents used a thermal imager from the street to detect heat patterns consistent with indoor grow lamps, and the Court held that “where the Government uses a device that is not in general public use, to explore details of a private home that would previously have been unknowable without physical intrusion, the surveillance is a Fourth Amendment ‘search,’ and is presumptively unreasonable without a warrant.”5Justia U.S. Supreme Court Center. Kyllo v. United States, 533 U.S. 27 (2001)
Kyllo does not overrule Riley; it sits alongside it. Naked-eye observation from legal airspace remains outside the warrant requirement. Sense-enhancing technology that reveals what could not otherwise be seen without physical intrusion does not. The Court reinforced that direction in Carpenter v. United States, 585 U.S. 296 (2018), stating that courts must “assure preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted.”6Supreme Court of the United States. Carpenter v. United States, 585 U.S. 296 (2018)
What Riley Means for Drones
Riley predates consumer and police drones by decades, but the case still supplies the governing framework. Under FAA Part 107 rules, commercial and government drones may fly up to 400 feet above ground level, the same altitude the helicopter used in Riley.7Federal Aviation Administration. Small Unmanned Aircraft Systems (UAS) Regulations (Part 107) The Supreme Court has not yet decided a warrantless drone surveillance case.
The two Riley tests point different ways when applied to drones. The plurality’s altitude-legality rule would likely permit a drone at a lawful height. O’Connor’s regularity test cuts the other way, because sustained drone hovering over a private yard is not something the public does with any frequency. Brennan’s concern about surveillance platforms unavailable to ordinary citizens looks different now that consumer drones are cheap and widely owned.
Several states have not waited for the Court to sort this out. At least a dozen require police to obtain a warrant before using drones for surveillance, including Florida, Idaho, Illinois, Montana, Oregon, Tennessee, and Texas. Those statutes operate independently of the federal Fourth Amendment question. Until the Supreme Court revisits aerial surveillance, the controlling doctrine is Riley’s plurality, O’Connor’s concurrence, and the technology boundary drawn by Kyllo, applied against whatever state law adds on top.