The answer to this question is important in light of the California legislature's recent passage of a bill restricting the use of drones by law enforcement agencies. If existing California law already requires police to obtain a warrant for aerial surveillance, then this bill would not seem to make much of a difference.
Normally, I would quickly answer that police are not constitutionally required to obtain a warrant before engaging in aerial surveillance. But after some searching of California case law, I found that the answer is a little bit more complicated -- although I am still fairly sure that the answer is no.
My Initial Answer
My initial instinct to conclude that police in California do not need a warrant to carry out aerial surveillance is based on the 1989 Supreme Court case of Florida v. Riley. There, the Court held that officers did not carry out a Fourth Amendment search when they photographed a suspect's backyard from a helicopter that they were flying within navigable airspace. The plurality held that people do not have a reasonable expectation of privacy from this sort of surveillance, and O'Connor's concurrence stressed the importance that the helicopter be flown in a manner consistent with the common flying habits of the public at large.
For this reason, I've been of the opinion that police in California can conduct aerial surveillance of suspects so long as their plane or helicopter is flown in navigable airspace in accordance with the applicable regulations.
California's Initial Case Law
During an informative Twitter exchange earlier today, I learned about a California Supreme Court case that potentially complicates the answer to this question. In People v. Cook, the California Supreme Court held that a person has a reasonable expectation of privacy in his or her fenced-in backyard. The court held that aerial surveillance of the yard -- even from 1,000 feet in the air -- intrudes on the curtilage of the home and violates a reasonable expectation of privacy. The Court held that this was unconstitutional under Article 1, Section 13 of the California Constitution.
Cook was decided in 1986. It was later narrowed by the California Supreme Court in People v. Mayoff, where the Court held that aerial surveillance of open fields was permitted under the California Constitution. The Cook holding was further narrowed by People v. Venghiattis, where the California Court of Appeal held that Cook's reasoning did not apply to an unfenced yard where marijuana was being grown 100 feet from the suspect's house.
The Impact of Proposition 8: The "Truth in Evidence" Rule
More important than these factual distinctions, however, are the legal developments that took place in California after Cook. Soon after Cook, Proposition 8 added the Truth in Evidence Rule to the California Constitution, which can now be found at Article 1, Section 28(f)(2). In In re Lance W, the California Supreme Court construed this rule to require that evidence can only be excluded in a criminal trial if it is obtained in violation of the Federal, rather than the state, Constitution. Mayoff and Venghattis noted this change, and concluded that Cook no longer applied, since its holding was based on the California Constitution.
The California Court of Appeal took further note of the impact of the Truth in Evidence Rule in the 1986 case of People v. Sabo. While the Sabo court held that evidence obtained from a low-flying helicopter violated a reasonable expectation of privacy, the court reached this conclusion because the helicopter was not within navigable airspace. The court noted in footnote one of its opinion that it was basing its opinion on the recent Supreme Court case, California v. Ciraolo, and that Cook would not warrant the exclusion of evidence if the admission of the evidence was consistent with the holdings of the United States Supreme Court.
In the 1988 case of People v. Romo, the California Court of Appeal confronted a case very similar to Cook. There, an officer conducted aerial surveillance of a suspect's fenced-in yard, and took pictures of the yard where the suspect was growing marijuana. The court noted that the marijuana was farther from the suspect's house than the marijuana in Cook, but further noted that Cook did not mandate exclusion of the evidence because its holding was based on the California, rather than the Federal, Constitution. In light of the Truth in Evidence Rule and the interpretation of that rule in Lance W., the Romo court held that Cook no longer applied.
This series of cases explains the California Court of Appeal's decision in People v. McKim. There, the court held that no Fourth Amendment search took place when officers spotted marijuana growing "30-40 yards" from the suspect's "rural residence." While the court could have relied on Venghiattis to factually distinguish Cook, the court cited neither Cook nor Venghiattis, and focused instead on the first case I mentioned in this post: the United States Supreme Court case of Florida v. Riley.
In light of these cases and legal developments, I am inclined to conclude that California law does not restrict aerial surveillance by government officers beyond any restrictions required by the Supreme Court's existing case law.
Admittedly, I could not find any California Supreme Court case that explicitly overturns Cook. And all of the California Court of Appeals cases that noted the significance of the Truth in Evidence Rule as overturning Cook involved scenarios that could have been factually distinguished from the Cook case.
But in light of the Truth in Evidence Rule, I don't think that Cook would apply if a court were to consider the same facts today. While the California cases have involved facts that are potentially distinguishable from Cook, Florida v. Riley involved a case where marijuana was growing a mere "10 to 20 feet" from the defendant's home in the defendant's fenced-in yard -- a fact pattern that is extremely similar to that in the Cook case. California courts have consistently held that the Fourth Amendment, rather than Article 1, Section 13 of the California Constitution, governs whether the police carry out a search. If the aerial surveillance in Riley was not a search, then the aerial surveillance in Cook would probably not be considered a search.
All of this means that government officers can conduct aerial surveillance of people as long as they are flying within navigable airspace. This also means that California's AB 1327 -- which would require the government to obtain a warrant before using drones for surveillance in criminal cases -- could have a meaningful impact on the law governing searches and surveillance.
UPDATE - 8/28/2014
I have made changes to the sentence describing Justice O'Connor's concurrence in Florida v. Riley.