From the Congressional Research Service, by Richard M. Thompson II:
… the constitutionality of domestic drone surveillance may depend upon the context in which such surveillance takes place. Whether a targeted individual is at home, in his backyard, in the public square, or near a national border will play a large role in determining whether he is entitled to privacy. Equally important is the sophistication of the technology used by law enforcement and the duration of the surveillance. Both of these factors will likely inform a reviewing court’s reasoning as to whether the government’s surveillance constitutes an unreasonable search in violation of the Fourth Amendment.
Read the full report on FAS.