Astronauts on the Apollo 11, 14, and 15 missions left retroreflectors on the Moon as part of the Lunar Laser Ranging Experiment. The Soviet "Lunokhod 1" and "Lunokhod 2" rovers also carried smaller arrays. Reflected signals were initially received from "Lunokhod 1", but no return signals were detected from 1971 until 2010, at least in part due to some uncertainty in its location on the Moon. In 2010, it was found in Lunar Reconnaissance Orbiter photographs and the retroreflectors have been used again. "Lunokhod 2's" array continues to return signals to Earth. Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally occurring photons challenging.