set of Vela Satellites in the Hotel series, DARPA began work on the next phase of the Vela satellite program, namely, on Vela Sierra, which was intended to monitor the atmosphere for covert nuclear tests. Following the deployment of the Hotel series, Los Alamos resumed development on the satellites, this time working towards the Advanced Vela series. These new satellites were created with the intention that they would replace the Hotel Series, as well as function as the satellites that would complete the objectives of the Vela Sierra subset. The new set of Advanced Vela satellites were equipped with the same detection equipment as the Hotel series, those being the 12 X-ray detectors and 18 internal Neutron and Gamma-Ray detectors, but were also equipped with a new set of instruments. The Advanced Vela series came equipped with two non-imaging photodiode sensors called bhangmeters. These measured light levels over sub-millisecond intervals, and could detect a nuclear explosion to within 3,000 miles. This addition was necessary due to the singular effects that atmospheric nuclear explosions produce, called either the "Double Flash" or the "Double-Humped Curve," which is a pair of flashes of light caused by an explosion, one lasting a millisecond which is quite intense and bright, the other far more prolonged and less intense. This "Double Flash" is what distinguishes atmospheric nuclear explosions from other kinds of explosions in the atmosphere, along with radiation, since the only feasible event that could theoretically produce a double event like an atmospheric nuclear explosion would be something incredibly rare like a meteoroid generated lightning superbolt. However, radiation in the atmosphere is far less easy to calculate, as the Van Allen radiation belts can mask radioactive wavelengths and signatures. The Advanced Vela satellites were additionally fitted with larger solar panels, since the new design required 120 watts as opposed to the Hotel series' 90 watts, as well as electromagnetic sensors for detecting an electromagnetic pulse from an in-atmosphere detonation. The new set of satellites had a design life of 18 months, but instead lasted on average 7 years, with the longest lasting of the set (Vehicle 9) being shut down in 1984, lasting around 15 years before being shut down. Vela Sierra's most famous actual event was the "Double Flash" detected on September 22, 1979 by Vela satellite 6911 near the Prince Edward Islands. However, due to the lack of any corroborating evidence that a