Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. USA-225, also known as the Rapid Pathfinder Prototype (RPP) and NRO Launch 66 (NROL-66), is an American satellite which was launched in 2011. The satellite is being used to perform technology demonstration and development experiments, including advanced dosimeters to characterize the space environment from a 1,200 kilometer low Earth orbit. It is operated by the United States National Reconnaissance Office.Rapid Pathfinder was developed for less than US$ 20 million over a period of less than two years. Its dimensions are 0.5 m times 0.5 m times 0.5 m, and its mass including payload is 235 kg.