CEI (Connected Element Interferometry, also known as “short baseline interferometry”) is a type of interferometry technique with a baseline length typically ranging from tens of kilometers. Through the measurement of carrier phase delay, it can accurately determine the precise angular position of a target relative to the baseline vector in real-time, making it suitable for high-precision orbit determination and relative positioning of medium and high-orbit satellites. This book focuses on introducing the fundamental principles of CEI technology, the design and construction of CEI systems, the key technological breakthroughs achieved in implementing CEI, as well as engineering application examples of CEI technology. It is a comprehensive and balanced resource, encompassing both theoretical and practical aspects.