Many applications have required the positioning accuracy of a Global Navigation Satellite System (GNSS). Some applications exist in environments that attenuate GNSS signals, and, consequently, the received GNSS signals become very weak. Examples of such applications are wireless device positioning, positioning in sensor networks that detect natural disasters, and orbit determination of geostationary and high earth orbit (HEO) satellites. Conventional GNSS receivers are not designed to work with weak signals. This book presents novel GNSS receiver algorithms that are designed to work with very weak signals.