Today's society has a vast dependence of accurate and reliable time and position from global navigation satellite systems (GNSS). A lot has happened in GNSS since the turn of the millennium, driven by both military and commercial interests. GPS and GLONASS are undergoing major modernizations and both Galileo and Beidou has emerged as new systems. The competition between systems is beneficial for the users as large amounts are invested to improve the systems and to make them relevant for the future. Both civilian and military users should stay up-to-date with the options available in the civilian market and the opportunities they offer. This report provides a survey of the four global navigation satellite systems used today: GPS, GLONASS, Galileo and BeiDou. The report focuses on the open signals and services but also provides an introduction to the history and architecture of each system.