说明：  Software-defined radios (SDRs) have been around for more than a decade. The first complete Global Positioning System (GPS) implementation was described by Dennis Akos in 1997. Since then several research groups have presented their contributions.We therefore find it timely to publish an up-to-date text on the subject and at the same time include Galileo, the forthcoming European satellitebased navigation system. Both GPS and Galileo belong to the category of Global Navigation Satellite Systems (GNSS).