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 sub-
ject and at the same time include Galileo, the forthcoming European satellite-
based navigation system. Both GPS and Galileo belong to the category of Global
Navigation Satellite Systems (GNSS).
