Galileo is designed to be interoperable but independent from GPS. Interoperability means that user receiver equipment should be able to take advantage of both systems to increase accuracy, integrity, and overall performance. Compatibility, on the other hand, implies that the two systems should operate concurrently but independently, with as little impact on each other as possible. The inclusion of Galileo should not degrade GPS standalone performance, and conversely GPS should not impact Galileo processing. The interoperability, despite frequency overlap, is accomplished through the use of different signal structures and codes. The frequency overlap, or essentially using the same carrier frequency, greatly simplifies the radio frequency (RF) design and reduces the cost for hybrid receivers; the same antenna and front end can be used to receive signals from both systems. The digital signal processing of the signals will, however, differ. This paper presents the design and implementation of a hybrid software GPS/Galileo receiver, together with testing and performance results using a GPS/Galileo software intermediate frequency (IF) simulator. INTRODUCTION Software defined radio (SDR) is a concept for transceivers in which the signal processing is accomplished via a programmable general-purpose microprocessor or digital signal processor (DSP), as opposed to an application-specific integrated circuit (ASIC). A fully software Global Navigation Satellite System (GNSS) receiver implementation holds many advantages over traditional receiver designs. The most significant of these advantages include the ability to do dynamic reconfiguration; the ability to reuse hardware in system design; and rapid design and prototyping cycles. The complete list is quite extensive, and is an important factor in light of upcoming expansions in GNSS service, including not only Galileo but also the forthcoming modernized GPS signals (Block IIR-M) and the QuasiZenith Satellite System (QZSS). Software GNSS receivers are a reality today and are receiving market acceptance as a feasible alternative to traditional hardware designs in certain applications. The authors demonstrated one of the first fully real-time software GPS receivers in 2000 [2] and have continued to develop software receiver technology further at NordNav Technologies. At present, a handful of research institutes are developing real-time software receivers [4,5], with efforts concentrating on multiple frequencies and wide bandwidth processing. Many are using software receivers as core tools in their research [7,8]. GALILEO AND GPS Galileo and GPS interoperability means that hybrid receivers will offer improved performance in several ways compared to GPSor Galileo-only receivers. The availability of satellites (29 GPS and 27 Galileo) will offer significant improvements in acquisition and tracking in demanding environments such as cities, where satellites are often blocked by buildings. Additionally, hybrid receivers can select those subsets of visible satellites that improve multipath performance for a given antenna location. Integrity performance will also benefit from the presence of the two independent systems.