## 1 Introduction  
Since Global Navigation Satellite System (GNSS) sensors (e.g., Global Positioning System or GPS of the US [1,2], Galileo of Europe, GLONASS of Russia, and Compass of China) are widely used in many fields, including navigation, surveillance and precise timing, their vulnerability to radio frequency interference (RFI) is drawing significant attention. GPS RFI [3-5] is especially problematic for GPS-based safety-of-life services such as aviation. For example, GPS RFI was detected during the installation of a Ground-Based Augmentation System (GBAS) [6-10] ground monitor at Newark airport (EWR) on 23 November 2009 [11]. The installed GBAS can support GPS-based precision landing approach of aircraft down to 200 ft above the runway. Since then, many other GPS RFI events have been detected at Newark airport and the certification of that GBAS has been delayed. The RFI sources turned out to be personal \"privacy jammers\" in vehicles on the nearby highway, which are designed to overwhelm weak GPS signals by transmitting signals in the GPS L1 frequency (1.575 GHz). In order to guarantee the integrity and availability of civil aviation under GPS RFI, the United States Federal Aviation Administration (FAA) has recently initiated an effort called Alternative Position Navigation and Timing (APNT) to provide a navigation system for aviation that can provide many of the operational capabilities that will be enabled by GPS [12,13]. The aim of APNT is to provide robustness for the US National Airspace to GPS outages due to RFI and other events. Among various GPS anti-jamming technologies, an adaptive beamsteering GPS receiver has been proposed for robust time synchronization between terrestrial assets used by APNT, such as Ground Based Transceivers (GBTs), even under GPS RFI [14,15]. Since ground stations are stationary, in principle one tracked satellite is adequate for time transfer. However, if a beamsteering GPS receiver can steer directed beams towards all GPS satellites in view and reject RFI adaptively as presented in this paper (Figure 1), it could be potentially used for many other applications such as hardening GBAS ground facilities. Mitigation of RFI for generic GPS applications using adaptive antenna arrays has been previously studied (e.g., [16-18]). In order to use a controlled reception pattern antenna (CRPA) array for high-integrity GPS applications such as aircraft landing guidance [19], the system should satisfy four quality requirements which are accuracy, integrity, availability, and continuity [20]. Several researchers have studied the feasibility of using CRPA for high-integrity GPS applications [21-23]. The development of GPS software-defined radio (SDR) [24, 25] has been beneficial for studying adaptive beamsteering GPS receiver technology as well [26]. A software implementation of a GPS receiver is more desirable than a traditional hardware implementation using field-programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs) for multiple reasons. First, such an implementation has higher flexibility and shorter development time, allowing new algorithms to be quickly implemented and tested. Second, a software implementation that can run on commercial off-the-shelf (COTS) general-purpose processors makes the technology more cost effective and accessible for the civil consumer market. Although a real-time GPS SDR requires extensive computational power, real-time GPS L1 SDRs using COTS general-purpose processors has been developed by several researchers (e.g., four tracking channels for GPS in 2001 [27] and 24 tracking channels for GPS and Galileo in 2006 [28]). However, development of a real-time GPS SDR for CRPA is still challenging because adaptive GPS beamsteering processing requires much more computational power than a conventional GPS receiver. We are not aware of a previous real-time GPS SDR development for CRPA with all-in-view satellite tracking capability. [PERSON] _et al._[29] presented a real-time beamsteering GPS SDR for robust time transfer for APNT.