OBJECTIVE: Develop a GPS anti-jam antenna that interoperates with both GPS pseudolites and Blue Force Electronic Attack (BFEA) interference sources. DESCRIPTION: The signals transmitted by GPS satellites reach the surface of the earth at extremely low power levels, and as a result, the signals are susceptible to intentional and unintentional interference. Sources of intentional interference, known as GPS jammers, are becoming increasingly easier to obtain and use, and consequently, their use has become more pervasive. [1, 2] Military GPS receivers can be outfitted with special purpose antennas to help mitigate the effect of jamming. These antennas, or arrays of several antennas, create nulls in the direction of interference sources to cancel the incoming noise. One such antenna technology, known as the Controlled Radiation Pattern Antenna (CRPA), consists of an antenna array and a processing unit that performs a phase-destructive sum of the incoming interference signals.  GPS pseudolites (pseudo satellites) are another technique utilized by the military to help mitigate intentional GPS interference. A GPS pseudolite is a terrestrial or airborne platform that transmits GPS signals at power levels strong enough to be received in a noisy environment. Pseudolites can be deployed in a wide Area of Operation (AoO), and compatible military GPS receivers can navigate using the signals transmitted by pseudolites. [4, 5] Blue Force Electronic Attack (BFEA) interference sources have the ability to deny the use of GPS and other satellite based navigation systems (collectively known as GNSS) to hostile forces, while simultaneously maintaining service to Military GPS User Equipment (MGUE). To this end, the BFEA interference sources will broadcast waveforms that are designed to preserve specific military GPS signals while denying access to civilian GPS and GNSS signals. Both of these technologies, GPS anti-jam antennas and GPS pseudolites, are effective at mitigating interference to GPS, or in the case of BFEA, denying its use to hostile forces, but they are largely incompatible. That is, a receiver with a GPS anti-jam antenna could null the strong signals produced by a GPS pseudolite, mistaking it as an interference source. The goal of this SBIR effort is to develop a GPS anti-jam antenna that interoperates with both GPS pseudolites and Blue Force Electronic Attack (BFEA) interference sources. The antenna shall be capable of receiving military GPS signals, civilian GPS signals, and GPS pseudolite signals. The antenna shall also be capable of nulling several hostile interference sources, and ignoring any BFEA interference sources. The antenna shall accomplish these tasks simultaneously, that is, it shall receive GPS signals from satellites or pseudolites, while ignoring hostile and BFEA interference sources. The initial intent is to deploy these anti-jam antennas on ground and air platforms, specifically platforms that will emit the pseudolite or BFEA signals. Antennas co-mounted on these platforms shall be capable of ignoring the strong emissions mounted nearby. PHASE I: Design a novel military GPS anti-jam antenna capable of simultaneously receiving military GPS signals, civilian GPS signals, GPS pseudolite signals, and nulling or ignoring interference sources. Develop the overall antenna design and antenna processing unit. PHASE II: Develop a prototype GPS anti-jam antenna capable of simultaneously receiving military GPS signals, civilian GPS signals, GPS pseudolite signals, and nulling or ignoring interference sources. Demonstrate this capability in a controlled laboratory environment. PHASE III: This anti-jam antenna system could potentially be used in a broad range of military applications where access to GPS would otherwise be denied or degraded due to hostile interference. For example, if the Army is conducting operations in an area where GPS signals are denied as a result of hostile interference sources, but GPS pseudolites are deployed to help mitigate this issue, the anti-jam antenna system would allow a military GPS receiver access to the pseudolite signals while at the same time, it will nullify any interference sources in the area. This will give the receiver a much greater advantage over using either one of those technologies alone. The anti-jam antenna system could also be installed on GPS pseudolites, since they will likely be used in environments where GPS is denied or degraded and where BFEA sources are deployed. Commercial applications include government agencies employing GPS devices in the pursuit of criminals using GPS jammers, or transportation systems that employ GPS augmentation signals for improved tracking.