Rockwell Collins' Flexible Digital Anti-Jam Architecture (2003)

Until quite recently, most GPS anti-jam technology fielded traces back to technology developed in the late '70's. These analog-based anti-jam systems control the reception pattern of the GPS antenna system using a controlled radiation pattern antenna and associated electronics, mitigating the effects of interference by nulling out the offending interference energy. Analog systems are generally implemented using a set of analog phase shifters to adaptively weight the output of each antenna element. Phase control has limited precision due to the analog nature of the phase shifters. Furthermore, the weights must be calculated using an iterative algorithm for which the anti-jam system performance can suffer in high dynamic environments. Due to the size, power consumption, weight, and cost associated with these analog systems, their use has not grown across a wide base of potential users. Hence, an alternative solution is needed to more adequately address the needs of the GPS user community. With the advent of modern digital technology, Rockwell Collins, Inc. has embraced a digital architecture for spatial GPS anti-jam. This paper will describe why Rockwell Collins has made this choice. One consideration was the performance aspect of digital anti-jam, but just as important were the real-world practicalities important to almost any customer: size, weight, power, and cost. Also described is Rockwell Collins' unique ability to provide a fully integrated GPS and anti-jam system. It is this unique ability that allows Rockwell Collins to provide a high performance yet low cost anti-jam system that fully addresses the practical implementation issues. With digital signal processing, precise control of signal phase is possible when combining information from each of the antenna elements. This leads to the ability for improved control of the antenna pattern by accurately applying the desired weights, yielding both deeper nulls and increased steering gain for systems that implement beamsteering. Precise phase control can nearly eliminate in-phase and quadrature-phase (I/Q) imbalance which is a known limiter of nulling performance in analog systems. The I/Q signals necessary for the anti-jam processing can be generated digitally with a Hilbert filter which can achieve almost any desired level of negative image rejection. Adaptation rates of the complex weights and hence the convergence times of the nulling performance are significantly faster in a digital system. This allows the digital anti-jam processing to respond to the jamming environment much faster than an analog system. Fast adaptation rates are important for responding to changes in spatial orientation of the jamming environment as well as changes in amplitude of the jamming environment. Also important is the fact that digital systems allow the adaptive weights to be applied to the same signal from which they are calculated, which leads to deeper null depths in a jammer environment in which the spatial orientation of the jammer signals is changing rapidly. Building on significant past experience with both analog and digital anti-jam systems, Rockwell Collins has moved forward with a scalable, digital anti-jam architecture that meets the current industry needs and incorporates the flexibility to address the rapidly changing GPS anti-jam market. There exists an industry need to provide a small and inexpensive anti-jam system for the low to medium (20-40 dB) anti-jam range. The ability to incorporate this level of anti-jam performance is important to protect GPS reception in platforms such as guided munitions, artillery, and handheld applications. This paper describes the first production version of Rockwell Collins digital anti-jam technology, a multi-element digital nuller. Test results of this system will be provided and described, including performance against both broadband and narrowband jammers. This anti-jam system is both inexpensive and small, which will allow for integration on the same card with Rockwell Collins' industry leading SAASM-based GPS receivers. This paper will describe how this integration occurs and the associated benefits, including benefits that cannot be realized in existing standalone digital architectures. In addition to the reduction in cost and size of GPS anti-jam hardware, integration allows for a more intelligent anti-jam system that responds optimally to the jamming environment. Through an improved GPS/anti-jam interface, a more robust solution exists that has the ability to synchronize processing between the anti-jam and GPS processor. Furthermore, the potential exists for next generation of Rockwell Collins' GPS receiver to host the anti-jam software on the same processor as the GPS receiver software. While the current version of Rockwell Collins' digital architecture addresses low to medium anti-jam threats, the architecture scales readily both in terms of number of elements and number of complex weights per element to extend capabilities for the number of jammers and null depth. This paper will address this scalability as well as Rockwell Collins' path to further reduce the size and cost of digital anti-jam systems, implementing ASIC, MMIC, FPGA, and other recently developing technologies. Further reductions in size result in smaller form factors for the existing system or allow for the incorporation of more capability into the same form factor. Also discussed are trends towards even more flexibility in terms of reconfigurable hardware and software.