How do I design an anti-jam GPS receiver for military applications?

Designing an anti-jam GPS receiver for military applications requires a controlled reception pattern antenna (CRPA) system that can electronically null interfering signals while maintaining gain toward GPS satellites. A CRPA system consists of an array of antenna elements (typically 4-7 elements in a circular arrangement), a GPS receiver, and a digital beamformer that adaptively weights the element signals to place pattern nulls in the directions of jammer sources. Since GPS signals are approximately 20-30 dB below the thermal noise floor, even relatively weak jammers can disrupt reception, so the anti-jam system must provide 30-70 dB of jammer suppression depending on the threat level. The number of independent nulls the array can form equals one less than the number of elements (N-1), so a 7-element CRPA can null up to 6 simultaneous jammers. Additional anti-jam techniques include using the GPS military code (M-code), which provides a more jam-resistant modulation, narrow bandpass filtering to reject out-of-band jammers, and inertial navigation system (INS) aiding that allows continued navigation during brief GPS outages.