Secure, reliable position and time information is indispensable for many civil GPS applications such as guiding aircraft, tracking freight, synchronizing power grids and cellular networks, and time-stamping financial transactions. This paper introduces a signal authentication architecture based on a network of cooperative receivers. A receiver in the network correlates its received military P(Y) signal with those received by other receivers (hereinafter referred to as crosscheck receivers) so as to detect spoofing attacks. This paper describes a candidate structures to implement this architecture. Our theoretical analysis shows that the signal-to-noise-ratio significantly affects pair-wise check performance, and the final authentication performance improves exponentially with increasing number of cross-check receivers. We have conducted two field experiments to evaluate pair-wise check performance in different spatial conditions (urban canyon and open space) and different transport modes (static and moving). The experiments shows that pair-wise check performance is sensitive to spatial conditions, but insensitive to transport modes.