GPS Chimera: a software receiver implementation

The need of introducing authentication features in future civil navigation signals has been and still is subject of extensive discussions within the GNSS community. As for the GPS system, Chimera is the authentication enhancement designed for the L1C signal. The name is an acronym and stands for Chips-Message Robust Authentication, stressing that it is able to jointly implement navigation message and spreading code authentication, acting both at the data bits and spreading code chips levels. Chimera is now under a testing phase, carried out by the Air Force Research Laboratory, while its first experimental version is foreseen to be broadcast in 2023, from the Navigation Technology Satellite-3 experimental satellite. This paper addresses the innovative implementation of the Chimera protocol verification inside a GPS software receiver. The core concept of Chimera is based on the authentication markers, i.e., authentication chips that puncture the nominal spreading code. They are cryptographically generated using a key derived either from the digital signature of the navigation message, or obtained from an out-of-band source. The paper gives particular emphasis to the implementation of the Chimera markers verification, which is based on the correlation between the ADC signal samples previously stored and the marker samples generated inside the receiver. In addition, the paper also includes the description of the software tool for the generation of the L1C signal with the Chimera enhancement. Due to the absence of live signals, in fact, the development of a proper software generator has been needed to test the receiver.