  Many vehicle platforms typically use sensors such as LiDAR or camera for
locally-referenced navigation with GPS for globally-referenced navigation.
However, due to the unencrypted nature of GPS signals, all civilian users are
vulner-able to spoofing attacks, where a malicious spoofer broadcasts
fabricated signals and causes the user to track a false position fix. To
protect against such GPS spoofing attacks, Chips-Message Robust Authentication
(Chimera) has been developed and will be tested on the Navigation Technology
Satellite 3 (NTS-3) satellite being launched later this year. However, Chimera
authentication is not continuously available and may not provide sufficient
protection for vehicles which rely on more frequent GPS measurements. In this
paper, we propose a factor graph-based state estimation framework which
integrates LiDAR and GPS while simultaneously detecting and mitigating spoofing
attacks experienced between consecutive Chimera authentications. Our proposed
framework combines GPS pseudorange measurements with LiDAR odometry to provide
a robust navigation solution. A chi-squared detector, based on pseudorange
residuals, is used to detect and mitigate any potential GPS spoofing attacks.
We evaluate our method using real-world LiDAR data from the KITTI dataset and
simulated GPS measurements, both nominal and with spoofing. Across multiple
trajectories and Monte Carlo runs, our method consistently achieves position
errors under 5 m during nominal conditions, and successfully bounds positioning
error to within odometry drift levels during spoofed conditions.
