Systems and a related method for internally monitored navigation replace one or more inertial reference units (IRU) of an aircraft navigation system with GNSS-assisted multi-mode receivers (MMR) including dual-antenna receivers. Each MMR may validate inertial position data generated by a remaining IRU by detecting drift errors in the inertial position data or latent faults in the IRU. The system may include, in place of one or more IRUs, attitude heading and reference systems (AHRS) incorporating lower-grade high-performance inertial sensors. Internal monitoring of the remaining IRUs or the inertial position data generated thereby may alternatively be carried out by the AHRS. Based on internal monitoring by the MMRs and/or AHRS, user display and flight control systems of an aircraft can exclude a faulty IRU, preventing the use of position solutions incorporating erroneous inertial data.