Modern Inertial Reference Units [0] (IRU/IRS/INS) are fitted to most airliners and business jets, and are an alternative to GPS. IRU accuracy is less than GPS,

Modern Inertial Reference Units [0] (IRU/IRS/INS) are fitted to most airliners and business jets, and are an alternative to GPS. IRU accuracy is less than GPS, and older units have issues with drift. The FAA hasn't approved any IRU for ADS-B[1] backup. More importantly, ATC can use the Mode-S[2] transponder without GPS to locate the aircraft. Secondary Surveillance Radar won't be phased out in 2020. ADS-B will be required by the FAA in 2020. [0] https://en.wikipedia.org/wiki/Inertial_navigation_system https://en.wikipedia.org/wiki/Inertial_navigation_system [1] https://en.wikipedia.org/wiki/Automatic_dependent_surveillance_%E2%80%93_broadcast https://en.wikipedia.org/wiki/Automatic_dependent_surveillan... [2] https://en.wikipedia.org/wiki/Secondary_surveillance_radar#Mode_S https://en.wikipedia.org/wiki/Secondary_surveillance_radar#M...