Alternative terminal navigation based on modified airport multilateration system

In this paper, an alternative to Global Navigation System (GPS) based navigation in the terminal airspace is proposed to improve the resilience of our nation´s Next-Generation air transportation system (NextGen) against man-made and natural threats to the GPS. GPS is a critical element of the future aviation infrastructure and interruptions of GPS service are known to have measurable impacts on NextGen procedures in use today. We propose that, by modifying airports´ multilateration surveillance systems to broadcast new navigation signals, seamless operation during GPS outages is possible. In this new concept, synchronized MLAT transceivers broadcast pseudo-randomly staggered messages so that onboard avionics can estimate an aircraft´s position from the Time Difference of Arrival (TDOA) between messages received from multiple pairs of MLAT transceivers. Also, MLAT transceivers are modified to reply to TCAS interrogations or spontaneously respond to MODE-S Downlink messages allowing aircraft to measure the round-trip delay for range calculation. Using either multiple TDOAs, multiple ranges, or a combination of both from either a single airport or multiple airports, onboard avionics are able to calculate ownship position for navigation.