US Patent 11698461 GPS denial detection and reporting and mitigation

A system and method of identifying and responding to a GPS denial of service includes: configuring a mode S transponder for transmitting a GPS time-of-day message as a downlink format message using a BDS register, and configuring an aircraft surveillance system for receiving one or more GPS time-of-day messages transmitted as a downlink format message. The surveillance system compares the received time-of-day message(s) from the aircraft to a comparison time of day, and validates reception of authentic GPS signals by the aircraft when the received time-of-day message is within a threshold amount of the comparison time of day. The comparison time of day may be the GPS time of day of one of a plurality of aircraft in the surveillance volume or may be the GPS time of day determined by the aircraft surveillance system. An indicator on the transponder indicates counterfeit GPS signals, permitting mitigation od induced navigation error.