A system for verifying ADS-B messages for an aircraft provided with Automatic Dependent Surveillance - Broadcast systems "ADS-B"; the system comprising: a receiver module configured to demodulate and decode signals received from a Mode S transponder, wherein the receiver module determines the type of message received (18, 19, 20) and then extracts and parses the information from each kind of message; the message being an ADS-B message, a request message or a response message; a processor module configured to: gather ADS-B message information extracted and parsed by the receiver module for a periodic time window from other aircraft within ADS-B range; the information being at least: an aircraft ID, an aircraft position for said aircraft ID and a timestamp for each aircraft ID; and record the information into a table; check if for each aircraft within ADS-B range there is at least four timestamps gathered from other aircraft; if so, the processor module is further configured to perform multilateration calculations to determine the position of each aircraft and to compare the calculated position with the position of the aircraft contained within the ADS-B message, being a truthful ADS-B message if both match; if there is not at least four timestamps gathered from other aircraft, check whether a request message from other aircraft within ADS-B range is received within a predetermined time delay; if affirmative, sort the table by the aircraft ID, determine a transmission timeslot wherein the transmission time slot corresponds to the aircraft own position in the sorted table, determine a random delay, and broadcast a response message during the transmission timeslot when the random delay is over, the response message including the ADS-B message information gathered for the periodic time window; if negative, broadcast a request message after the predetermined time delay to each aircraft within ADS-B range; the system further comprising a transmitter module configured to format the request message and the response message for sending said request message and response message to the Mode S transponder. a receiver module configured to demodulate and decode signals received from a Mode S transponder, wherein the receiver module determines the type of message received (18, 19, 20) and then extracts and parses the information from each kind of message; the message being an ADS-B message, a request message or a response message; a processor module configured to: gather ADS-B message information extracted and parsed by the receiver module for a periodic time window from other aircraft within ADS-B range; the information being at least: an aircraft ID, an aircraft position for said aircraft ID and a timestamp for each aircraft ID; and record the information into a table; check if for each aircraft within ADS-B range there is at least four timestamps gathered from other aircraft; if so, the processor module is further configured to perform multilateration calculations to determine the position of each aircraft and to compare the calculated position with the position of the aircraft contained within the ADS-B message, being a truthful ADS-B message if both match; if there is not at least four timestamps gathered from other aircraft, check whether a request message from other aircraft within ADS-B range is received within a predetermined time delay; if affirmative, sort the table by the aircraft ID, determine a transmission timeslot wherein the transmission time slot corresponds to the aircraft own position in the sorted table, determine a random delay, and broadcast a response message during the transmission timeslot when the random delay is over, the response message including the ADS-B message information gathered for the periodic time window; if negative, gather ADS-B message information extracted and parsed by the receiver module for a periodic time window from other aircraft within ADS-B range; the information being at least: an aircraft ID, an aircraft position for said aircraft ID and a timestamp for each aircraft ID; and record the information into a table; check if for each aircraft within ADS-B range there is at least four timestamps gathered from other aircraft; if so, the processor module is further configured to perform multilateration calculations to determine the position of each aircraft and to compare the calculated position with the position of the aircraft contained within the ADS-B message, being a truthful ADS-B message if both match; if there is not at least four timestamps gathered from other aircraft, check whether a request message from other aircraft within ADS-B range is received within a predetermined time delay; if affirmative, sort the table by the aircraft ID, determine a transmission timeslot wherein the transmission time slot corresponds to the aircraft own position in the sorted table, determine a random delay, and broadcast a response message during the transmission timeslot when the random delay is over, the response message including the ADS-B message information gathered for the periodic time window; if negative, broadcast a request message after the predetermined time delay to each aircraft within ADS-B range; the system further comprising aircraft within ADS-B range; the system further comprising a transmitter module configured to format the request message and the response message for sending said request message and response message to the Mode S transponder.