Aviation detect and avoid method and system

A detect-and-avoid system for an ownship aircraft comprising:

• a control station in communication with an ownship aircraft for controlling the ownship air-craft;
• at the ownship aircraft, a Passive Secondary Surveillance Radar (PSSR) system in communication with the control station, the PSSR comprising:
• a first receiver for receiving a reply from a target object wherein said reply is responsive to an interrogation signal comprising P1 and P3 pulses sent by a narrow-beam antenna of a Secondary Surveillance Radar (SSR) to said target object;
• a second receiver for receiving a stream of P2 pulses from the SSR, said P2 pulses being transmitted in a staggered pattern by a wide-beam antenna of the SSR;
• a first processor coupled to said first receiver and said second receiver,
• a second processor for executing instructions to:
• determine a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P1, P2, P3) of the SSR, the P2 and P3 pulses being synchronized to respective P1 pulses with respective first and second predefined time gaps;
• estimate a transmit time of said interrogation signal based on a reception time of said reply, and the PRF pattern;
• determine the position of the target object based on an altitude information of the target object provided in said reply, a location of the SSR, said transmit time of said interrogation signal, and said reception time of said reply;
• and
• during a time-window where the PSSR is within range of the wide-beam antenna:
• detect successive P2 pulses;
• form a time-ordered sequence of intervals separating the successive P2 pulses;
• identify at least two successive congruent segments of the time-ordered sequence;
• and
• determine a pulse repetition pattern of P2 pulses as one of the segments subject to the constraint that a number of intervals of said one of the segments is within predefined lower and upper bounds.