Aviation detect and avoid method and system

1. A method for detecting and avoiding a target object, comprising:

determining a position of the target object, comprising:

at a Passive Secondary Surveillance Radar (PSSR) spaced apart from a Secondary Surveillance Radar (SSR):

determining a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P 1 , P 2 , P 3 ) of a Secondary Surveillance Radar (SSR), the P 2 and P 3 pulses being synchronized to respective P 1 pulses with respective first and second predefined time gaps;

receiving a reply from the target object in response to an interrogation signal comprising a P 3 pulse sent by the SSR to said target object;

estimating a transmit time of said P 3 pulse of the interrogation signal based on a reception time of said reply, and the PRF pattern; and

determining the position of the target object using an altitude information h of the target object contained in said reply, a location of the SSR, said estimated transmit time of said P 3 pulse of the interrogation signal, and said reception time of said reply;

said determining said PRF pattern comprising:

during a time-window where the PSSR is within range of a wide-beam antenna of the SSR:

detecting successive P 2 pulses;

forming a time-ordered sequence of intervals separating the successive P 2 pulses;

identifying at least two successive congruent segments of the time-ordered sequence;

determining a pulse repetition pattern of P 2 pulses as one of the segments subject to the constraint that a number of intervals of said one of the segments is within an initial length lower bound and a predefined length upper bound;

deriving the PRF pattern based on the pulse repetition pattern of P 2 pulses and corresponding values of the first and second predefined time gaps;

and

controlling the target object so as to avoid the target object.

2. The method of claim 1 further comprising:

during a time-window where the PSSR is within range of a narrow-beam antenna of the SSR:

detecting P 1 and P 3 pulses; and

determining a value of the second predefined time gap between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR.

3. The method of claim 1 wherein said forming comprises:

initializing an array of said intervals and

initializing a first pointer of said array.

4. The method of claim 1 wherein said identifying comprises:

finding a primary string of adjoining intervals, of said time-ordered sequence, in which a first interval is distinct from any other interval with a last interval preceding an interval that equals said first interval;

examining a candidate string of adjoining intervals of maximum congruence to said first string, following said last interval; and

subject to a determination that said candidate string is fully congruent with said first string, determining said first string as said PRF pattern.

5. The method of claim 4 further comprising, subject to a determination that said candidate string is not fully congruent with said first string:

appending said candidate string to said primary string; and

updating said last interval to be the end interval of the candidate string.

6. The method of claim 4 further comprising continually storing said primary string and said candidate string in said array.

7. The method of claim 4 further comprising terminating said determining of the PRF pattern subject to a determination that at least one of said primary string and candidate string comprises a number of intervals exceeding said predefined length upper bound.

8. The method of claim 1 further comprising:

using said pulse repetition pattern of P 2 pulses as a reference string, and a number of intervals of the reference string as a reference length;

continuing to receive P 2 pulses;

forming consecutive strings of intervals between successive pulses, each consecutive string comprising a number of intervals equal to the reference length;

determining a number of consecutive strings that are congruent with the reference string; and

subject to a determination that said number of consecutive strings at least equals a predefined congruence lower bound, ascertaining correctness of detected pattern.

9. The method of claim 8 further comprising, subject to a determination that said number of consecutive strings is less than said congruence lower bound:

resetting said initial length lower bound to a higher value not exceeding said predefined length upper bound; and

repeating said determining said PRF pattern.

10. The method of claim 8 wherein said determining a number of consecutive strings that are congruent comprises sequentially determining congruence of two successive strings, starting with said reference string.

11. The method of claim 10 wherein said determining congruence comprises:

determining a respective absolute value of a difference between each interval of one of the strings and an interval of a corresponding positions of the other string; and

ascertaining congruence subject to a determination that said respective absolute value is below a first prescribed tolerance level.

12. 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 P 1 and P 3 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 P 2 pulses from the SSR, said P 2 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 (P 1 , P 2 , P 3 ) of the SSR, the P 2 and P 3 pulses being synchronized to respective P 1 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 P 2 pulses;

form a time-ordered sequence of intervals separating the successive P 2 pulses;

identify at least two successive congruent segments of the time-ordered sequence; and

determine a pulse repetition pattern of P 2 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.

13. The detect-and-avoid system of claim 12 wherein said instructions further cause the second processor to:

during a time-window where the PSSR is within range of the narrow-beam antenna:

detect P 1 and P 3 pulses; and

determine a value of the second predefined time gap between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR; and

derive the PRF pattern based on the pulse repetition pattern of P 2 pulses and corresponding values of the first and second predefined time gaps.

14. The detect-and-avoid system of claim 12 wherein:

the first processor continually determines inter-pulse intervals and stores the intervals in a buffer; and

the second processor independently reads individual inter-pulse intervals and executes said instructions,

thereby decoupling pulse acquisition and inter-pulse measurement timescale from processing timescale.

15. The detect-and-avoid system of claim 12 wherein said second processor is configured to ensure that a mean execution time per interval does not exceed a mean inter-pulse interval.

16. The detect-and-avoid system of claim 12 further comprising ground-based PSSR equipment installed within a ground-based surveillance system, said PSSR equipment being communicatively coupled to respective interface equipment within said control station.

17. The detect-and-avoid system of claim 14 further comprising a comparator unit communicatively coupled to said respective interface, the comparator unit comprising a respective processor configured to:

receive data relevant to safety of the ownship generated at the PSSR system of the ownship;

receive data relevant to safety of the ownship generated at the ground-based PSSR equipment;

perform comparative data analysis to enhance safety measures.

18. The detect-and-avoid system of claim 12 wherein said instructions further cause the second processor to:

during a time-window where the PSSR is within range of the narrow-beam antenna:

initialize an array of said intervals; and

initialize a first pointer of said array.

19. The detect-and-avoid system of claim 12 wherein said instructions to identify further cause the second processor to:

during a time-window where the PSSR is within range of the narrow-beam antenna:

find a primary string of adjoining intervals, of said time-ordered sequence, in which a first interval is distinct from any other interval with a last interval preceding an interval that equals said first interval;

examine a candidate string of adjoining intervals of maximum congruence to said first string, following said last interval; and

subject to a determination that said candidate string is fully congruent with said first string, determining said first string as said PRF pattern.

20. The detect-and-avoid system of claim 12 wherein said instructions further cause the second processor to:

during a time-window where the PSSR is within range of the narrow-beam antenna:

use said pulse repetition pattern of P 2 pulses as a reference string, and a number of intervals of the reference string as a reference length;

continue to receive P 2 pulses;

form consecutive strings of intervals between successive pulses, each consecutive string comprising a number of intervals equal to the reference length;

determine a number of consecutive strings that are congruent with the reference string; and

subject to a determination that said number of consecutive strings at least equals a predefined congruence lower bound, ascertaining correctness of detected pattern.