Method and system for tracking objects using passive secondary surveillance radar

1. A method for determining a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P 1 , P 2 , P 3 ) of a Secondary Surveillance Radar (SSR), the SSR comprising a main antenna and a wide-beam antenna whose beam-width is wider than a beam-width of the main antenna, pulses P 1 and P 3 being generated by the main antenna, and pulses P 2 being generated by the wide-beam antenna, each of the P 2 and P 3 pulses being synchronized with a respective P 1 pulse with respective first and second predefined time delays, the method comprising:

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

(i) during a time window when the PSSR is within the beam-width of the wide-beam antenna, and thereby P 2 pulses become detectable at the PSSR, detecting successive P 2 pulses;

(ii) forming a time-ordered sequence of P 2 pulse intervals derived from the P 2 pulses detected in the step (i);

(iii) determining a pulse repetition pattern of P 2 pulses using the time-ordered sequence of P 2 pulse intervals, comprising:

(iii-1) provided at least one P 2 pulse interval for a next detected P 2 pulse is already present in the time-ordered sequence determined in the step (iii), and

the time-ordered sequence of P 2 pulse intervals starts repeating, determining the pulse repetition pattern of P 2 pulses using the time-ordered sequence;

(iii-2) otherwise, repeating the steps (i) to (iii-1); and

(iv) when the PSSR is within the beam-width of the main antenna, detecting P 1 and P 3 pulses, and determining a value of the second predefined time delay between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR;

(v) obtaining a value of the first predefined time delay, and deriving the PRF pattern for the staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR based on the pulse repetition pattern of P 2 pulses determined in the step (iii), and corresponding values of the first and second predefined time delays.

2. The method of claim 1 , further comprising predicting a transmit time fora P 3 pulse based on said PRF pattern provided the PSSR is outside the beam-width of the wide-beam antenna, thereby determining the transmit time for the P 3 pulse when P 2 pulses from the wide-beam antenna are not detectable.

3. The method of claim 2 , further comprising determining a position of a target object using the transmit time of the P 3 pulse and a reply message from said target object received at said PSSR, wherein said reply message is in response to an interrogation from the SSR.

4. The method of claim 3 , wherein determining the position comprises determining a position of an aerial, nautical or ground object.

5. The method of claim 1 , wherein the step (i) comprises detecting a first and second successive P 2 pulses, each having a respective pulse detection time, and the step (ii) comprises determining a first pulse interval between the first and second successive P 2 pulses, and storing the first pulse interval as the time-ordered sequence of P 2 pulses.

6. The method of claim 5 , wherein the step (iii-1) further comprises:

(iii-1) determining a new pulse interval between a new P 2 pulse and a last received P 2 pulse; and

(iii-1b) provided said new pulse interval does not match the first pulse interval, adding said new pulse interval to the time-ordered sequence of P 2 pulse intervals, and repeating the steps (iii-1a) to (iii-1b).

7. The method of claim 6 , wherein the step (iii-1) further comprises:

provided said new pulse interval matches the first pulse interval, and the time-ordered sequence repeats upon detecting further P 2 pulses, determining the pulse repetition pattern of P 2 pulses based on the repeating time-ordered sequence.

8. The method of claim 1 , wherein the step (iii) further comprises applying statistical processing or averaging for updating said PRF pattern.

9. The method of claim 1 , wherein the PSSR is one of the following:

a stationary PSSR;

a mobile PSSR.

10. The method of claim 1 further comprising determining an interrogation mode of the PRF pattern, comprising:

(i) determining an interrogation mode sequence of said main antenna using P 1 and P 3 pulse combinations; and

(ii) matching said interrogation mode sequence in the PRF pattern.

11. The method of claim 1 further comprising performing a calibration operation to compensate for a time drift caused by electronics in said PSSR, thereby improving accuracy of determining the PRF pattern.

12. The method of claim 1 , further comprising processing multiple reflected interrogation signals for improving signal-to-noise ratio.

13. The method of claim 1 , wherein the first predefined time delay is 2 μs, and the second predefined time delay is one of the 8 μs and 21 μs.

14. The method of claim 1 , further comprising determining a position of a transponder-equipped target object, when a communication between the SSR and the PSSR is blocked, resulting in the PSSR being capable of receiving a reply from the transponder-equipped target object in response to an interrogation signal from the SSR, while not being capable of receiving the interrogation signal from the SSR, the method comprising:

storing the PRF in a memory;

at the PSSR:

detecting a receipt time of the reply from the transponder-equipped target object;

back calculating a transmit time of the interrogation signal using the receipt time and the stored PRF; and

determining the position of the transponder-equipped target object comprising using the receipt time of the reply and the back calculated transmit time of the interrogation signal, thereby determining the position of the transponder-equipped target object when the communication between the SSR and the PSSR is blocked.

15. The method of claim 14 , further comprising transmitting the interrogation signal at 1030 MHz, and the reply at 1090 MHz.

16. A Passive Secondary Surveillance Radar (PSSR) for determining a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P 1 , P 2 , P 3 ) of a Secondary Surveillance Radar (SSR), the SSR comprising a main antenna and a wide-beam antenna whose beam-width is wider than a beam-width of the main antenna, pulses P 1 and P 3 are generated by the main antenna, and pulses P 2 are generated by the wide-beam antenna, each of the P 2 and P 3 pulses being synchronized with a respective P 1 pulse with respective first and second predefined time delays, the PSSR comprising:

a memory coupled to a processor and having computer executable instructions stored thereon, causing the processor to:

detect successive P 2 pulses when the PSSR is within the beam-width of the wide-beam antenna;

form a time-ordered sequence of the detected P 2 pulse intervals;

(iii) determine a pulse repetition pattern of P 2 pulses using the time-ordered sequence of P 2 pulse intervals, comprising:

(iii-1) provided at least one P 2 pulse interval for a next detected P 2 pulse is already present in the time-ordered sequence, and the time-ordered sequence of P 2 pulse intervals starts repeating, determining the pulse repetition pattern of P 2 pulses using the time-ordered sequence;

(iii-2) otherwise, repeating (i) to (iii-1);

(iv) when the PSSR is within the beam-width of the main antenna, detect P 1 and P 3 pulses, and determine a value of the second predefined time delay between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR;

(v) obtain a value of the first predefined time delay, and derive the PRF pattern for the staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR based on the pulse repetition pattern of P 2 pulses, and values of the first and second predefined time delays.

17. The PSSR of claim 16 , wherein the computer executable instructions further cause the processor to determine an interrogation pattern of said PRF pattern based on P 1 and P 3 pulse combinations.

18. The PSSR of claim 16 , wherein the computer executable instructions further cause the processor to predict a transmit time for P 3 pulse based on said PRF pattern provided the PSSR is outside the beam-width of the wide-beam antenna.

19. The PSSR of claim 16 , wherein the computer executable instructions further cause the processor to determine a position of a target object using a transmit time of an interrogation.

20. The PSSR of claim 16 , wherein the target object is one of an aerial, nautical or ground object.

21. A method for determining a position of a target object, comprising:

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

(a) determining a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR, the SSR comprising a main antenna and a wide-beam antenna whose beam-width is wider than a beam-width of the main antenna, pulses P 1 and P 3 being generated by the main antenna, and pulses P 2 being generated by the wide-beam antenna, each of the P 2 and P 3 pulses being synchronized with a respective P 1 pulse with respective first and second predefined time delays, comprising:

(i) during a time interval when the PSSR is within the beam-width of the wide-beam antenna, and thereby P 2 pulses are detectable, detecting successive P 2 pulses;

(ii) forming a time-ordered sequence of P 2 pulse intervals derived from the P 2 pulses detected in the step (i);

(iii) determining a pulse repetition pattern of P 2 pulses using the time-ordered sequence of P 2 pulse intervals, comprising:

(iii-1) provided at least one P 2 pulse interval for a next detected P 2 pulse is already present in the time-ordered sequence determined in the step (iii), and

the time-ordered sequence of P 2 pulse intervals starts repeating, determining the pulse repetition pattern of P 2 pulses using the time-ordered sequence;

(iii-2) otherwise, repeating the steps (i) to (iii-1);

(iv) when the PSSR is within the beam-width of the main antenna, detecting P 1 and P 3 pulses, and determining a value of the second predefined time delay between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR; and

(v) obtaining a value of the first predefined time delay, and deriving the PRF pattern for the staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR, comprising using the pulse repetition pattern of P 2 pulses determined in the step (iii), and values of the first and second predefined time delays;

(b) 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;

(c) 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

(d) 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.

22. The method of claim 21 further comprising determining an interrogation pattern of said PRF pattern based on P 1 and P 3 pulse combinations.

23. The method of claim 21 , wherein the step (a) of determining the PRF pattern of the SSR is performed prior to interrogating the target object.

24. A Passive Secondary Surveillance Radar (PSSR) system for determining a position of a target object, comprising:

(a) a first receiver for receiving a reply from the target object wherein said reply is responsive to an interrogation signal comprising a P 1 and a P 3 pulse sent by a main antenna of a Secondary Surveillance Radar (SSR) to said target object;

(b) a second receiver for receiving a plurality 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 having a beam-width wider than a beam-width of the main antenna; and

a memory coupled to a processor and having computer executable instructions stored thereon, causing the processor to:

(c) determine a pulse repetition frequency (PRF) pattern for staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR, comprising:

(i) detecting successive P 2 pulses when the PSSR is within the beam-width of the wide-beam antenna;

(ii) forming a time-ordered sequence of the detected P 2 pulse intervals;

(iii) determining a pulse repetition pattern of P 2 pulses using the time-ordered sequence of P 2 pulse intervals, comprising:

(iii-1) provided at least one P 2 pulse interval for a next detected P 2 pulse is already present in the time-ordered sequence, and the time-ordered sequence of P 2 pulse intervals starts repeating, determining the pulse repetition pattern of P 2 pulses using the time-ordered sequence;

(iii-2) otherwise, repeating (i) to (iii-1);

(iv) when the PSSR is within the beam-width of the main antenna, detecting P 1 and P 3 pulses, and determining a value of the second predefined time delay between P 3 and P 1 pulses, thereby determining an interrogation mode of the SSR; and

(v) obtaining a value of the first predefined time delay, and deriving the PRF pattern for the staggered interrogation pulses (P 1 , P 2 , P 3 ) of the SSR, based on the pulse repetition pattern of P 2 pulses, and values of the first and second predefined time delays;

(d) estimate a transmit time of said interrogation signal based on a reception time of said reply, and the PRF pattern; and

(e) determine the position of the target object based on an altitude information of the target object present on said reply, a location of the SSR, said transmit time of said interrogation signal, and said reception time of said reply.

25. The PSSR system of claim 24 further comprising:

a mixer and a local oscillator for translating the reply into an intermediate frequency band reply signal and for translating the P 2 pulses into an intermediate frequency band P 2 pulses; and

a single channel high-speed Analog-to-Digital Converter (ADC) for digitizing said intermediate frequency band reply signal and said intermediate frequency band P 2 pulses and transmitting the digitized intermediate frequency band reply signal and the digitized intermediate frequency band P 2 pulses to said processor.

26. The PSSR system of claim 24 further comprising:

a first mixer and a first local oscillator for translating the reply into a base band reply signal;

a second mixer and a second local oscillator for translating the P 2 pulses into a base band P 2 pulses; and

a dual channel high-speed Analog-to-Digital Converter (ADC) for sampling said base band reply signal and said base band P 2 pulses, and transmitting sampled base band reply signal and sampled base band P 2 pulses to said processor.

27. The PSSR system of claim 24 wherein the location of the PSSR is determined using a global positioning (GPS) receiver and wherein the location of the SSR is a fixed location known to the PSSR system.

28. The PSSR system of claim 24 , wherein the computer readable instructions causing the processor to determine the PRF pattern of the SSR are configured to be executed prior to interrogating the target object.