SECONDARY RADAR DEFRUITING SYSTEM

• US 3,705,402 A
• Filed: 05/27/1970
• Issued: 12/05/1972
• Est. Priority Date: 05/27/1970
• Status: Expired due to Term

• Current Assignee: Jack R. Ballantyne, Melvin E. Houck
• Original Assignee: Jack R. Ballantyne, Melvin E. Houck
• Inventors: Ballantyne, Jack R., Houck, Melvin E.
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US05/040,984
• Time in Patent Office: 923 Days
• Field of Search: 343/6.5R,6.5LC
• US Class Current: 342/40
• CPC Class Codes: G01S 13/784 Coders or decoders therefor; Degarbling systems; Defruiting systems

Abstract

A defruiter system useful in secondary radar applications for enabling local ground station equipment to distinguish target reply messages responsive to its interrogate signal from reply messages responsive to interrogate signals of remote ground stations. In secondary radar systems, a target transponder may at any one time be within the range of several interrogating ground stations, and therefore may be interrogated by and transmit replies to all of them. At any one ground station, the replies responsive to its own interrogation will appear at the same range during successive sweeps. However, replies initiated by interrogations from remote stations will be at unsynchronized sweep rates and will appear at the one station at different ranges during successive sweeps. This type of interference is referred to as '"'"''"'"''"'"''"'"'fruit.'"'"''"'"''"'"''"'"' The subject defruiter system eliminates this fruit interference by comparing the time occurrence of framing (bracket) pulses during successive sweeps. When framing pulses during two successive sweeps match in time, then the data bits therebetween are read and the entire reply message is interpreted as being responsive to a local interrogation. Sweepto-sweep framing pulse comparisons are performed by delaying each received framing pulse by one full sweep interval and then logically gating it with the signal received during the subsequent sweep interval.

Claims

1. A system useful in secondary radar systems for comparing the range associated with reply messages received during two successive sweep intervals wherein each of said reply messages includes a framing pulse, said system including:

means for detecting a framing pulse;

delay means responsive to the detection of a framing pulse for subsequently providing a delayed framing pulse delayed by the duration of one sweep interval, said delay means including a shifT register comprised of stages 1, 2, 3, . . . n and wherein said delayed framing pulse is represented by the state of stage n, said delay means being coupled to said means for detecting a framing pulse, for receiving a binary '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' in said stage 1;

means selectively providing shift pulses to said shift register at a rate to shift a bit from stage 1 to stage n during one sweep interval; and

utilization means responsive to the detection of a framing pulse concurrently with the provision of a delayed framing pulse.

2. The system of claim 1 wherein each of said reply messages is comprised of a plurality of bits represented by the occurrence or non-occurrence of a pulse in each of a plurality of successive time slots including first and second framing bit time slots spaced by a group of data bit time slots;

and wherein said means selectively providing shift pulses includes control means for providing said shift pulses at a rate less than the rate at which said bit time slots occur.

3. The system of claim 1 including:

sweep timing means providing one range zero signal during each of said sweep intervals; and

wherein said means selectively providing shift pulses includes clock means responsive to said range zero signal for applying said shift pulses to said shift register.

4. The system of claim 3 including:

range select means for defining a desired maximum range count;

range counter means for counting said shift pulses occurring subsequent to a range zero signal; and

means for preventing the application of said shift pulses to said shift register after said range counter means counts to said maximum range count.

5. The system of claim 4 wherein the bit length of said shift register is related to said maximum range count defined by said range select means.

6. A system useful in secondary radar applications for separating a reply message received at one station in response to local interrogations from reply messages received in response to interrogations initiated by other stations wherein said reply messages are each comprised of a plurality of bits represented by the occurrence or non-occurrence of a pulse in each of a plurality of successive time slots including first and second framing bit time slots spaced by a group of data bit time slots, said system including:

a shift register having a plurality of stages respectively identified as 1, 3, . . . n;

sweep timing means successively defining sweep intervals and including means for generating an interrogate signal and a range zero during each of said sweep intervals;

clock means responsive to said range zero signal for subsequently providing shift pulses for selective application to said shift register for shifting a bit through said n shift register stages in one sweep interval;

means for detecting the occurrence of a framing bit pulse including gating means responsive to he detection of a framing bit pulse for applying a binary '"'"''"'"''"'"''"'"'1'"'"''"'"''"'"''"'"' to the shift register stage 1; and

utilization means responsive to the detection of a framing bit pulse while shift register stage n concurrently stores a binary '"'"''"'"''"'"''"'"'1.'"'"''"'"''"'"''"'"'

7. The system of claim 6 including:

range select means for defining a desired maximum range count;

range counter means for counting said shift pulses occurring subsequent to a range zero signal; and

means for preventing application of said shift pulses to said shift register after said range counter means counts to said maximum range count.

8. The system of claim 7 including means for varying the length of said shift register dependent on the value of said maximum range count defined by said range select means.