SECONDARY RADAR SYSTEM FOR TIME-SHARED TARGET IDENTIFICATION

• US 3,801,980 A
• Filed: 09/13/1972
• Issued: 04/02/1974
• Est. Priority Date: 09/14/1971
• Status: Expired due to Term

• Current Assignee: Albert A. Janex, Gerard G. Martin-Garin, Jacques J. Danton
• Original Assignee: Albert A. Janex, Gerard G. Martin-Garin, Jacques J. Danton
• Inventors: Danton, Jacques J., Janex, Albert A., Martin-Garin, Gerard G.
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US05/288,784
• Time in Patent Office: 566 Days
• Field of Search: 343/6.5R,6.5LC
• US Class Current: 342/45
• CPC Class Codes: G01S 13/781 Secondary Surveillance Radar [SSR] in general

Abstract

An aircraft identification (IFF) system involving ground interrogating stations and multiple airborne transponders. At least one interrogating digital code preceded by a synchronizing pulse sets up a sequence whereby replies are transmitted in the order of their reception at each airborne transmitter. Thus, multiple interrogation can be handled with higher response probability than heretofore. Digital logic is provided for selection of predetermined reply codes and their order and time of transmission from each airborne transponder.

Claims

1. A secondary radar system, operable in synchronizing with a ground primary radar, for target identification, including ground interrogation and receiving equipment and an airborne transponder beacon for each identifiable aircraft, comprising the combination of:

first means within said ground equipment for cyclically transmitting signal sequences, each sequence beginning with synchronization pulses, and thereafter changing to a first digital interrogation code;

second means within a transponder equipped aircraft for receiving said interrogation pulses and for initiating a synchronous event timing cycle in response to said synchronizing pulses of said signal sequences and for decoding said digital interrogation code;

third means including a plurality of memories each storing a predetermined reply code each representative of discrete information to be selectively provided from said ground station to an aircraft;

fourth means responsive to the decoded interrogation provided by said second means to select a discrete reply code from the memories of said third means; and

fifth means responsive to said second means for programming the transmission of said selected reply code to said ground station at a time in accordance with the time of reception of the corresponding ground interrogation code received by said second means from each ground transmitter interrogating said aircraft.

2. A secondary radar system for target identification comprising:

a central ground station which transmits coded interrogations synchronous with the pulses of an associated primary radar, said pulses being of period Ts and which includes a receiver and a decoder for received replies to its interrogations, the said interrogations comprising at least two pulses P1 and P3 which enclose a plurality of pulses, the number of those pulses and their time intervals defining predetermined interrogation modes;

a plurality of transponders, each associated with a target to be identified and comprising means for decoding interrogations to recognize said interrogation mode and for transmitting predetermined replies starting with a predetermined delay ta after reception of pulse P3 of an interrogation according to one of said modes;

first means within said ground station for cyclically transmitting sequences of (1 + N) interrogations whose pulses P3 are spaced from each other by time interval Ts, the first interrogation being for synchronization, the N following interrogations being those of one of a plurality of predetermined modes used for identifying the S targets;

said first means including (1 + C) encoding memories, each storing digits of the binary number which characterizes each of the (1 + C) coded interrogations;

a plurality of devices for primary selection of encoding memories, controlled by a synchronous counter which counts synchronization pulses of period Ts, to select synchronization interrogation mode memory and interrogation of one of the C modes and operating during a time duration NTs; and

a code pulse generator of period theta i which synchronizes operation of an encoder whose output delivers at tIme intervals multiple of theta i, the pulses of the coded interrogation stored in the selected memory, said predetermined advance being such that pulse P3 is transmitted at a predetermined time Ta with respect to said synchronization pulse of period Ts.

3. Apparatus according to claim 2 in which said first means includes sequence generating means for transmitting (1 + N + M) sequences, where M is an additional interrogation sequence differing from that provided by the corresponding N sequence, said ground station is defined as producing (1 + C + M) codes, comprising said coded interrogations, and including (2 + M) devices for said primary selection of encoding memories, whereby said M interrogations operate to select from M memories containing standard interrogation codes for further recognition security.

4. Apparatus according to claim 3 wherein each transponder, corresponding to one of said targets, includes decoding means for discretely recognizing said synchronization interrogations and those of each of the C identification modes and means for generating recognition pulses and applying them to a corresponding one of (1 + C) time-separated reply channels.