Patent GB-1136442-A (GB)
Title: Improved digital code extractor
IPC: G01S13/78
Filed: 1966-07-19
Abstract: 1.136.442. Pulse radar. SOC. NOUVELLE D'ELECTRONIQUE ET DE LA RADIO INDUSTRIE. 19 July, 1966 [12 Aug., 1965], No. 32448/66. Heading H4D. On the arrival of the initial pulse of a multiposition digital code at the input of a code extraction apparatus, the code pulse repetition rate being R, a first train of pulses E, Fig. 10 (not shown), of repetition rate R and having its first pulse starting at the instant of the arrival of the initial code pulse, is generated, and a second train of pulses E<SP>1</SP>, also of repetition rate R but out of phase by a predetermined amount relative to the train E, is generated; the phases of code pulses subsequent to the initial one are compared with trains E and E<SP>1</SP> to direct code pulses into a first or second channel. The invention is particularly described as applied to a secondary pulse radar system in which an aircraft transmits a coded reply, Fig. 1, consisting of a thirteen-position code group between a pair of framing pulses F1, F2, and a pulse SPI which may be introduced manually by the aircraft operator; for civil aircraft the seventh of the thirteen positions is left blank. Garbling may exist between replies from different aircraft: in Fig. 3 code groups C, C<SP>1</SP> are separate but one pulse from each may be spaced by the F 1  to F 2  spacing; in Fig. 4 the code groups are in overlapping relationship and garbling may be of two kinds: "interleaving" Fig. 5, where pulses I 1 , I 2 belong to group C and pulse I<SP>1</SP> 1 to group C<SP>1</SP>, or "phase-garbling" ("pulse-overlap"), Fig. 6, where I and I<SP>1</SP> pulses overlap. General system description. Fig. 7.-Incoming response pulses from the radar receiver are applied to input 1 of analyzer 2: on receiving an initial code pulse it is directed into processing channel I and subsequent pulses also if they are found to be part of the same code group. If a subsequent pulse at input 1 is out-of-phase therewith it is directed into processing channel II and subsequent pulses found to be part of the out-of-phase group are also directed thereto. This selective channelling is actuated by a sampling circuit 100 which generates the E, E<SP>1</SP> pulse trains. Coincidence gates 22, 29 are connected to input 1 and to respective pulse trains E, E<SP>1</SP> and their outputs go to respective shift registers 3, 3<SP>1</SP>. Each processing channel.transfers its complete code group in parallel from its shift register to a buffer storage arrangement 102, 1021 where the group is stored for a prescribed period during which a " garble analyzer logic" 9 determines whether the group in channel I is garbled and, if so, to enter a garble signal G into the buffer store 102. At the end of the storage period the contents of the buffer stores are transferred to a conventional decoder not described. No garble signal need be associated with a group stored in channel II since if present it is necessarily garbled. Coincidence gates 4, 4<SP>1</SP> feed respective units 12, 12<SP>1</SP> which issue a "stop" signal A, A<SP>1</SP> on sensing a coincidence between the framing pulses F1, F2 of a group in the associated shift register if the other register is at that time empty, or on sensing that both shift registers have been emptied of their contents after having failed to sense such coincidence indicative of a truncated or mutilated group in the shift register. A "stop" signal arrests the operation of unit 100. Detailed arrangements.-The analyzer 2 of Fig. 7 includes a clock pulse generator of repetition rate R/16, a counter, a decoder matrix, and associated binary pairs, AND, and OR gates, Fig. 8 (not shown). There is an input synchronous with the primary pulse transmitted from the ground radar, which influences binary pairs to ensure that the initial response pulse from the target will be directed to shift register 3; the decoder matrix produces shifting pulses for both registers to afford requisite timing, Fig. 11 (not shown). Interleaved code pulse trains are directed to the appropriate shift register. The outputs of the shift registers are processed as described with reference to Fig. 12 (not shown), which relates to one channel and which also includes a "stop command" circuit and the garble analyzer logic; a "stop command" signal issues when mutilated code groups are present in both shift registers, the system operation ceasing when both registers have been emptied. The garble analyzer logic includes AND gates and binary pairs to perform garbleindicating code pulse timing coincidence checks; Figs. 13, 14 (neither shown). More than two channels may be provided.