Patent GB-1278451-A (GB)
Title: Discriminating signaling system
IPC: G01S13/933
Filed: 1969-06-05
Abstract: 1278451 Radar RCA CORPORATION 5 June 1969 [10 June 1968] 28535/69 Heading H4D In a signalling system there are asynchronous stations some at least of which transmit interrogating signals of which each successive signal is characterized by a pseudo-random or random code, transponder means at at least one other station responsive to such interrogating signals to transmit successive reply signals each also characterized by the same code, and detection means at an interrogating station extracting from all reply signals received tnerby those reply signals derived in response to interrogating signals transmitted by this station. The simplest transponder comprises a receiver producing from an interrogation signal consisting of bursts of a first carrier frequency and a second carrier frequency corresponding first and second outputs A, A which cause answering pulses to be emitted on third and fourth carrier frequencies, Fig. 1 (not shown). In another transponder, Fig. 2, outputs A, A and data code outputs B, B cause outputs on four corresponding carrier frequencies; a voice radio is added. The embodiment is a collision warning system for aircraft, an interrogator-responder on at least one aircraft incorporating the apparatus illustrated generally in Fig. 3, and an airport radar including an interrogator. In the apparatus of Fig. 3 switch 302 is in the position shown-first mode-when not close to an airport but is put into its other position-second mode-when close to an airport. FIRST MODE A pseudo-random binary code of (2<SP>10</SP>-1) bits is produced by a ten-stage shift register in conjunction with exclusive OR logic, Fig. 4 (not shown), in response to one-microsecond clock pulses, recurring at one-millisecond intervalsthe " bit period "-from generator 304. (An n-stage shift register gives a pseudo-random code of (2<SP>n</SP>-1) bits and n = 10 is preferred in the embodiment). A pseudo-random bit is transmitted by a two-frequency code during a clock pulse presence. Receiver 316 derives at its output bursts of frequency f 3 , f 4 , f 5  or f 6  in accordance with the signal from each other transponding craft (see below); decoder 320 includes filters tuned to those frequencies and logic means whereby simultaneous pulses are applied to conductors A, A, B, B as follows: Range correlation discriminator 310 inputs are conductors A, A, the output of pseudo-random code generator 306 and the output of frequency divider 324, which is a single output pulse for every 2<SP>10</SP> pulses from clock pulse generator 304; the binary value of each interrogation pulse is correlated over 2<SP>10</SP> consecutive bit periods with the binary value of each reply pulse: a high degree of correlation exists only for responses to " own " interrogations. In one form, Fig. 5 the A, A outputs from decoder 320 are fed to a summing circuit 504 via switch means 500 which has its contacts as shown when the binary value " one " occurs in the output of pseudorandom code generator 306 and changed-over when that binary value is " zero ", the summing circuit additionally has an input from a recirculation loop 506 to which is fed the phasedetected output of a delay line 514 the total loop delay (most of which is in the delay line) being one bit period. All the integrated information stored in line 514 during the preceding 1024 bit periods is read out, utilizing the output of frequency divider 324 (Fig. 3), and the process repeated. (Delay line carrier source 510 makes possible the storage of bits of positive and negative polarity). Inverter 502 provides for the negative signs in a specified correlation function. A modified arrangement utilizing two delay line loops is described, Fig. 6 (not shown). The recirculating delay line systems may be replaced by, e.g. storage cathode-ray tubes. Range derivation means 328 has an input integrator 708, Fig. 7, the output from which is applied directly, and via a delay means 702 and inverter 704, to a summing circuit 700. A detector 706 produces an output when its input is of a given polarity and of greater than a preset amplitude. Range and alarm gate means 322 incorporates an automatic range tracker in which the variable time delay in means 800, Fig. 8, is controlled by the voltage output of means 802, the latter including automatic tracking, e.g. by sampling oppositely-sloped sawtooth signals on the occurrence of the signal from the range derivation means. Pulses from means 800 are applied to a " data gate " 806 and to an analogue-todigital converter 330, Fig. 3, and a gate 804 provides for audible and/or visible warning that another craft is dangerously close. Data transmission, Fig. 10.-A binary data code is arranged such that a synchronizing character cannot be imitated by arranging that the eighth, fourteenth &c. bit positions are always binary zeroes. Display means 336, Fig. 3, may include digital indicator gas tubes and automatic pilot control means. In the second mode, i.e. when close to an airport, no interrogation pulses are transmitted by the aircraft but are transmitted by an airport station which includes part of the apparatus of Fig. 3 and a primary scanning radar with a P.P.I. display having a range marker from range gate means similar to that of Fig. 3 so as to indicate the range of a craft interrogated, Fig. 11 (not shown). An aircraft in this mode can monitor the reply pulses which other craft transmit in response to the airport interrogator, and afford displays as in the first mode. The airport aerial for received reply signals is directional in azimuth and elevation and the airport has the data from those reply signal to enable it to pass relevant information to aircraft, even those not equipped with the interrogator-responder apparatus of Fig. 3.