939,278. Radar. CUBIC CORPORATION. Nov. 6, 1961, No. 39679/61. Class 40 (7). In the tracking of C.W. transponders; on aircraft by measuring for each craft its slant range and two direction-cosines a different sub-frequency is assigned at the ground radar-direction finder for each aircraft to be tracked and range measurement is carried out sequentially as the sub-frequencies are radiated in turn. Only the aircraft corresponding to a particular sub-frequency is transponding during the transmission thereof; and in the embodiment described (1) there are two aircraft, each responding on the same carrier frequency, itself different from the radar carrier frequency; (2) range is determined by phase comparison of five different frequencies frequency-modulated on to the radar carrier frequency and their reply signals; (3) the direction-cosines are determined by a phase-comparison arrangement utilizing two orthogonal sets of spaced aerials; and (4) one aircraft is called by including a frequency of 100 kc/s. in the composite ground transmitter modulation signal and the other aircraft by substituting that frequency by one of 115 kc/s. Range measurement.- Ambiguity is resolved by using five frequencies. Each range readout unit includes five channels each of which takes the corresponding delayed range signal and the original range reference signal and compares their phases by a resolver whose shaft displacement, at a point of zero resolver output, represents the phase displacement between its two input signals; all the resolver shafts are coupled to a gear train driven by a servo at step-down ratios corresponding to respective frequency ratios. Channel selection is afforded by continuously sensing the resolver error in each channel and switching the lowest range frequency channel (i.e. that having the greatest ambiguity-resolving ability) to control the servo whenever its resolver error reaches a predetermined magnitude. During an interval when the corresponding transponder is quiescent the servo action is effectively disabled; but the sequencing rate employed is sufficiently high to ensure inappreciable servo data lag. Transponders, Fig. 3.-The ground-transmitted signals are applied via a mixer 42 to a discriminator 45 the output signal from which is fed to a filter 46 and an amplifier 47 sharply tuned to each of the range signal frequencies thereby reducing noise; amplifier 47 is applied to one input of a phase modulator 48. Filter 46 is tuned in one transponder to 100 kc/s. and, in the other, to 115 kc/s., so that each transponder transmits sequentially as " called " by the ground station. Degenerative feedback from amplifier 58 to mixer 42 is stated to minimize range signal phase shifts incurred in passing through the transponder circuits. Angle Measuring Apparatus, Fig. 4.-The y-axis direction-cosine measuring unit 22 includes pairs of aerials with spacings of 50 #, 5 # and 4.5 #. Oscillator 24 produces a 1 kc/s. reference signal and a pair of R.F. signals differing by 1 kc/s. A.M. detector 75 produces an output signal representing the directioncosine; ambiguity is resolved firstly by the 5 # system and then by a #/2 system derived from the 5 # and 4.5 # systems. The x-axis direction-cosine measurement is made in a similar manner by utilizing aerial pairs 84, 85 and 86. Unit 26 routes the A.M. detector outputs to the appropriate x- and y-axis servo readout units on the basis of the sequencing signal being modulated at any instant on to the radar carrier signal. Sequence control. The sequencing control unit incorporates a 40 c/s. free-running multivibrator, a register which includes flip-flops, gated 100 kc/s. and 115 kc/s. oscillators, and diode switches, and a monostable circuit to delay the passage of range and direction-cosine signals to their respective readouts after each oscillator switching operation in order to permit the signal transmitted by the other transponder to complete its passage.