Patent GB-696847-A (GB)
Title: Ground controlled aircraft landing system
IPC: G01S13/91
Filed: 1951-04-03
Abstract: 696,847. Radio navigation. BENDIX AVIATION CORPORATION. April 3, 1951 [April 28, 1950], No. 7743/51. Class 40 (vii). A ground-controlled aircraft landing system comprises a radar system scanning sectors in azimuth and elevation, each containing a predetermined glide path and means for comparing the phase of a received echo signal with (a) the phase of the transmitted signal to actuate a first servo loop driving a range indicator; (b) the phase of the horizontal scanning to actuate a second servo loop driving an azimuth indicator; and (c) the phase of the elevation scanning to actuate a third servo loop driving an elevation indicator. The invention is described with respect to a system for handling up to ten planes at once (spaced apart one mile) over the final ten miles of an approach path. The transmitter 10 is connected alternately by switch 15 driven by motor 16 to azimuth scanning aerial 13 and elevation scanning aerial 14. The output of receiver 11 is fed through range gate 18, the selected signal determining the phase # 2  of the output of oscillator 21. A reference phase signal (which controls the transmitter 10) of the same frequency is supplied by oscillator 22 through phase splitter 23 to synchro-resolver 24, whose output signal is arranged to have a phase # 3  corresponding to range. Range indicator 331 is coupled to the resolver. Comparator 25 fed with # 3  and # 2  signals gives an error signal adjusting servo-motor 27 which positions the rotor of a further synchro resolver 29 whose stator is fed with quadrature # 3  signals. The output of this resolver has a phase # 4  corresponding to the speed of the craft under control which is combined with the # 2  signal and the resultant combined with the # 3  signal in phase comparator 30, giving an output which controls the position of the rotor of resolver 24 by means of servo-motor 32. This motor also controls the positioning of the range gate applied to circuit 18 which is arranged to progressively decrease in width as the craft approaches the landing point. The range determining servo loop is described in greater detail in connection with the circuit of Fig. 2 (not shown). The azimuth indicating servo loop indicates phase splitter 40, to which is applied the signal of phase # 2  from oscillator 21, coupled to resolver 41 whose rotor is moved in synchronism with the aerial scanning. The output of the resolver is converted into pulse form to give pulses 43 applied to azimuth gate circuit 44. The gating pulses applied to this circuit are timed in accordance with the output of synchro resolver 47, fed with # 2  signals and set in accordance with the craft azimuth by servo 52 driven by phase comparator 50. The remainder of the loop comprises an oscillator whose output is phased in accordance with the pulses passing gate circuit 44 and is compared at 50 with the phase of the output of resolver 47. An azimuth indicator 53 is also operated by resolver 47 and azimuth adjustment may be obtained by manual or automatic means 541 to correct for the radar being to one side of the glide path. The elevation loop illustrated at the bottom of Fig. 1 is substantially similar to the azimuth servo loop just described. Further details of the azimuth loop circuit are described with respect to the circuit of Fig. 3 (not shown). The range azimuth and elevation information may be displayed on a single instrument by projecting crossed lines 83, 84 whose position is dependent on the horizontal and vertical displacements of the craft from the desired glide path on to a screen 80 having calibrated scales 81 and 82. A counter 90 is preferably used to indicate range. A modified range loop circuit is described with reference to Fig. 6 (not shown), in which a receiver system is rendered selective with respect to target velocity as well as target range. The velocity sensitive portion of the circuit is disabled until the apparatus has actually commenced to follow a craft. The velocity sensitive circuit enables some simplification in the azimuth and elevation servo loops to be effected. So far a single set of apparatus has been described for the control of a single craft. A system using 11 sets of apparatus, Figs. 7a, 7b and 7c (not shown), may be used to control the approach of up to 10 craft at a time and in such a system connecting links with phase shifters equivalent to one mile separation are introduced between the range synchro resolvers of each set so that all the sets are in series and register from 10 by one mile steps to 20 miles when no craft are under control. Cam switches are arranged to come into operation as each resolver reaches the 10-mile range position disconnecting the link with the resolver of the previous set and conditioning it to accept and follow an echo signal. Thus there is always a set ready and available to an incoming craft as soon as the craft in front has reached the nine-mile position. When the craft under control by a given set reaches zero range the cam switches operate to re-establish control with one mile delay behind the resolver of the previous set in the series. Each of the 11 sets of apparatus of the system may be provided with its own transmitter for transmitting range, azimuth, and elevation information to the craft under control. Information thus provided may be fed to an auto-pilot on the craft or presented on a display of the type previously described. Any of the communication channels may be temporarily interrupted and used for verbal contact with the craft under control. To avoid interference and cross-talk due to the use of eleven transmitters time sharing arrangements such as are described in Specification 686,580 may be used.