Patent GB-709029-A (GB)
Title: Improvements in and relating to radio beacons
IPC: G05D1/06
Filed: 1951-12-06
Abstract: 709,029. Radio navigation. BRITISH THOMS ON-HOUSTON CO., Ltd. Dec. 6, 1951 [Dec. 6, 1950], No. 28639/51. Class 40 (7). [Also in Group XXXVI] In a G.C.A. system in which the horizontal and vertical deviations of an aircraft from a landing path are determined by a ground pulse radar and communicated to the plane by radio telephone, the radar beam scans sequentially Up, Down, Left, Right and the R.T. carrier is modulated by an additional "orienting" signal whose phase is changed abruptly by 90 degrees as the radar beam sweeps through the landing path so that the phase of the orienting signal is 0 degrees, 90 degrees, 180 degrees or 270 degrees when the instantaneous direction of the radar beam is above, below, left or right of the landing path and the airborne receiver comprises means responsive jointly to the orienting signal and the received radar pulses to give a further signal indicating the deviation of the aircraft from the landing path, which signal is applied to control a visual display, e.g. a cathoderay tube, and/or an automatic pilot. As described the frequency of the orienting signal is synchronized with the radar pulse recurrence frequency and comprises the second harmonic thereof so that it is higher than the upper limit of the transmitted speech frequencies and the signal is applied to a circuit, Figs. 4a and 4b (not shown) producing four outputs in phase quadrature which are selectively applied to modulate the RT carrier by a switch synchronized with the radar aerial scan. At the aircraft receiver, Fig. 5 (not shown), the orienting signal is separated from the speech signals and applied in plane quadrature to a cathode-ray tube giving a circular trace which is intensity modulated by the received radar pulses so that during the azimuthal scan of the radar aerial a bright spot is produced at one or two diametrically opposed positions on the circular trace (depending on the sense of the horizontal deviation of the aircraft from the landing path) and during the elevation scan a further bright spot is produced at one of two further diametrically opposed positions at right angles to said first positions. Due to the finite angular width of the radar beam, four quadrantly spaced spots will appear when the aircraft is on the landing path. In a modification, Fig. 6 (not shown), the circular trace is deflection modulated by applying the received radar pulses to adjacent X and Y plates and in a further modification, Fig. 7, the orienting signal is applied in phase quadrature to the four deflector plates 33, 34 of the cathode-ray tube 25 via normally non-conductive double-triodes 44, 47 any one section of which is rendered conductive when a received radar pulse is coincident with the positive peak of the corresponding orienting signal component by applying positive radar pulses from receiver 13 to the cathodes of V44, V47 via inverter valves 48, 49. Thus during the azimuthal aerial scan a pulse is applied to either the left or right deflector plate 34 (depending on the sense of the horizontal deviation of the aircraft from the landing path) producing a horizontal radial line 51 and during the deviation aerial scan a vertical radial line is produced, two perpendicular diametrical lines being produced when the aircraft is on the landing path. The circular trace may be tested by connecting the grids of V48, V49 to a positive voltage by means of switch 50.