Patent GB-595022-A (GB)
Title: Aircraft radio guiding system
IPC: G01S1/02
Filed: 1945-04-20
Abstract: 595,022. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. April 20, 1945, No. 10042. Convention date. April 26, 1944. [Class 40 (v)] An aircraft radio guiding system comprises airport equipment consisting of a plurality of radio beacons energized successively and cyclically at the same carrier frequency for non-overlapping periods ; the signals from each beacon have a different distinctive characteristic and on the craft a single receiver receives and detects all the signals which are then separated and applied to indicators by selector means responsive to the different characteristics. The system consists of a number of localiser beacons with associated glide path beacons and a number of course beacons ; only selected ones of the localiser and glide path beacons being operative at any one time according to which runways are in use at the airport, having regard to wind direction. A rotary beacon and a pulse repeater for enabling a craft to determine its direction and distance from the airport may also be included in the system and the indicators on the craft may combine the information of two or more beacons, possibly together with artificial horizon and gyro compass data, on one unit. An airport layout, Fig. 1, consists of 3 runways, 8, 9, 10, provided with localiser and glide path beacons, 11, 12, 13, 14, 15, 16, and course beacons 17 and 18, rotary beacon 19 and pulse repeater 20. All the equipment is linked together and operates on the same carrier frequency ; adjacent airports require different carriers and a selector switch for tuning the receiver to the frequency used by the airport being approached is provided on the craft. A second selector switch determines which localiser and glide path, or course beacon, signals are applied to the indicators and is normally set in accordance with instructions from the airport control centre. There are two main indicators, the landing beam indicator and the position indicator, and these, together with a bearing indicator indicating the direction of the selected approach path or localiser beacon, and a gyro-compass provide all the information required. When flying along an approach course the position indicator, Fig. 5, is covered by a screen 57 and the only information given is the distance to the airport. On arriving in the vicinity of the airport the screen 57 is lifted and reveals a map of the airport on which the position of the craft is marked as a bright spot and its heading by small arrows. With the aid of this indicator the craft may be guided along a course, e.g. 25, Fig. 1, to an orbit path 26 which it may be flown round until the runway is clear for landing, when a turn 27 is made, bringing the craft into line with the runway. The landing beam indicator then enables the landing to be made, this consists of a spot and ring pattern, and the craft must be flown so that the spot is maintained central in the ring. The relative position of the spot and ring shows deviation from the correct landing path and in addition the spot and ring pattern may move as a whole from its normal position in the centre of the indicator to denote incorrect heading of the craft either up or down or to right or left. A bar passing across the landing indicator through the spot indicates the angle of bank of the craft. The rotary beacon 71 shown with repeater 72 in Fig. 9 radiates pairs of pulses to define a sweep line, which is made to rotate 12 times a second, and also an omnidirectional reference pulse every other time the sweep line passes through north. The sweep line is defined by radiating 5 Á sec. pulses in a figure of eight pattern and 3 Á sec. pulses in a cardioid pattern with the null of the cardioid aligned with one of the nulls of the figure of eight; the line along which 3 Á sec. pulses can be detected without 5 Á sec. pulses is the sweep line. The 5 Á sec. and 3 Á sec. pulses are generated by pulse generator 76 which feeds transmitter 77 and also controls switching unit 78 to feed the 5 Á sec. pulse through goniometer 85 to antennas 80-83, radiating the figure of eight pattern, and the 3 Á sec. pulse through goniometer 79 to antennµ 80-83 and simultaneously through cam switch 86 to central antenna 87 to radiate the cardioid pattern. Goniometers 79 and 85 serve to rotate the sweep line and are driven by motor 115; this also drives at half speed cam-operated switches 89A and 86. Switch 89A triggers generator 88A to radiate the 12 Á sec. omnidirectional reference pulse over central antenna 87 only, switch 86 disconnecting this antenna from the others at that time and also earthing generator 76 over line 89 to prevent generation of 5 Á sec. and 3 Á sec. pulses for 12 milliseconds. All the localiser, glide path, and course beacon signals are radiated during this 12 millisecond interval. In the aircraft equipment, Fig. 8, the signals pass through receiver 91 and are separated by pulse width selectors 92, 93, 94. The 5 Á sec. pulses are passed by selector 92 and are used to block selector 94, preventing 3 Á sec. pulses from passing this selector except for the short interval during which the sweep line passes the craft. The 3 Á sec. pulses passed during the period synchronize 12 cycles per sec. oscillator 95 feeding goniometer 62 through phase shifter 96. The 12 Á sec. reference pulses are selected by unit 93 and pass to circuit 97 unblocking discriminator 98 for the duration of the pulse. The movable search coil of goniometer 62 is connected to discriminator 98 and the alternating voltage produced in this coil by the rotating field, when passed by discriminator 98, produces a surge of current through motor 61 and drives the motor in a direction dependent on the polarity of the current, causing a rotation of the search coil until,. after a number of reference pulses, it reaches and maintains itself in a position in which it delivers zero signal to the. discriminator at the time it is unblocked. Deflection coil 60 of cathode ray tube 59 is mounted on the search coil shaft and thus is maintained in a position corresponding to the bearing of the craft from the airport. The distance determining equipment consists of ¢ Á sec. pulse generator 100 triggering transmitter 99 which works on its own special frequency. Signals radiated from the craft are received by repeater unit 72 at the airport and retransmitted at the airport carrier frequency. They are received by receiver 91 passed by pulse width selector 105 and fed to the control grid of the cathode-ray tube 59. The sweep current in deflection coil 60 is derived from circuit 101 triggered by generator 100 and so, as the amount of sweep occurring before the retransmitted pulse is received depends on the distance between the craft and the airport, the spot will appear at a distance from the centre of the tube depending on this distance. As coil 60 is placed according to the bearing of the craft from the airport by means of the rotary beacon signals the spot represents the craft in both azimuth and distance. In the Position Inaicator, Fig. 5, the tube 59 is mounted behind a half silvered mirror 63. On glass support 64 is placed a transparent map card having two maps traced on it in different coloured transparent inks and representing the airport, runways, and surroundings, on two different scales (Figs. 6 and 7, not shown). Two coloured bulbs in lamp 67 may be switched on by switch 68 alternatively, and whichever colour is chosen renders the map of the other colour visible when looking into half silvered mirror 63. Superimposed on the reflection of the maps the bright spot of the tube 59 can be seen indicating the craft's position. Switch 68 also controls the rate of increase of the current in sweep coil 60 so that the spot is correctly located whichever scale of map is used. Gyro-compass 66 supports a transparent disc 65 having black arrows marked on it, these are projected on to the map to indicate the heading of the craft. Glide path, localiser, and course, beacons. In order that the various glide path, localiser and course beacon signals may be identified and separated by the receiving system they are radiated successively during the 12 millisecond period when the rotary beacon is silenced and are modulated with different tone signals. For a system having three runways and four approach beacons in use, twenty tone frequencies are required and these are made of the order of 1,000 kc. and grouped so that when heterodyned with the correct one of four local oscillations they give beat frequencies of 16, 28, 40 or 52 kc. The two overlapping patterns of a beacon are radiated successively so that ten correctly timed triggering pulses must be generated to control the beacon timing. In the switching and timing unit, Fig. 10 (not shown) the timing pulses are generated in a switch unit driven by motor 115 (Fig. 9) through shaft 114. The timing pulses are passed over selector switches arranged so that any three sets of localiser and glide path beacons out of six may be chosen. In order to set the localiser and glide path beacons to transmit the correct line frequencies according to whether the runway they mark is being regarded as runway 1, 2 or 3 control signals, of +D.C., - D.C. or 60 cycles A.C. superimposed on the trigger pulses are also passed to them through the switches. Fig. 11 shows a localiser beacon, in this the control signals are applied to relay 160 to operate switches 161, 162 determining the modulation tones used. The timing pulse on which these signals are superimposed triggers triple pulse generator 163. This generator first applies a 200 Á sec. pulse to line 164, then a shorter switching pulse to line 180 and finally a 200 Á sec. pulse to line 183 commencing 200 Á sec. after the end of the first. The first pulse is applied to unblock circuit 165 to allow the two tones constantly generated in balanced modulator 166 to be applied to transformer 169 which is tuned to select one of them. T