Patent GB-992543-A (GB)
Title: Improvements in aircraft landing system
IPC: G05D1/06
Filed: 1961-05-01
Abstract: 992,543. Radio navigation. CUTLERHAMMER Inc. May 1, 1961 [May 6, 1960], No. 15654/61. Heading H4D. In a radio navigation system for aiding aircraft landing comprising two vertically scanning beams 15(a) and 16(b), Fig. 1, and a localizer beams 21(c), Fig. 2, scanning in azimuth, the three beams being modulated by pulse signals in a code indicative of their angular positions and also for beam identification purposes, the time spacing of all coding pulse transmissions is varied (as shown in Fig. 4) as a function of the beam angles indicated thereby. This arrangement ensures that position information is available to the aircraft of a high rate for small beam angles (i.e. aircraft close to runway) and, at the same time, allows a lower average PRF (for the same beam scanning rate) to be employed with a consequent reduction in power requirements of the transmitters. As shown in Fig. 4, the transmission on beam (b) consist of single pulses 28 whereas beams (a) and (c) are modulated by pulse pairs of different spacing for beam identification purposes in addition to providing beam angle information. Also, in the case of beam (c) left and right hand beam sweeps are identified by different spacings of the pulses of the pairs. The angle coding pulses are produced by the arrangement shown in Fig. 6 (similar arrangements being provided for each beam) which comprises a pick-up device 47 (e.g. of the photo-electric or magnetic type) supplying signals indicative of increments of e.g. 0.01 degree of the angle of scan of the aerial 45. These signals are supplied to a storage counter 48 the total count in which is transferred periodically via matrix 51 to a variable interval counter 52 the count in which, together with any angle increment signals (fed over line 64) occurring when the transfer matrix is not operative, is counted down to zero by means of clock pulses from oscillator 54 (of high repetition frequency relative to that of angle increment signals) at which count a pulse is supplied over line 62 to modulator 63 in which appropriate identification pulses are also produced. Before the count down commences gate 55 is closed and a gate 56 which is opened via switch circuit 57 supplies clock pulses to a fixed interval counter 58 which sets a minimum time interval (16 microseconds) in the transmission of angle information. At the end of this interval transfer matrix 51 and switch 57 are operated and the count down commences gate 55 now being open. The cycle then repeates on the occurrence of a pulse from counter 52 which operates switch 57 and reverses the condition of gates 55, 56. In this manner angle coding pulses are produced at intervals which increase with the absolute magnitude of the angle of scan. Switches 49, 49<SP>1</SP> operate to cause counter 48 to count up and down in accordance with the direction of scan for beams (a) and (b) and in accordance with left and right scans for beam (c). At an airborne receiver (Fig. 8) angle coding pulses 99, 991 (Fig. 9) received during the passage of the beams over the aircraft are fed via gate 96 to gate 97 to initiate gate pulses 101, 1011 the leading edges of which arrest the sweep of a linear sweep generator 98 for a period determined by the duration of such pulses the trailing edges 102 of which are produced by pulses 99 fed to gate 97 via line 113, gate 112, lines 114, 115, switch 116 and line 117 and are adjustable for calibration purposes. Gate 96 is then closed by pulses 101 to prevent operation by identification pulses in the case of beams (a) and (c) or other extraneous pulses and at the end of pulses 101 is further held closed by pulses 108 derived by differentiation of the linear sweep waveform in differentiator 107. The amplitude of the sweep waveform thus increases with increasing intervals between pulses 99, 99<SP>1</SP> &c. (see Fig. 9 (c)) the voltage level attained, which is a measure of the beam angle, being fed to a tracking comparator 111 together with any previously stored angle (memory) voltage from memory control 156, switch 158 and line 119 and with pulses 99 (line 117) via amplifier 118. Any error between the actual and memory angle voltages is then fed after integration in 125 and amplification in 135, 136 as a new memory voltage to the angle memory devices 141, 142, 143 via a control circuit 137 which introduces a delay determined by generators 146, 147 under. the control of pulses 99 after integration in 144 to compensate for the fact that the error voltage, after integration in 125, reaches its correct value only after the beam has passed over the aircraft. Memory control 156 supplies the stored signals from devices 141, 142 or 143 in accordance with scan control signals from 166 operated by gates 163, 164 controlled by the beam identity decoder 162 responsive to the (a) and (c) beams the (b) beam not carrying identification pulses. When gate 96 is closed by pulses 101 after the reception of an angle coding pulse 99 (as described above) further angle coding pulses which may be received pass through tracking gate 112, which is opened by pulses 177 (Fig. 9) produced in discriminator 178 by comparison of the linear sweep voltage (line 100) and the memory voltage (line 119 and switch 158) to enable the data recovery to continue. To prevent the linear sweep generator 98 from causing an erroneous tracking error signal to be developed should the amplitude of the sweep rise more than it should owing to a missing angle coding pulse (e.g. due to misfiring of the transmitter) the sweep is stopped by a hold gate 235 (Fig. 9) from generator 180 the leading edge of which coincides with the trailing edge of the tracking gate pulses 177 but the generation of which is inhibited by the arrival of a pulse from gate 96 or through line 117. Before a tracking condition has been established and the memory voltages built up to the proper values the tracking gate pulses 177 do not occur at the correct times for the reception of angle coding pulses (pulses 99) and to allow for this a search gate 181 passes the pulses 99 (on line 161) tc line 115 via line 183. Gate 181 which is closed during retrace and tracking gate intervals by means of pulses on lines 109 and 182 is other wise held open by the output of a memory circuit 185 supplied with pulses 99 from gate 112 and with similar pulses which pass through gate 181 and integrator 184 the output from 185 being maintained until all three beams are being tracked. Gate 181 then closes and tracking gate 112 then allows the operation to continue with the immunity from interference provided by such gate. Calibration of the angle de-coding circuits is effected during intervals between the passage of each beam over the aircraft. For this purpose a pulse train is generated by generator 201 the repetition frequency of which is equal to that corresponding to angle coding pulses representative of e.g. 0 degrees for one of the beams, e.g. for beam (b), so that by eliminating certain pulses from the train further trains corresponding to different angle codings may be produced and for each pulse train a voltage equal to that which the angle de-coding circuit should produce (assuming correct operation) is generated by a calibration voltage generator 202. During calibration intervals the cessation of outputs on line 200 from 146, 147 allows switches 116 and 158 to switch to the full line positions and the memory voltage in 156 is utilized via line 208 and units 209, 204 to operate unit 203 to select a calibration voltage output from 202 which is nearest to the memory voltage. The selected calibration voltage is then fed via line 205 and switch 158 to the tracking comparator 111 and also over line 225 to gate generator 207a which controls test pulse gate 206 to produce a pulse train (by the elimination of certain of the pulses from generator 201) corresponding to the selected calibration voltage, such pulse train being fed via switch 116 and line 117 to control sweep generator 98 in the same manner as the angle coding pulses 99 and via amplifier 118 to the tracking comparator 111. Any inaccuracy in the decoding process then results in the production of an error voltage output from comparator 111 which is applied via units 125, 135 and 136 and line 231 to servo gate 232 (opened by the cessation of voltage on line 200) which operates a width servo integrator 233 controlling the position of the trailing edge 102 of pulses 101 (Fig. 9). The commencement of the linear sweep in generator 98 is thus corrected until the error voltage on line 231 is reduced to zero. At the end of the calibration period (i.e. when the beams again sweep over the aircraft) a gate pulse produced in generator 195 by the first received pulse 99 and continuously renewed by subsequent pulses 991 &c. switches off (i.e. closes) gate 232 and operates switches 116, 158 back to the positions shown by dotted lines so that normal operation follows a delay line 95 ensuring that the latter switching occurs before received pulses 99 reach the decoding circuit. Instead of using delay line 95 the sensitivity of the channel including generator 195 may be increased over that of the decoding channel so that switching takes place before pulses of sufficient amplitude to actuate the decoding circuits are received. The angle information derived by the receiver may be utilized to compute a desired landing path as described in Specification 940,509 and the computer outfit may be used in an automatic pilot arrangement for landing the aircraft or to produce a visual indication of departures from the desired (computed) path. Instead of coding the beams as a function of the angle itself a function of the tangent may be utilized as described in the above mentioned Specification.