Patent GB-940509-A (GB)
Title: Aircraft radio landing system
IPC: G01S1/64
Filed: 1960-04-11
Abstract: 940,509. Radio navigation. CUTLERHAMMER Inc. April 11, 1960 [July 7, 1959], No. 12809/60. Heading H4D. An aircraft landing system comprises at least two radio beacons spaced apart a distance D longitudinally with respect to the desired landing path, each beacon transmitting a vertically scanning horizontal fan beam which is modulated with pulse signals coded in accordance with the instantaneous elevation of the beam and means being provided on the aircraft for decoding the beam signals to give corresponding signals defining the angular elevation of the aircraft from the beacons and for deriving from said elevation signals an error signal indicating deviation of the aircraft from a predetermined landing path. The error signal may be applied to a pilot indicator or to actuate an automatic pilot. As described the two beacons scan sequentially and each beam is modulated by (1) recurrent pairs of " sliding " and reference pulse signals whose spacing varies with the tangent of the elevation angle and (2) increment pulses 51, Fig. 10 (a) and (i) harmonically related to the reference pulse signals. Each reference pulse signal comprises a pulse pair 56, 61, Fig. 10 (c) and (i), whose spacing, 12 or 16 Ásecs., is characteristic of the beacon and each " sliding " pulse signal comprises a pulse pair 74, 78, Fig. 10 (g) and (i), whose spacing, 14 Ásec., is the same for each beacon. At the aircraft receiver the increment pulses are used to interpolate between the discrete values of elevation provided by the sliding and reference pulse signals. The beacons must be offset from the runway, Fig. 1 (not shown), to prevent obstruction but they may be sufficiently near to it so that the angles of elevation of the aircraft from the beacons are substantially the same as the angles of elevation from points 13<SP>1</SP>, 14<SP>1</SP>, Fig. 6, on the runway (denoted by the X axis) opposite the beacons. The tangents of the angles of elevation of the aircraft from the points 13<SP>1</SP> and 14<SP>1</SP> are denoted in Fig. 6 by a and b respectively and signals and apparatus corresponding to these points will be identified by these letters, the points 13<SP>1</SP> and 14<SP>1</SP> being referred to as the effective " a " and " b " beacon sites. As shown in Fig. 6, the preferred form of landing path comprises an initial straight glide path followed by a curved flare-out path, the transition point 24 between glide and flare-out being a predetermined distance F in front of the touch-down point 21 which is a predetermined distance L in front of the effective " b " beacon site 14<SP>1</SP>. The glide path is aimed at the effective " a " beacon site 13<SP>1</SP> or at a point 33 in front of the point 13<SP>1</SP> and during this phase of the landing the aircraft is flown so that its elevation from the glide aiming point is constant or so that the rate of change of this angle, or of its tangent, is maintained equal to zero. The flare-out path is defined by the equation where :- x<SP>1</SP> is the computed horizontal distance of the aircraft from the effective " b " beacon site 14<SP>1</SP> after the aircraft has passed the transition point 24. z is the height of the aircraft computed from the equation Z=b x<SP>1</SP>. S is the distance of the transition point in front of the glide aiming point. Means are described for deriving an error signal corresponding to deviation of the aircraft from such a path in terms of the height z and its rate of change z, Fig. 16, or in terms of the tangent b of the elevation of the aircraft from the effective " b " beacon site 14<SP>1</SP> and its rate of change b. Beacon transmitter. Figs. 9, 10 and Fig. 8 (not shown).-Each beacon, e.g. the " a " beacon having an effective site 13<SP>1</SP>, Fig. 6, includes a generator 43, Fig. 9, producing a shaft rotation proportional to the tangent a of the instantaneous beam elevation and a generator 44 producing an increment pulse for each equal increment in a corresponding to an angular increment of approximately 0.01 degrees, the scan magnitude, e.g. 20 degrees, and the rate of scan, e.g. 10 c/s., being such that the average recurrence frequency of the increment pulses is approximately 20 kc/s. The increment pulses are delayed at 52 by an amount characteristic of the beacon (e.g. 12 Ásecs. for beacon " a " and 16 Ásecs. for beacon " b ") giving a pulse train 51, Fig. 10 (a), and are also applied to a counter 55 producing an output pulse 56, Fig. 10 (b), for every five increment pulses. The pulses 51 and 56 are applied to an encoder 58 so that each pulse 56 produces a reference pulse pair 56, 61 of characteristic spacing and each pulse 56 triggers a sweep generator 62 whose sawtooth output 63, Fig. 10 (d), is applied to a voltage comparator 68 together with a D.C. voltage obtained by adding the outputs 60 and 85, Fig. 10 (e), from two potentiometers 71, 72 controlled by the tangent generator 43 so that during each forward stroke of the waveform 63 the comparator 68 produces a pulse 74, Fig. 10 (f), which is converted in an encoder 77 into a pulse pair 74, 78 (spacing 14 Áecs.), Fig. 10 (g) and (i), the resistive network and voltage sources V, -V1 and -V2 associated with the potentiometers 71, 72 being such that the time interval 84, Fig. 10 (i) between the second pulse 78 of a sliding pair 74, 78 and the second pulse 61 of the succeeding reference pair 56, 61 represents the tangent of the elevation of the axis of the beam at the instant when the second pulse 61 of the reference pair 56, 61 is generated. Each pulse in the increment pulse train output, Fig. 10 (a), from the delay circuit 52 which coincides with a second pulse 61 of a reference pulse pair 56, 61 is blanked out in a circuit 53 by means of a blanking pulse derived from an output pulse 56, Fig. 10 (b), from the counter 55. Similarly, any pulse in the increment pulse train which precedes a sliding pair 74, 78 by less than 18 Ásecs., or, in the absence of such a preceding increment pulse, the first increment pulse following the sliding pair, is blanked out in the circuit 53 by a blanking pulse 83<SP>1</SP>, Fig. 10 (h), initiated by a trigger pulse 83 and terminated by the next increment pulse, the trigger pulse 83 being generated by applying to a voltage comparator 67 the sweep wave 63 and the combined outputs from potentiometer 72 and a potentiometer 73 similar to potentiometer 71. The resultant output from the blanking circuit 53 is combined with the reference and sliding pulse pairs from the encoders 58 and 77 to give a resultant pulse train, Fig. 10 (i), which modulates the transmitter. Aircraft receiver, Figs. 11, 13, 15 and Figs. 12 and 14 (not shown).-Each time the beam from a beacon sweeps through the receiver, the receiver will receive an RF pulse train corresponding to the transmitted composite pulse train, Fig. 10 (i), but modulated in amplitude in accordance with the shape of the beam as shown in different degrees of detail in Fig. 13 (a), (b) and (c). Single sliding pulses and " a " and " b " reference pulses coinciding with the second pulses of the corresponding pulse pairs are derived from the output of the video amplifier 98, Fig. 11, by means of a discriminator 112 and corresponding coincidence detectors 122, 113, 114. Each sliding pulse triggers a sweep gate generator 123 whose output, Fig. 13 (e), gates on a sweep generator 124 to produce a linear sweep output 105, Fig. 13 (f), which is terminated by a read gate pulse 108, Fig. 13 (g), initiated in a generator 125 by the next succeeding reference pulse signal. The read gate. pulse 108 also opens a gate 129 to apply the terminal voltage 107, Fig. 13 (f), of the sweep 105 to a pulse counting integrator 132 to which the output from the video amplifier 98 is applied so that the D.C. output from the integrator 132 increases from said terminal voltage level 107 in small steps 102<SP>1</SP>, 102<SP>11</SP>, Fig. 13 (d), in response to each increment pulse or to each reference or sliding pulse pair from the video amplifier 98. When the gate 129 is opened by the next read gate pulse 108, the output from the integrator is reset to the new terminal sweep level and the process is repeated so that the resultant output voltage 103, Fig. 13 (d), and Fig. 15, of the integrator 132 represents the variation of the tangent of the elevation of the beam axis as the beam sweeps through the receiver. The required tangent of the elevation of the aircraft is given by the magnitude of the voltage 103 at the mid-point of the modulation envelope 99; Fig. 13 (a) and (b) and Fig. 15, of the received pulses and a D.C. voltage 147, Fig. 15, of such magnitude is produced in a memory circuit 135 by applying the voltages 103 and 147 and the video pulses from the video amplifier 98 to a comparator 134, Fig. 14 (not shown), which amplifies the video pulses in accordance with the algebraic difference between the instantaneous amplitudes of the voltages 103, 147 so that the polarity of the output from the comparator 134 reverses at the instant corresponding to the intersection of the voltage curves 103, 147. The positive and negative portions of the output are integrated and subtracted in a track error integrator 142 to give an error voltage which is applied through an amplifier 155 and charging amplifier 156 to the memory circuit 135 to vary the output therefrom until the positive and negative components of the output from the comparator 134 are of equal magnitude as shown in the third section of Fig. 15. Separate memory circuits 135, 136 are provided for the " a " and " b " signals and the inputs to and outputs from the memory circuits 135, 136 are selected by switching circuits 157, 137 which are changed over after each beam has swept through the receiver by means of a control pulse, Fig. 12 (not shown), generated at 161 in response to a delayed output from the video amplifier 98, the direction of switching being determined by outputs from the " a