Patent GB-1405874-A (GB)
Title: Landing system with selectable angular path
IPC: G01S1/60
Filed: 1974-03-14
Abstract: 1405874 Radio-navigation OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AEROSPATIALES 14 March 1974 [14 March 1973] 11428/74 Heading H4D A system for directing an aircraft along a selectable angular path comprises a ground station having an aerial from which a radiation lobe is emitted and is oscillated with a reciprocating scan movement at a steady rate that is reversed at each maximum and minimum excursion of the lobe. The signal from the aerial is an RF carrier frequency signal which is modulated by a signal whose frequency varies in unison with the scanning movement of the radiation lobe from the aerial, but with a phase delay relative thereto. There are two angularly displaced positions of said lobe which correspond to the same value of modulation frequency, one being on the outward scan, and one being on the inward scan. An aircraft receiver will receive signals of equal amplitude on that frequency if it is angularly located along the bisector of those two positions. Hence, within a useful part of the scanned angle, an aircraft may be directed along a particular angular path by adjusting its receiver for the corresponding modulation frequency, or it may determine its angular direction from the ground station by observing the modulation frequency which corresponds to the equi-amplitude condition (Figs. 1, 2, not shown). The system may be applied to elevational or azimuthal guidance, or to both. The cycles of modulation frequency variation may be modified by eliminating unusable portions adjacent their maxima and minima, e.g. using a trapezoidal rather than a triangular frequency/time plot (Fig. 5, not shown). An airborne receiver for transmissions in which the carrier is amplitude modulated by a frequency modulated modulating signal is shown in Fig. 3. A detector 6 extracts the modulating signal for application via an amplifier 7 to one input of a phase-comparator 8. Also at the output of the detector 6 there is a D.C. voltage whose amplitude is a function of the intensity of the R.F. signal received, i.e., proportional to the intensity level of the transmit lobe for the angular direction corresponding to the modulating frequency received, and this is applied to respective first inputs of AND gates 14, 15. A variable low frequency local oscillator 9 which can, for example, be pre-set to the value of modulation frequency corresponding to the elevational angle of a glide slope, is connected to the other input of the phase-comparator 8. Output from the said phase-comparator is coupled through respective threshold circuits 10, 11 and monostable flip-flops 12, 13 at the second inputs of the AND gates 14, 15. The comparator 8 gives a positive or negative output signal when the frequency of the signal coming from the amplifier is greater or smaller than that of the signal from the local oscillator. In timing diagram (a), (t 1 -t 2 ), (t 1 <SP>1</SP>-t 2 <SP>1</SP>), (t 1 <SP>11</SP>-t 2 <SP>11</SP>), &c. are parts of the cycle where the received frequency is greater than the local oscillator frequency, and (t 2 -t 1 <SP>1</SP>), (t 2 <SP>1</SP>-t 1 <SP>11</SP>), &c. are parts where it is smaller. Diagram (b) shows the output from the comparator, which is applied to the threshold circuits 10, 11, the former of which saturates on positive signals and the latter on negative signals. Outputs from the threshold circuits 10, 11 as shown in diagrams (c), (d) are applied to the flip-flops 12, 13, which operate on the rising edges thereof, and produce respective trains of short constant-width output pulses as shown in diagrams (e), (f). Hence, the AND gates 14, 15 are enabled at respective instants t 1 , t 1 <SP>1</SP>, &c. and t 2 , t 2 <SP>1</SP>, &c. when the D.C. voltage in the output of the detector 6 is sampled, and is coupled to respective inputs of a differential amplifier 16, which incorporates integrating circuit means. The output side of the amplifier 16 is connected to a zero-centred indicator 17, which remains undeflected when the aircraft is on course. In another embodiment, the carrier signal itself is frequency modulated. A variable R.F. local oscillator 18, Fig. 4, is provided, and an L.F. oscillator 19 is connected to one input of the phase-comparator 8. Output from the I.F. amplifier 5 is fed to both the detector 6 and the L.F. amplifier 7. The detector 6 provides output only to the first inputs of the AND gates 14, 15.