Patent GB-646738-A (GB)
Title: Improvements in and relating to radio navigational aids
Inventor: SOPER DENIS COURTNEY
IPC: G01S1/02
Filed: 1948-03-15
Abstract: 646,738. Radio-navigation. MINISTER OF SUPPLY. March 15, 1948, No. 7797. (Class 40 (vii)] A radio-navigational aid for the control of aircraft approaching or departing from an airfield comprises, on the ground, a beacon transmitter operating on a first radio-frequency and transmitting periodic synchronizing pulses, each followed by a plurality of range-marking pulses, and a transponder operating on a second radio-frequency, and, on each aircraft co-operating with the ground station, a receiver for receiving the synchronizing and range-marking signals, a transmitter triggered by the synchronizing signals to retransmit pulse signals on the second frequency, and means whereby the synchronizing signals initiate a time-base for an indicator on which range-marking signals and signals received from the ground transponder are displayed. The display appears as shown in Fig. 1, the range-marking pulses producing " boxes " as shown at B. Pulses re-radiated from the ground transponder and initiated by re-transmission of the synchronizing pulses from aircraft at various ranges are displayed as shown at R. Each aircraft flies so as to maintain its own indication R within a respective " box," while the timing of the range-marking signals is made to vary with respect to the synchronizing pulses in such a manner that the " boxes " move to decrease their ranges. The aircraft accordingly approach the airfield at a controlled rate and are spaced apart in time by predetermined intervals. The invention may be similarly employed to organize the controlled despatch of aircraft froin an airfield. The ground beacon comprises a transmitter 1 controlled by a modulator 2 and wave-form generator 3 which generates a train of synchronizing pulses each of which is followed by a series of marker pulses. The recurrence frequency is determined by master oscillator 4 and the rate of change of timing of the marker pulses is determined by the unit 5 which incorporates a manual control enabling an operator to vary the rate of change. Signals transmitted from 1 are received on an aircraft by a receiver 9. Discriminator 10 selects the synchronizing pulses which are passed via switch S and modulator 11 to trigger transmitter 12 which radiates interrogating signals on a different radio-frequency. These signals excite transponder 6, 7, the re-transmitted signals being received by receiver 13. The output of receiver 13 is applied, together with the range-marking pulses from discriminator 10, to indicator 14, the time-base for which is supplied from unit 15 which is triggered by the synchronizing pulses. A type A display, as shown in Fig. 1, is accordingly produced on the indicator 14. To enable the pilot of an aircraft to identify the indication R due to his own craft switch S is set to connect the oscillator 16 to the time-base and interrogator circuits. The relevant indication will be that which remains fixed in position on the timebase. The system consequently provides a plurality of annular zones about the beacon, as shown in Fig. 2, the zones closing in on the beacon at a steady rate and the aircraft within the zones similarly approaching the beacon. When an aircraft arrives at a predetermined range from the beacon, means is provided to enable it to orbit the beacon until it reaches the point G from which it enters the glidepath leading to the runway L. The ground station may also comprise a radar system 8 having a P.P.I. display to enable the controller to obtain an overall picture of the distribution of aircraft about the airfield, and an indicator 25 which presents a copy of the display shown at 14. A left-right indication may also be provided on the aircraft. The synchronizing signal is applied to variable delay unit 18 the output from which triggers a strobe generator 17. The strobe pulse is set to embrace the aircraft's own " box " and response pulses whereupon comparison circuit 19 delivers an output which is fed back to delay circuit 18 to produce automatic following of the selected pulse. The selected " box " and response pulses are fed to comparison circuit 19a over one channel and the strobe pulses are fed to the same circuit over another channel. The output of circuit 19a is applied, via a switch 20, to a left-right indicator 21 which indicates whether the selected response pulse is set centrally in its " box ". The switch 20 is actuated by a pulse derived from the delay circuit 18 when the range decreases to a predetermined value, whereby the indicator is connected to the instrument-landing-system apparatus 22 which is used during the approach along the glide-path. To enable the ground controller to communicate selectively with the pilot of each aircraft using the system, means 23 are provided at the ground station for selectively modulating the appropriate rangemarking pulse, e.g. by time-variation of the trailing edge of the pulse. Means may alternatively be provided for modulating the appropriate response pulse from transmitter 7. In the aircraft demodulator 24 is connected to the output channel of strobe generator 17 carrying the response and " box " signals, to extract any modulation superimposed on the aircraft's own response or " box " signal. In a modification for accommodating approaching aircraft in more than one speed group, the separate speed groups are allocated different heights and are controlled by beacon systems operating on respective radio frequencies. In an alternative arrangement, the rate of movement of the " boxes " is caused to vary with azimuth, the transmitter 1 being provided with a continuously rotating narrow-beam aerial as indicated in dotted lines in Fig. 3. Each speed group is then allocated a different azimuth of approach. In this arrangement, indicator 25 may be made to provide a P.P.I. display and radar system 8 may be dispensed with. The system using a rotating directive aerial may be used alternatively to vary the rate of movement of the " boxes " to compensate for the effects of the wind, or to allow for the greater distance that aircraft approaching from one direction must fly over those approaching from another direction to reach the point G.