601,401. Radio navigation valve circuits. BARBER, L. C. Sept. 5, 1945, No. 22916. [Classes 40 (vi) and 40 (vii)] Navigating apparatus, for maintaining an aircraft on an orbital track at a predetermined distance from a responder beacon, comprises a range indicating meter 9 of the centre-zero type, operated by a radar transmitter-receiver 1 and also fed with an adjustable pre-set potential so as to give a predetermined indication when the aircraft is following the desired orbital course. The exploring and echo pulses are fed to a cathode-ray display unit of the,type described in Specification 600,804, and also to the unit 3 which delivers a voltage proportional to range to the range meter 4 and to the meter 9. A strobe pulse generated in the unit 3 is maintained in coincidence with the echo signal, whereby the voltage proportional to range is developed. The output from a potentiometer unit 5 with which is associated a slider calibrated in terms of range is also applied to the meter 9, so that a central indication is obtained when the desired course, as indicated by the slider, is being followed. Detailed operation. A trigger pulse, Fig. 3a, coinciding with the exploring pulse is applied to the anode of the valve V15 in the Phantastron circuit 11, Fig. 4, which is described in Specification 582,758. A square pulse, Fig. 3b, is taken from the screen grid, the leading edge coinciding with the trigger pulse and the length depending upon the standing voltage applied at the cathode of the diode D1. The trailing edge of the pulse triggers the blocking oscillator V17, in the grid circuit of which is an opencircuited delay line DL1, the reflected pulse from which cuts the valve off again. The blocking oscillator output, which comprises the directly generated pulse, and a pulse delayed by the delay line DL2, consists of a double strobe, as shown in Fig. 3c. Each half of the strobe is fed to a respective control grid of the push-pull pair V1, V2, in the common cathode circuit of which is the valve V3, to the control grid of which is applied the echo signal, Fig. 3d. When the echo and strobe signals overlap, a current pulse is developed in the secondary of the transformer T2, its direction depending upon which of the two strobe pulses coincides with the echo, and when the echo is straddled by the strobe no resultant current flows. The pulse current is rectified by the double-diode D2, the output from which is smoothed and fed to the control grid of the valve V12, the anode and grid of which are resistively coupled together in such a fashion that current flows to or from the control grid of the Miller time-base valve V13 in accordance with the sign of the output of the rectifier D2. No current flows to or from the grid of V13 when the strobe straddles the echo pulse, so that the anode voltage of V13 is stabilised, thereby stabilising the cathode voltage at a value which maintains the strobe and echo pulses coincident. The anode voltage of V13, which is consequently proportional to range is applied to the cathode follower range meter circuit 4, and also to the control grid of the valve V19 together with an opposing negative voltage from the potentiometer unit 5. The anode and grid of V19 are resistively coupled together in such a manner that current flows to or from the control grid to maintain the grid voltage constant, current also flowing through the centre-zero meter 9a in one direction or the other, according as to whether the actual range is greater or smaller than the selected orbiting radius. A central indication is obtained on the meter when the desired course is being followed. Scanning in range. This is obtained by opening and closing the switches S1, S2 alternately. When an echo signal coincides with the strobe, as may be seen on the display unit 2, Fig. 1, the switches are opened either manually or automatically by means of relays operated by the current output from the transformer T2. Alternative embodiment. In an alternative embodiment disclosed in the Provisional Specification, a pulse generator produces marker pulses corresponding to range intervals of one mile. The marker pulses are used to produce a square wave whose length, corresponding to an integral number of miles, is under the control of the pilot. The square wave is compared with a further square wave, obtained from the circuit which provides the range voltage, in a suitable valve circuit the output of which gives an appropriate indication. The Provisional Specification also describes a circuit for providing a " turn signal " for an automatic pilot, as disclosed in Specification 594,484. Specification 582,503 also is referred to in the Provisional Specification.