578,953. Directive wireless signalling. SPERRY GYROSCOPE CO., Inc. March 10, 1941, No. 3265. Convention date, March 8, 1940. [Class 40 (v)] Apparatus for measuring the distance between two stations comprises radio means at the home transmitting station for transmitting to the distant station periodic signals in the form of periodic modulations of predetermined frequency imposed on a carrier wave, radio means at the distant station for receiving from the home station the transmitted signals and for retransmitting them back to the home station, radio means at the home station for receiving from the distant station the retransmitted signals and means for determining the distance by comparing and indicating at the home station the phase difference between the transmitted. and received signals. Fig. 1 shows an arrangement for determining the distance between stations 1 and 2 comprising transmitters 3, 3<1> and receivers 10, 10<1>. Transmitter 3 is modulated by a single phase winding of a phase'shifter 5 supplied by a generator 6. The phase supplied to the transmitter can be varied by a crank 7 co-operating with an indicator 9. The demodulated output from receiver 10 is fed through a calibrator 11 and switch 4 to one pair of plates of a cathode ray tube 12 the other pair of which are fed from one phase of the output of the generator 6. Station 2 has similar components indicated by primed reference numerals. To calibrate the system switch 4 is set to the left-hand position and switch 4<1> to the right. The transmitters and receivers are tuned to the same 'wave length and the calibrators 11, 11<1> are adjusted to give straight line patterns on the cathode ray tubes showing the transmitted and received signals to be in phase. To make a determination from station 1 of the distance of station 2' switch 4 is set to the left-hand position, switch 4<1> is set to the right-hand position and the transmitters are set for different wavelengths. Crank 7 is turned to give a straight line pattern and their indicator 9 shows how much phase difference has been introduced in order to correct the phase lag due to the transit of the signal. Dial 9 is' calibrated to read directly in distances. Fig. 2 shows an arrangement in which the distance is read directly on a cathode-ray tube 19. A two-phase generator 17 serves to modulate 'transmitter 3 and also to supply in quadrature the deflecting plates of the cathode-ray tube 19.which is biassed to cut-off by grid 20. Energy from transmitter 3, after reception at 10<1>, retransmission on a different wavelength at 3<1>, and reception at 10, is supplied to a relaxation oscillator 21 which only passes current on the peaks of the output of 10. The output of 21 is fed through transformer 22 to unblodk the tube 19 on the peaks of the output from 10. Since tube 19 if unblocked would produce a circular trace, it follows that when unblocked by the output from 21 it produces a spot of light whose position on the circle depends on the phase lag due to transmission and hence on the distance of the remote station. Tube 19 can thus be calibrated to read distances directly. The frequency of generator 17 is chosen so that 360 degrees phase lag corresponds to the largest distance to be measured. Thus, 5000 cycles gives 360 degrees lag in 18 miles. In a modification (Fig. 3, not shown), an additional modulation frequency ten times as great is also supplied to the transmitter. At the receiver these two frequencies are supplied alternately by means of a commutator to the cathode-ray tube to give two traces, one inside the other, one spot of light giving a reading in miles and the other in tenths of miles. In a further modification, in addition to the two distance indications, there is also exhibited on the cathode-ray tube a direction reading from a directional receiver of the kind described in Specification 496,521, [Group XXXVIII], the several traces occurring successively. but appearing simultaneous due to persistence of vision.