Patent GB-590460-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S1/24
Filed: 1943-12-10
Abstract: 590,460. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Dec. 10, 1943, No. 20748. Convention date, June 28, 1941. [Class 40 (v)] A beacon system for providing a course or bearing line comprises two spaced transmitters emitting pulses in such phase relationship that a craft on the desired position line receives the pulses from both transmitters with a definite time delay between them, the timing of the received pulses being observed by the comparision of harmonics in the received pulse signals. Two transmitters A, B, Fig. 1, with associated aerials 10, 11 are spaced apart and each supplied with a recurrent pulse signal, spaced in time with respect to one another. Lines of constant time displacement between received pulses will be hyperbolµ which at considerable distances from the transmitters are effectively radii through the mid-point between the aerials 10, 11. Preferably, a single pulse generator 12 is employed controlling transmitter B while an aerial 10<1> and receiver B receive pulses from transmitter B which after suitable time delay in delay means 13, are employed to control transmitter A. A monitoring circuit 9 may be provided to ensure that the timing of pulses from both transmitters A and B is correct. Alternatively, the pulse generator 12 could control both transmitters over lines. The disposition of the pulses of recurrence period T is shown in Fig. 2, the B pulse being transmitted an interval bo after the A pulse, while the pulses suffer delays of rA and rB in reaching a particular receiving point so that the interval between received pulses is b, which for the desired course is made equal to T/4. The output of a receiver comprising R.F. and I.F. stages 14 and limiting and detecting means 15, 16 is fed to filters 17-20 which select the fundamental, an even harmonic and various odd harmonics of the pulse recurrence frequency. Voltmeters 22, 23 and 24 indicate the differences between the fundamental and third harmonic, and the fifth and third harmonics and also indicate the second harmonic. The values of these quantities z31, z35 and y2 are shown in Fig. 4 plotted against the received pulse separation b and it can be seen that when b=T/4 all these quantities are zero, z35 giving the sharpest indication, while the second harmonic y2 is zero for this particular value of b only, and prevents incorrect zero values for the other quantities being selected as defining the desired course. Any desired course or bearing line complying with the requirement b=T/4 may be set up by adjustment of the transmitted time delay bo by means of the delay means 13, a suitable indicating means 21 being provided to observe and calibrate the course directivity. Control of course may also be effected at the receiver by delay means which alters the timing of one received pulse with respect to the other. The use of a limiter in the receiver causes any reflection or other disturbing effects to be substantially eliminated. The signal to noise ratio may also be improved, since the maximum pulse displacement needed in the receiver is T/2, by half-wave rectifying some of the fundamental frequency and utilizing the rectified energy to control blocking of the receiver during the unused portion of the recurrence period.