Patent GB-810966-A (GB)
Title: Loran equipment
Inventor: PALMER WINSLOW
IPC: G01S1/24
Filed: 1956-08-10
Abstract: 810,966. Radio navigation. SPERRY RAND CORPORATION. Aug. 10, 1956, No. 24625/56. Class 40 (7). Relates to a L.F. Loran system comprising a master station transmitting two interleaved pulse trains X and Z, Fig. 2A, each of recurrence frequency f = 50 c./s. and the spaced slave stations transmitting respectively pulse trains Y and W also of recurrence frequency f, the position of a mobile receiver relative to the stations being determined at the receiver by measuring the time intervals X-Y and Z-W; in order to identify the X pulses a ghost pulse X<SP>1</SP> displaced 1 msec. from the X pulse is transmitted every third cycle. According to the present invention, at the mobile receiver local X, X<SP>1</SP> and Z pulses derived by frequency dividers from a 100 kc./s. oscillator 1, Fig. 3, are automatically synchronized with received X, X<SP>1</SP> and Z pulses by comparing local and received Z pulses in a discriminator 5 and applying the output therefrom to control the frequency of oscillator 1 via gates 11 and 12 which are opened only when the local and received X and X<SP>1</SP> pulses coincide thereby ensuring that the local Z pulses lock only on to the Z components of the received pulses; variably delayed local Y and W pulses derived from the outputs from the frequency dividers and the local X and Z pulses are automatically aligned with the received Y and W pulses by servos 29, 29<SP>1</SP> controlled by discriminators 27, 27<SP>1</SP>, the time delays X-Y and Z-W being indicated by counters 9; 9<SP>1</SP> and ganged to the servos 29, 29<SP>1</SP>. As shown in Fig. 3, a precision local Z pulse, Fig. 2 F, is generated by applying the output from the 100 kc/s. oscillator and through a chain of frequency dividers 2 to trigger a square wave generator 3 of recurrence frequency f whose output, Fig. 2 B, is differentiated in a pulse generator 3<SP>1</SP>, the negative pulses of the resultant output, Fig. 2 C, being applied to trigger on a bi-stable trigger circuit 4 which is triggered off by the next succeeding pulse of the 20 kc/s. pulse train, Fig. 2 E, from a pulse generator 7 triggered by the 20 kc/s. output from the first frequency divider 2. The local Z pulse, Fig. 2 F, is derived from the trailing edge of the output, Fig. 2 D, from the bi-stable circuit 4 and is applied together with the received pulses from the receiver 6 to the coincidence circuit 5 whose output is applied through the gates 11 and 12 to a reactance valve 13 which controls the frequency of the oscillator 1 so as to automatically maintain the local and received Z pulses in coincidence. The positive pulses of the output, Fig. 2 C, from the pulse generator 31 are used as the local X pulses and local X<SP>1</SP> pulses are derived therefrom by a 3: 1 frequency divider 15 and 1 msec. delay 1b, the gates 11 and 12 being opened only when the local and received X and X<SP>1</SP> pulses are substantially coincident as determined, by coincidence circuits 18, 19. A variably delayed local Y pulse, Fig. 2 I, is derived from the trailing edge of the pulse output, Fig. 2 G, from a bi-stable trigger circuit 21 which is triggered on by a local X pulse (positive pulse, Fig. 2 C) and triggered off by the next succeeding pulse in a. variably delayed 200 c/s. pulse train, Fig. 2 H, generated as in Specifications 654,979 and 644,579, by applying outputs from the first four frequency dividers 2 through geared phase shifters 22 ... 25 to a mixer 26. The local Y pulse, Fig. 2 I, is automatically maintained coincident with the receive Y pulse by adjusting the phase shifters 22 ... 25. by means of a servo 29 controlled by the output from a coincidence circuit 27 to which the local Y pulses and the received pulses are applied. The Z-W delay is derived in a similar manner by corresponding components 9<SP>1</SP>, 21<SP>1</SP> ... 271 and 29<SP>1</SP>, the local W pulse, Fig. 2 L, being derived from the trailing edge of a pulse, Fig. 2 J, initiated by a negative pulse in the output, Fig. 2 C, from the pulse generator 3<SP>1</SP> and terminated by the next succeeding pulse of a variably delayed 200 c/s. pulse train, Fig. 2 K, from the mixer 26<SP>1</SP>. As shown'in Fig. 4, the coincidence circuit 5, Fig. 3, comprises a heptode 30, whose anode circuit is earthed, the local and received pulses being applied respectively to the first and third grids thereof, and the time constant of the input circuit 35, 36 to the third grid being greater than the duration of the received pulses 40, Fig. 6. The bias E 1  on the heptode is adjusted by a potentiometer 39 so that in the absence of received pulses the heptode is cut off and the negative anode output E o corresponds to a point B, Fig. 5, on the E 0 -E 1  characteristic of the heptode, the inductance 14 in the tuned circuit of the oscillator 1 being adjusted so that under these conditions the recurrence frequency of the local pulses is less than that of the received pulses. Thus when pulses 40, Fig. 6, are received from the receiver 6 the local pulses will drift during successive recurrence periods towards the leading edge of the received pulses and when coincidence is effected the heptode will become conductive to an extent depending on the amplitude of the portion of the received pulses being sampled by the local pulse. The resultant decrease in anode voltage is integrated by capacitor 31 and resistor 32 giving an average output of increased negative amplitude which increases the frequency of the oscillator 1 and an equilibrium condition is established when the amplitude of the sampled portion of the received pulse 40 is A<SP>1</SP>, Fig. 6, corresponding to a point A, Fig. 5, on the heptode characteristic. Capacitor 33 and resistor 34 serve to suppress oscillations in the frequency control loop. The coincidence circuits 18, 19, 27 and 271, Fig. 3, are all preferably as in Fig. 4 in which case the gates 11 and 12 may be relays which are closed only when the corresponding control voltages from the coincidence circuits 18 and 19 are between the limits O and P, Fig. 5, and the servos 29 and 291 may be made to operate about the level of the voltage E 0  corresponding to the point A, Fig. 5, by providing a bucking voltage of this amount. Specifications 661,954 and 684,831. also are referred to.