Patent GB-651259-A (GB)
Title: Navigation aiding radio systems
IPC: G01S1/24
Filed: 1947-07-09
Abstract: 651,259. Radio navigation ; saw - tooth generating circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 9, 1947, No. 18195. Convention date, July 10, 1946. [Classes 40 (vi) and 40 (vii)] The invention relates to apparatus for measuring the time interval between pairs of periodically-recurring pulses such as are received from a pulse hyperbolic radio navigation system of the Loran type and comprises means for scanning a cathode-ray tube screen at the pulse recurrence frequency of the system and for causing successive pulses from the two stations of the system to appear on the scanning trace with one of them in a predetermined position and means for producing an index marker on the trace which may be moved into co-incidence with the other of the two pulses. Various pairs of ground stations characterized by different pulse recurrence frequencies may be selected at will by adjusting the recurrence frequency of the cathode-ray tube time base trace. Referring to the pulses as A and B pulses (the B pulse occurring after the mid-point of the A pulse period) these pulses are displayed on two cathode-ray traces each of half the repetition period duration and the scanning rate is then altered so as to drift the B pulse to the left end of the lower trace. The A pulse then appears on the same half trace and the index marker is located under it by adjusting coarse and fine multivibrator trigger circuits. For more accurate adjustment the cathode-ray indicator is switched so that the A and B pulses appear on parallel fast traces. One of these commences at the start of the slow-speed trace and the other at the time the index marker occurs so that by further adjustment of the trigger circuits exact alignment can be obtained. An amplitude balance circuit is provided to enable the A and B pulses to be brought to the same amplitude to assist in this alignment. The time difference between the A and B pulses is then equal to the time difference between the starts of the fast sweeps and this time difference is measured by counting timing marks provided on the trace. First using the slow-speed trace and 1000 Á sec. timing marks the time between the index marker and the end of the trace is determined approximately and then the fast sweep is used for accurate determination of the remainder with the aid of 100 and 10 Á sec. marks. The fast sweep used itself includes a high-speed expanded portion close to, but delayed slightly after, the commencement of the sweep and it is on this expanded portion that both the final alignment of the A and B pulses and the final time interval measurement is made ,the 10 Á sec. timing pulses being visible separately on this portion of the trace. The actual fraction of 1000 Á sec. interval is read between the " reading point " which is the 1000 Á sec. mark appearing nearest the expanded portion of the trace and the " cross hair " mark occurring 50 Á sec. after the commencement of the high-speed trace. Circuit arrangements. The sweep trigger pulses, timing marker pulses, and index marker pulses are derived from a pulse generator unit and frequency-divider chain. This chain is provided with pulse feed-back arrangements to two of the dividers for count subtraction purposes so that any of a number of pulse recurrence frequencies may be selected at will. The generator and count subtration circuits are similar to those described in Specification 633,762. The fast sweep generator described with reference to Fig. 3 of that Specification may be used in connection with the present invention but an improved generator is also described (Fig. 5a). In the improved circuit an additional valve 116a is employed so that the condensers in the timing circuit recharge very quickly and the circuit recovers and is ready for re-operation more rapidly. In this circuit network 117b provides the fast sweep while network 117a provides the high-speed expanded portion. The latter is delayed slightly after the commencement of the sweep by delay circuit 117c. Application of a pulse at J cuts off both valves and triggers the sweep circuit.