GB1051373A - - Google Patents

Abstract

1,051,373. Synchronizing remotely positioned timing devices. Mc DONNELL AIRCRAFT CORPORATION. Sept.14, 1964, No.37528/64. Heading G3T. In a system for synchronizing a number of remotely positioned timing devices, for use, e.g. in satellites, in aircraft collision avoidance systems, or for navigation, the timing devices are of the kind such as those using a crystal oscillator or the like and capable of keeping time in very precise small increments. The system comprises means for assigning a different distinct group of increments to each timing device for synchronizing purposes, means for comparing the time of occurence of the assigned group of increments of each timing device with the time of occurence of the same increment group at another timing device to see if the compared groups occur simultaneously, and means for shifting the time of occurence of one of said compared groups at one of the timing devices relative to the time of occurence of the same corresponding increment groups at the other timing device in a direction to make similar later occurences of the same corresponding increment groups take place simultaneously at both of the timing devices. In one system for use between aircraft and a ground station and using a return signal, the aircraft carries a count-up counter 28, Fig. 1 (not shown) and the ground station carries both a count-up counter 28 and a count-down counter 30. When the countdown counter reaches the same count as that which was in its count-up counter on receipt of a signal, it transmits the return signal. This will be received displaced from the synchronization time by 2#T, where #T is the actual phase error. The propagation time between the aircraft and ground is automatically cancelled Figs.2 to 4, (not shown). Alternatively, count-up counters only may be used, that in the ground station being allowed to count up to a predetermined number of increments before a return signal is sent. Circuit means utilizing known devices, for performing the synchronization in the respective cases where both count-up and count-down counters are used or where count-up counters only are used, are disclosed, Figs. 5 and 9 (not shown). Particular groups of increments or "message slots" may be alloted to particular aircraft. There may e.g. be three thousand message slots in a three second period, so that each aircraft, up to a total number of three thousand, is synchronized once every three seconds.