Today, '''time synchronization''' can occur between systems around the world through [[satellite navigation]] signals and other [[time and frequency transfer]] techniques.
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== Navigation and railways ==
Time-keeping and synchronization of clocks is a critical problem in long-distance ocean navigation. Before [[radio navigation]] and [[Radionavigation-satellite service|satellite-based navigation]], navigators required accurate [[time]] in conjunction with astronomical observations to determine [[History of longitude|how far east or west]] their vessel traveled. The invention of an accurate [[marine chronometer]] revolutionized marine navigation. By the end of the 19th century, important ports provided [[time signal]]s in the form of a signal gun, flag, or dropping [[time ball]] so that mariners could check and correct their chronometers for error.
Synchronization was important in the operation of 19th-century railways, these being the first major means of transport fast enough for differences in [[local mean time]] between nearby towns to be noticeable. Each line handled the problem by synchronizing all its stations to headquarters as a standard [[railway time]]. In some territories, companies shared a single railroad track and needed to avoid collisions. The need for strict timekeeping led the companies to settle on one standard, and civil authorities eventually abandoned local mean time in favor of railway time.
