Editing Time and frequency transfer (section)

==Two-way==
In a two-way time transfer system, the two peers will both transmit and receive each other's messages, thus performing two one-way time transfers to determine the difference between the remote clock and the local clock.<ref name="Jones"/>{{rp|118}} The sum of these time differences is the [[round-trip delay]] between the two nodes. It is often assumed that this delay is evenly distributed between the directions between the peers. Under this assumption, half the round-trip delay is the propagation delay to be compensated. A drawback is that the two-way propagation delay must be measured and used to calculate a delay correction. That function can be implemented in the reference source, in which case the source capacity limits the number of clients that can be served, or by software in each client. The [[NIST]] provides a time reference service to computer users on the Internet,<ref>{{cite web|url=https://www.nist.gov/physlab/div847/grp40/its.cfm|title=Set your computer clock via the Internet using tools built into the operating system|publisher=National Institute of Standards and Technology|access-date=2012-12-22}}</ref> based on Java applets loaded by each client.<ref>{{cite book | last=Novick| first=Andrew N. | title=Time Distribution Using the World Wide Web |url=http://tf.nist.gov/timefreq/general/pdf/1499.pdf |display-authors=etal |archive-url=https://web.archive.org/web/20160303180104/http://tf.nist.gov/timefreq/general/pdf/1499.pdf |archive-date=2016-03-03}}</ref> The [[two-way satellite time and frequency transfer]] (TWSTFT) system being used in comparison among some time laboratories uses a satellite for a common link between the laboratories. The [[Network Time Protocol]] uses packet-based messages over an IP network.

Historically, the [[History_of_longitude#Land_surveying_and_telegraphy|telegraphic determination of longitude]] was an important way to connect two points. It could be used one-way or two-way, with each observatory potentially correcting the other's time or position. [[Telegraphy]] methods of the 19th century established many of the same techniques used in modern times, including round-trip time delay calculations and time synchronization in the 15 to 25 millisecond range.<ref>{{cite EB9 |wstitle = Measurement of Time |volume= XXIII |last=  Dreyer |first= John Louis Emil |author-link= John Louis Emil Dreyer |pages= 392-396 |short= 1}}</ref>