Editing Longwave (section)

===Time signals===
There are institutional broadcast stations in the range that transmit coded time signals to radio clocks. For example:
*[[WWVB]] in [[Colorado]], United States, on 60&nbsp;kHz, 70 kW [[Effective radiated power|ERP]]
*[[DCF77]] in [[Frankfurt]], [[Germany]], on 77.5&nbsp;kHz, 50 kW
*[[JJY]] in Japan, on 40 & 60&nbsp;kHz, 50 kW
*[[RBU_(radio_station)|RBU]] 66.66&nbsp;kHz in [[Taldom transmitter]], Russia, 10 kW
*[[BPC (time signal)|BPC]] in [[Shangqiu]], [[China]], 68.5&nbsp;kHz, 90 kW
*[[Time from NPL|MSF]] time and 60&nbsp;kHz frequency standard transmitted from [[Anthorn radio station|Anthorn]] in the UK, 17 kW ERP.
*[[ALS162_time_signal|ALS162]] from [[Allouis]], [[France]], on 162&nbsp;kHz, 800 kW

[[Radio clock|Radio-controlled clocks]] receive their time calibration signals with built-in long-wave receivers. They use long-wave, rather than [[shortwave|short-wave]] or [[mediumwave|medium-wave]], because long-wave signals from the transmitter to the receiver always travel along the same direct path across the surface of the [[Earth]], so the time delay correction for the signal travel time from the transmitting station to the receiver is always the same for any one receiving location.

Longwaves travel by [[groundwave]]s that hug the surface of the Earth, unlike [[mediumwave]]s and [[shortwave radio|shortwaves]]. Those higher-frequency signals do not follow the surface of the [[Earth]] beyond a few kilometers, but can travel as [[skywave]]s, '[[refraction|bouncing]]' off different layers of the [[ionosphere]] at different times of day. These different [[radio propagation|propagation]] paths can make the time lag different for every signal received. The delay between when the long-wave signal was sent from the [[transmitter]] (when the coded time was correct) and when the signal is received by the clock (when the coded time is slightly late) depends on the overland distance between the clock and the transmitter and the [[speed of light#In a medium|speed of light through the air]], which is also very nearly constant. Since the time lag is essentially the same, a single constant shift forward from the time coded in the signal can compensate for all long-wave signals received at any one location from the same time signal station.