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Discussion Starter #1 (Edited)
Accuracy.JPG I have just bought a cheap Saint Leonhard radio controlled watch (time signal from German longwave transmitter DCF77) which is an analog watch, with an additional digital window. As you can see from the attached graph, it loses about 1-1.5 secs per day, but is deemed accurate (!) because it is synchroniseds daily at 3 o'clock W European time. Questions:
1. the station DCF transmits permamently a signal, so why is my watch only synchronised at 3am?
2. I took the painful decision to get up from bed at 5 to 3am, to watch the change, and the following happened..... At 3am precisely the 3 analog hands showed exactly 3am and remained locked to this time, and the digital seconds continued showing a 'slowness' of 1 second. At about 4-5 minutes past 3, suddenly the analog hands moved to the correct position and the 3 hands were exact. The digital seconds had also corrected itself to the exact time. All values were checked with a HERMLE RCC2000 radio-controlled clock, which is synchronised hourly. Question: What's going on, and why did the analog hands stop for 4-5 minutes?
 

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I don't know the answer to you question but do you mean to tell me that there is more than one 3:00 O-clock in the day? :-d
 

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Just a guess but the hands probably stop when the radio receiver gets turned on. Transmissions are likely slow and/or intermittent so it takes a couple minutes before the watch synchronizes and starts running again. The reciever likely requires a relatively large amount of power which is why it only updates one per day. Stopping the hands during receiving may be just to inform the user, it may be an attempt to limit total power during recieving, or it may be necessary to resynchronize the movement.
 

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Discussion Starter #4
Just a guess but the hands probably stop when the radio receiver gets turned on. Transmissions are likely slow and/or intermittent so it takes a couple minutes before the watch synchronizes and starts running again. The reciever likely requires a relatively large amount of power which is why it only updates one per day. Stopping the hands during receiving may be just to inform the user, it may be an attempt to limit total power during recieving, or it may be necessary to resynchronize the movement.
I agree with the above, but I still find it strange, that for the 4-5 minutes after 3am, the digital second display continues working, whereas the analog hands don't move (minute & second hands at 12 and hour hand at 3 position). It is also strange that if I wish, I can automatically set the time at ANY time of day, but left on its own, the watch only corrects the time (using the time signal) at 3am each day.
 

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Atomic Casio are programed to only update once each day after local midnight. Once it updates, it is good until the next local 0001.

So it is not strange, just programming. Best to read the instructions for your particular module.



 

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Both the Leonhard watch and Hermle clock probably drift a lot between updates so you may be comparing 2 moving targets. Those devices typically don't have highly accurate quartz movements because they are updated daily. To check their accuracy compare each time keeper to one of the atomic clock websites.

The update takes several minutes because the data transmission rates are very slow and if the transmission is not successful the update may be attempted again. It also takes time for the watch to process and adjust time. I suspect that your watch freezes the movement because it is easier and cheaper to program an update to a fixed target. I recall reading that the Casio Oceanus freezes some functions during this process as well. Most rc time keepers begin looking for a signal after midnight when signal strength is highest and ambient noise level is lower.

The NIST website has an excellent description of how the radio control system works, how clocks and watches are updated and provides signal propagation maps.
Radio-controlled clocks
 

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Discussion Starter #7
John[/QUOTE said:
Thanks John, tha seems to have answered it very well.
As far as the watch's instructions are concerned, there is only a minimum described.
 

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I assume there is a cost in terms of battery as well each time this is done, so once a day at night when the watch is probably stationary makes sense. My Seiko Astron GPS Solar handles this differently I believe, in that it detects when the ambient light increases i.e. there is a good chance the wearer is outside with clear sky above them, with better GPS signal reception and will try to trigger a synchronization then.
 

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Programming is the answer I think. Running the motors to rotate the hands is very power hungry so if they delay the hands for 5 minutes they know that even if the watch was fast, it will only have to step forward by a few minutes. The alternative for a fast watch would be to rotate the hands completely round the dial.

I have a Citizen that synchronises at 2am - and yes I have stayed up (once) to watch it when the clocks went on an hour. On the Citizen the hands will move independently though I believe some RC controlled watches have the minute hand rotate the full 60 minutes when the hour is changed.
 
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