It's odd, though: I got a cheap one for Christmas, and the manual
*said* it synchronised to Germany -- but it's synched itself properly to GMT, and I can only guess that the unit itself is set for Rugby, but that the manual that came with it wasn't changed.
(Unless the German source knows that I'm calling in from the UK and adjusts accordingly -- but I doubt that. If it's using the German source, though, it's still synchronising fine for me.)
DCF77. The signals can be a bit weak as you move further north and west in the UK.
For positioning, yes, but I understand that there is also a time stamp in the transmissions. I believe that the receiver is able to deduce a reasonably accurate time from the signals.
It can become reasonably respectable. You can configure your software to work with multiple time servers. With the better software, each peering is done with a digital phase locked loop and the short term jitter between you and the time server can be averaged out. This is then accounted for as an offset and offsets between peerings compared to produce a fairly accurate time after a few hours of operation.
No it isn't the same as having your own caesium time standard equipment, but you can generally get to few tens of mS accuracy, which is generally OK for purposes apart from precision science.
Even if it's "bounced" off a satellite, the geostationary are about 22,500 miles high, giving a round trip if 50,000 miles.
at propagation speed, around 186,000 miles per second, that only puts in about a quarter of a second of the delay, so much more of it is due to processing
Probably that, although I suppose it could be set up to subtract an hour when sold in the UK market and still use DCF77.
Even so, the methods of coding the time signal used by MSF Rugby and DCF77 Mainflingen are fairly similar, so the basic electronics will probably be switchable to either according to market. The radio part is probably tuned differently - 77.5kHz vs. MSF's 60kHz
At Northern Europe latitudes (assuming uplink and reception) about
600mS is usually considered to be the round trip time allowing for various modulation, demodulation and other comms equipment.
The additional delays are as a result of the MPEG1 layer 2 compression and the various forms of error correction and other processing employed in the transmission path.
Play around with the clock buttons a bit - I did earlier today (accidentally!) and found one display of -1. When I set that to zero the clock showed CET. I guess they are ready set for the UK market at -1 before despatch.
I used to have a clock that I made that used this Droitwich R4 transmitter when it was on 200kHz as a frequency reference. It worked extremely well because the carrier was a national standard with a caesium (or maybe it was rubidium) reference. Then they shifted to
198kHz. The clock was discrete logic based and so not particularly easy to modify to work at the new frequency,
"Bob Eager" wrote in news: snipped-for-privacy@rikki.tavi.co.uk:
Never thought of that, you'd have to be at the right place on the equator.
However, given the diameter of the earth at roughly 4000 miles, and allowing tangential sight (to give me a right angle triangle, it's 50 years since school) it couldn't be as much a 27000, which wouldn't add much,
My point, which my brain wishes it hadn't tried to show off with, was that all the processing causes most of the lag - now I need a bevvy and a lie down
I'll give that a try, but I fiddled around a bit and didn't see an obvious setting. (There's a +12 to -12 setting for an alternative time zone, but that doesn't seem to apply to the main display.)
What makes me think that it's a "wrong manual" thing is that I also can't seem to get a 2400 hour display -- just an am/pm -- even though the manual uses an illustrative time of 16:20. I'll play around a bit more...
(So far, though, I'm quite tickled with it: it displays indoor and outdoor temperatures, with the outdoor temperature through a radio sender. I'm a happy little gadget man.....)
There's certainly something going on with the factory settings. It has a C/F display switch, but so far I've not found a way to show a 24-hour clock, just am/pm (even though the manual illustrates with a time of
The GPS receiver has to find the distance from each of four satellites to get a fix on your position. It does this by measuring the time taken for the signal to travel from the satellite to the receiver. In order to do this the receiver *must* know the exact time with nanosecond accuracy.
The satellites have atomic clocks and send a message to the GPS receiver which includes the exact time it was sent. By a very ingeneous method the receiver examines the data from at least four satellites and calculates its position and the exact time.
If you want more details:
formatting link
So the bottom line is that the GPS receiver gives the time with atomic clock accuracy.
Err, I thought the earth was about 8000 miles diameter?
My mathematics is a bit on the rusty side now, but Australia is about
12,000 miles around the surface, so mother earth is about 24000 circumference.
As circumference = PI x diameter then circumference / PI = diameter, which is 24000 / 3.14, approx 8000.
Professor PoP
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