BBC Greenwich Time Signal inaccurate

50Twuncle

Has anyone else noticed that the GTS broadcast by BBC (the clock) is always 10 seconds slow (compared to a radio controlled (atomic) clock ?
This is supposedly due to the bounce time for a signal on Sky - but I thought that radio signals travelled at almost the speed of light, so should take milliseconds to travel this distance ?
Whats going on ?

hans_2

The encoding and decoding of the digital signal causes a delay, of usually between 2 and 8 seconds. In the case of satellite broadcasting, the travel time of the signal to and from the satellite adds about another 0.25 seconds.

https://en.wikipedia.org/wiki/Greenwich_Time_Signal

EssexExile

Processing digital signals takes time so digital broadcasts are always a little behind real time. I have two TVs downstairs & if they're both on the same channel they're about a second apart, I don't know how far they are both behind real time.
From memory it takes about a quarter of a second for a signal to get from a geo-stationary satellite to earth. Or is that there & back? I'll let someone else look it up.

Edit: Thanks Hans!

50Twuncle

So - is terrestrial TV time signal correct ?

forgotmyname

Light travels at the speed of light, not broadcast signals.

Some TV's can induce a lag also.

fenlander_uk

If you line up an FM radio, a DAB radio, an internet radio feed, a Freeview TV and a cable TV (and presumably a satellite TV - I haven't got one of those) you can enjoy a prolonged unsynchronised symphony of time signals. I always assume it's the FM signal that is closest to being correct. Don't throw away that old transistor radio yet, folks!

PasturesNew

I never know/care what the time is. My PC clock's never right, my car clock is an hour out at the moment (it's right half the year). My kitchen clock stopped at 3.45pm about 6 months ago (need a ladder to get up there to change the battery, so never bother). I've two small alarm clocks in the bedroom - they don't resemble real time at all, they're both different and hours out.

I'd not be bothered by 10 seconds.

I bought a radio controlled clock from a car boot - best thing ever. One clock that actually tells the right time (most of the time). I'd not buy a clock in the future that's not radio controlled to be honest... it's too hard working out if a clock's right or not.

unforeseen

forgotmyname wrote: »

Light travels at the speed of light, not broadcast signals.

Nope broadcast signals also travel at the speed of light.

Light and broadcast signals are all RF. It is just that light is a lot higher in the frequency band than broadcast signals.

Undervalued

You don't of course get this problem with good old analogue radio!

Generally progress has a snag somewhere!

50Twuncle

I had assumed that the delay was put in on purpose to stop us seeing live events if something goes wrong - and someone dies ?

alanwsg

The most accurate clock available to most people is a locked-on GPS receiver.
Check your time signals against that.