[time-nuts] Time syncing WiFi routers using FM radio

Bob Camp kb8tq at n1k.org
Wed Nov 11 18:13:25 EST 2015


Hi

There are a number of issues with what they are proposing:

1) Inside a building you have reflections. Those reflections create dead spots. A “no antenna” FM radio
is not very good at sorting that stuff out. If you get the station and I don’t … that’s a problem.

2) If you have radios from different manufacturers, the sensitivity will vary quite a bit. What I can hear easily
on my Sony is not even out of the noise on my USB radio stick. 

3) If the station is like most, the RDSS tags are .. ummm …. errr .. a bit random. I sync to this, you sync to 
that and we don’t have the same time. 

4) Even if we both are on the same station, you have wonderful things like multi path and IF delay that are going to
skew the time outputs. If tenth second timing is ok, this isn’t a problem. Most TDMA systems move a lot faster 
than that. 

5) At least around here, if I start at the bottom of the band (as they propose), I’m picking up low power FM stations 
run by college dorms and the like. They probably are the least likely of any stations out there to be reliable sources
of timing. 

That’s just with a few minutes thought ….

For time sync, broadcast signals are a pretty well studied topic. Sync signals from TV stations are a much better 
“target” if you want to do local timing sync. You still have all the multi path issues, but working out what’s what is 
a bit easier. 

Bob

> On Nov 11, 2015, at 3:16 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Mark,
> What's the inherent problem with the idea?  In the early days of radio, they used commercial radio transmissions as an encryption mechanism, so there is some history of using commercial radio as a reference source.  Is it simply a matter of different propagation delays and multi-path issues at different locations?
> 
> Bob
> 
>      From: Mark Sims <holrum at hotmail.com>
> To: "time-nuts at febo.com" <time-nuts at febo.com> 
> Sent: Wednesday, November 11, 2015 10:59 AM
> Subject: [time-nuts] Time syncing WiFi routers using FM radio
> 
> Something tells me these guys haven't a clue...
> http://www.theregister.co.uk/2015/11/11/boffins_teach_routers_to_tune_in_and_dance/                         
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