Hacker News Reader

Comment by toomuchtodo | Hacker News Reader

It’s a shame there aren’t more time tower sites broadcasting in a distributed manner for redundancy.

Like the 6 independent 24-satellite (plus spare) constellations that constantly orbit Earth and output synchronized beacons accurate to nanoseconds?

How effective is receiving precision time from a GNSS constellation within structures, urban areas, and other challenging SNR envs? Another comment mentions cell towers broadcast this unencrypted and without the need for service, so perhaps the solution is to instead require strong SLAs around that service, with the gov potentially paying for that service level?

https://news.ycombinator.com/item?id=40388421

Yep, reception is a real problem with straight GNSS. WWVB suffered from it too, and cellular (which is locked to GPS) is a big upgrade. I suppose that now the thing to do is wait for the $0.10 cellular clock chip that makes it easy.

My interest is as a bit of a precision time enthusiast and an advocate of public goods. We’re awash in “good time”, GNSS, RF, NTP over IP, cellular, but it still feels like there is a gap to fill to make precision time super cheap and ubiquitous. Getting time over mains power or other previously convenient but potentially low precision sources should be over, but getting precision time should be as easy and reliable as mains power. Thanks for riffing with me!

I feel like once you're concerned about SLAs for precision timekeeping, you need to start worrying about spoofing as well, which is tricky to combat in an inherently unidirectional protocol.

Galileo has an interesting signal authentication service [1], but even that is subject to replay attacks by definition (although you can't skip ahead a receiver's time).

In comparison, networked bidirectional protocols like NTP are almost trivially secure-able, by including a challenge in the request and a cryptographic signature over the response.

[1] https://berthub.eu/articles/posts/galileos-authentication-al...

Noted my car got time from GPS the other day and it got me thinking, why aren't there unencrypted packets on all wifi that distribute NTP time so my oven/microwave/alarm/etc can grab it from any wifi it can see (even if it's not paired to them) - bonus points if APs also hand out local TZ data

Phone networks can already do this. And its true broadcast, you can decode it passively without ever connecting to the network, so no SIM/transmission required.

I have no idea this is actually enabled in networks in practice, but the specification exists.

And in my experience, it's often wrong on some towers. I can't enable the setting on my phone to automatically set the time from the network, because if I do, it gets several minutes out of sync with the real time, which breaks TOTP generation.

Yeah, a large German cell provider had their time off by a couple of minutes some years back, which made the news as it resulted in people's alarms ringing late and them being late to work/school etc.

Hopefully modern phones prioritize GPS or NTP derived time and timezone information over NITZ (which is how networks provide it). At least Android has the capability to do so, but it's ultimately up to device manufacturers, as far as I understand [1].

[1] https://source.android.com/docs/core/connect/time

Could the detection and reporting of this inaccuracy be crowdsourced, either with mobile devices or perhaps SDRs? Fetch tower time, fetch real time over GNSS or NTP, report delta with tower ID and drift to a central server? Perhaps even include a map, like the map of GPS error rates derived from ADS-B data feeds.

I'm almost certain that at least Apple already has such a database on the operator level, i.e. a list of MNCs and the (non-)trustworthiness of NITZ. I vaguely remember seeing some carrier settings keys to that extent somewhere.

Why bother? If you can download data from a central server you have an internet connection, so you can also retrieve time via NTP instead.

Distributed monitoring and centralized reporting would be used to correct anomalous towers providing bad time and can provide independent verification of precision. It is not enough to provide a service; you must also have a system to validate it, and correct when needed.

Then you'd need Internet access to get the data, and if you had that, you could just use NTP.

Unencrypted broadcast NTP timestamps sound like a good idea until your neighbor starts broadcasting false timestamps to prevent your loud smart leaf blower from charging overnight :)

Best practice would be to listen to all the local APs and choose what most of them say

Making one AP look like thousands is trivial.

It's probably about time for an update that can work with more compact receivers.

Governments(Or an industry group, or really anyone) could publish digitally signed timestamps every second with a a key that doesn't change, and you could have a BLE profile for sharing them.

Reply on news.ycombinator.com