> a more popular idea seem to be to piggy back onto GNSS signals

Don’t all GNSS systems provide a highly accurate time signal (maybe needed for computing the distance to satellite)? I know GPS does at least.

Yes, the GPS signal in a way is primarily a very precise time signal :) It’s a TDOA system!

I was referring to piggying back an emergency signal onto that!

Yes, it is needed to compute a location. More accurate time = more accurate location.

As mentioned somewhere else, these time signals also carry information about DST and adapt for leap seconds, both of which GPS doesn't do as far as I know. I remember that earlier versions of Android always set the phone clock to GPS which was then off by a number of seconds.

GPS navigation messages do include information about leap seconds and other stuff necessary to convert between GPS time and UTC. See GPS Interface Specification IS-GPS-200, 30.3.3.6.2 - UTC and GPS Time

> These days though, a more popular idea seem to be to piggy back onto GNSS signals, which are also broadly available (no idea how well they compare in practice to low frequency time beacons), and for which importantly many mobile devices already have a receiver built-in

GPS chips are very very power intensive as the compute power involved to decode a signal out of something that would normally be way below the noise threshold is still very high, despite some two decades worth of optimization. And they're pretty useless outside of direct line of sight towards the sky.

> In fact, I can't imagine many events other than weather (which is usually localized) that warrant such a low-entropy signal on a nationwide basis, and everything I can imagine has very limited actionability. ("Asteroid inbound, prepare to hide in the basement for a few thousand years"?)

It's a trigger signal for a low-power receiver that can then turn on a higher-bandwidth (and thus, higher power) device to detect what is actually going on. Say you're a country at war like Ukraine - have the "emergency bit" on nationwide during Russian air raids so that receivers can listen for area-specific signals and blare horns if affected.

No matter what, we have to prepare for war, and that includes having robust technology that is hard to take out on the sender side (which cellphone stations aren't, they're easy targets in a cyber war!) and, most importantly, can be stored in everyone's garden shed and live on a single battery for years.

>Say you're a country at war like Ukraine - have the "emergency bit" on nationwide during Russian air raids so that receivers can listen for area-specific signals and blare horns if affected.

And what would prevent Russia from continuously broadcasting this emergency bit to cause chaos?

> And what would prevent Russia from continuously broadcasting this emergency bit to cause chaos?

So what, all that would do in my scenario is drain the batteries of the civilian populations' alarms a bit faster as they'd keep their more power-intensive CPU awake.

DCF77 is a natural for this because it already includes a phase-modulated signal (as well as the 1bps amplitude-modulation). At the moment it's a 512-bit pseudo random sequence, but could obviously carry a data stream instead.