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Comment by Havoc | Hacker News Reader

Why don’t cities have ground beacons for this?

Much cheaper than satellites and would be guaranteed to see heavy use

Some places have free RTK networks their citizens can use. Michigan's department of transportation, for instance, runs a CORS network that anyone can request access to. (At least as far as I can tell. The signup form doesn't ask for affiliation or payment.) It's just distributed over the internet, the assumption being that you probably have internet access already.

One of these days I'll figure out how to set up a free NTRIP caster on my Galmon station so it can do double-duty. The trick then is advertising and discovery.

It would be lovely to have, say, a standard wifi SSID or a standard LORA channel that your local corrections network would broadcast on. That way you could have a large number of client devices not each needing their own internet access SIM card or whatever. I wonder if the corrections stream would fit into an FM RDS payload or something.

Trouble is, there's so much money in the L-band corrections services, and so little money in replacing them for free...

Oh, yeah, the cryptocurrency folks have weighed in, there's a thing called "goodnet" which appears to be microtransactions in exchange for NTRIP streams over some medium. I haven't looked further into it.

Do you mean GEODNET?

Oops, indeed I do.

While we're at it, could we also broadcast navigation map updates via that FM stream?

Why over a special radio system? Why not over the internet?

Thanks - that's a lot of good info :)

Sparkfun page suggests one can do some of these adjustments via software too:

https://learn.sparkfun.com/tutorials/what-is-gps-rtk/all

>1 cm accuracy is also possible with a few lower cost receivers (such as the NEO-M8T) by capturing raw streams from the GPS satellites and then post processing the logs with an open source program called RTKLIB.

Post-processing is wild. Lots of detail here: https://rtkexplorer.com/

There are even cheaper raw-data receivers, this one is popular among Galmon stations: https://www.aliexpress.us/item/2251832630341954.html

Why a special wifi? Why not just multicast streaming over the internet?

So you can deploy a bazillion devices without each one needing internet access, which typically has a cost because there's administrative overhead to handling abuse complaints, etc, that come along with full internet access.

If all you got was a one-way stream of data with no internet access (assume your CORS virtual station location is the AP's location, so no need to even pick your own mountpoint) then the abuse potential is basically nil. Just like with FM RDS and stuff, it's just a broadcast that you receive, you can't do anything bad with it. It's not internet, it's just one-way data. Difference being that a lot of microcontrollers now have wifi MAC/PHY built in, whereas FM would need more silicon.

Advertising and discovery: rtk2go.com or similar. Public casters.

GPS beacons would be stupidly expensive, as you'd need tens of thousands of them per city, and each one of them would need a very accurate atomic clock.

It would make far more sense (but still unviable) to go for Eurobalise-style RFID tags embedded in the road surface.

https://en.wikipedia.org/wiki/Eurobalise Cool.

>you'd need tens of thousands of them per city

I was thinking more find a tall building and throw a single one there. Or maybe tallest 3.

As I understand it even a small number of extra satellites with line of sight can improve results a fair bit.

So aiming at low hanging fruit rather than blanket city

>very accurate atomic clock.

Indeed, but I'd think managing that on ground is easier than in space.

What happens when the building sways multiple feet in the wind? We don't normally think about them moving, but for accurate positioning I'd think the additional distance to the beacon could be a problem.

no, you arn't understanding. Those beacons would just be fixed places that know what their position is according to the GPS network.

You're tihnking of someone tkaing a GPS sattilite and putting it somewhere.

That's kinda what RTK does, you have a ground station and transfer correction data for the visible satellites from it to your GNSS receiver. It doesn't transmit a GNSS signal itself but functionally the effect is vaguely comparable to a "satellite on the ground", without interfering on the actual signals.

The newest TV broadcast standard (ATSC 3.0) does include positioning information which is far, far, far harder to jam than GPS signals.

https://www.nab.org/bps/

As with all things ATSC 3, the question becomes that while it is possible... a) is it actually enabled on stations?, b) is it DRM encumbered?, c) is it patent encumbered?

5G has fairly sophisticated positioning support, especially in the latest releases. Base stations generally are in precisely known locations and have good timebases. Urban locations which are challenging for gnss usually have good 5G coverage

https://www.ericsson.com/en/blog/2024/11/5g-advanced-positio...

Nextnav (no affiliation) is trying to do something similar. https://nextnav.com/

Edit: not affiliated with the company

There's no need with hybrid positioning in cellular location chipsets that use cell towers, one or more GNSS constellations, and/or a WiFi SSID GIS database (like Skyhook).

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