Comment by mapt

Can we agree that with close to 10,000 Starlink birds in the air and counting, that whatever succeeds GPS needs to be a much larger constellation than the 30-unit GPS constellation in MEO, flying much lower?

It is absolutely insane to me that Japan would be trying to economize using Molniya orbits or geosynchronous orbits in 2025.

Some BOTE math:

There are ~40k square degrees in a sphere. If I'm in a dead-end alley in an urban canyon and I have access to a 40 degree by 100 degree viewshed of sky (~4000 square degrees), that's 10% of the sky. Surface area of a 900km sphere is ~10 million square kilometers, 10% of that is 1 million square kilometers, Earth's radius is ~6400km, the orbital radius at 1000km is ~7400km, surface area of a 7400km radius sphere is ~700 million square kilometers, 1 million / 700 million ~= 1/700. Fly 3500 birds and you'll on average see five of them at a time in a 40 by 100 degree viewshed. But you'll have such a large angular parallax between their positions, and so little of the ionosphere in the way, that you get extremely high accuracy.

...

Many countries have their own SBAS to correct for ionospheric ephemera using a combination of regional ground stations and low flying satellites - The US calls their WAAS, Europe uses EGNOS, Japan uses MSAS, Russia and China have versions for their respective constellations, etc.

QZSS is a related but distinct idea that poses a little more like a localized addition to the GPS constellation.