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I don't think the GPS approach can work with ground stations - what GPS has going for it is line-of-sight to the satellites. Multipath would be a hell of an issue in an urban environment.

That said, here's a paper describing various positioning methods - http://emits.esa.int/emits-doc/1-5200-RD25-Signal-processing...

EOTD looks rather interesting - https://en.wikipedia.org/wiki/E-OTD

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WAAS is a combination of GPS and ground stations. It is accurate to about 6 feet. Widely used in aviation.

The ground stations don't transmit to your device. WAAS still needs the satellite signals to work and so won't help indoor reception problems.

Correct. The ground reference stations multiplex their data to the WAAS satellite, which then pushes the data on WAAS-specific channels (hence, why your GPS receiver specifically needs to be built to support WAAS)

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