I'm scratching my head at that China GPS problem you had.. there are only two reasons I can think of. One I experienced myself, with a 3rd GPS receiver. It didn't work in Beijing. I'm pretty sure the reason was because of reception problems due to lots of high buildings, even though I tested it in probably the widest avenue in the city (if there had been lanes all the way across I guess there was room for at least 8 lanes. That's how wide it was, and I was in the middle).

For this particular receiver I think the problem was simply that it's a very crappy receiver, not sensitive at all. Old too. It has problems in deep valleys also.

The other possibility I can think of would be interference, or even some kind of jamming signal. It sounds odd I know, but other than reception problems as described above I don't know what it could be.