Yes there are other satellites appearing such as the Galileo, but it's not working yet. Also it is becoming increasingly easy to put that lights up into the sky so before long there will be hundreds of thousands of them.
In the meantime, as far as local triangulation is concerned, I have a idea. This could very well actually not work though.
There is one Rover and a number of base stations in a field. The Base stations send out to signals simultaneously, one at 50 kHz (ultrasonic) and the other at 2 GHz (rf).
The first signal is slower than the second, so the second signal arrives first and acts as a kind of stop watch, telling the rover to expect a 50 kHz signal very soon. The time difference between the two signals is proportional to the actual physical distance . Make adjustments for humidity, pressure and temperature and now do a triangulation and we might have an accurate fix to less than 1 cm? Any comments?