There is a network of hundreds of (static) GPS receivers to monitor seismic activity. They are post-processed to identify and remove the ionospheric effects yo

There is a network of hundreds of (static) GPS receivers to monitor seismic activity. They are post-processed to identify and remove the ionospheric effects you mention. They used to (ca. 2000) get ~1mm accuracy in the plane of the Earth, and ~1cm accuracy in the radial direction. The accuracy seems to have increased in the meantime, and it appears to be ~0.1mm in the plane of the Earth. GPS is combined with other sensors like strain meters, etc., into something called Plate Boundary Observatory: http://pbo.unavco.org/instruments/gps http://pbo.unavco.org/instruments/gps