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GNSS and clock comparisons measure slightly different things and are complementary. GNSS gives the position of the receiver in space relative to some defined co
by fsh 6y ago
GNSS and clock comparisons measure slightly different things and are complementary. GNSS gives the position of the receiver in space relative to some defined coordinate system. This allows accurate mapping of the earth's surface and measurements of continental drift. Clock comparisons measure (via the gravitational red shift) the difference in gravitational potential between the locations of the clocks. This is affected not only by the height difference, but also by changes in the mass distribution within the earth. Transportable optical clocks would allow real-time mapping of the earth's gravitational equipotential surface (geoid). Currently, the geoid has to be determined by painstakingly levelling accross the continents point by point. This can be surprisingly accurate (cm-level) but is obviously much too slow to observe fast variations, for example due to earthquakes.
- geenew 6y agoGNSS and clock comparisons measure different things, one is a based on radio wave travel time between a receiver and the satellites in the constellation, the other is a physical measurement. That's what I was referring to wrt independence: a gravity measurement would avoid any potential error source in the whole chain of the GNSS system (which is quite lengthy). The geoid has been partially satellite-derived for some time, see the GRAIL, GRACE, and GRACE-FO missions. The US is also running a nationwide airborne gravity program at the moment, too, which an impressive undertaking and should be done soon-ish. Comparing a measurement of actual local gravity against the geoid-stated gravity should be enough to give the elevation relative to the geoid, which is the same output that you get from GNSS. That's my intuition, anyway.