5 ms·
The potential land surveying application of high-accuracy clocks is quite interesting. It would provide a measure of elevation that is (in the vertical scale, a
by geenew 6y ago
The potential land surveying application of high-accuracy clocks is quite interesting. It would provide a measure of elevation that is (in the vertical scale, at least) completely independent of GNSS, at an accuracy comparable to what can be achieved by current field-deployed GNSS receivers, at ~0.02m
I wonder how far that could be stretched. Current permanent ground-station GNSS receivers have vertical accuracies in the mm scale. They achieve that by solving for measurements taken over very long periods (~6 months). Would atomic clock height measurement tools be able to achieve similar accuracies, if the datasets for a single station were made over a similar length of time?
- tgb 6y agoIt sounds like it would more be used for relative elevation differences. Is GNSS the gold-standard for that? I'd have thought you could do better with a laser and a level.
- AnimalMuppet 6y agoI'm not sure that we have levels that are accurate enough for that.
- tgb 6y agoReally? I can't tell what accuracy a good theodolite gets, but this consumer device [1] claims 3/16ths of an inch at 30ft. Obviously that won't scale to 1km distance like they're doing in this, a professional one seems competitive with the quoted ~2cm accuracy. [1] https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/dp/B07DMK1NB8 https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/d...
- WJW 6y agoAbsolute gold standard for use in a laboratory, probably not. But GNSS is definitely the instrument of choice for 99% of surveying and navigational purposes.
- fsh 6y agoGNSS 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.