GPS MEGATHREAD

The Something Awful Forums > Discussion > RSF: The Great Space Space > GPS MEGATHREAD

pumped up for school: Nov 24, 2010

Phanatic posted:

One thing worth mentioning about the satellites is that since everything is based on very precise timing (a precision of 20-30 nanoseconds), for GPS to work at all there need to be two correction factors applied to the clocks.

The first is due to special relativity: Since the satellites are moving so rapidly relative to an earthbound observer, the observer will see a GPS satellite's clock as ticking slower than his, by roughly 7 microseconds per day. The second is due to general relativity: Since the satellites are at a very high altitude, farther away from the Earth's gravity, spacetime is flatter there, and to an observer on the ground a GPS satellite's clock would tick faster than his by about 45 microseconds per day. So without corrections for this, the satellite clocks would run faster than clocks on the ground by 35 microseconds per day, which means about 10 kilometers in error, per day.

The GR correction is built into the reference clocks on board the satellites, they're just designed at a slower rate here on the ground so that once you put them in orbit they tick at the correct frequency. The SR corrections are done at the receiver level, based on which satellites it's using for its calculation.

I didn't think about the clock speeds. Neato. Waiting for the "oops" day when a technician puts the wrong freq. clock on the bird.

I use GPS clocks on the ground for syncing remote instruments. Better than the old method of quartz clocks, and a side-effect of a technology not really known by general public.

# ¿ Feb 24, 2021 21:26

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# ¿ Apr 24, 2024 04:38

pumped up for school: Nov 24, 2010

Warbird posted:

This is more on the GIS side of things, but folks here might be in the know. Is there a reasonable way for your garden variety moron to get a list of existing surveying points in an area? I do a bit of drone based mapping for shits and giggles and I can't justify spending oodles of money for even a cheap whatchamahoozit to make a GCP for my maps. I understand that this work would have already been done for most urban/semi urban areas and figure I can plot down a marker on top of a nail/marker if someone else has done the work.

I'm going to get on the horn with the county's GIS folks but I figured I'd ask here for content's sake if nothing else.

Let me know what you find out. I know when I'm local, I can get control from my county's website. They have a clunky interface but it works.
https://explore-washoe.opendata.arcgis.com/datasets/survey-monuments?geometry=-129.085%2C38.973%2C-109.529%2C41.899
But I run into trouble working, when nothing is local to me and I'm navigating (sometimes very bad and dated) foreign pages trying to get the same info. Last week I found an old Corps monument, a county monument, and one set by the utility district (this is on a hydro dam). The utility gave me their info in no time, but the Corps monument isn't matching up, and it isn't an old datum shift or anything. I'm almost scared to get the county data, and have 3 different and unreconcilable data sets.

# ¿ Apr 27, 2021 17:35

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