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Snafuperman
Post  Post subject: GPS calculations very sophisticated due to Earth Tides  |  Posted: Wed May 25, 2016 1:29 am

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First off, let me be clear about what I mean by "Earth Tides" versus ocean tides: https://en.wikipedia.org/wiki/Tide#Earth_tides

Quote:
The Earth's crust shifts (in/out, east/west, north/south) in response to lunar and solar gravitation, ocean tides, and atmospheric loading. While negligible for most human activities, terrestrial tides' semi-diurnal amplitude can reach about 55 centimetres (22 in) at the equator—15 centimetres (5.9 in) due to the sun—which is important in GPS calibration and VLBI measurements. Precise astronomical angular measurements require knowledge of the Earth's rotation rate and nutation, both of which are influenced by Earth tides. The semi-diurnal M2 Earth tides are nearly in phase with the moon with a lag of about two hours.


In another book I read, Annals of the Former World, it was stated that the continents, riding on tectonic plates, rise and fall up to 1 foot due to Moon and Sun tidal pull.

So, not only do GPS satellites have to take into effect General Relativity time differences due to the gravitational well, but also have to allow for the Earth's continents rising and falling. I'm sure this doesn't get in the way of the normal GPS that we use in our cars but it must play hell with GPS measurements of mountain uplift due to colliding plates and also GPS measurements of plate lateral movements.

Does anyone here work with this stuff? If so, any insights?


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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Wed May 25, 2016 2:23 am

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I guess what I have a problem with is . . . Occasionally I see a documentary on plate tectonics and there will be some Earth Sciences student up the Himalayan mountains measuring the rate of range lifting each year and I wonder how confident they are in the GPS calculations that measurement is based on?


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Lynx_Fox
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 7:00 pm

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Snafuperman wrote:
In another book I read, Annals of the Former World, it was stated that the continents, riding on tectonic plates, rise and fall up to 1 foot due to Moon and Sun tidal pull.

So, not only do GPS satellites have to take into effect General Relativity time differences due to the gravitational well, but also have to allow for the Earth's continents rising and falling. I'm sure this doesn't get in the way of the normal GPS that we use in our cars but it must play hell with GPS measurements of mountain uplift due to colliding plates and also GPS measurements of plate lateral movements.

Does anyone here work with this stuff? If so, any insights?


There's such a vast size and temporal difference in scale between diurnal variations and geological uplifting that I doubt it's much of a problem at all.


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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 7:40 pm

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When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

And that 1 foot per day I mentioned is just an average. Earth tides vary just as ocean tides do, depending on the positions of the Earth, Moon, and Sun (which varies) and their relative distances apart (which also varies).

What is your background? Are you a geologist? A geophysicist?


Last edited by Snafuperman on Fri May 27, 2016 7:47 pm, edited 1 time in total.


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Robittybob1
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 7:45 pm

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Snafuperman wrote:
When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

Logically a rise and a fall takes you back to the original position but uplift is up and stays up.


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iNow
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 7:50 pm
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You fail to account for the magnitude of the shifts. Unless the upward and downward motions are equal in scale it will not be returned to original starting position.

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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 8:00 pm

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iNow wrote:
You fail to account for the magnitude of the shifts. Unless the upward and downward motions are equal in scale it will not be returned to original starting position.

I assume your replying to Rob's post. I am not going to reply to his post since he doesn't understand the problem.


Last edited by Snafuperman on Fri May 27, 2016 8:03 pm, edited 1 time in total.


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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 8:02 pm

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Example: You take a measurement today (somewhere in the Himalayas) and you get an "exact" altitude/height. You go back next year at the exact same day and time, so that the Sun is (pretty much) in the exact same position relative to Earth. Even this you would have to take into account that the Earth year is not exactly 365 days. But the Moon would almost certainly be in a different position than it was the previous year. So the continental rise due to Earth Tide would be different from the previous year, maybe even inches different. Do you see the problem here?

Basically, my OP question is: is this variance accounted for in the GPS software or do the geologists making these measurements have to do the math necessary to "smooth out" the Earth tide so that accurate comparisons can be made?


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iNow
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 8:19 pm
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That probably depends on the actual software being used and how their specific (usually proprietary) algorithm works. Hard to say what is and what is not accounted for mathematically without direct access to their code.

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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 8:24 pm

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Well, I was hoping someone here might be in that sub-field and would know the answer. I've looked, but have never found a good geology-only forum. Hence, I asked here.

I've searched the net.

I would even accept an answer like "Yes, that is taken into account by a complicated math algorithm in the GPS software" by someone in the know, a geophysicist or tech in that sub-field.


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Lynx_Fox
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 9:21 pm

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Snafuperman wrote:
When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

And that 1 foot per day I mentioned is just an average. Earth tides vary just as ocean tides do, depending on the positions of the Earth, Moon, and Sun (which varies) and their relative distances apart (which also varies).

What is your background? Are you a geologist? A geophysicist?



Don't see it as an issue at all. I'm a meteorologist and this sort of calculation, while not trivial, is routinely done for many types of observations-- it's a matter of averaging across the scales you want to analyze--most of the complications are working within the limitations of the instruments to account for their precision, accuracy and systemic differences from their placement and degradation with time and having solid algorithms to filter (or just investigate) anomalous data.


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Snafuperman
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri May 27, 2016 10:07 pm

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Robittybob1 wrote:
Snafuperman wrote:
When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

Logically a rise and a fall takes you back to the original position but uplift is up and stays up.

Rob, looking back, your answer was just as good as the others -- sorry I belittled your answer. You understand the problem just as well as the others.

Thanks for all the responses.


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Robittybob1
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sat May 28, 2016 12:25 am

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Snafuperman wrote:
Robittybob1 wrote:
Snafuperman wrote:
When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

Logically a rise and a fall takes you back to the original position but uplift is up and stays up.

Rob, looking back, your answer was just as good as the others -- sorry I belittled your answer. You understand the problem just as well as the others.

Thanks for all the responses.

It could have written with a few more words but I tried to get the idea across as simple as I could. Thanks for the apology, it is greatfully accepted.


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Lynx_Fox
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sun May 29, 2016 2:18 pm

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Snafuperman wrote:
Robittybob1 wrote:
Snafuperman wrote:
When the continents are rising and falling a foot per day, you don't see a problem with trying to measure geologic upthrust (example Himalayas) when the upthrust may only be 1 or two centimeters per year? Why so?

Logically a rise and a fall takes you back to the original position but uplift is up and stays up.

Rob, looking back, your answer was just as good as the others -- sorry I belittled your answer. You understand the problem just as well as the others.

Thanks for all the responses.


Unfortunately his answer was wrong. The amplitude and period of the diurnal cycle from day to day is NOT going to return to the same level-because astronomical forcing is constantly changing. Far more robust analysis is required to determine averages and trends.

Below is an example from observed Earth Tide:
[EDIT]MODNOTE: Repaced IMG tags with THUMB tags. Click image to enlarge[/EDIT]

Image

(taken from http://www.okgeosurvey1.gov/tide.html)


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Robittybob1
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sun May 29, 2016 5:20 pm

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Lynx_Fox wrote:
Snafuperman wrote:
Robittybob1 wrote:
Logically a rise and a fall takes you back to the original position but uplift is up and stays up.

Rob, looking back, your answer was just as good as the others -- sorry I belittled your answer. You understand the problem just as well as the others.

Thanks for all the responses.


Unfortunately his answer was wrong. The amplitude and period of the diurnal cycle from day to day is NOT going to return to the same level-because astronomical forcing is constantly changing. Far more robust analysis is required to determine averages and trends.

...

I was pretty brief "Logically a rise and a fall takes you back to the original position but uplift is up and stays up", it is not bad but as you point out I need to add something about a long period of time. This is a modification.
"Logically over a long period of time just rises and falls takes you back to the original position but uplift is up so the trendline stays up"


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Pong
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sun Jun 12, 2016 10:22 am
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As always Lynx_Fox's graphic contribution owns the discussion.

Though I thought it sea tides at first glance.
Lynx_Fox wrote:
Far more robust analysis is required to determine averages and trends.

Parable: There's a lake in Japan that became a reservoir, so water level would rise and fall substantially at odd times over the year. Community on the lake wanted a boat dock. Problem: how to keep the boat dock precisely at water level, whatever that might be? Their solution involves sensors, and hydraulics to raise and lower the dock. It's really an impressive work of machinery, and now a minor tourist attraction. Like all docks the ramp is hinged so you can walk down from the shore above, though some reason it was gated maybe due to technical difficulties, so I couldn't marvel up close at this ingenious application of brute-force machinery.

Lynx_Fox, you've been around marinas. You know what my parable's about.

So this "far more robust analysis required"... it's brute force isn't it? Isn't there a more direct way?


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paleoichneum
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Wed Jun 15, 2016 2:44 pm
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Pong wrote:
As always Lynx_Fox's graphic contribution owns the discussion.

Though I thought it sea tides at first glance.
Lynx_Fox wrote:
Far more robust analysis is required to determine averages and trends.

Parable: There's a lake in Japan that became a reservoir, so water level would rise and fall substantially at odd times over the year. Community on the lake wanted a boat dock. Problem: how to keep the boat dock precisely at water level, whatever that might be? Their solution involves sensors, and hydraulics to raise and lower the dock. It's really an impressive work of machinery, and now a minor tourist attraction. Like all docks the ramp is hinged so you can walk down from the shore above, though some reason it was gated maybe due to technical difficulties, so I couldn't marvel up close at this ingenious application of brute-force machinery.

Lynx_Fox, you've been around marinas. You know what my parable's about.

So this "far more robust analysis required"... it's brute force isn't it? Isn't there a more direct way?


Pong, do you have backing published data, or just parables?

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Pong
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Fri Jul 08, 2016 5:51 am
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paleoichneum wrote:
Pong, do you have backing published data, or just parables?

It's not a claim, it's a question.

I illustrated how complicated solutions are sometimes unnecessary.

What do you think? Can you suggest an elegant solution to the problem of compensating for Earth tides?


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Lynx_Fox
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sat Jul 09, 2016 6:49 pm

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Most of the statistical methods involved are the "ELEGANT approach," most of the methods being developed during the 19th century. It shouldn't be considered even remotely hard by anyone science minded.


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Pong
Post  Post subject: Re: GPS calculations very sophisticated due to Earth Tides  |  Posted: Sat Jul 09, 2016 8:24 pm
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Thanks, you clearly understand my question. :)


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