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Earthquakes, Tsunamis And Gps


Neo_Geo
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Brian Williams on NBC News Tonight reported that the earthquake that caused the tsunami in the Indian Ocean actually affected the Earth's rotation - slowing it down "a fraction of a second". While this sounds newsworthy to a layman, in terms of GPS, which deals with nanoseconds, it sounds to me like it could have impacted the accuracy of the entire system as a whole!

 

I'm not talking about land shifting, plate techtonics, or any localized errors due to movement of the earth's crust. I'm talking about systemic errors due to the shift in the Earth's rotation caused by the jolt! The GPS system relies on the predicted rotation of the Earth, and if the rotation is interrupted for "a fraction of a second", I would think that the whole system would've been a little bit off.

 

I have not had occasion to use my GPSr lately, so I have no idea. Has anyone noticed any anomalies in accuracy - specifically Saturday night or Sunday morning?

Edited by Neo_Geo
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Okay, I just took my 60C outside for a few minutes. With an EPE of 11 feet, I marked a new waypoint at my front doorstep in Annandale, Virginia. The new waypoint is offset from a pre-existing waypoint of the same location by 46 feet bearing 134° (SW). :D

 

Anyone else have any similar or different results?

Edited by Neo_Geo
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Ok - du you really think the US invest billions of $ in a system which can be seriously disturbed by any earthquake? GPS is a military system and it's a little bit too expensive to break that simple. There're several bases around the world to ensure the system works properly

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Yes, I could see the possibility of momentary disruption of accuracy, but the system automatically compensates for such timing discrepancies. The satellite signals are received by ground stations at precisely known locations allowing any errors to be detected and corrections to be sent back to the satellites.

 

Of course coordinates of locations near the actual site of the quake may have changed slightly due to relative motions of the tectonic plates.

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Ok - du you really think the US invest billions of $ in a system which can be seriously disturbed by any earthquake? GPS is a military system and it's a little bit too expensive to break that simple. There're several bases around the world to ensure the system works properly

This wasn't just "any earthquake" - this was a magnitude 9.0. I doubt that designers of GPS back in the 70s knew everything there is to know about seismic science back then and 9.0 magnitude quakes don't happen every day!!

 

Anyway, I took my GPSr to work with me this morning - turned it on and set it outside 5 minutes before I left. My tracks were no different from previous tracks before the quake. Guess I had bad satellite geometry last night.

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Whoa!!! Wait just a minute there, Skippy! :D

 

You don't know thing one about me or my feelings for (or lack of feelings for) the victims of the disaster! It just so happens that I have been watching the Web and television with great interest since the beginning, and with each new revision of the death toll, I become more shocked horrified! My heart truly goes out to the families of the victims and I can't imagine the suffering the surviviors are facing! Their loved ones washing over into the next town, and being buried before they even know for sure if they are alive or dead... Battling the stench of death to hastily bury loved ones and strangers in mass graves in an effort to combat disease (which will no doubt break out in the next few days). The fact that the dead are probably the LUCKY ones...

 

I have been thinking about all that every waking moment, dude! Now with that said, tell me... How is all THAT relevant to the "GPS Units and Software" forum? :D

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Whoa!!! Wait just a minute there, Skippy! :laughing:

 

You don't know thing one about me or my feelings for (or lack of feelings for) the victims of the disaster! It just so happens that I have been watching the Web and television with great interest since the beginning, and with each new revision of the death toll, I become more shocked horrified! My heart truly goes out to the families of the victims and I can't imagine the suffering the surviviors are facing! Their loved ones washing over into the next town, and being buried before they even know for sure if they are alive or dead... Battling the stench of death to hastily bury loved ones and strangers in mass graves in an effort to combat disease (which will no doubt break out in the next few days). The fact that the dead are probably the LUCKY ones...

 

I have been thinking about all that every waking moment, dude! Now with that said, tell me... How is all THAT relevant to the "GPS Units and Software" forum? :D

Hmmm....so where do Tsunamis belong in this forum? To quote yourself...

 

I may not know where I am, but I know where I've been and I know where I'm going!

 

Do you now?

 

I just thought that mentioning the effect of Tsunamis and massive earthquakes on GPS at this time was a bit below the belt....but that is my opinion obviously not yours. I would stick to a map anyway as they are probably not effected. Still....have a happy New Year and happy cache hunting. :D

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so where do Tsunamis belong in this forum? To quote yourself...

If you read my original post in this thread, you might have a clue and would NOT have needlessly asked this question. For your benefit, I will repost the relevent part here again:

 

the earthquake that caused the tsunami in the Indian Ocean actually affected the Earth's rotation - slowing it down "a fraction of a second".

bla-blah-blah

in terms of GPS, which deals with nanoseconds, it sounds to me like it could have impacted the accuracy of the entire system as a whole!

 

Get it now???

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NeoGeo brought up an interesting point. Neo is curious, as are many of us, about this sudden shift in planet rotation affecting our GPS accuracy. Garmin Guy jumped the gun here with his accusation-which is tactless to assume lack of compassion on someone else. With these kinds of responses, we will soon be prefacing every post with our compassion quotient before we can ask an innocent question.

Garmin Guy-there are discussion boards and chat rooms out there where the participients are discussing this human tragedy. Find one, please.

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I think your comments are a bit tactless....I mean there maybe 100,000 dead in the Asian region and all you are worried about is your GPS....missing a heartbeat! come on have some commonsense before you post. :D who really cares if you take a turning too early!

Whoaaa! :D

 

You seem to be missing the use of this discussion forum. It concerns the topic of gps's and geocaching. I don't think that the original poster of this topic was trying to be cold hearted. He/She had a valid concern and this is the place to voice this type of concern. You may want to find a current events forum if you want to hear peoples feeling about this devastation.

 

Bring out the firehoses! :laughing:

 

PEACHESandDIESEL

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correct me if i'm wrong!!!!!

 

I don't recall setting my gps clock!!

 

So where does this info come from ...does it come from the sats.

If so...can't they adjust the time there and fix the entire issue.

 

 

oh...and my gps was right on the money for my past 6 caches!!!!

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correct me if i'm wrong!!!!!

 

I don't recall setting my gps clock!!

 

So where does this info come from ...does it come from the sats.

If so...can't they adjust the time there and fix the entire issue.

 

 

oh...and my gps was right on the money for my past 6 caches!!!!

You are right....the time is received from the SAT!

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From what I am hearing, the affect to the overall earth was a slight change in the 'wobble'. (The earth spins on it's axis but the poles shift as well)

 

Checking for info on the web, I think this guy will have the best answer. I'm sure he'll be interested in the affect.

 

article from May 24th, 2004

"Now, however, Geoff Blewitt, University of Nevada research geophysicist, has an explanation for this mysterious geo-wobble.

 

"The theory, which my colleagues and I have proven using GPS observations of the Earth, is that it's likely to be caused by the surface matter being redistributed,..........."

Edited by BlueDeuce
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So where does this info come from ...does it come from the sats.

If so...can't they adjust the time there and fix the entire issue.

Your right you don't set the time on your GPSr, that is one of the things that is computed and initially programmed the first time you start it up (you know the time it took up to 5 minutes to get a fix) and when there is a cold start. Most GSPr have a clock to keep track of time between starts and used that time to reseed the solution of the GPS equations. Time is one of the variables that is solved in the GPS equations, that is why you need 3 satellites to get a 2-D fix and 4 for a 3-D fix (location plus time with time required for each).

 

Also, yes the earth’s rotation slowed down slightly about 3 milliseconds is what I recall as the current estimate. The shape of the geoid also changed slightly and the poles moved, I believe I heard that they move a little over an inch.

 

Also the ground stations recomputed the orbital information I believe daily and upload those corrections to the satellites, this is part of the block of information that the satellites transmit individually.

 

Finally, an earlier comment about having gone outside and check a previous way point and it appeared to move. Yes I know that later it was rechecked and it back as the writer thought it should be. They even commented on the geometry of the satellites. Just a point, the error displayed on your GSPr is 1 sigma. If the first fix was done with good geometry and had an error of 9 feet and the second with bad geometry and had an error of 20 feet; this does not mean that the actually location is in a circle that is 29 feet. The actual shape if the possible area is most likely very complex and is based on both sets of satellite geometries but it will generally be bounded by a sphere with the 1 sigma radius. Also the 1 sigma is at about 70% probability (assuming that my defective memory is corrected).

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I too heard about the possible effects on earth's orientation, so I took a look at the IERS daily bulletins to see if there was any noticable kink. It looks like the normal daily variation in the length of day is a few tenths of milliseconds, and there doesn't seem to be any major blip in the data yet. They say it takes about 18 hours to process the data so it should have shown up in the daily datasets by now, I would think. The daily time variation is part of the normal GPS processing.

 

The data also contains measurements of the orientation of the earth's axis. I can't see anything other than the normal daily variation of about a milliarcsecond. If uncorrected, that would amount to a surface positional error of about 3 cm, and that's the earth's normal daily fidget.

 

Other factors such as atmospheric and broadcast ephemeris errors are several orders of magnitude greater.

 

I'm not going to worry about my handheld GPS being affected.

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my understanding is, as mentioned above, is that the errors picked up due to instability of the earths orbit would be small in relation to other ionospheric interferences. eventually these errors would be eliminated once the earths rotation stablized by reprogramming the system parameters re:orbitial info. im unsure as to the longterm effect, i would think that we are essentially behind rotationally due to the quake by milliseconds and correction isnt possible or even necessary.

 

the other types of error that could be introduced would be actual physicial movements of the earth. here there are 3 options:

1)actual land mass movement, generally slips/thrusts

2)alteration of the earths shape

3)mass wasting of the contential landmass

 

ive heard numbers of as much as 40m horizontal movements on some south pacific islands, this type of error would only affect mapping and other documented land positions, the gps would infact give the correct 'new' location.

the mass wasting of the landmass has been documented, and depending on quanties could introduce errors specifically in elevation and possibly with the estimate position errors function.

i have not heard any statements as to the post event shape of the earth, however, i would think that the amount of change in shape would intorduce less errors than those picked by the geoid representation of the earth.

 

since this magnitude event has only been recorded 3 or 4 times we are in unknown territory and most scientific statements are educated guesses based on what we know.

 

one thing i havent heard yet is the effect on sea level, anyone?

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There is an article that I found that talks about the magnitiude of the wobble, and it sets it at about 1 inch. Hardly anything all that signifcant a shift.

 

But the issue that is probably confounding you is the GPSr is just recieving data from satellites, not sending any, and not calculating any base distances from anywhere. They just send datastreams of satellite ID and the time, accurate to 10^-6 or so.

 

The GPSr is regularily updating an almanac that lists the space coordinates of all the satellites, and preforms a calculation given the locations of the satellites, and the change in time of the recieved signal allows the GPSr to plot its location.

 

This is a real basic explaination of how GPS works.

 

Because of this, there would be no change in the accuracy of the GPS network. The satellites were probably completely unaffected by the earthquake, if for know other reason than the are in space. Think about it.

 

Rob

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Okay, I just took my 60C outside for a few minutes. With an EPE of 11 feet, I marked a new waypoint at my front doorstep in Annandale, Virginia. The new waypoint is offset from a pre-existing waypoint of the same location by 46 feet bearing 134° (SW). :D

 

Anyone else have any similar or different results?

This has nothing to say about the accuracy of the unit after the earthquake. Maybe the accuracy was off when you first took the point.

 

I don't know what the EPE is reading and what confidance level it refers to, but that can be unrelated to your problem. How was the wander than the unit showed? In other words how much did the location move about? That's a clearer understanding of error, but not related to distance without careful conversion.

 

You don't describe what the terrain was outside your house, but just standing near it can cause for multipath error to occur, where the signal bounces off the house before it is detected by the reciever.

 

Rob

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...

I don't know what the EPE is reading and what confidance level it refers to, but that can be unrelated to your problem. How was the wander than the unit showed? In other words how much did the location move about? That's a clearer understanding of error, but not related to distance without careful conversion.

...

the epe is the estimated error due to mathematicial calculations. it is not related errors picked up via signal transfer to the gps from the sats or other 'external' influences.

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Ok - du you really think the US invest billions of $ in a system which can be seriously disturbed by any earthquake? GPS is a military system and it's a little bit too expensive to break that simple. There're several bases around the world to ensure the system works properly

As a matter of fact, this isn't too far fetched when dealing with contractors building things at the lowest possible price. Corners get cut. Look at the first computers the government used... they were that limited. Yes we've learned a lot since then, but don't discount the fact that there is a distinct possibility that the oldest satellites that are up may have this issue.

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:D:D:D:D:huh::o:D:huh::o

OK, I have an easy fix for the first problem, but it's the second that I am afraid of.

 

1st fix: If every cache owner goes to each of his/her cache sites, and moves the cache 0.0000000000023176" to the left, everything will be back to normal as far as geocaching goes.

 

Now, it's the second problem that is scary.

Have you noticed that it is slightly warmer today than normal. The earth is off its axis. If you don't believe me, watch the Twilight Zone episode on New Years Day. Everyone will be sweating soon. Save some water, people will be killing for it.

:P:DB);):o:huh::)B):DB)

 

 

 

OK, don't flame me. Just a little humor. Yes, I feel very bad for those poor people effected. It's just some of the posts in this thread that set me off.

 

:lol:

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...

I don't know what the EPE is reading and what confidance level it refers to, but that can be unrelated to your problem. How was the wander than the unit showed? In other words how much did the location move about? That's a clearer understanding of error, but not related to distance without careful conversion.

...

the epe is the estimated error due to mathematicial calculations. it is not related errors picked up via signal transfer to the gps from the sats or other 'external' influences.

I would love to understand how Garmin calculates EPE, as PDOP is what should be calculated. This article is the best explaination of PDOP and GPS I have ever seen. PDOP is dimensionless, how EPE comes up with a dimension I don't know.

 

http://www.pobonline.com/CDA/ArticleInform...8,72407,00.html

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I would love to understand how Garmin calculates EPE, as PDOP is what should be calculated. This article is the best explaination of PDOP and GPS I have ever seen. PDOP is dimensionless, how EPE comes up with a dimension I don't know.

 

http://www.pobonline.com/CDA/ArticleInform...8,72407,00.html

I answer my own question by querying Google:

 

http://gpsinformation.net/main/epenew.txt

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