Earthquake: effect on GPS

The Japan earthquake has reportedly moved the whole country ~2 meters from where it was before. I also saw a report somewhere that it had affected the earth's rotation very slightly; the figure I saw quoted was that there had been a time change of 1 microsecond.

I don't know if either of these figures is accurate; quite possibly not. But they raise an interesting point, which maybe an expert on the GPS system could answer.

The essence of GPS is to keep extremely accurate time, as light/radio waves travel very fast (186,000 miles per second). In 1 microsecond, light will travel approx 1/5 mile, or ~1,000 feet. How will they reset the GPS system to correct for this change? Will it be automatic, or does someone in the US Defense Dept. or somewhere have to reset the timing on the GPS satellites? Has anyone noticed inaccuracies in their GPS since the earthquake?

I have Google Maps on my cellphone, and I was interested to see it was about 20 feet off when I checked it yesterday.

Japan moved 13 feet closer to the US and lost 2 feet in elevation (making the tsunami worse). Earth's rotation speed up 1.6 us and more importantly shifted axis by 6.5 inches.

The DoD will probably update/orient the satellites once the Earth settles down, it's still wobbling.

http://www.ioc-sealevelmonitoring.org/station.php

click last 7 days.

USAF Space Command will have to update the "info" to update the satellites. Also, INS systems (LNS) will have to be updated as well, but that's controlled by the Navy IIRC. (The "Back-up" to GPS).

mellowman said:

Japan moved 13 feet closer to the US and lost 2 feet in elevation (making the tsunami worse). Earth's rotation speed up 1.6 us and more importantly shifted axis by 6.5 inches.

The DoD will probably update/orient the satellites once the Earth settles down, it's still wobbling.

http://www.ioc-sealevelmonitoring.org/station.php


click last 7 days.

Click to expand...

Wow those are some scary numbers (rushes off to check Google maps on his 'phone).

I'm not sure it will have much impact on GPS at all. GPS operates on triangulation of signals between earth stations and orbiting satellites. The computers will compensate easily for the differences in position of earth stations..the same way they do when they compensate for the movement of a GPS receiver in a car or boat.

I think what they meant by GPS being off, is physically in Japan. All the waypoints, road, etc are set by co-ordinates, which have now all shifted. SO all those Caches will not be where the original coordinates say they should be ( In Japan).

For GPS systems that include maps, I think there is no way to automatically correct them. They will need to be updated with new maps. GPS coordinates will not change due to the earthquake, but the way GPS coordinates map to the land will.

Well, on the bright side it means the next time I visit Japan it'll be a shorter plane ride. Japan is a wonderful country with lots of good things to eat and an incredibly different culture. Hopefully they recover quickly from this string of bad events.

DM51 said:

I have Google Maps on my cellphone, and I was interested to see it was about 20 feet off when I checked it yesterday.

Click to expand...

An extra layer of complexity is that rather than having dedicated GPS receivers (devices that would take up more space and power), cells phones can rely on a kind of secondary GPS. The software in the phone triangulates angle and distance to 2+ cell towers, which themselves are triangulated to 2+ satellites. Errors due to changes in any measurement would relay downstream.

ElectronGuru said:

An extra layer of complexity is that rather than having dedicated GPS receivers (devices that would take up more space and power), cells phones can rely on a kind of secondary GPS. The software in the phone triangulates angle and distance to 2+ cell towers, which themselves are triangulated to 2+ satellites. Errors due to changes in any measurement would relay downstream.

Click to expand...

Add to the fact that the code (that the satellites receive) get's scrambled x number of times a day, to prevent it from falling into "bad guys" hands, and if you don't know when that is (which you won't...), you can't rely on GPS 100% of the time anyways.