Hacker News Reader

Comment by jevinskie | Hacker News Reader

Does this affect the orbit of satellites? Do satellite location services like GPS need recalibration?

It will not greatly affect the orbit of any satellite, because the Earth's center of gravity can't be shifted by any action internal to the Earth itself by conservation of momentum, excepting somehow raising some part of Earth above the relevant orbits which no earthquake is going to do. Otherwise to change the trajectory of Earth's center of gravity requires an external force of some sort. Some small effect from a different mass distribution will be caused and it will have to be taken into effect, but when you care about these effects as far as I know you have to measure them empirically anyhow, because if you care at that level of detail it turns out there's a lot of noise as the Earth warms and cools and has various flows within it and as water moves hither and yon and so on. So while it will have some effect it is merely one of very many such things.

Technically speaking, it won't require any recalibration either, because when the satellites tell you that you are 2.4 meters away from where you were yesterday when standing in the "same location", they're right. (Assuming this number is reliable.) GPS doesn't really tell you where you are "on the surface", they tell you where you are within the sphere they encompass and from there we map that back to the surface location with other knowledge our systems have. If Madagascar took it upon itself to go dock with Australia tomorrow, the GPS satellite system itself would not need to be updated. All the things that use GPS and have some reason to expect Madagascar to be in a certain location would need to be updated.

> All the things that use GPS and have some reason to expect Madagascar to be in a certain location would need to be updated.

In this case, various street turn-by-turn GPS based systems (for locations in JP) would now be off by 2.4m (assuming the article is accurate). Likewise the ground coordinates for runways would be shifted slightly (if I'm following this correctly).

> It will not greatly affect the orbit of any satellite, because the Earth's center of gravity can't be shifted ...

Even if the center of gravity is not moved, there is an impact on trajectories. We are used to use a simplification that consider only the gravity center (using this theorem http://en.wikipedia.org/wiki/Shell_theorem) but this theorem doesn't apply if we want to be really precise.

Is it the case that you might explain such a thing with words that look something like "Some small effect from a different mass distribution will be caused and it will have to be taken into effect, but when you care about these effects as far as I know you have to measure them empirically anyhow, because if you care at that level of detail it turns out there's a lot of noise as the Earth warms and cools and has various flows within it and as water moves hither and yon and so on. So while it will have some effect it is merely one of very many such things."? If only I had thought to add such words to my original post. Alas.

Earthquakes definitely alter the Earth's gravity field, because they change the Earth's mass distribution. These changes have been routinely noticed by people who track satellites.

In the early 2000s there was a mission called GRACE to precisely measure the gravity field and its temporal variation. It's important to know it exactly for all sorts of purposes.

Here's some nice results from GRACE:

http://www.nasa.gov/vision/earth/lookingatearth/grace-images...

The second item there shows a change in the acceleration due to gravity that was caused by the Sumatra earthquake of December 2004. It is measured in nano m/s^2.

The movie above that item shows the changes in gravity due (mostly) to sloshing water on a global scale.

Thank you so much for this information! This is a fascinating mission.

I'm not educated on satellites at all myself but wondered the same thing. Surely satellites have beacons or towers around the earth to calibrate itself?

Kind of. Its called WAAS (Wide Area Augmentation System): http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System

uh no.

the problems WAAS solves are not big ones for spacecraft, and the accuracy it provides isn't particularly necessary for most of them.

lots of them don't even have GPS, and use other means (star tracking) of positioning themselves.

Guess I mis-read the parent's question last night. You're right.

I am not even sure if civilian GPS's posses the ability to be accurate to ten inches.

Here's a surprising fact. If you allow after-the-fact reprocessing of the GPS and significantly increased modeling technology, you can get millimeter accuracy from civilian GPS.

For more on this global network of re-analyzed GPS, see:

http://sideshow.jpl.nasa.gov/mbh/series.html

Here's a particular time series, near Claremont, CA, a few tens of miles east of LA proper:

http://sideshow.jpl.nasa.gov/mbh/all/plots/CLAR.jpg

These plots are lat/lon/height (in centimeter units) versus time. In the height plot, you can see a several-cm drift which has annual/seasonal features. This is due to subsidence due to pumping of ground water in the summer. You can also see some green lines which I believe are large earthquakes.

Besides airborne and (coming soon) satellite radar interferometry, reprocessed GPS is one of the main ways we have to observe seismic displacement fields.

Surely its accurate to 2.4 meteres though?

According to http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C... normal GPS systems will be accurate to within 2.5 m 95% of the time. Better accuracy can be achieved using differential GPS systems and things like WAAS, but they rely on ground-based systems and thus might not be a good way to measure how much the ground shifted.

My GIS certificate is ancient at this point and I have never managed to work in the field I trained for, but my recollection is that they are accurate to within only about 10 meters, whether personal or professional, unless you spend quite a lot of time gathering data from multiple satellites (on the order of several hours, depending upon the level of accuracy you are seeking). EDIT: In other words, "I just turned mine on" shouldn't get any reading on any system that is more accurate than about 10 meters. It takes time to get accurate readings with GPS.

Someone with a more recent credential/professional experience want to confirm or deny that recollection -- or even update it with more recent facts? (For reference to my question of "more recent", IIRC, I got my certificate in 2002.)

Turned on my GPS. Works fine.

Reply on news.ycombinator.com