# If super quake slows earth rotation slightly, how does GPS system compensate? **URL:** \ **Category:** Factual Questions\ **Created:** [December 28, 2004, 4:39am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888 "2004-12-28T04:39:50Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 28, 2004, 4:39am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/1 "2004-12-28T04:39:50Z")
Per the story below. Will a slowdown of say, 3 microseconds, mess up the super precise GPS satellite programming calculations? [Quake rattled Earth orbit, changed map of Asia: US geophysicist](http://sg.news.yahoo.com/041228/1/3pim1.html) > [@](#): > > “That earthquake has changed the map,” US Geological Survey expert Ken Hudnut told AFP. “Based on seismic modeling, some of the smaller islands off the southwest coast of Sumatra may have moved to the southwest by about 20 meters. That is a lot of slip.” The northwestern tip of the Indonesian territory of Sumatra may also have shifted to the southwest by around 36 meters (120 feet), Hudnut said. > > In addition, the energy released as the two sides of the undersea fault slipped against each other made the Earth wobble on its axis, Hudnut said. > “We can detect very slight motions of the Earth and I would expect that the Earth wobbled in its orbit when the earthquake occurred due the massive amount of energy exerted and the sudden shift in mass,” Hudnut said. > > Another USGS research geophysicist agreed that the Earth would have got a “little jog,” and that the islands off Sumatra would have been moved by the quake. ---
**Author:** ![treis](https://avatars.discourse-cdn.com/v4/letter/t/bc79bd/32.png) [@treis](https://boards.straightdope.com/u/treis)\ **Post date:** [December 28, 2004, 7:35am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/2 "2004-12-28T07:35:06Z")
I am not an expert on GPS but I know that they have ground stations to ‘calibrate’ the satilites. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 28, 2004, 2:56pm UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/3 "2004-12-28T14:56:29Z")
> [@treis](#): > > I am not an expert on GPS but I know that they have ground stations to ‘calibrate’ the satilites. I know about the ground stations per a previous thread. What I’m wondering (and I should have been much clearer in the OP) f this is something they _have_ to deal with, or the if the earths rotation period is a non-issue for GPS calculations. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [December 28, 2004, 3:06pm UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/4 "2004-12-28T15:06:46Z")
A GPS receiver is fundamentally calcultaing its location with respect to the GPS satellites. It uses satellite ephemeris information to relate this to a position on (or near) the earth. So if the earth’s position with respect to the satellites changes, it seems correct to assume that the satellites’ ephemeris data must be updated to account for this. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [December 29, 2004, 12:17am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/5 "2004-12-29T00:17:55Z")
According to [a Chicago astronomer](http://www.chicagotribune.com/news/local/chi-0412280221dec28,1,6119845.column?coll=chi-news-hed) (registration required), the day is a bit shorter. > [@](#): > > It’s analogous to the increase in rotational speed that a twirling ice skater experiences when he or she draws in their arms. It is estimated that during the Sumatran quake, a block of material roughly 600 miles in length and 100 miles in width fell 30 feet closer to the Earth’s axis of rotation. The planet has responded by rotating more rapidly, albeit ever so slightly, and our 24-hour days are now one ten-thousandth second shorter. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [December 29, 2004, 12:41am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/6 "2004-12-29T00:41:08Z")
> [@Xema](#): > > A GPS receiver is fundamentally calcultaing its location with respect to the GPS satellites. It uses satellite ephemeris information to relate this to a position on (or near) the earth. So if the earth’s position with respect to the satellites changes, it seems correct to assume that the satellites’ ephemeris data must be updated to account for this. The GPS [atomic clocks are regularly updated](http://www.ntp-time-server.com/gps-time-server/gps-time-server.htm) to keep them in step with the earths varying rotation rate. There are also some [rather surprising reasons](http://www.sciencedaily.com/releases/2003/03/030306075514.htm) for that varying rate. ---
**Author:** ![Tripler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripler/32/21807_2.png) [@Tripler](https://boards.straightdope.com/u/Tripler)\ **Post date:** [December 29, 2004, 12:55am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/7 "2004-12-29T00:55:10Z")
I’ll second **treis** , in that I know for a fact that things will be corrected. There are systems in place for events such as this. Tripler Despite the Iraqi war, I’d ask that you have _some_ faith in the U.S. Government . . . ---
**Author:** ![rfgdxm](https://avatars.discourse-cdn.com/v4/letter/r/c0e974/32.png) [@rfgdxm](https://boards.straightdope.com/u/rfgdxm)\ **Post date:** [December 29, 2004, 1:42am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/8 "2004-12-29T01:42:30Z")
No. GPS will just indicate some places in the world have moved. As they have. ---
**Author:** ![threemae](https://avatars.discourse-cdn.com/v4/letter/t/5f9b8f/32.png) [@threemae](https://boards.straightdope.com/u/threemae)\ **Post date:** [December 29, 2004, 4:32am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/9 "2004-12-29T04:32:34Z")
Frankly, there would be a lot more support for a 25 hour day than taking whatever we have now and shortening it. Needless to say, this doesn’t help. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [December 29, 2004, 5:40am UTC](https://boards.straightdope.com/t/if-super-quake-slows-earth-rotation-slightly-how-does-gps-system-compensate/281888/10 "2004-12-29T05:40:39Z")
> [@David Simmons](#): > > The GPS [atomic clocks are regularly updated](http://www.ntp-time-server.com/gps-time-server/gps-time-server.htm) to keep them in step with the earths varying rotation rate. For a look at the baffling complexity encompassed by **DS** ’s sentence, check the website of the [International Earth Rotation and Reference Systems Service](http://www.iers.org/) (IERS), and the [Earth Orientation Department of the U.S. Naval Observatory](http://maia.usno.navy.mil/eo/). The navy site has a good summary of what’s meant by [Earth Oriention here](http://maia.usno.navy.mil/eo/whatiseop.html), and the [2003 annual report of the IERS](http://www.iers.org/iers/publications/reports/2003/) contains a [subreport (pdf)](http://www.iers.org/iers/publications/reports/2003/ar2003_341.pdf) from the International GPS Service (IGS) which includes a map of the location of the 362 ground tracking stations used by the GPS service. There’s a station at the southern tip of Sumatra, and another near Madras India.