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# Physics/Relativity in GPS

Question
QUESTION: Hi,

I read an article regarding error corrections in GPS in Trimble site. Trimble is a company which works on GPS. Please see the link below.

http://www.trimble.com/gps_tutorial/howgps-error.aspx

I don't see any error corrections due to relativity being used in GPS in the above link. Please explain.

Regards

ANSWER: They go on and on about atomic clocks being needed for ultra-precise timing.  They use that and the tricks that are used to avoid having atomic clocks in small GPS receivers to get around needing to explain the complex details of relativity...that's some well-written stuff they have, from a standpoint of sidestepping complex topics so that the general public reading their page can understand it more easily, to be sure.  But if relativistic effects weren't important (and they have been definitely measured to be important without the extra satellite measurement they mention to get around needing them for normal GPS), they wouldn't be needed in the satellites and ground transmitters at all.  GPS is corrected for relativity using, it appears with the timing trick they mention.  If you can tell me how measuring precise time relative to another satellite would *not* correct for that, I'm all ears.

---------- FOLLOW-UP ----------

QUESTION: Hi,

Can't the satellites clocks be synchronized with earth's time at regular intervals to keep all the satellites clocks and earth's clocks synchronized. Once satellites and earth clocks are synchronized, why do we need relativistic corrections. Assuming that relativity is used in GPS, I would like to know how the correction is done in satellite. Are the satellites clocks reset remotely from earth at regular intervals or is the relativistic calculations done in the receiver's end, please clarify.

Regards,

ANSWER: Sure they can, but why would we not need relativistic corrections?  I mean, the correction is done via the fact that the satellites all lose the same relative amount of time. How would earth-bound receivers with no transmitting capability possible reset an atomic clock in orbit?  I'm very confused by the nature of that question.

---------- FOLLOW-UP ----------

QUESTION: Hi,

I am a non-physicist. I read about Einstein's relativity and I find it illogical and bizarre. I am skeptical about relativity being used in GPS. Please see the below link.

One of the advisory member, Ron Hatch himself denies relativity being used in GPS. I am confused why the scientific community say that relativity is used in GPS. Please clarify.

OK, you're not a physicist.  I AM a physicist, and it takes a long time to understand relativity...but I did that.  I spent that time, did the math, and put in the work.  You did not and this argument you're attempting in no way is about GPS itself but about how you don't understand Relativity.  Let's run through the history of the above conversation in brief:

1)  You sent me a website with an erroneous analysis of relativity in GPS.  I explained.  I used to work in space science on the same hall in the same building as the guys who developed that system.
2)  I'm an expert and you want my opinion, which I gave you.  You came back with a statement about synchronizing clocks.  That's necessary every so often, but it has nothing to do with the relativity issue as I explained it in my first reply.  The synchronization is necessary and does happen regularly, but the relativistic corrections using a fourth satellite are another matter entirely.  Nothing to do with the synchronization, just a trick for devices to not need an atomic clocks themselves in order to do the relativistic corrections they would otherwise absolutely need with only three satellites.
3)  You state that you are not an expert and you have asked me for a third time about GPS with a website that contains a link to a profile.  In that link or that profile it in no way says that the man thinks relativity is irrelevant to GPS.  Here's a link from the same website explaining relativity and how important it is in all the satellite navigation systems (deep space and otherwise): http://www.gps.gov/cgsic/meetings/2012/weiss2.pdf  It's technical, so you'll probably ignore it, but that's beside the point.  If you have a website where he says it's not used in GPS receivers, then it's not used because a 4th satellite measurement is substituted like I previously said.
4)  This is not about GPS, you said you find relativity bizarre and illogical.  If you have a direct attack on relativity that flies in the face of all the many, many measurements made of the theory on Earth and in space, you may reply.  If you continue to ignore what I told you and try to pick apart GPS based on ignoring the "4th satellite means a receiver doesn't have to calculate corrections exactly, it can approximate from the 4th satellite measurement" response that covers this question, please take your question to another physicist.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.