Talk:Global Positioning System

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Doesn't work indoors[edit]

I have just recently acquired my first GPS device - a smartphone. For half an hour it has been lying on my desk helplessly trying to get a lock. That's something that I didn't know: GPS doesn't work inside buildings. It's a basic and important fact that is not in the current article, unless I missed it.

I do know that some wavelengths do pass through concrete or other materials, others don't. Some reflect on surrounding buildings, on cliffs, on water, others don't. GPS is in the 20-30cm range. Somewhere the transparency and reflection such waves should be specified.

More generally, is there a page that sums up in a practical way the transparency and reflection data for various wavelengths? Electromagnetic spectrum is too theory-oriented. I just want to know: will waves of a certain frequency enter buildings (through a direct route, through the windows?), will they pass the upper atmosphere (or perhaps be reflected), or through clouds, how far will they go underwater?

David Olivier (talk) 09:56, 5 September 2013 (UTC)

Most of your questions belong on our wp:reference desk/science — see wp:talk page guidelines. However, your remark that the article should mention the basic fact that GPS doesn't work inside buildings, is of course very on topic on this talk page. Go ahead, be wp:bold and fix it. Wikipedia is yours. DVdm (talk) 10:50, 5 September 2013 (UTC)
The wavelength of the GPS L1 carrier signal is 19 cm (1575 MHz). The primary reason GPS doesn't work indoors is that the signal is extremely weak in the first place-- it can be blocked by trees, fences, people standing in the way (happened numerous times doing data collection on Pike's Peak...), or even sometimes waving your hand in front of the antenna. Moreover, reflected waves are not helpful in GPS as they take a longer path than the one straight from the satellite to you, and will confuse the receiver (they are also weaker as well). There are, indeed, only certain wavelengths that penetrate the atmosphere ([1]), the L-band (1-2 GHz) was selected for GPS for this reason. If you're curious about these topics, I recommended the articles on penetration depth, path loss, and radio propagation. siafu (talk) 13:51, 5 September 2013 (UTC)
Thanks for your answer. Yes, I'm curious about such topics, generally, but that wasn't the point of my question, which was a practical one. Yes, I could be bold and add a section ("Availability", perhaps), but that would imply a lot of research on points that I guess most editors of this page already know. For instance: is the service available (with sufficient quality) at all latitudes? I gather from the section "3.1 Space segment" that it is, but that should be stated more clearly somewhere. OK, the signal is weak, but still some things will be more transparent than others. My experience is that it usually will work inside a bus. It doesn't work in most buildings, but does in my mother's house that is of wood with a tiled roof. But if I say that on the page someone will jump on me for inserting OR :D Will it work on a street surrounded by many skyscrapers? Also: does the weather affect the signal? Maybe I will go ahead sometime and be bold, and put in all I know and think I know and see what happens. David Olivier (talk) 13:58, 14 September 2013 (UTC)
To reduce the chances of being reverted for OR, find yourself some sources with this little tool —producing for instance this— and then be bold :-) - Cheers - DVdm (talk) 18:48, 14 September 2013 (UTC)

Section on Regulatory spectrum issues concerning GPS receivers is quite biased towards LightSquared[edit]

Having lived through the LightSquared testing while in the GPS industry, I find many omissions from the posting to make LightSquared appear to be a victim. Most notably, that GPS is first and foremost a military system that is owned and operated by the US Air Force and paid for by tax dollars from US citizens. It is not a private system. Military receivers would also be subject to jamming from a high-powered close-in signal, but obviously this was not widely discussed. Any major changes to the GPS system would be paid by citizens whereas new civilian receivers would have to be purchased by consumers. If LightSquared had been allowed to go forward, private GPS receiver companies actually would have made a lot of money by having to restock all GPS receivers in the US with new, more expensive receivers containing more complex filters. Customers outside of the US would still be able to use simple, inexpensive GPS receivers since the LightSquared contract is only for the US. It would have been painfully ironic if the GPS system, one of the greatest "public goods" (to use the economic term) provided by the US government, were to operate poorly in the US while operating very well in the rest of the world.

Other items that were left out of this section include: Section 911 of the 2012 Defense Appropriations Act, signed on Dec 31, 2011, forbade the FCC from lifting the prohibition against terrestrial operations (related to LightSquared) until concerns of interference with GPS have been resolved. Any proposed resolution must pass committees in the US House of Representatives and the US Senate. The spectrum that LightSquared owns is Mobile Satellite Service in the L-band, allocated by the FCC for satellite to earth communications. The FCC had a clause that allowed ATC or Ancillary Terrestrial Communications, but this was intended only to augment MSS, not to create a nationwide terrestrial network. LightSquared obtained the L-band spectrum via the purchase of SkyTerra in March of 2011 for $280M. This price was *far* below the price of terrestrial spectrum of similar bandwidth. In effect, LightSquared was attempting to buy terrestrial bandwidth at a satellite to earth price. If the LightSquared proposition was really financially sound, it would have been able to make money with their service by purchasing appropriate terrestrial bandwidth.

108.213.70.59 (talk) 04:55, 22 October 2013 (UTC)

I believe there was an article in GPS Solutions in 2012 that basically laid all this out, in fact. Don't have it to hand, though. siafu (talk) 05:16, 22 October 2013 (UTC)

section "Navigation Equations". a mistake in pseudorange equation?[edit]

currently:

p_i = \left ( t_\text{r} - t_i \right )c

shouldn't t_r be replaced by t_r tilde ? — Preceding unsigned comment added by 77.125.77.35 (talk) 10:17, 17 December 2013 (UTC)

Blacklisted Links Found on the Main Page[edit]

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Yes check.svg Fixed See [2]. - DVdm (talk) 13:19, 3 April 2014 (UTC)

Accuracy?[edit]

This is an excellent article, but I came to this page hoping to find out how accurate GPS is, and cannot find this information which I would have thought basic. I can see that there are sections and linked articles on improving accuracy, but a top-level summary of accuracy would be very useful. Even better would be a summary of how the accuracy of GPS has changed during its development, and/or how accuracy is affect by the number of satellites viewed, how accurate can it be? Also, how does GPS accuracy compare with Galileo/ Glonass etc? Gebjon (talk) 10:51, 9 April 2014 (UTC)

I have added some basic figures in the section Global Positioning System#Accuracy enhancement and surveying. Feel free to hone and add more. - DVdm (talk) 11:32, 9 April 2014 (UTC)