Talk:Global Positioning System/Archive 7

This is an archive of past discussions about Global Positioning System. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

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Archive 5

Links to commercial site

For the time being I have removed two links to a commercial site per wp:ELNO #5. I'm not sure whether saying to exeptionally disagree because the sources are excellent readings to make probably too long already a list of external links even longer by adding two links on a commercial site — even if the articles are really excellent. I think that by allowing this, we risk getting flooded with more manufacturers with websites having more technical articles. DVdm (talk) 13:40, 10 January 2011 (UTC)

I understand your concern. I am aware that the list of external links is already quite long, and in this regard have carefully considered the appropriateness of the links. My conclusion was that the benefits outweigh the concern about a commercial offering for the following reasons: The compendium document succeeds in providing an excellent introduction and overview of the technical concepts, an aspect in which the current article is a disgrace. Several of the current external links discuss very specific technical issues, and none of them provide a good overview or historical introduction. Finally, I think that wp:ELNO #5 does not exactly apply - obviously, any company has financial interests. However, the linked documents hardly contain any explicit advertisements or promotion. The document presenting the origins of GPS contains nothing additional but the copyright of the company. Of course I respect the opinion of others, so let's see what consensus we can get. Nageh (talk) 14:04, 10 January 2011 (UTC)

Good idea. I personally think that thay are good sources and so do you, but in the end, who are we to decide? :-) Cheers - DVdm (talk) 14:19, 10 January 2011 (UTC)

General Relativity

re: "In 1956 Friedwardt Winterberg proposed a test of general relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks."

Wouldn't be better to move this from "History" to the section "Error sources and analysis", along with a short discussion describing the use of general relativity to correct the atomic clocks? Psalm 119:105 (talk) 17:54, 16 January 2011 (UTC)

Maybe. I am not really happy with the current layout of the article anyway. I think it should start with an introductory (1) Principles of satellite navigation, where the basic concepts are described, then go on with (2) Development of the GPS, which discusses both historical development and more advanced concepts (including the influence of time drifting due to relativistic effects, which is significant for GPS applications, btw), then describe (3) System structure and (4) Applications, next (5) Technical specifications, and finally (6) Accuracy enhancement techniques (and (7) Other systems), leaving Navigation equations as an advanced topic in a separate article (with basics described in sections 1 and 2). Something like this. Nageh (talk) 18:58, 16 January 2011 (UTC)

Now why would anyone want to put in something as uninteresting as technical specifications yet leave out the most interesting part of the entire document, Navigation Equations. The mathematics used in the navigation equations is certainly not advanced. It is very basic and fundamental. The physics is very basic and fundamental. We certainly don't need to dumb down the article to such an extent that everything that in any way resembles mathematics or physics goes into an advanced section. RHB100 (talk) 20:30, 18 January 2011 (UTC)

This is nowhere near what I intended. Sorry, my bad. I was somehow assuming that Methods of solution of navigation equations would discuss advanced techniques, similar to section Accuracy enhancement and surveying, without taking a second look. I agree that these are basics that need to be described in the article. If not right in section (1), which was my actual intention, of the structure I proposed, then in a separate section (where best?). Still, advanced calculations are possible pertaining to accuracy enhancement techniques; they are certainly best discussed in separate articles (e.g., the GPS augmentation articles). Regarding technical specification, as a mathematician you may find them uninteresting yet they are an important aspect. I moved them pretty much to the end in my list, anyway. Hope that clarifies things. (Btw, I'm surprised you suspected me to dumb down articles. I have recently argued from the opposite position on the Scientific guidelines talk page.) Nageh (talk) 21:14, 18 January 2011 (UTC)

With regard to specifications, I think the opening paragraph describing the capabilities of GPS covers a good part these speicifications. Most of the remaining part can best be covered in an error analysis. Incorporating ""Error analysis for the Global Positioning System"" into the GPS main article would complete the technical specifications and at the same time make the article more interesting. RHB100 (talk) 19:38, 19 January 2011 (UTC)

The "error analysis" material was in this article (see this version). In October, the extensive discussion of relativistic effects was removed from this article and is now in Error analysis for the Global Positioning System. Its relevance isn't limited to error analysis, though. This article should include at least a summary of the applicability of relativity to GPS: that the satellites orbit so high and move so fast that relativistic effects are unusually large; that the system accuracy depends on precision timing to the microsecond level, so the tolerance for error is unusually small; and that, as a result of these factors, GPS system design must -- and does -- take account of relativity. We could mention that there are some smaller relativistic effects (velocity and elevation of receiving stations, for example) that aren't large enough to worry about, and refer the reader to Error analysis for the Global Positioning System for further detail on that subject. I agree with Psalm 119:105 that such a section could also include the 1956 proposal and the actual test that was conducted.

Many aspects of the treatment of relativity were discussed in great detail over the summer in this thread and the next few after it in the archive. JamesMLane t c 01:17, 21 January 2011 (UTC)

The article, Error analysis for the Global Positioning System, contains material which is even more important for GPS than the material on relativity. For example the material on the calculation and derivation of geometric dilution of precision (GDOP) is also contained in this article. I think the contents of the article, Error analysis for the Global Positioning System, should again be made a part of the GPS article. RHB100 (talk) 19:53, 21 April 2011 (UTC)

Satellite positioning inconsistency in figure

The "visual example of the GPS constellation in motion" figure seems to be inconsistent with the article. The article states "the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees" but the angular difference in the figure is 90 degrees for all satellites.

This inconsistency should be resolved one way or the other. I have also added an equivalent note to the discussion page for the figure. 134.134.139.74 (talk) 19:50, 17 February 2011 (UTC)

I don't know how you are able to determine that "the angular difference in the figure is 90 degrees for all satellites". First of all the figure provides a two dimensional depiction of three dimensional motion. It is hard to measure angles accurately in this situation. Second the satellites are continually moving. There are not only difficulties in measuring the angles between moving objects in a given plane but also the distraction of the motion in other planes. But you may have found a way to measure the angles in the figure. If so, I would like to see an explanation of how you do it. RHB100 (talk) 20:21, 22 April 2011 (UTC)

Doppler shift discovered with Sputnik? I don't think so.

As of 4/5/2011 the article reads:

A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. **They discovered that,** because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that because they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite)).

The phrase "They discovered that," is completely out of place. These scientists did not have to receive Sputnik to "discover" Doppler shift of radio signals. Doppler is taught in applied math 101. If no Doppler shift had been observed, then *that* would have been a significant discovery. ;-)

Therefore, please, someone, move this phrase alone to the next sentence. I suggest: "A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. Because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. Because they knew their exact location on the globe, they discovered that they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite))."

-Al Roxburgh —Preceding unsigned comment added by 98.174.141.194 (talk) 21:48, 5 April 2011 (UTC)

Multidimensional Newton-Raphson calculations

As a licensed professional engineer with advanced engineering degrees from both the University of Arkansas and UCLA, I would like to point out that there are both advantages and disadvantages associated with the Multidimensional Newton-Raphson approach. As the licensed professional engineer who documented this method for Wikipedia I am uniquely well qualified to make this statement. It is generally true that all methods have advantages and disadvantages. The fact that a method works well certainly provides no exception. In the case of a one dimensional root finding method you know for a continuous function that when positive and negative values are found a root lies somewhere in between. For a multidimensional root finding method, you never know in the process of iteration that a root even exists except in the unusual case that you hit upon the exact solution. Almost all methods of solution occasionally fail. It is theefore important that we be aware of all disadvantages. We can make a judgement that it might be wise to disregard the disadvantage but we should never make the mistake of assuming that a disadvantage does not exist. RHB100 (talk) 01:35, 10 May 2011 (UTC)

The sweeping statement that "There are no good general methods for solving systems of more than one nonlinear equations." is almost vacuous and therefore nonsensical and irrelevant. Multidimensional non-linear equations are routinely being solved quite dependably in a host of engineering environments. For the problem on hand there does not seem to be a problem in the application of NR, especially since a good starting value is easily available. Perhaps the most pessimistic that can be said in this context is "there is no guarantee of NR's convergence in all cases". −Woodstone (talk) 06:30, 10 May 2011 (UTC)

The statement is a direct quote from the book "Numerical Recipes". It is obviously true since you cannot bound a multidimensional root. Nothing in the "Numerical Recipes" statement of this disadvantage contradicts what you have said after your first sentence. And on the other hand nothing you have said contradicts this disadvantage. You need to come up with a sourced statement which backs up your statements above and add this as an advantage of this method. The statement "there does not seem to be a problem in the application of NR" may well be true. But you have got to come up with a source. Such a source would not eliminate the disadvantage but it would be an advantage that shoulod be listed' RHB100 (talk) 02:49, 11 May 2011 (UTC)

Even if "there are no good general methods for X" would be true, it does not imply that "there is no specific good method for a specific case of X". Your conclusion is invalid. Since NR is used in many implementations of GPS, it is evidently a good method for it. See for example " Determination of GPS receiver position using Multivariate Newton-Raphson Technique for over specified cases, where NR is concluded to be as good as Bankcroft's method. −Woodstone (talk) 08:24, 11 May 2011 (UTC)

I agree with you Woodstone that the paper you cite indicates an important advantage of the multidimensional Newton-Raphson method. So important that I have added this advantage to the relevant section in the GPS article. I thought about waiting to let you add this advantage but I decided it was so important that I should add it without further delay. RHB100 (talk) 20:07, 11 May 2011 (UTC)

Glad you appreciate the cite. Your latest addition makes the view rather contradictory: like "it cannot work, but performs as well as the best algorithm". Anyway, we should clarify that the NR is not applied to the direct equations, but to a least squares optimisation. −Woodstone (talk) 10:38, 12 May 2011 (UTC)

Listing both the advantages and disadvantages is not contradictory. It is instead just reallity. On the one hand it has certain advantages but on the other hand it has disadvantages. I think your statement, "it cannot work", is a misinterpretation of the disadvatage. RHB100 (talk) 23:37, 12 May 2011 (UTC)

In what cases do any of these methods actually fail to converge? Using a simple least squares method, you can literally set the initial guess to {0, 0, 0} and still get converges in 2-3 iterations. The linear range of the equations is sufficiently large as to be extremely robust, in my experience, and I've only seen a failure when something is grossly wrong in other ways (e.g., bad hardware, massive iono disturbances, &c.). I'm concerned about implying to the lay reader that these solution methods are somehow unreliable. siafu (talk) 02:38, 14 May 2011 (UTC)

Well the language used certainly does not suggest that the multidimensional Newton-Raphson method is somehow unreliable. However, if you can add a sourced statement regarding the reliability of this method, I think it might be a useful addition. RHB100 (talk) 03:14, 17 May 2011 (UTC)

Solving the location in X,Y and Z or any other variables is multidimensional. Solving the location using a digital computer as all gps systems do is numerical. So all solutions do a numerical multidimensional solution. Even implementations of algebraic solutions are implemented as numerical solutions. So if there is a 'general' disadvantage it affects all digitally implemented solutions tackling this problem. Crazy Software Productions (talk) 13:22, 3 June 2011 (UTC)

An addition was made to split off methods for more than 4 satellites. That is not quite correct. Even in the case of 4 satellites, the system is overdetermined. There are 4 equations and 4 unknowns, but that does not imply there is a solution. Actually, even is there is no clock error, other errors in the measurements or the model would make an exact solution almost sure not to exist. Three satellites determine two possible solutions. The fourth one will practically never match precisely. Instead of solving for equality, an optimisation needs to be done. There is no sharp distinction between using 4 satellites or more. −Woodstone (talk) 05:56, 7 June 2011 (UTC)

The above is not quite correct. First I am assuming a normal GPS receiver with a quartz clock. Normally a quartz clock has an accuracy of less than 1 in 1 000 000. So that clock can not be used for accurate positioning. Then 4 satellites are needed. There are 4 equations and 4 unknows. Mathematically (for the GPS situation) this will result in 1 or 2 exact solutions.

The clock is only used for timing the differences of the reception time.

One satellite does not give any position at all.
The solution for receiving the signals of two satellites is a plane with the shape of an hyperboloid.
The solution for receiving the signals of three satellites is the intersection of two hyperboloids. This is a closed or an open curve.
The solution for receiving the signals of four satellites is the intersection of th closed or the open curve with an extra hyperboloids. If the curve was closed this will result in two solutions. If the curve was open this will result in one or two solutions.

Mathematically these solutions are exact. In reality the errors in the signals (and timing between the signals) will give small positional errors, but mathematically there are exact 1 or 2 solutions.

2D solution. Using a fixed height (fixed distance from the centre of the earth). Three satellites put you on a curve. This curve will intersect the earth a two points. Allthough it is possible that the curve exactly touches the earth, this will be rare and only be for an infinite small time. So with three satellites and the earth there are mathematically two solutions. Only one of the solutions will be stable (Over time). (A curve that intersects with a sphere has to intersect at at least two points, one point can be considered the entry point and on point can be considered the exit point. Mathematically there are endless curves which can intersect a sphere at only one point, but this type of curve isn't the type produced by the intersection of two hyperboloids.)

3D solution. Again starting with the curve of three satellites. Introducing a fourth satellite. This introduces another hyperboloid the curve will intersect this hyperboloid in one or two points. Only one of the points will be stable.

When there are two solutions, the method to calculate these solutions does not alter that. With Bancroft both solutions are provided. With pseudorange calculation the first estimate will determine which point it comes up with. With pseudorange calculations the calculation will automatically fix on the stable point after a few seconds.

Four satellites is not an overdetermined system.

Crazy Software Productions (talk) 19:24, 18 June 2011 (UTC)

NAVSTAR name or acronym still unresolved

I may have been a bit hasty reverting the latest addition of NAVSTAR as being an acronym, or not. This was last discussed incompletely, and therefore unsatisfactorily here and here. Could the imperious authority "John Walsh" be a hoax? —EncMstr (talk) 20:18, 20 June 2011 (UTC)

The NAVSTAR acronym definition (NAVigation System Timing And Ranging) was created by Rockwell International Space Division as published the "GPS NAVSTAR Space Vehicle Description Handbook" when they manufactured the GPS satellites in Seal Beach, CA. It was the GPS JPO that started referencing the satellites as GPS vs. NAVSTAR GPS.Robapodaca (talk) 00:04, 21 June 2011 (UTC)

Cost

What were the cost of each semgment of the GPS? Who paid for it? Article does not answer this basic question!!! — Preceding unsigned comment added by 79.80.138.51 (talk) 19:01, 22 June 2011 (UTC)

There will probably never be a full accounting for the costs. It was 1) a U.S. government project, and 2) a military project. It might be possible to dig up some contributing costs, like the cost of launching a satellite, but any price quoted by NASA is highly questionable since government accountants are in a completely different world, where even the conversion rate to ours is unknowable; besides, they don't really have to account for their costs. That combined with various off-the-books costs for security and secrecy reasons mean that any hard number presented would be only the tip of the iceberg and attract various wild guess multipliers to get the true figure (probably varying from 50 to 1000). —EncMstr (talk) 19:27, 22 June 2011 (UTC)

That's a bit of a silly answer. It's not completely mysterious, just complicated, and someone would have to do a more detailed investigation. Most of the costs are not classified; for example, the recently announced contract for the new control segment gives a figure of $1.5 billion [1]. There will be some variability in estimates, because there remain elements of the payload that are classified, but this would account for a relatively small part of the budget. I'm not aware off the top of my head of any actual estimates, but I may be able to find something in a couple days. siafu (talk) 20:09, 22 June 2011 (UTC)

"L4 Band" and Ionospheric Correction

I have been looking for a reliable citation for the note in the 'Satellite Frequencies' section about the use of an 'L4' transmitted frequency for ionospheric correction.

I can find no independent reference to the existence of an L4 transmitted frequency; the most recent official 'Global Positioning System / Standard Positioning Service / Performance Standard' dated Sept 2008 (http://www.pnt.gov/public/docs/2008/spsps2008.pdf) makes no mention of L4, only L1 and L2 (and future L1C, L2C and L5 signals).

However I have found references to a calculated L4 signal in several papers about ionospheric correction of GPS signals where L4 is some kind of differential error signal calculated from L1 and L2. Example: 'Optimal Noise Filtering for the Ionospheric Correction of GPS Radio Occultation Signals' http://nldr.library.ucar.edu/repository/assets/osgc/OSGC-000-000-001-401.pdf

Is there some confusion here between a fictitious calculated signal (that by convention is referred to as L4) used purely for correction purposes and a real signal that does not actually exist?

Hughdel (talk) 17:20, 25 July 2011 (UTC)

As far as I understand there was an internal study in the 90's on how the signal structure of the GPS system could be enhanced, and part of this study suggested the introduction of a new carrier (L4), which should be made available to the civil sector and was also intended for ionospheric correction. Current research papers discuss alternate ways for carrying out ionospheric correction, but make reference to the L4 carrier by equally calling the linear combination of L1 and L2 as such. In other words, there are two different meanings for L4, one is the actual separate carrier from L1 and L2, and the other is the linear combination of L1 and L2 (i.e., a "virtual" carrier). Here is a reference that discusses the introduction of the L4 band: [2]. And here are some slides stating the frequency of the carrier: [3]. Nageh (talk) 18:33, 25 July 2011 (UTC)

The iono-free linear combination of L1 and L2 is almost always called L3, and not L4 (see Misra & Enge, e.g.). Prior to looking at your posted sources, Nageh, I've never seen the use of the term L4 anywhere. siafu (talk) 02:39, 26 July 2011 (UTC)

Images instead of text

The images used for some of the mathematics are not rendered well on my system. Part of the problem is that I have the pages enlarged somewhat to compensate for poor vision. Could someone replace these images with text? If that is not practical, it would help if the images were redone in .SVG. SlowJog (talk) 23:42, 6 October 2011 (UTC)

Before anyone changes this, try adjusting your Math rendering option of your user preferences on the Appearance tab (at the bottom of the page). I show six options in Firefox 7.0. If you are using an older browser or a non-standards complying one (like Internet Explorer pre-8.0 or so), consider upgrading. Be sure to WP:CLEAR your browser cache after saving a new setting. —EncMstr (talk) 23:56, 6 October 2011 (UTC)

The definition of variable should be corrected

Some definitions of variable and equations in this article should be corrected. The symbols of correct and apparent variables should be distinguished.

My suggestion is as follows: (1) $\,{\tilde {t}}_{\text{r}}$ : the apparent time of signal reception, which is indicated by a GPS receiver. (2) $\,t_{\text{r}}$ : the time of signal reception (unknown). (3) $\,(x_{\text{r}},y_{\text{r}},z_{\text{r}})$ : the receiver position (unknown). (4) $\,p_{i}:=c({\tilde {t}}_{\text{r}}-t_{i})$ : pseudorange, which is observed by a GPS receiver. (5) $\,b:={\tilde {t}}_{\text{r}}-t_{\text{r}}$ : the clock advance of a GPS receiver (unknown). (6) relations: $\,t_{\text{r}}=p_{i}/c+t_{i}-b$ . $\,(x_{\text{r}}-x_{i})^{2}+(y_{\text{r}}-y_{i})^{2}+(z_{\text{r}}-z_{i})^{2}=c^{2}(t_{\text{r}}-t_{i})^{2}$ . Kkddkkdd (talk) 14:52, 18 September 2011 (UTC)

We already have a variable for the clock bias called b. Adding another variable for essentially the same thing would do harm rather than good. RHB100 (talk) 01:48, 9 November 2011 (UTC)

Undue weight on Bancroft

Three times clearly wp:UNSOURCED wp:POV phrases, inserted by user RHB100 (talk · contribs) here and here, about Bancroft's method ("the most important method of solving the navigation equations because it involves an algebraic as opposed to numerical method," and that it "has the advantage that it can be used for the case of four satelites or for the case of more than four satellites."), were removed or rephrased by 78.147.75.16 (talk · contribs) here and by myself (here and here). I have left a warning on the user talk page about insertion of wp:POV.

On the other hand, do we really need this in a separate section? Shouldn't we instead just mention the name "Bancroft" once in the section about the least squares method? - DVdm (talk) 08:11, 9 November 2011 (UTC) Surely this books search has a proper source that is more suitable than the current source, a student assigment, sitting here. For instance, this book source says that Bancroft, Krause, Abel and Chaffee, and Hoshen developed non-iterative closed-form solutions to the nonlinear GPS pseudorange equations. Rather than having the current section, based on a weak source and merely stating the obvious, shouldn't a short referenced mention along these lines be largely sufficient? Any other suggestions? - DVdm (talk) 12:37, 9 November 2011 (UTC)

Bancroft, S.; , "An Algebraic Solution of the GPS Equations," Aerospace and Electronic Systems, IEEE Transactions on , vol.AES-21, no.1, pp.56-59, Jan. 1985, doi: 10.1109/TAES.1985.310538. This is the original paper, found here, though not freely available online, this would clearly be the best source for the actual method. There's nothing at all wrong with using print sources, why not just stick with this? siafu (talk) 17:44, 10 November 2011 (UTC)

Good. I have added the source. No problem if it is not free - see wp:SOURCEACCESS. Thanks. - DVdm (talk) 18:08, 10 November 2011 (UTC)

The IEEE, like many (most?) professional societies, allows authors to post papers on the their personal or work web site, subject to certain restrictions. Has anyone asked Bancroft if he'd be willing to do this? LouScheffer (talk) 18:20, 10 November 2011 (UTC)

Bancroft method made even more neutral, current reference retained, new may be added

So long as the section "Bancroft's method" remains as it is now, I have no objections to removing "Bancroft's method is perhaps the most important method of solving the navigation equations". I don't think it was a violation of the neutrality policy since it is like saying, I think GPS is perhaps a better navigation system than dead reckoning. RHB100 (talk) 22:25, 9 November 2011 (UTC)

The referebce, http://www.macalester.edu/~halverson/math36/GPS.pdf, must be retained. It is the best written and most clear description of the Bancroft method of any free source I have been able to find on the internet. Additional references may be added but it is important that this source be retained. To judge the best source, it is necessary to read and comprehend the material. I don't believe DVdm has read the source material with sufficient comprehension to understand the Bancroft method. RHB100 (talk) 22:42, 9 November 2011 (UTC)

If I would not have a masters degree in mathematics (and another one in IT, by the way), my understanding of the Bancroft method would be just as irrelevant as it is now (see your talk page and this old WQA) . It does not matter whether you and I understand it, or whether you and I are qualified in anything. It matters whether Wikipedia text is wp:verifiable by means of wp:reliable sources. Trust me, I do understand the equations, and I am quailified to judge that, and they are good—very good. There is no question about that, but for Wikipedia our judgment is irrelevant. My point is just this: having noticed that this particular source is merely a student assignment (and therefore not peer-reviewed), albeit, as you said "on the website of a highly respected Canadian university", and although 100% OK for you and for me, I wondered whether we didn't have "Wikipedia-better" sources, i.e. published books or peer reviewed articles. Don't be afraid, I will not remove "your" source, but if someone removes it and replaces it with something more solid (in the Wikipedia sense), I will support that action. That said, what do you (and others) think about adding the statement I suggested in the previous section, together with the book-source I provided? - DVdm (talk) 17:24, 10 November 2011 (UTC)

Meanwhile, ref to original added - see prv section. - DVdm (talk) 18:09, 10 November 2011 (UTC)

The reference resides on the website of a Minnesota college, http://macalester.edu, not a Canadian university, 'highly respected' or otherwise. - Pirround (talk) 00:43, 7 February 2012 (UTC)

I don't know what statement in the previous section you are talking about. RHB100 (talk) 00:55, 11 November 2011 (UTC)

potential WSJ resource

Beijing Launches Its Own GPS Rival by Jeremy Page 28. December.2011 (page A9 in print). 99.181.153.29 (talk) 02:12, 29 December 2011 (UTC)

Possible problematic edits

I noticed that an editor who made several edits to this article may be linked to a PR company (see WP:COIN#Qorvis for background). Could someone with more knowledge of GPS check the edits that WeatherBug17 (talk · contribs) made to the article? Some of the content has already been removed but some remains. Thanks SmartSE (talk) 21:38, 30 December 2011 (UTC)

Similarly, this section in GPS modernization and the entirety of Joint Polar Satellite System was written by them if anyone has the time to check them over. SmartSE (talk) 21:44, 30 December 2011 (UTC)

Opening Comment

The comment, "This article's use of external links may not follow Wikipedia's policies or guidelines. Please improve this article by removing excessive or inappropriate external links, and converting useful links where appropriate into footnote references. (January 2012)", appears more likely to cause harm than to result in benefits. I recommend that it be removed. RHB100 (talk) 02:32, 22 February 2012 (UTC)

I agree with that comment, and recommend that it be followed in stead of removed. See wp:ELNO. - DVdm (talk) 07:30, 22 February 2012 (UTC)

Neutrality of LightSquared section

Whoever wrote the section documenting the LightSquared/Coalition to Save Our GPS controversy is clearly biased in favour of the coalition. These two lines particularly caught my attention: "In the face of demonstrated disruption to GPS operations, LightSquared has turned to a strategy of blaming GPS manufacturers for building receiving equipment which "..looks into their (LightSquared) spectrum". This, despite the fact that the spectrum in question was never envisioned as being used for terrestrial broadcast." The language of this section is not neutral enough for Wikipedia, and should be substantially rewritten or removed. --The Editor 18:17, 26 October 2011 (UTC)

Reality is somewhat biased on this issue. It's certainly true that LightSquared has been trying to place the responsibility on the GPS community by insisting that GPS users add filters to their receivers, and it's also true that this part of the spectrum was never envisioned as being used for terrestrial broadcast in that the entire neighboring band was used for satellite communications. siafu (talk) 18:48, 26 October 2011 (UTC)

I just wanted to add my thoughts - Siafu, the statement in this section is indeed factually correct, however, the wording is quite biased. I think phrasing like "LightSquared says that GPS manufacturers are to blame for building receiving equipment which "..looks into their (LightSquared) spectrum". They say that the GPS industry has had almost 10 years to prepare or object, but has chosen not to until recently. However, the spectrum in question was never envisioned as being used for terrestrial broadcast." would be much more neutral. I'm leaving this for someone else to make any actual changes, but wanted to help. If no one changes this in the near future, I will make it myself. (I'm leaving it for others because: a. I have never made a change before and am somewhat uncomfortable doing so, and b. because I'm not actually happy with my version, though I think it would be a big improvement from the existing wording.) — Preceding unsigned comment added by 65.46.168.178 (talk) 05:51, 13 December 2011 (UTC)

LightSquared's public statements have been technically preposterous, only sounding vaguly plausible to someone completely unfamiliar with radio frequency communications. It is flatly not possible to build a receiver with 100% rejection of out-of-band signals, and GPS receivers are particularly problematic because sharp cutoff filters necessarily have messy phase response, which corrupts the very precise timing required for GPS to work. Higher-resolution receivers, such as used in surveying and automated tractors, generally have the highest receiver bandwidths.

Later processing stages, particularly the spread-sectrum demodulation, do an excellent job of ignoring unwanted signals, but they are limited by the dynamic range of the front end. While another satellite signal (which is what the spectrum is reserved for) would not be a problem, LS wanted to use the frequencies for terrestrial broadcasts, which have far more power available, and are enormously closer to the receivers. The net recevied power would be a million to a billion times (60–90 dB) more powerful than the GPS signal.

This would saturate the automatic gain control circuity, and require a 20+-bit ADC (impossible to build at the necessary data rates) to digitize the GPS signal in the presence of LS's interference. For the receiver designer, this is almost the same as deliberate jamming, which civilian GPS receviers are not designed to resist.

LightSquared bought the spectrum cheap precisely because it was reserved for low-powered satellite applications. Then they said they wanted to use it for terrestrial transmitters. The FCC thought it was impossible to not interfere with existing GPS receivers, but LightSquared insisted it was. So the FCC let them try. And, because the laws of physics still apply, they fell on their faces.

It's like buying land zoned low-density and asking for permission to build a syscraper because you can do it without casting shadows on the adjacent houses. And then whining because, after being given an opportunity to demonstrate this miraculous ability and failing, your application for a zoning variance is denied. 71.41.210.146 (talk) 18:50, 28 March 2012 (UTC)

I've added an update on the current status of LightSquared before the FCC. LightSquared is discussed in two places in this article, which I think should be consolidated. Also the first paragraph's discussion of Part 15 oversimplifies the situation. The fact that consumer grade GPS receivers carry the must accept interference notice does not mean that anyone is free to radiate signals that interfere with GPS. --agr (talk) 11:23, 2 May 2012 (UTC)

You may in fact add that intentionally jamming in the GPS band is not just not allowed, but is in fact a felony in the US. siafu (talk) 16:12, 2 May 2012 (UTC)

SuperGPS

Added in

"* SuperGPS - a form of GPS for land-based navigation^[1]" at other systems.

A new seperate article should be made about it. The system proposed is allot more accurate (upto 10 cm), country-independant, impossible to jam and best of all, uses no satellites (easier to repair, lower costs). The only downside seems to be that it doesn't work at sea (needs substations).

91.182.205.137 (talk) 11:25, 12 April 2012 (UTC)

I removed this from the list of other systems, primarily because AFAICT it only exists as a proposal as of now, and does not appear to be even being funded for implementation. Also, from that powerpoint, it seems that this system is only for time and frequency transfer, and not navigation or positioning. As an aside, that proposal makes a number of claims about GPS that are quite misleading, so I would also challenge its reliability. siafu (talk) 19:32, 12 April 2012 (UTC)

Criticism

The article do not contain any criticism of the US GPS system. The article on Galileo comments that the GPS system can be shut down at the behest of the US president. Should this not be in there? Gnurkel (talk) 08:50, 11 April 2012 (UTC)

This criticism is a bit overblown, based solely on the fact that the US government could, in theory, shut down the GPS system if it decided to. Per a 1996 Presidential Policy Directive signed by President Clinton ([4]), since reiterated by all subsequent presidents: "We will continue to provide the GPS Standard Positioning Service for peaceful civil, commercial and scientific use on a continuous, worldwide basis, free of direct user fees." The more recent document ([5]), signed by President Obama, states: "[the United States shall] Provide continuous worldwide access, for peaceful civil uses, to the Global Positioning System (GPS) and its government-provided augmentations, free of direct user charges." So they've made it a matter of national policy that GPS will be free and not shut off, and there's the simple fact that disabling the GPS service without warning would be crippling to the economy, and certainly not in the national interests of the United States. So, sure, in principle it could be done, but this is simply a result of GPS being controlled by a single government, whereas Gallileo represents an international cooperative effort including both government and business; the EU could, in theory, kill Gallileo at any point just by withdrawing official support just as easily as the US government could disable GPS. siafu (talk) 15:37, 11 April 2012 (UTC)

I agree. The article makes it pretty clear that the system is controlled and operated by the U.S. which conceivably could switch it off at a whim. But it also makes it pretty clear that the system is heavily relied upon by many important users, U.S. and others. Anyone capable of understanding the major points of the article can easily infer the latter. —EncMstr (talk) 20:13, 11 April 2012 (UTC)

I feel like I should add that I think that comment DOES belong in the article about Galileo, as American military control of GPS was one of the stated motivators for the development of the EU system. This does not mean that intentional disabling of GPS is anything but a fringe possibility, against the stated intent of all parties responsible for its operation and maintenance, and definitely does not mean that it merits mentioning in this article. siafu (talk) 19:37, 12 April 2012 (UTC)

Has anyone pointed out that the Galileo consortium could also switch off Galileo at a whim? Why should this not be pointed out in the GPS article as a reasoning for folks to use GPS? The system is clearly controlled and operated by the EU. The EU could easily make a case that in order to preserve their safety in a time of war, that it would be necessary to shut down the Galileo system every bit as "off" as the US could turn off the GPS. 14 June 2012 — Preceding unsigned comment added by 132.3.57.68 (talk) 17:13, 14 June 2012 (UTC)

The reason that US ownership of GPS is important is because it was one of the stated reasons for the creation (funding, development, etc.) of Galileo in the first place. The reverse statement is not true, and it's not really an important reason to "rely" on GPS-- it is most likely that in the future, GNSS receivers will avail themselves of all the satellite signals that they can use, including GPS, Galileo, GLONASS, and potentially Beidou/COMPASS as well. siafu (talk) 18:19, 14 June 2012 (UTC)

correction! It will not be "shut down"! the implementation allow to turn back selective signal aka turning off the high accuracy for non military users and that can be turn on/off anytime in any part of the globe (selective. Iraq for example) without affecting other parts of the system. 72.185.61.209 (talk) 00:27, 25 July 2012 (UTC)

wayback of a dead link

wayback machine has an archive of a dead link, specifically, citation #40. I would change it myself but i'm only familiar with very, very simple wikipedia markdown x.x

link: http://web.archive.org/web/20081114182739/http://www.navmanwireless.com/uploads/EK/C8/EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters.pdf — Preceding unsigned comment added by 76.67.36.49 (talk) 01:20, 16 April 2012 (UTC)

Done I've updated it to the version from March 28, 2009. - M0rphzone (talk) 05:52, 5 May 2012 (UTC)

Damage

It appears that the section on how GPS operates has been substantially damaged by back-and-forth edits and reverts. Someone who is an expert on the material should review and provide references. Someone more comfortable with reading edit histories than I should, for the time being, pull text from an earlier version of the page so the section is at least not broken. This is not an appropriate place for people's opinions of how they think GPS works. — Preceding unsigned comment added by 192.76.175.3 (talk) 18:30, 24 July 2012 (UTC)

The experts are providing references.LouScheffer (talk) 20:33, 24 July 2012 (UTC)

People, let's not get into an edit war. Let's come to a consensus here first before we start madly editing the article. With the references being provided, that shouldn't be a problem. In the mean time, you could add a Disputed-section to the article if necessary. Martijn Meijering (talk) 22:21, 24 July 2012 (UTC)

Disruptive editing

The anonymous IP keeps reinserting his point of view against a consensus of other editors. This is clearly against the rules. It is also pointless, because if this is escalated, as it will be eventually, he will most certainly lose as these rules are enforced very strictly. The way to influence an article in case of a dispute is to engage constructively on Talk and to try to achieve a consensus. The alternative will end in a ban and no influence at all. Martijn Meijering (talk) 18:29, 25 July 2012 (UTC)

hey sock puppet, where is the consensus and voting? This is not how free wiki works.

72.185.61.209 (talk) 02:50, 26 July 2012 (UTC)

Your 3 satellite version has been reverted by 4 different editors (a total of 5 times). Your concern has been discussed pn the Talk page, but no-one else seems to agree with you. It looks like a concensus to me. Meters (talk) 03:26, 26 July 2012 (UTC)

You want to accept a positional error of hundreds of metres? Then by all means, use just three satellites. But if you don't want that error, you will need to have your receiver's clock corrected, which means you need four. -- Denelson 83 07:28, 26 July 2012 (UTC)

In practice, you need to correct your clock even for a three satellite fix. You need a microsecond accurate clock even to compute distances to 300 meters or so, since light goes 300 meters in a microsecond. Your basic crystal clock can't do this, so you solve for a clock offset just as in the 4 satellite case. It's just that you can do this with one less variable since you have one less unknown. You'll get the degraded position and also a degraded clock adjustment, good to perhaps a microsecond.

"In practice, you need to correct your clock even for a three satellite fix." This is basically saying that you need extra information; i.e., if you already know the exact atomic time, you don't need to solve for it. This is not "basic" GPS operation, but assisted GPS-- equivalent to already knowing with certainty what your ECEF Z-coordinate is, for example. Without knowing, or just assuming, one of the four values which need to be solved for, it is mathematically impossible to arrive at a position solution using only three satellites. siafu (talk) 13:55, 26 July 2012 (UTC)

Yes, it's just another way of saying the same thing. You could actually do the calculation without using a local clock at all. Using the differences in the received times puts your position somewhere on a curve that is the intersection of two hyperboloids. Then you use your extra information to find the spot on your curve, and you've found your location without ever explicitly computing an accurate time. The accurate time is still available - take your final location, then add the time of flight to the time of the transmitted signal - but it was never explicitly computed during the process. LouScheffer (talk) 14:36, 26 July 2012 (UTC)

It looks as if the IP has a history of edit warring and being blocked for it. I think we can see where this will end. Martijn Meijering (talk) 17:08, 26 July 2012 (UTC)

Basic operation

The basic mode of operation of GPS is to use (at least) 4 satellites to solve for x,y,z,t. See, for example, this explanation and many others. If you believe otherwise, please provide a reference. Thanks, LouScheffer (talk) 13:27, 24 July 2012 (UTC)

That is incorrect! THREE satellites are needed for 2D location fix (every satellite sends location and precise ATOMIC time, besides other information: Ephemeris and many more) , fourth satellite needed for altitude only. Furthermore GPS sattelites or Earth are not static (nothing in Universe is). I recommend some reading of GPS SIGNAL and Trilateration article. 72.185.61.209 (talk) 18:44, 24 July 2012 (UTC)

Are you seriously using some mathematical example (formula) created by teachers from Penn state university? Its a formula example how to solve mathematical problem not how GPS actually works. Time signal is send by EVERY GPS satellite (its actually precise ATOMIC clock time)

here is a real source Operation Guide for DAGR: http://webcache.googleusercontent.com/search?q=cache:WPAfxtkXAOsJ:www.i-mef.usmc.mil/external/wss/deployable_virtual_trianing_environment/dvte_handouts/gensim/tech_manuals/dagr/DAGR%2520Pocket%2520Guide.pdf+&cd=3&hl=en&ct=clnk&gl=us#47 72.185.61.209 (talk) 18:54, 24 July 2012 (UTC)

The operation guide says at least four satellites are needed. The only way that three could possibly work is if the receiver's elevation/altitude is known by another means. Otherwise, a GPS receiver must solve for three dimensions plus time. That requires at least four satellite signals. More than four are useful for extra precision and when some are have close angularity. —EncMstr (talk) 19:33, 24 July 2012 (UTC)

Here is from The Future of the Global Positioning System, straight from the Defense Department (who built GPS):

The GPS receiver uses the position and time information, broadcast in the navigation messages and traveling at the speed of light, to calculate approximate ranges to each of the satellites within line of sight to its antenna. These approximate ranges are called pseudoranges, since biases in the user receiver clocks prevent the precise individual ranges from being measured directly. The pseudorange from each individual satellite for a specific but unknown value of user clock error defines a sphere on which a user may be located in three-dimensional space. The intersection of three spheres defines a point, though the intersection is imprecise due to the aforementioned biases in the receiver clock (which in nearly all cases is not an atomic clock) and to effects of ionosphere and atmosphere on the signal transit time. Addition of a pseudorange from a fourth satellite allows calculation of the user receiver clock error and permits computation of the three physical dimensions of the precise intersection, as well as precise time.

Or try this one Introduction to GPS (italics and bold in the original):

GPS receivers are equipped with crystal clocks that do not keep the same time as the more stable satellite clocks (the satellite clocks can be nearly synchronised to GPST using the clock correction model transmitted in the navigation message). Consequently each range is contaminated by the receiver clock error. This range quantity is therefore referred to as pseudo-range, and in order for the user to derive position from pseudo-range data, the receiver equipment is required to track (a minimum of) four satellites, and solve for four unknown quantities: the three-dimensional position components and the receiver-clock offset (from GPST) -- see Section 1.3.5. This is the basis of GPS real-time navigation, and why GPS could be considered an example of a time-difference-of-arrival system.

Or many, many more.LouScheffer (talk) 20:39, 24 July 2012 (UTC)

darling you need to READ the article. THREE dimensional fix (four satellites) is needed for planes and superman (and you). Here is another source.

Three satellites are needed for 2D (basic) fix. http://www.gpsinformation.org/dale/gpsfix.htm 72.185.61.209 (talk) 20:44, 24 July 2012 (UTC)

LouScheffer is correct; four satellites are required for an unassisted GPS receiver-- unassisted in the sense of not being provided any PNT data outside of GPS signals. Probably the most standard refernce, IMO, is Global Positioning System: Signals, Measurements, and Performance by Pratap Misra and Per Enge. In my copy (1st ed.), on p. 23:

A user, therefore, needs a minimum of four satellites in view to estimate his four-dimensional position: three coordinates of spatial position, plus time.

You may be getting confused with the idea of a "2-D" position; all position fixes are, in fact, 3-dimensional, since we live in 3-D universe. Receivers that do "2-D" fixes are taking advantage of extra information, namely the known elevation map of the Earth's surface; for example, car GPS receivers take advantage of the fact that they can assume that the car is on the road, and not anywhere off-road, and can constrain the problem that way. This is essentially equivalent to assisted GPS, mentioned by EncMstr. However, almost all receivers built and designed, including the ones in your cellphones and cars, require 4 satellites in view to function, and the basic positioning problem requires 4 satellites in view. siafu (talk) 20:57, 24 July 2012 (UTC)

Did you even bother to read the source article? : http://www.gpsinformation.org/dale/gpsfix.htm

@72.185.61.209: Unless you are a Flat Earther, you should realize that any position along the surface of a sphere is a position in 3 space. 2D solutions are more apt to apply to short range positioning systems like the defunct LORAN system. —EncMstr (talk) 21:16, 24 July 2012 (UTC)

We use navigation generally for 2D maps! (latitude, longitude) Unless military or flying the plane (then we use altitude as well). Lets get real here. GPS units gets a fix from THREE satellites. That is a must have minimum! Furthermore in the real life scenario six or more satellites are used for increased accuracy. That does not change the fact, that a basic fix is 2D (most maps are in 2D not 3D) is just from 3 satellites using Trilateration. We are talking here about GPS satellite fix (from cold start), not general use. Lets not mistake those two! 72.185.61.209 (talk) 00:40, 25 July 2012 (UTC)

There are many practical reasons not to use the three satellite solution. First, note the error in position will be comparable to any error in elevation. Just to name a few:

You need a map that has the elevation in every part of the world, otherwise you can be off by kilometers. Most GPS implementations don't have the storage room for such a map.
If you at the bottom of a cliff, or anywhere the elevation changes rapidly, position may be way off.
If you are in a city with skyscrapers, what altitude do you use?
The same GPS might be used by a rafter on a river and a driver on the bridge high above. What altitude do you use?
If you use it in a hot-air balloon, or an airplane, you get the wrong location.

Furthermore, all of these happen without warning, since the receiver has no way to check. Also, such a solution may well be outside the accuracy limit for cell-phone 911 calls. All these are reasons that 3 satellite is a special case, not normal operation. LouScheffer (talk) 01:28, 25 July 2012 (UTC)

@72.185.61.209: Please indent comments which are responses to a previous comment. See WP:TALK.

I am not sure what kind of map you are referring to—paper or stored digital. Even a GPS which does not display elevation/altitude must do 3D calculations to determine where it is unless it is explicitly told its elevation (or told once where it is so that it can deduce elevation). General purpose GPSs do not have the luxury of assuming an elevation as they may be carried by someone on board a ship or in an airplane. The difference of 60,000 feet (18,000 m) which the same GPS receiver might experience has to be accounted for. —EncMstr (talk) 01:46, 25 July 2012 (UTC)

From a Trimble GPS tutorial: Trimble Navigation is one of the oldest and technically strongest of the GPS companies. Presumably they know how their own receivers work. (Emphasis added) LouScheffer (talk) 02:54, 25 July 2012 (UTC)

The secret to perfect timing is to make an extra satellite measurement.

That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

This idea is so fundamental to the working of GPS that we have a separate illustrated section that shows how it works. If you have time, cruise through that.

If our receiver's clocks were perfect, then all our satellite ranges would intersect at a single point (which is our position). But with imperfect clocks, a fourth measurement, done as a cross-check, will NOT intersect with the first three.

So the receiver's computer says "Uh-oh! there is a discrepancy in my measurements. I must not be perfectly synced with universal time." Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point.

That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand. Once it has that correction it applies to all the rest of its measurements and now we've got precise positioning.

One consequence of this principle is that any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously.

how more simply can I explain this? you don't need elevation for navigation (unless airborne) general maps USE TWO DIMENSIONs. Navigation was, and still is done , by latitude and longitude. GPS receiver is correcting time with every signal received from the GPS satellite. Stop talking about nonsense about some mystery fourth satellite sending "time" only. They all do! And every GPS satellite knows exactly how far from the Earth it is located.(takes about 130ns for the signal to travel from GPS satelite. so time correction would be irrelevant from your mystery fourth satellite.Correction of time is done many times in second by THREE satellite signals. Care to read GPS SIGNAL article before discussing this even further? Those satellites are not just some simple beacon. They are sending lot more data. Thats why THREE of them are only needed for basic operation or cold fix. I am not saying that that is used in real life scenario. That would be 6 and more GPS satellites. BUT you are grossly wrong about assuming that FOUR satellites are needed for navigation or fix.

72.185.61.209 (talk) 05:06, 25 July 2012 (UTC)

I certainly agree all GPS signal contains the exact time they were sent. However, the receiver needs to know the exact time they were RECEIVED. That's because it takes the difference between these times, divides by the speed of light, and uses that to find the distance to the satellite. It does the receiver no good at all to know the signal was transmitted at 13.000001234 seconds after the hour unless it knows the received time to comparable accuracy.

The internal clock in a GPS is not nearly good enough for this purpose. Furthermore, the receiver cannot just use the times it gets from the satellites directly, since they will all be different - they have all traveled different differences, and suffered different delays. The net result is that the receiver must calculate the accurate received time, using the data (including the sent time) from the satellites, finding a location in space (x,y,z or lat,long,elevation) and an accurate time, that agrees with all the data received. That's four variables in four unknowns, so four satellites are needed.

As the article notes, you can do this with 3 satellites if you assume an elevation. But the receiver still needs to compute the super accurate time, because the time is needed to compute the delay, which is needed for the distances, which are needed for even a 2-D fix.

The reason this is not normally done is that assuming an elevation is error-prone. For example, from A US government publication on using GPS during wildfires

There are several different types of errors that can occur when using a GPS receiver, for example:

...

Unknowingly relying on a 2D position instead of a 3D position for determining position coordinates. This mistake can result in distance errors in excess of a mile.

So while a GPS system can work with three satellites, it then has to assume an altitude, which is error-prone. This is a desperation measure by a GPS when it can't find 4 satellites. Four satellites requires no assumptions and is much more accurate. This is why it's the normal mode of operation. LouScheffer (talk) 10:53, 25 July 2012 (UTC)

you are the one who is Assuimng. GPS receiver has time and location from 3 satellites and that is good enough to calculate location for normal human being on earth. its even good for emergency location and for cold fix. We are not talking about accuracy here. You are missing the point. four sattelite is needed for altitude measurement. there is no question about it. You dont need to know you altitude for location fix on the map. and again i have to repeat my self Trilateration do some reading about it. tri- means 3 BTW

72.185.61.209 (talk) 17:26, 25 July 2012 (UTC)

Yes, Tri means three - three distances. How does it find the distances? It knows what time they were sent, and what time they were received. How does it know what time they were received? It has to solve for that, that make four (4) measurements in order to do trilateration. (Or assume one of this distances, such as the altitude).

If you don't think GPS receivers need an accurate clock, how do you think they calculate the distance from each satellite? If it does need accurate times, how do you propose the GPS box figures out what they are? LouScheffer (talk) 18:06, 25 July 2012 (UTC)

gps receivers (used by general public) do not have atomic clock. but they have very good time correction from signal send by every satelite with location data.that signal travels abut 130ns so that makes every receiver very correct clock. So you theory about fourth satelite needed for time correction is incorect. every gps receiver does time correction with every signal received. And again, I have to repeat my self. Do some reading and educate your self about GPS SIGNAL

72.185.61.209 (talk) 02:48, 26 July 2012 (UTC)

I'm quite sure we don't need to continue this discussion any further. It's very clear that this anonymous contributor is not familiar with the functioning of GPS receivers, and we've shown enough references to convince anyone who is eager to learn. Further changes to the basic operation section by this contributor to the effect of 3 satellites being all that are necessary should be considered disruptive at this point. siafu (talk) 18:25, 25 July 2012 (UTC)

so to sum this up. just because i dont have account created i have nothing to say? stop saying anonymous.you yourself are using anonymous name. or is Siafu the name you pay your taxes with? didnt think so! I have provided clear references showing that 3 satellites are needed for cold fix and for latitude and longitude navigation. that is a fact. Educate your self about GPS SIGNAL before trying to insult somone!

72.185.61.209 (talk) 02:48, 26 July 2012 (UTC)

You should be aware that a GPS receiver does not directly calculate latitude and longitude. The position is always first calculated using a three-dimensional rectangular coordinate system, and then converted into whatever coordinate format is selected using whatever geodetic datum is selected. -- Denelson 83 07:42, 25 July 2012 (UTC)

Reverted him again, and templated him for edit warring. Meters (talk) 03:35, 26 July 2012 (UTC)

and that makes you a hero?

72.185.61.209 (talk) 03:43, 26 July 2012 (UTC)

Answer me this. How do you think the GPS calculates how far away it is from the satellite? Obviously, this is needed for even a 2D fix. You just need a few words, not a formula or anything.

It is certainly true the GPS signals contain the time they were sent (which is exceedingly accurate) but they have differing delays from 65 to 85 milliseconds to the receiver, depending on where the satellite is in the sky. This is fine for your setting your watch, where you could just choose one, or an average or something. But please explain how it enables you to find the distance to each satellite. LouScheffer (talk) 10:42, 26 July 2012 (UTC)

No no, be specific. I'd like to see the math that can solve 4 unknowns from just 3 equations. siafu (talk) 13:50, 26 July 2012 (UTC)

The use of terms like 3 equations in 3 unknowns or 4 equations with 4 unknowns is an inappropriate over simplification of the GPS problem. The navigation equations are nonlinear and the attempt to apply rules for linear equations to these nonlinear equations indicates a superficial understanding of the problem. RHB100 (talk) 20:58, 5 September 2012 (UTC)

Most solution algorithms for the navigation equations (e.g. batch filter, kalman filter) require linearization of the equations. Also, it is not at all possible to solve an underdetermined system of non-linear equations any more than it is for a system of linear equations; your statement is effectively irrelevant. I'd thank you to keep comments about other editors' expertise out of the discussion. siafu (talk) 21:47, 5 September 2012 (UTC)

Siafu, the Bancroft method solves the nonlinear equations directly. Kalman filter is not a soloution method. Solution methods are listed in the section, "Navigation Equations". Batch filter is not listed. The statement, "Also, it is not at all possible to solve an underdetermined system of non-linear equations any more than it is for a system of linear equations", is meaningless to me. I hold advanced engineering degrees from both the University of Arkansas and UCLA. I am a Licensed Professional Engineer. Do you have any degrees from top quality engineeering schools? RHB100 (talk) 00:21, 6 September 2012 (UTC)

I do, in fact, hold such degrees, and I am an expert in GPS, specializing in GPS radio occultation. The batch filter is essentially a least squares method for solving for a position solution; it, along with the Kalman filter, are common in statistical orbit determination and precision GPS applications (e.g. seismic monitoring). The Bancroft method is interesting, but in my experience, very rarely referenced or used since linearization is more than sufficient to determine an accurate and precise solution. In addition, the Bancroft method is still going to be unable to solve this system of equations with only three measurements, since an ambiguity remains here just as it does in the linearized system. Congrats on your credentials, also, but they don't hold much weight here on wikipedia, I'm afraid, since there's no way to prove that they are real. siafu (talk) 04:15, 6 September 2012 (UTC)

Even if there was a way to prove that these credentials are real, they are 100% irrelevant here. RHB100 should know that - he was warned about this many times on this artcle talk page and on his own talk page, which he recently cleaned out. RHB100, please do not go that way again, but back your edits with reliable sources, not with an authoritative appeal to your personal experience. Thanks. - DVdm (talk) 08:07, 6 September 2012 (UTC)

I think the most recent changes that Lou Scheffer has contributed has the section "Basic concept of GPS" pretty much the way it should be as far as I can tell at this time. He has done a good job of utilizing the best parts of other's contributions along with his own contributions to come up with a well written section. Let's not make it worse by making poorly thought out edits. RHB100 (talk) 20:03, 6 September 2012 (UTC)

The clock bias

In the section of navigation equation, following to the GPS/GNSS convention, the clock bias should be its advance. The signal transit time, thus, should be $t_{\text{r}}-b-t_{i}$ .

Kkddkkdd (talk) 15:42, 14 August 2012 (UTC)

Two sensible solutions with 3 satellites

This is just an academic point, but it seems to me it should be possible in some odd cases to get two sensible solutions with 3 satellites. The solution you get will be reflected through the plane of the satellites to get the other solution. In general this will be in outer space, since all satellites are above you. But if the three satellites are all in one GPS orbital plane, this plane cuts through the center of the earth, hence the reflected point will also be on the surface of the Earth. (In general there are 4 satellites in one orbital plane, so if they were evenly spaced you cannot see three at once. But they are unevenly spaced, and there may be spares, so maybe you could see three at once.) Then if you are stationary (with respect to the Earth's surface) then the other point will be moving with twice the Earth's rotational velocity at that point, and hence in general not be likely. But if you are in the far North, or far South, then this velocity can be reasonable as well. So in the very special case of three satellites, all the same plane, and a position in the far North or South, you could get two sensible solutions. Is this reasoning correct? LouScheffer (talk) 11:56, 7 September 2012 (UTC)

The 3 satellites do not have to be co-orbital to allow two solutions on the surface. In the archive of this talk, user "crazy software productions" shows a constructive proof. The "other" solution is always moving rather fast in time, so real confusion will be unlikely.Woodstone (talk) 16:39, 7 September 2012 (UTC)

His case gives two solutions near the Earth, but one solution will be moving with a good fraction of orbital velocity - this is why I call that solution not 'sensible'. In the case I outline, both solutions are near the surface *and* moving slowly, so you can't pick between them based on nearness to the Earth's surface, or speed. LouScheffer (talk) 17:29, 7 September 2012 (UTC)

This could still be useful for launch vehicles, which nowadays use GPS in addition to inertial navigation (I think), but unless we have a source describing it, shouldn't we avoid putting it in the article? Martijn Meijering (talk) 09:57, 8 September 2012 (UTC)

It looks like we agree. Now how about editing out the section trilateration (and 1-D root finding), which describes a theoretical method for exactly 4 satellites which I suspect is not used in commercial GPS devices. −Woodstone (talk) 04:48, 8 September 2012 (UTC)

Basic concept of GPS needs additional sourcing and explanation

The following is stated in Basic concept of GPS, "Three satellites might seem enough to solve for position since three measured distances define just two points". It occurs to me as I further review this section that there is a need for additional sourcing and explanation. How do we know that three measured distances define just two points? I think it is somewhat beyond being obvious. I think that to make this statement we need a source justification. That justification can be provided in a fairly easy and straightforward manner by mentioning that the two points are the intersections of the surfaces of three spheres and then referring to trilateration which shows that there are typically two intersections. RHB100 (talk) 04:12, 9 September 2012 (UTC)

The section has become too complicated. Its title Basic concept is a strong hint for readers there is no detailed nor rigorous theory of operation, and certainly they do not need to be prepared for or endure an in-depth geometric analysis or heavy duty physics. The section should only give the general mechanism of system operation in a conceptual way. It is intended for the intelligent layman as well as an accessible introduction for the detailed treatment later. In its original version, I have found it effective to squelch common myths:

myth: satellites (therefore governments, companies, etc.) can track a person using a GPS receiver
myth: GPS receivers transmit a signal
myth: only one satellite signal is needed
myth: that speed is directly determined by the unit
myth: that setting the GPS calendar/clock is necessary for it to work
myth: that once locked, a GPS receiver works in a tunnel or aboard a submarine
myth: that wind affects accuracy
why it can't tell where it is from a deep shaft or surrounded by tall buildings
why altitude information is usually less accurate than horizontal position

It is sufficient to speak of distances to satellites and precise satellite positions as those principles can be applied intuitively. Mentioning the importance of clock accuracy is a less important concept for understanding the system, but it has been there since the inception; appropriately, it has explained how the system measures the distance from the receiver to each satellite. Unfortunately, that discussion has encouraged tinkering for rigorous accuracy which now seems to be leading to an increasingly rigorous geometric treatment. —EncMstr (talk) 19:33, 9 September 2012 (UTC)

There is one more issue that I have observed which needs to be taken care of. There is a note in a reference which says in part "The two positions are symmetrical through the plane of the satellites". This statement is quite true. However, Wikipedia requires that we have a source for the facts we state. Although the statement is true, it is not obvious. How do we know that it is true. We know because the article, trilateration, provides the source reference we need. In trilateration it is shown that the two solutions are symmetrical about the plane containing the three sphere centers. therefore I think the note should be modified so that a source reference is given as to the reason for the symmetry. RHB100 (talk) 23:24, 9 September 2012 (UTC)

I have gone ahead and implemented the suggestions I have made above. I will let somebody else take care of the suggestions of EncMstr. RHB100 (talk) 00:33, 10 September 2012 (UTC)

EncMstr has complained that the Basic concept section is too complicated. Although I do not fully agree with EncMstr, I think there is one way in which the section could be simplified. We say "Three satellites might seem enough to solve for position" and we then explain why three satellites might seem like enough and explain why three satellites is actually not enough. Going through and understanding this might be an unnecessary intelectual exercise. A better way might be to eliminate the remarks on "Three satellites might seem enough to solve for position" and instead focus the arguments on why four satellites is enough. RHB100 (talk) 18:18, 10 September 2012 (UTC)

I sympathize with RHB100, but agree with EncMstr. The basic operation section has again gotten too complicated. When most readers encounter something like "the intersection of the surface of three spheres", they will stop paying attention. It's not wrong, and maybe they *should* understand it, but many will be turned off and read no further. LouScheffer (talk) 11:26, 11 September 2012 (UTC)

Please do not leave out explaining the basic concept of triangulation in that section. I had no idea how GPS worked until several years when I attended a presentation on the workings of the GPS system, and once the method of triangulation was described the basic concept of GPS became so obvious. Really, this is fundamental, and it is just high-school math after all. The details are complicated, the basics are not. I would even suggest that the basic section should come as the first section in the article, before the history section. Do not dumb the article down to a "how can I use GPS with my new iPhone" style content. As for the myths presented by EncMstr, these would need to be reliably sourced if they are really common myths in order to be addressed in the article. Nageh (talk) 13:18, 11 September 2012 (UTC)

OK, I tried a re-write, with an analogy to make it (I hope) simpler, and address most of the points of EncMstr. Feel free to edit/change/revise/revert etc... LouScheffer (talk) 13:44, 11 September 2012 (UTC)

Not a bad rewrite, but it sweeps under the carpet that in the 2-D case, there are two intersections of the circles. I don't see an easy way around that. And it misses the opportunity of showing that if the watch is off by just a second, the error is in the order of 300 m. −Woodstone (talk) 16:59, 11 September 2012 (UTC)

As for the two intersections, I think it could be said that you can resolve this either by knowing your approximate location (you'll usually know whether you are on the northern or the southern hemisphere) or by using another lateration, which guarantees a unique solution. Nageh (talk) 17:09, 11 September 2012 (UTC)

Of course, the current proposed text has very much the form of a how-to. I think what would be needed is a simple example for 2D triangulation (possibly using light rather than sound waves), then explain how this can be amended to not require synchronized clocks, and then generalize to the 3D case. I am a bit disappointed that no good diagrams for trilateration are available on commons; it would be simple to explain the basic concepts using some good pictures. Nageh (talk) 17:05, 11 September 2012 (UTC)

I think LouScheffer has succedeeded in dumbing down the section. People in general are not as dumb as LouScheffer thinks. Practically everyone is familiar with the fact that we live on an approximately spherical earth, but Scheffer thinks the concept of a sphere would be just too difficult for people to understand. And these big words like intersect, LouScheffer would never want people to learn what for some people would be a new word. LouScheffer apparently wants to keep everybody as dumb as possible. This dumbing down of everything is disgusting. RHB100 (talk) 19:15, 11 September 2012 (UTC)

Personal attacks and commenting on editors instead of content is also pretty unhelpful and "disgusting". We do need a simplified section for the lay reader, and providing such a thing does not mean we have to do without more complicated and in-depth explanations elsewhere. This section is entitled "Basic Operation", and it should be really basic. siafu (talk) 19:22, 11 September 2012 (UTC)

Alright as siafu says this simplified section for the lay reader does not mean we have to do without more complicated and in-depth explanations elsewhere. Scheffer in providing a 2D analogy should not have removed the more realistic and clearly explained explanation ln terms of spheres and their intersections. I think there are a lot of people if not a majority of the readers of the section "Basic concept of GPS" who want to know how GPS really works. I think that for the readers of this section, the statement, "When most readers encounter something like "the intersection of the surface of three spheres", they will stop paying attention." is untrue. We should separate the 2D analogy from the "Bssic concept of GPS" section. RHB100 (talk) 05:00, 12 September 2012 (UTC)

Another thing to think about is the level of English of readers. Wikipedia is used quite a bit by students and those for whom English is not a first language. "Sphere" and "intersect" are not common words - "intersect" in particular in not even in the 10,000 most common words. Wiktionary's list. So I think these should be avoided in the "basic" section. My personal preference is for the simplest explanation that is not wrong in the basic section, and the full explanation in the "Navigation equations" section. But of course each person's opinion may vary... LouScheffer (talk) 00:49, 12 September 2012 (UTC)

This 2D example certainly doesn't qualify as an explanation of basic concepts. Even if it were correct it would not qualify as an explanation. Failure to think clearly in terms of number of intersections has caused errors in this example. RHB100 (talk) 05:00, 12 September 2012 (UTC)

The 2D example involving Church bells was most incorrect. This example implied that two circles intersect at one point but this is of course incorrect since they typically intersect at two points. The Basic concept of GPS section was therefore replaced with a correct description. RHB100 (talk) 06:09, 12 September 2012 (UTC)

In response to Nageh above, I with the help of others have two pictures showing how 2 spher surfaces intersect and how a third sphere surface intersects the other two but somebody took them down. They could be brough back if this appears to be the proper thing to do. Also there is an excellent diagram at the beginning of the trilateration article. RHB100 (talk) 01:56, 14 September 2012 (UTC)

Overanalysis of the 4 satellites minimum

I contracted:

Four sphere surfaces typically do not intersect. This can be seen from the fact that three sphere surfaces typically intersect at two points as shown in trilateration and for a fourth sphere surface to intersect the other three, it would have to go through one of the points at which the other three intersect. This is a special case not the general situation.^[2] But we know that the four sphere surfaces corresponding to the four satellites do intersect at a point, namely the position of the receiver. Thus we can say with confidence that when we solve the navigation equations, the solution gives us the position of the receiver along with accurate time thereby eliminating the need for a very large, expensive, and power hungry clock.

into:

From these signals, position, altitude and precise time can be computed.

because I felt that the original explanation was obtuse and User:RHB100 reverted that change. I'd like to discuss the change here.

I do not understand why we need an explanation of why these 4 spheres will intersect because: (1) Obviously they will intersect because they are all defined to contain your current position and (2) They won't intersect exactly because of lack of precision, noise, bad signal, reflection, etc. so you will have to guess at the most likely position based on the imperfect data you get.

In general, it seems like this section is trying to describe the theory behind how someone could triangulate position with perfect signal and no assumptions (like assuming a driver is on the surface of the earth, say). I think this is not very valuable and instead the minimum satellite number should be referenced to an actual specification/measurement. For example, does the US government specify how many satellites should be needed to accurately measure position? Even better would be if they have information on how much accuracy can be expected based on number of satellites we're reading from!

Anyway, even if you guys want to keep this section completely theoretical, I think the explanation could be made much less verbose. Perhaps something like:

Without any assumptions (like the user being on the surface of the Earth), for distances are needed for perfect three-dimensional trilateration. In addition to position and altitude, solving the navigation equations will also provide accurate time.

Rather than trying to explain why 4 distances uniquely determine 3d position, I'd suggest we just add a reference so that people who want to dig deeper, can. Cheers, — sligocki (talk) 20:52, 21 September 2012 (UTC)

The 4 satellite requirement is not a matter of an expert opinion, but a simple mathematical constraint. Four unknowns requires four measurements to determine. Specifically, statements like:

For example, does the US government specify how many satellites should be needed to accurately measure position? Even better would be if they have information on how much accuracy can be expected based on number of satellites we're reading from!

Don't really make much sense; without four satellites the accuracy will be zero in a theoretical sense because it's not possible to solve the system of equations involved. As an aside, of course the US Gov says that you need four satellites (in the GPS ICD, e.g., which can be found here), the same way that the designers of cars say that you need to put fuel in them to make them run. In the non-basic sense, there are special cases where you can use fewer satellites, but this requires additional data source(s) to substitute for the missing satellite measurements. I agree that the intersecting spheres discussion can get very obtuse very quickly, and isn't really needed, however, and I very much endorse a much less verbose description. siafu (talk) 21:27, 21 September 2012 (UTC)

We are trying to describe the basic concept of how GPS works. There are many people with scientific curiosity who want to understand the basic concepts of how GPS works not just that it does work. We have already achieved a great simplification by going from explaining why three satellites are insufficient to a concentration on explaining why four satellites are sufficient. The key to a clear explanation is to use sufficient words to explain with clarity. When you try to be cute by explaining too many things with too few words, the explanation ends up being unclear and confusing to many readers. RHB100 (talk) 22:00, 22 September 2012 (UTC)

Explaining basic concepts for all readers

There are some who have indicated that they desire to see less explanation of the fundamentak principles. Yet others have expressed an interest in understanding how GPS works. In order to satisfy these conflicting desires, some of the explanatory material has been moved to notes. RHB100 (talk) 20:28, 23 September 2012 (UTC)

Even greater conciseness was achieved through editing amd eliminating a sentence. RHB100 (talk) 23:04, 23 September 2012 (UTC)

GLONASS

Why is the NA in navigation capitalized on the N in GLONASS? Thanks!! If you have the answer tell me on my talk page.

--Miquaz1 (talk) 23:35, 26 September 2012 (UTC)

According to the page wikipedia has on GLONASS, the "GLO" is for Globalnaya ("Global"), so it would seem that someone at the Russian Federal Space Agency way back when just decided to capitalize the whole thing. siafu (talk) 00:38, 27 September 2012 (UTC)

You know, I might have been the one that did that years ago, but since then a) the GLONASS page itself doesn't show the mixed capitalization, and b) I've rarely seen a page other than the main article explain why something is acron-ized the way it is. I'll change it now but Miquaz1 was good to question it. - Davandron | Talk 04:31, 18 December 2012 (UTC)

Section needed

There needs to be a section on getting lost by following GPS. Some people get lost by asking for the shortest route, like this couple did: [6]. They were looking for the shortest route from Oregon to Jackpot, Nevada. --Auric (talk) 02:10, 3 October 2012 (UTC)

This is not relevant to this (GPS) article. If such content deserves to go anywhere, it would be in GPS receiver in a section on misuse, or–more likely, in an article of notable examples of this kind of tragedy: incidents involving GPS receiver misuse, failure to use good judgment, blind faith in technology, following sheep over the cliff, or the like. —EncMstr (talk) 04:28, 3 October 2012 (UTC)

Thanks. I'll do just that.--Auric (talk) 04:50, 3 October 2012 (UTC)

External link to GPS simulation

I restored a link to a Java based simulation of GPS and GLONASS that provides a graphical depiction of space vehicle motion and illustrates the variation of dilution of precision (DOP) with varying constellation configuration. The latter is hard to appreciate and compare between the two systems with out observing it over time. This link is certainly relevant to the article and is devoid of any advertizing or other commercial content. The link existed in the article for a number of years and was recently deleted without any specific explanation. Its purpose was explained on this talk page (since archived) and no counter comments were made. Roesser (talk) 00:22, 26 October 2012 (UTC)

Events on GPS

GPS was disabled during Georgia conflict. (it should have been mentioned) — Preceding unsigned comment added by 189.113.75.27 (talk) 19:36, 8 November 2012 (UTC)

Not finding any detailed refs on this but there are mentions: 2008 South Ossetia war, [7]. -—Kvng 00:44, 24 November 2012 (UTC)

It was not disabled, it was just locally jammed. You can jam the public GPS band but if you don't have an alternate frequency to use you're basically without positioning as well. OTAN military generally jam public GPS in the area of operation but they can still use the encrypted military band when they do. Shutting down the whole GPS system would cause worldwide trouble. 62.42.72.17 (talk) 22:03, 7 December 2012 (UTC)

In Navigation equations Why replace $\,t_{r}$ with $\,{\tilde {t}}_{\text{r}}$ ?

Why replace $\,t_{r}$ with $\,{\tilde {t}}_{\text{r}}$ ? It adds nothing to understanding yet it creates more cumbersome notation. RHB100 (talk) 21:27, 16 January 2013 (UTC) hai

Clock bias

The sentence "The clock error or bias, b, is the amount that the receiver's clock is off. " in Navigation equations seems redundant. Kkddkkdd (talk) 17:39, 30 March 2013 (UTC)

I agree, I suggest you delete it Roesser (talk) 20:22, 30 March 2013 (UTC)

Threat to aircraft and ships from car based jammers.

Hi all, I don't think there is any ref to this issue in the article. See here [8] for some material. Is this a non-story I wonder (ie do the jammers actually not work?) Anyone have some good information? Springnuts (talk) 16:55, 13 April 2013 (UTC)

I've seen a few talks on the issue of interference and jammers. This is a definite issue, though, since because of the low power of the GPS signals, jammers do in fact work; there was a documented incident with a problem caused by a trucker using a GPS jammer at Newark airport, for example ([9],[10], pretty sure this was because of interference with a WAAS base station). In addition, it is illegal in the United States to buy, sell, or operate a device that produces interference in the GPS frequencies (L-band). siafu (talk) 17:11, 13 April 2013 (UTC)

^ SuperGPS concept of VU Amsterdam's LaserLab
^ Also the fact that two intersections are typical for three sphere surfaces, we can say that three satellites are inadequate.

[1] SuperGPS concept of VU Amsterdam's LaserLab

[2] Also the fact that two intersections are typical for three sphere surfaces, we can say that three satellites are inadequate.

[1]

[2]