Talk:Global Positioning System/Archive 7

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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.

1. One satellite does not give any position at all.
2. The solution for receiving the signals of two satellites is a plane with the shape of an hyperboloid.
3. The solution for receiving the signals of three satellites is the intersection of two hyperboloids. This is a closed or an open curve.
4. 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" 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. --  Denelson83  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)