Talk:Global Positioning System

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)

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 ([1]), 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 ([2]), 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)

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)

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)

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)

@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)

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)