Talk:Global Positioning System/Archive 5

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Number of satellites needed?

I'm a little cofused about the following sentence: "A typical GPS receiver calculates its position using the signals from four or more GPS satellites." - it contradicts with the info found in [4]: "In order to make this simple calculation, then, the GPS receiver has to know two things: * The location of at least three satellites above you * The distance between you and each of those satellites " Tauntz (talk) 09:49, 23 April 2008 (UTC)

If the GPS receiver had an atomic clock, it could calculate its position using only three satellites because it would have an accurate time reference. However, consumer models typically use quartz clocks which only have short-term accuracies at the precision needed for this application. So a fourth satellite is needed so that the current time can be calculated to a usable level of accuracy. — Val42 (talk) 05:39, 24 April 2008 (UTC)

Additional: The location of at least three satellites above you and the information [error, read distance instead of information Crazy Software Productions (talk) 16:02, 20 June 2008 (UTC)] between you and each satellite is indeed enough for calculation of your position. The problem with this is that you do not know the distance to each satellite and can not determine that information from only the signals of three satellites. One possible solution would be that you have a very accurate clock (as suggested in the above anwser) which is synchronised to the satellites clocks. Another way to know the distance to each satellite is to use two way communication. But GPS does not work with two way communication. A very long measuring tape is not an option either. See the paragraaf "Number of satellites needed for positioning" in this discussion session for a further explanation. GPS uses three timedifferences to determine the location. To get three timedifferences, the signals of four satelites are needed.

How can three satellites be enough to locate you in space? The earth is not a plane, it is a sphere. To locate a point in space you mathematically need four reference points (not in the same plane) to measure your distance from. Think of the intersection of four spheres. Three are not enough (you get two intersection points). Don't confuse with the plane where three reference points are enough (if and only if they are not on the same line !) to calculate your position (three circles to get one intersection point). —Preceding unsigned comment added by 212.27.60.48 (talk) 07:54, 17 June 2009 (UTC)

Problem with a lot of sites and booklets explaining the workings of the GPS that those explanations are simplified, by assuming that an accurate clock is available. For a simplified explanation this is not a problem, but if this simplified explanation then is used the determine the number of satellites needed, all sort of wrong reasons turn up why a fourth satellite is needed.Crazy Software Productions (talk) 21:19, 26 April 2008 (UTC)

Even without an atomic clock, some consumer GPS navigation devices used on the ground should be able to calculate the current position, since they have reasonable accurate information of the position of the Earth's surface. --SmilingBoy (talk) 16:17, 15 May 2009 (UTC)

The earth is the fourth sphere. And there is no down loading of the geography. The GPSR treats the earth as a sphere and you are above or below sea level.

1 satellite, a circle on the ground of the earth (the other sphere)

Incorrect, with no absolute time (quartz clock) there is no way of estimating the size of the circle. With normal clockdrift the circle could be expanding (or contracting) with supersonic speed. This problem does extend to the 2 satellite scenario, where both circles can expand or contract with high speed. Crazy Software Productions (talk) 19:04, 7 July 2009 (UTC)

2 satellites, Conjunction of two circles which is two points on the ground.

3 satellites, a single point on the earth with no altitude correction.

With three satellites and a assumed or guessed hight (distance from the middle of the earth) the location can be calculated, but the accuracy depends on the difference between the actual and the assumed hight. (There can be more than a single solution, but there is only one solution which is stable.Crazy Software Productions (talk) 19:04, 7 July 2009 (UTC)

4 satellites, correct altitude as defined by delta from sea level.

I think both sentences in question are correct if the one talking about 3 sats is not talking about altitude which will put you on the right road with a GPSR containing a map. 4 satellites for the added benefit of altitude. which is really not necessary for location.

If no altitude is known, there can be no accurate calculation of the position. If an altitude is guessed (previous known altitude) or is assumed, the calculated position is dependend on how accurate the assumed altitude was.

2D position calculation, normaly uses the last known altitude of the GPS unit. If the unit is not moved and not reset, this altitude will be accurate enough to do the positional calculation in most situations.Crazy Software Productions (talk) 19:04, 7 July 2009 (UTC)

Personally, I like locks on eight or more satellites. ;-)MBCF (talk) 00:46, 18 June 2009 (UTC)

A lot of assumptions are required: is the GPS receiver on the ground? Is the "ground" a building, mountain, etc.? Last I checked, the surface contour data was hundreds of megabytes just for the U.S. While I suppose it could be added to a modern unit, it's hard to justify it given the other variables. —EncMstr (talk) 16:25, 15 May 2009 (UTC)

See my comments just above. No GPS tells you your distance from ground. It tells you distance from sea level. (You use an altimeter to tell you when you are going to hit the ground if parachuting but right before you take off the runway you set it to zero while standing on the ground.MBCF (talk) 00:46, 18 June 2009 (UTC)

Except, if you don't have an altimeter, GPS-computed elevation is terribly inaccurate. There is such a thing, however, called the Wide Area Augmentation System, that tries to make that elevation figure a little more accurate, but it only works if you're in North America. The equivalent in Europe is EGNOS. --  Denelson83  01:33, 19 June 2009 (UTC)

Reference number 4 is dead

Site not found. brandnewbrain (talk) 09:43, 2 February 2009 (UTC)

GPS/ Meteorological satellites

Surely if you could use meteorological satellites the way you can use GPS systems, you could fit a receiver onto a vehicle such as a car and use it to provide constant updates about the weather —Preceding unsigned comment added by 78.150.77.106 (talk) 17:35, 3 February 2009 (UTC)

It doesn't work that way. Weather satellites only send their readings to meteorological organizations, which then process the readings and distributes them to the public. They don't send their analyses over those satellites. --  Denelson83  01:11, 4 February 2009 (UTC)

And even if you would be able to receive the satellite signals, for example weather pictures, you would need quite an antenna setup on on vehicle, to be able to get enough signal. (Large antenna which has to be aimed at the satellite all the time.) The weather satellites transmit far more information than the GPS satellites and therefore you need a far better receiving station.83.85.120.147 (talk) 21:56, 8 February 2009 (UTC)

However, some GPS receivers, like the Garmin GPSMAP 478, allow an XM WX receiver to be connected so you can receive weather information that way. --  Denelson83  00:58, 9 February 2009 (UTC)

Disadvantage of Multidimensional Newton Method

I removed the following sentence about Newton's method, then it was reinserted:

A disadvantage of this method is that according to, "There are no good general methods for solving systems of more than one nonlinear equations."

While it is true that there are no good general methods for solving nonlinear systems, I don't think that this can be counted as a disadvantage of Newton's method, for two reasons:

• to solve GPS, we don't need a "good general method" for nonlinear systems, we only need a method that works well for the specialized quadratic systems arising in GPS. The given reference doesn't talk about the quality of Newton as it applies to these specific systems. I don't have a reason to believe that Newton is not a "good method" when applied to those systems.
• even if the reference claimed that there are no good methods to solve the quadratic systems arising in GPS, which it doesn't, then that criticism would apply to all methods, not just to Newton's method.

AxelBoldt (talk) 23:58, 10 February 2009 (UTC)

I am glad to see you have put your argument in the Discussion section. Perhaps we can both learn from the discussion. I think we are talking about the multidimensional Newton or Newton Raphson method. I put the quote from Numerical Recipes in for the purpose of giving both real advantages as well as disadvantages. It seems to me that the multidimensional case has the inherent disadvantage that you cannot bracket the solution between arguments that have a positive error and those which have a negative error. This of course differs from the single dimensional case where a solution can be bracketed. It may turn out that multidimensional Newton Raphson works well for finding the position and clock error corresponding to the approximate intersection of four spheres. But it seems to me that in view of the comment in Numerical Recipes, one should always be aware of the possibility of failure.

RHB100 (talk) 21:47, 12 February 2009 (UTC)

Commercial Aspect

Does the US Government charge any fee (royalty) from the producers of civilian GPS tracking devices? Does the US Government receive anything from the civilian use of GPS? Olegwiki (talk) 10:46, 11 March 2009 (UTC)

No they don't receive anything, other than I guess a warm fuzzy feeling for providing the service globally for free. PJohnson (talk) 07:20, 3 July 2009 (UTC)

So why don't they charge a royalty? Or is them just all terribly nice folks who don't care for the filthy lucre down yonder at the DOD? Maikel (talk) 11:33, 9 August 2009 (UTC)

In short, when the decision to make GPS publicly available was made, following the shoot-down of Korean Air Lines Flight 007, it was decided to make it free of charge, and no-one has since revised that policy. In a way this has worked out well; if they had set the royalties in 1983 while thinking of aircraft navigation, they might have set it quite high, compared to the $30 receivers we have nowerdays. Also, even though the government doesn't get direct royalties from GPS users, they benefit from the availability of cheap commercial-off-the-shelf receivers, the development of which is paid for by the consumer market. A license fee would also be difficult to enforce effectively. Mike1024 (t/c) 10:42, 10 August 2009 (UTC)

Contractors?

This is a very in-depth, technical article, but I find no mention of who the prime contractors are. That's odd, considering that most articles on space programs mention the contractors fairly prominently. 198.45.18.48 (talk) 14:28, 15 May 2009 (UTC)

That's a great suggestion. Please add them. —EncMstr (talk) 14:32, 15 May 2009 (UTC)

Countries forbidding GPS

I can’t find anything about the legal status of using GPS in the article. [1] tells that using GPS devices is forbidden in North Korea, Syria and Egypt. On OpenStreetMap I have heard that GPS devices were forbidden in China, but have been legalised in Egypt recently. I think this would be important information for the article. --84.153.8.183 (talk) 20:44, 17 May 2009 (UTC)

Data up to four hours old

I removed a sentence in the section on ephemeris errors stating that "data up to four hours old is consdiered valid", &c., since it's not, strictly speaking true. Ephemerides are uploaded on average every two hours, and are considered valid for the two hours BEFORE the time of ephemeris and two hours AFTER the TOE. At the end of that two hour window after TOE, the data is only two hours old, not four-- ephemerides more than two hours old are considered invalid. siafu (talk) 21:57, 28 May 2009 (UTC)

"Satellite numbers" table

The quasi-footnotes should be replaced with a non-numeric symbol because "0+12³" is misleading (I first read it as 0+1728). Perhaps * ** *** or * † ‡, etc. Msanford  T  13:35, 16 June 2009 (UTC)

I agree with your suggestion. Go ahead and do it. I suggest letters but it is up to you. RHB100 (talk) 18:29, 4 August 2009 (UTC)

External link to International GNSS Service

As done for the article on Global Navigation Satellite System (GNSS), I suggest to add an external link to the International GNSS Service (IGS) at <http://igscb.jpl.nasa.gov/>. Formerly known as the International GPS Service, the IGS is "a voluntary federation of more than 200 worldwide agencies that pool resources and permanent GPS & GLONASS station data to generate precise GPS & GLONASS products". The IGS provides the highest accuracy GPS satellite orbits and clocks available anywhere, free of charge, and at varying degrees of latency. Scientists and surveyors needing highest accuracy position, velocity, time, topospheric delays, etc, should be using IGS products. [Mike Craymer, Geodetic Survey Division, Natural Resources Canada]

— Preceding unsigned comment added by 132.156.28.124 (talk) 19:48, 6 July 2009 (UTC)

More on how satellites determine their positions

Only a casual reference to how a satellite knows its position is ever made. "Then the new ephemeris is uploaded and the satellite marked healthy again." I am personally unable to find reliable information about the process, but surely someone can fill in the article as to how often satellites need to be told their position, and how they calculate it in between these updates. —Preceding unsigned comment added by 72.89.225.174 (talk) 21:40, 29 July 2009 (UTC)

There is the statement, "The ephemeris is updated every 2 hours ...", in the section, Navigation signals. The ephemeris along with time enables the satellite to approximate its position. RHB100 (talk) 20:07, 1 August 2009 (UTC)

GPS / GNSS / NAVSTAR

This article is a bit two-faced, so to say. :) The intro states it's about one specific GNSS, namely NAVSTAR, but most of the article is about GNSS in general. Which of the two it should be about is a bit tricky, but a choice should be made.
I understand that, strictly speaking, GPS means NAVSTAR specifically. If it is to be about that, then a lot of (general) information should be moved to the GNSS article (or a new more in-depth article in some cases, given the amount of detail). But a much used consideration on Wikipedia is what people expect when they type in a word (or abbreviation). That would be the navigational bit, irrespective of which specific system is used. In which case a separate NAVSTAR article should be created (that link now redirects to this article). This will become more of an issue in the future, when more systems are being employed. People will most probably keep on calling that GPS, even when they use, say, Galileo. Another reason to make this the general article is that the term 'Global Positioning System' sounds rather generic.
I don't care much which decision is made, as long as one is made. Any thoughts? DirkvdM (talk) 07:06, 6 August 2009 (UTC)

Btw, this should also be addressed in other articles. For example, GNSS applications intermittently speaks of GNSS and GPS, even though (I assume) GNSS is always meant. And then there are the articles GNSS Road Pricing and GPS tracking, which are both about the generic system, but use different abbreviations. And is a GPS navigation device only for NAVSTAR? Or can several systems be used by one receiver (now or in the future)? So would that then have to be renamed to GNSS navigation device? DirkvdM (talk) 07:38, 6 August 2009 (UTC)

The article focuses on GPS, the GPS navigation signals, frequencies, etc and also covers fundamental principles. To cover GPS properly, it is necessary that these fundamental principles be included. But in so doing some material will probably be applicable to systems under development. But this is the nature of fundamental principles. This general information should remain a part of GPS. Articles covering new systems once they are developed may reference GPS or write an alternate description description of the general information. RHB100 (talk) 20:18, 6 August 2009 (UTC)

(indented your post)

Why should general information not be in an general article? Given the size of the article, such a split makes sense anyway, so why not solve the two issues at the same time? DirkvdM (talk) 07:22, 7 August 2009 (UTC)

What specifically are you talking about when you say general information? RHB100 (talk) 20:22, 7 August 2009 (UTC)

Well, the three best known GNSSes, GPS, GLONASS and Galileo, all rely on satellites in several orbital planes at medium earth orbit, with the satellites transmitting radio signals comprised of a transmission time and satellite position information; and receivers with access to at least 4 satellites are able to solve a system of equations to determine the time and receiver location, accurate to tens of meters. Both GPS and Galileo use CDMA, and GLONASS might move to CDMA in the future for compatibility. Compass may well follow the same design as well. Galileo and modernised GPS will boast very similar features, and are designed to be compatible. On the other hand, non-global satellite navigation systems like Beidou operate on different principles. I'm not sure if we should move the common features into a common article or not. Mike1024 (t/c) 10:00, 19 August 2009 (UTC)

Latest Launch

Current text: The most recent launch was on March 15, 2008.[11] The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.[12]

Correction: The most recent launch was on March 24, 2009. Citation: http://www.spaceflightnow.com/delta/d340/ —Preceding unsigned comment added by 130.221.224.5 (talk) 11:57, 13 August 2009 (UTC)

Update: The most recent launch was on Monday, August 17, 2009. Citation: http://www.spaceflightnow.com/delta/d343/ —Preceding unsigned comment added by 130.221.224.5 (talk) 16:47, 19 August 2009 (UTC)

Missing diagram

Position calculation introduction: "Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, illustrates the intersection."--There is no sign of the figure. I assumed it must be in the trilateration article, but it isn't. --Musiconeologist (talk) 18:48, 18 August 2009 (UTC)

This diagram has been in the Position calculation, advanced section for a long time. Up until a few months ago it was also in the Position calculation, introduction section but somebody removed it. I put it back in this morning so that it is now in both Position calculation sections again. RHB100 (talk) 19:19, 18 August 2009 (UTC)

Mathematical notation shoulld be kept large for readability

It is important that mathematical symbols be kept large since otherwise it is difficult to correctly read subscripts and superscripts. RHB100 (talk) 00:30, 20 October 2009 (UTC)

Very long template

This article is not unreasonably long. It already has been split with the addition of the introductory article. Any further split will likely interfere with it comprehensibility. I am therefore removing the very long template. Roesser (talk) 16:11, 20 October 2009 (UTC)

It is very long, and most readers won't read all of it. The details of position calculation should be moved to a sub-article like GPS position calculations. —EncMstr (talk) 16:57, 20 October 2009 (UTC)

After the split-off of the GPS Introductory article, this article is now intended (as I see it) for the technically minded reader who indeed will read most of it, especially the details of position calculation. It is more comprehenible to leave such details within the context of this article. There are various other technical details in the article, so where do we stop in splitting them off? I personnally learned a lot about GPS from this article and would hate to see it diminished by splitting off important details. Roesser (talk) 17:18, 20 October 2009 (UTC)

Somehow I missed the split off, and it took a rather concentrated effort to find the link (almost, but not quite at the top) to the split off article. I guess I expect it would be the other way around. This, Global Positioning System is the general overview and introduction, and another article has the nitty gritty details. —EncMstr (talk) 18:17, 20 October 2009 (UTC)

We already have the basic material covered in the section, Basic concept of GPS. I think one advantage of leaving it all together is that it allows readers to more easily decide what sections they want to read. I think it is great to have more detailed material available on navigation, frequencies, demodulation, decoding, and position calculation even though some readers may not be interested in these topics. RHB100 (talk) 18:59, 20 October 2009 (UTC)

Earlier this year we had a peer review of this article. I responded to many of the comments of the reviewers making changes in accordance with their suggestions so as to make the article better. Of all the comments the reviewers made, I do not recall a single one saying the article was too long. RHB100 (talk) 00:21, 21 October 2009 (UTC)

Vectors, scalars, GDOP, PDOP, and HDOP

A vector is characterized by a magnitude and a direction as defined at The Physics Department - Mechanics, Vectors. A quantity that has only size is called a scalar. A vector is not tied to a particular coordinate system but can be expressed in many coordinate system (e.g. body fixed, earth fixed, geocentric equatorial ... ). RHB100 (talk) 20:55, 16 November 2009 (UTC)

The quantities GDOP, HDOP, VDOP, TDOP defined in the section, Geometric dilution of precision computation (GDOP), are not vectors since there is no direction associated with these quantities. They are scalars. (PDOP, HDOP, VDOP, TDOP) is not a vector, it is an array of scalars. RHB100 (talk) 20:55, 16 November 2009 (UTC)

Can GPS been used at height or above of that of the GPS-Satellites?

Hi, I'm just curious to know, if it is possible to locate one's position, when the local altitude is equal or above that of the satellites. So could it been used for spacecraft applications? As far as I understand it, a sphere is needed for the projection of the coordinates. but has it be under the satellites, or is it still working above them? Thank you in advance! :) —Preceding unsigned comment added by 195.37.176.75 (talk) 19:13, 26 November 2009 (UTC)

That depends on how high an orbit you're trying to achieve. GPS satellites actually broadcast their signals in one direction (like a torch), rather than broadcasting in every direction (like a lightbulb). So if you were orbiting at the same altitude as GPS satellites do (20,000 km) you wouldn't get a signal because the 'torch' isn't pointed at you. On the other hand, not everything orbits that high! The international space station orbits at about 300 km, and the space shuttle can't get above 1000 km. At those sorts of altitudes the 'torch' would still be pointing at you so you could still get GPS - just go to a space equipment supplier like BAE Systems and you can buy a Space GPS Receiver and you're ready to go. Mike1024 (t/c) 17:45, 27 November 2009 (UTC)

The main problem with the position is the 'torch' effect of the GPS satellites as described above. They point towards the earth. But there are some satellites which are almost at the other side of the earth but are still visible, so their torches directed at the earth just mis the earth by a bit and might be received. Mathematically there is no problem of calculating the position if the receiver is higher than the GPS satellites if you have enough signal. I have seen (do not know where) a paper how the GPS satellites can be used beyond the distance of the 20000 km above earth. This paper did not give any indication if this was put into practice. One other point is that it can not be a 'civilian' reciever, because for civilian recievers they are not allowed to operate under these conditions. (Above a certain hight and above a certain speed).

Positioning of satellites at the moment is done the other way round. The satellite sends a signal which is received by several time synchronised stations. The same mathematics as in GPS is used to calculate the position of the satellite. Because the satellite does not take any sharp turns, and their trajectory is very predictable, this method of positioning the satellite is sufficient for knowing the position of the satetellite. Crazy Software Productions (talk) 12:35, 28 November 2009 (UTC)

GPS positioning is used in ISS operations these days (fairly new). It is used by both the ATV and the HTV cargo vessels (and ISS of course) for early approach. Final approach is usually infrarad or radar or something like that. And the space shuttle uses GPS to land since late 2007. It starts GPS tracking prior to the de orbit burn , and except during a short portion of deorbit, it uses it all the way until it lands. —TheDJ (talk • contribs) 14:14, 28 November 2009 (UTC)

The "see also" template should be removed

There is a template which says, "The length of this "see also" section may adversely affect readability." I don't know whether that is true or not. The length of the "see also" section has never bothered me in the slightest. However the template has certainly adversely affected the readability of the document for me. Whenever I right click on a reference so as to view it under a new tab, I don't see the reference but instead I am redirected to this uninformative template. RHB100 (talk) 21:05, 1 December 2009 (UTC)

I think there are cases where the template presents a far bigger problem than the problem the template is trying to correct and that this "see also" template is one of those caes. RHB100 (talk) 20:35, 1 December 2009 (UTC)