Talk:Global Positioning System
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Common vs. Public Good

From the third paragraph: "President Ronald Reagan issued a directive making the system available for free for civilian use as a common good." Shouldn't that be public good? Based on the common good (economics) page, GPS would appear to be categorized as non-rivalrous (one person's consumption of GPS does not pleclude another's consumption), which would mean it's a public good. Further, I would consider GPS perfectly analogous to free-to-air television, which is given as an example of a public good. Jtradke (talk) 13:05, 21 May 2008 (UTC)

Accuracy and error sources

Since GPS signals propagate nearly at the speed of light, this represents an error of about 3 meters.

Why "nearly" the speed of light? Being EM radiation, wouldn't they propagate at the speed of light? --Marainman 20:27, 24 January 2007 (UTC)

Well, believe it or not the speed of propagation changes depending on the material that the radiation is passing through. In the case of our atmosphere, it's pretty close to the speed of light in a vacuum but isn't exactly the same. Check out speed of light. - Davandron | Talk 23:37, 24 January 2007 (UTC)
OK, makes sense now. I wasn't thinking nearly "nearly" enough. I suppose when we're talking about comparing signal timings from only thousands of miles away, that little bit can make a big difference. --Marainman 17:03, 25 January 2007 (UTC)
The Speed of light is not a constant therefore when it travels through 'dense' materials like the earths atmosphere or air, it slows down, therefore the signal is traveling at the speed of light and can in no way travel faster than that, but it is traveling slower that light in a vacume. —Preceding unsigned comment added by 68.9.95.171 (talk) 00:27, 13 March 2008 (UTC)
How does that make sense? Given Davandron's explanation, the phrase should be changed either to "Since GPS signals propagate nearly at the speed of light in a vacuum" or to "Since GPS signals propagate at the speed of light". —The preceding unsigned comment was added by 68.163.39.247 (talk) 01:33, 4 April 2007 (UTC).
You're absolutely right, of course. Fun with words.
All that said, I just noticed the GPS ICD defines the speed of light as the SI value in a vacuum for all GPS calculations. So, in this case, I don't think it matters much which way it's stated. If you'd like, go ahead and change it to "...propagate at the speed of light." And perhaps wikilink it.
For the record and the confused, even at sea level light is still 99.971% of c. And most of the path between the ground and the satellite is not at sea level, so the path's propagation is probably better than 99.99% c. - Davandron | Talk 03:45, 4 April 2007 (UTC)


"Position accuracy can be improved by using the higher-speed P(Y) signal." This isn't clear to me - higher-speed than what? AndrewWTaylor 13:02, 1 February 2007 (UTC)

Higher speed than the C/A code. The P(Y) code is ten times faster, so has 1/10th of the bit-position error factor. I'll see if I can improve that paragraph. - Davandron | Talk 15:59, 1 February 2007 (UTC)
thanks - I think it's a bit confusing because the sentence comes right after the bit about signals propagating at near light speed. AndrewWTaylor 21:59, 1 February 2007 (UTC)
I was confused as well when I initially read the statement... altered wording to explicitly refer to the higher P(Y) frequency rather than "speed" Macboots 21:08, 14 May 2007 (UTC)

Can anybody suggest how to control the fuel consumption on a car with GPS?

One of my cars has GPS on it. And I wonder how can I control or check out of fuel consumption with the program working with that GPS.

Thanks for further. Am - 124.120.77.228 09:25, 13 February 2007 (UTC)

Hello 124.120.77.228, the wikipedia is not intended to be a general forum, so questions and discussions only loosely associated with the article are discouraged. You might try asking your question in a gps forum, like GPS Passion's Forum. Good luck! - Davandron | Talk 13:56, 13 February 2007 (UTC)

Sure we can do that, but since this section is not intended for public discussions and others, I will be glad to answer (124.120.77.228 and interested people) through through my mail.

Article structure

I think the 'Applications' section should be moved back to the top of the article. It was moved further down based on the idea the article should be arranged "What it is, How it works, When it came to be, How it's used.", but I have been unable to find any reference for this structure. It seems to me that Inverted pyramid, Spiral approach, and items on the current duscussion page all tend to indicate this section should be moved back near the top of the article. --Michaelfavor 23:47, 16 February 2007 (UTC)

The "list of applications" seems less encyclopedic and more what wiki is not than the remainder of the article. I'm curious to understand why you would put this at the top? Can you cite any similar Featured Article that was formated this way; with an applications section first, then info on what it is or how it works? In a quick search I found none in that format, however both Radar and Compact Cassette use the current format. - Davandron | Talk 05:37, 17 February 2007 (UTC)

Missing 15m Relativistic correction?

I'm probbaly being dense here, but I've read Neil Ashby's piece and I can't find mention of the cited 15m claim. (He does talk about an 8m correction for orbital eccentricity on the top of page 8.) Can anyone point me at where he says there is a 15m correction? --Fuchsia Groan 14:28, 2 March 2007 (UTC)

I was unable to reconcile the 15m note with that article, so you're not alone. I'd guess that the 15m figure was original referenced by the IEEE presentation in the following sentence. That said, if the physics today article covered all the relativistic effects, then it should be able to yield the same result. So, we probably need to clean this up (either the article is wrong, or one of the sources is wrong). - Davandron | Talk 03:40, 3 March 2007 (UTC)
In my edits I went ahead and pulled it. If we find a way to usefully incorporate it, we can put it back in. The chunk in question was
The error introduced by relativistic effects can be as much as 15 meters. The GPS system also makes adjustments for the relativistic drift of the atomic clocks in the satellites. Parts of this correction are carried out in the satellites and parts in the receiver.<ref>Deines, "Uncompensated relativity effects for a ground-based GPSA receiver", Position Location and Navigation Symposium, 1992. Record. '500 Years After Columbus — Navigation Challenges of Tomorrow'. IEEE PLANS '92.</ref>
- Davandron | Talk 05:10, 3 March 2007 (UTC)
That's good work. It would still be nice to have a distance figure - it gives you a "feel" for the significance of Relativistic effects. (In a dense English housing estate, 15m would be the wrong street.) BTW this is the IEEE Paper cited above. (Although I'm not a subscriber.)
--Fuchsia Groan 10:40, 6 March 2007 (UTC)
Thanks for the kind words and good find on the paper.
After the rewrite, one of my concerns with a section discussing "errors due to relativistic effects" is that all of the currently listed effects are compensated for; they shouldn't show up as errror in a position fix. The other error sources are all present, though some are addressed with certain techniques; ie dual frequency, WAAS, etc. At the same time, I think it's important to describe the relativistic impacts so that people know what they are and how they are compensated. Does that make sense? - Davandron | Talk 13:44, 6 March 2007 (UTC)
Yes, that makes sense - but there seems nowhere else to put it!
Anyway, Neil Ashby says that the corrections vary from manufacturer to manufactuer, with not everybody correcting for Sagnac, so it may be there are real relativistic errors in a position fix. And the corrections he outlines are only approximations, ignoring higher order terms, so there is a tiny residual relativistic error.
--Fuchsia Groan 19:04, 8 March 2007 (UTC)

Interesting RAND org article to maybe integrate

I just stumbled onto this article at the RAND organization and in a quick glance through it might have some good content to integrate. I'm a little busy at the moment, so if someone else can use this before I get to it then go for it. - Davandron | Talk 03:09, 6 March 2007 (UTC)

Wow - great paper you found there, Davandron! I'd suggest addding it as a link, rather than attempt to encorporate bits and pieces - because it is so incredibly thick full of information - in the history section. The other steve jobs 16:29, 11 June 2007 (UTC)

Question re GPS History

Greetings! It isn't clear to me from the article when exactly the restrictions were removed on civilian use of the GPS (so that everybody could receive the more accurate signals previously limited to the US military). Was this Clinton's 1996 change, or when exactly did this happen? It would be nice if someone would update the article to clarify this. Thanks!  :-) —Preceding unsigned comment added by 152.216.11.5 (talk • contribs) 15:27, 23 March 2007

I believe a civilian-usable signal was aways available; there were no restrictions on recieving it. I know that in 1983 Reagon said it would be made available to the public. What Clinton did was remove an intentional error source (called selective availability) which made the civilian signal significantly more incorrect than the military signal.
I'll see if I can make that a little more clear in the article. - Davandron | Talk 17:37, 23 March 2007 (UTC)

Wikiproject Aviation

I'm not overly familiar with the wikiproject program, but this article covers far more ground than the aviation group alone. Should every project be invited to tag the talk page and assess it to their criteria? Seems like we'd end up with a long list of tags, and conflicting quality ratings. The tag was applied as part of a mass tagging (almost looks like a script), not apparently by someone who felt it was appropriate for this specific article. I yanked the tag for the time being, and invite the organizers of the aviation group to discuss here how they would improve the article. - Davandron | Talk 13:51, 24 March 2007 (UTC)

I've re-added the {{WPAVIATION}} banner because it falls (at least in part) under that projects scope. If other projects feel the article falls under their scope as well they may add tags to. If it seems like there are too many tags then there is the {{WikiProjectBanners}} template to help combine them. This is all part of the Wikipedia Version 1.0 assessment drive. It may not cause an immediate improvement in this article, but will hopefully allow it to be seen by more than just those who are currently interested in editing it. - Trevor MacInnis (Contribs) 06:13, 25 March 2007 (UTC)
I respectfully disagree, but we'll see. If your team gets around to rating the article, within say a month, then the community will know you're serious and not just soaking up articles. In the mean time, Military technology and electronics seem very apt projects for this to be tagged with. - Davandron | Talk 16:32, 25 March 2007 (UTC)

Trilateration or Multilateration?

Does GPS use Trilateration or Multilateration? (aside: both of those wiki pages refer to GPS). AndrewRH 15:02, 30 March 2007 (UTC)

I think the multilateration is adding a condition to its definition that isn't universally agreed with; namely the absolute requirement of Trilateration. If thats the case, they are effectively the same thing, see Talk:Multilateration#Merge. - Davandron | Talk 19:51, 30 March 2007 (UTC)
I noticed this, and I believe the explanation under the heading 'Simplified method of operation' is incorrect. The receiver is only able to measure the relative distances between satellites, that is unless it depends on an internal clock being accurate enough to be compared with the satellite timestamp. If the internal clock is not used for comparison then the individual distance to each satellite will not be known unless it is specifically calculated after all of the relative measurements have been taken. I don't believe the absolute distances are actually involved in finding the current location. Confirmation by an expert would be useful here. --Shinglor 15:28, 3 August 2007 (UTC)

GPS for Wardriving

Should something be mentioned in the 'Applications' section about GPS being used for wardriving? I searched through the entire article but didn't find any mention of GPS devices being used for this purpose. 131.230.53.188 03:15, 11 April 2007 (UTC)

What to do with the "Applications" section??

This is getting out of hand. Every day the application section seems to gain another entry only related to this article by the fact it uses an off-the-shelf gps receiver module. It grows, looking less and less "encyclopedic" with each addition. Imagine if we started listing every application that uses a lightbulb or a wheel, in those articles? Editors, we need to resolve this sooner rather than later, so how should we do it??

  1. Should we split it into its own page, the way laser created laser applications, and create GNSS applications? It moves the content so it's no longer in our backyards, but the page looks pretty bad.
  2. Should we just slice it down aggressively and keep out the wikilinks? Seems like without tending it will just grow again (i.e. requires lots of maintenance).
  3. While a thought is to move wikilinks into the "See also" section, this again just creates a long list, who's function seems better served by people searching for what they want.

Looking for guidance I found:

  • Laser split theirs into laser applications
  • LED ending up with a long list of truncated text
  • Composite material has a long list of articles
  • Lightbulb is very short paragraph; not sure if thats intentional or not.
  • The wheel lucked out; it was obvious enough it didn't get an applications section.
  • Metal has a small applications section, not much guidance
  • Aluminum has a prose-based applications section, with most things in engineering

- Davandron | Talk 04:55, 11 April 2007 (UTC)

With the lack of discussion after two weeks, I'm simply going to follow WP:Bold; I'm going to move the detailed "applications" to its own article, link it with a main article, and replace with four paragraphs of prose describing applications. My goal will be to keep those paragraphs concise, which I hope will reduce the maintenance burden. - Davandron | Talk 13:31, 26 April 2007 (UTC)

Precision

"More than two dozen GPS satellites are in medium Earth orbit"

How many are there exactly? Is this an unknown number?

Ironcorona 20:25, 18 April 2007 (UTC)

The number of operational satellites varies as units fail and are replaced, but the article does state in the space section:
"As of April 2007, there are 30 actively broadcasting satellites in the GPS constellation." - Davandron | Talk 02:46, 19 April 2007 (UTC)

Picture

Is it me, or does the 'Artist's conception of GPS satellite in orbit seem like it's at too low an altitude? —Preceding unsigned comment added by Phooto (talk • contribs) 14:23, 19 April 2007

The Earth part of the picture might be from the shuttle or a similar vessel, which would be lower in orbit than the satellites. That said, I don't think the Earth component of the picture was the artist's focus. - Davandron | Talk 17:16, 19 April 2007 (UTC)


Putting things in a alphabetical order

Hey, there. I just got the following message on my talk page, which I would like to answer here.

Order of projects at GPS
Hey there, I'm reverting the order of projects at GPS. In the future, please be considerate and discuss your suggested changes in order or add your new projects to the end of the list. - Davandron | Talk 12:41, 9 May 2007 (UTC)

I apologize if I made a mistake here. I've put my project tag in at least two dozend other pull down menu's. But nowhere there was so much and so long information in this menu. So it seemed like a good idea to put that Systems tag a little higher on the list... to give the other projects all the time and space to use the further space in the pull down menu. I guess I made a bad judgement.

The thing I did was put the names in an alphabetical order. I guess the Military history WikiProject was first, does most of the improvement here, and want to remain first.

A thing I want to do also, is put this article in a systems category. In that case I would also put the category names under the article in alphabetical order. Because of this argument now, I thought I ask here first. So, what do you think of that? - Mdd 13:11, 9 May 2007 (UTC)

I've just been bold and changed it anyway. Keep up the good work here. - Mdd 14:16, 9 May 2007 (UTC)


Leap Seconds

The statement "GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC." is misleading. It leads the reader to think that his GPS receiver won't get leap second information from the satellite.

As the next paragraph specifies, "The GPS navigation message includes the difference between GPS time and UTC" and "Receivers subtract this offset from GPS time to calculate UTC." When the leap second occurs the GPS time doesn't change, but the UTC correction data that is part of the GPS signal jumps to a different integer number of seconds. The information is the same, but the the format is different.

The statement "GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC." does not clearly convey the fact that it only refers to the GPS time portion of the GPS signal, and that the GPS signal does contain "other corrections which are periodically added to UTC." Also, "added to" should be "added to or subtracted from" when talking about leap seconds.

References:

Hewlett Packard Application Note 1289: The Science of Timekeeping http://www.allanstime.com/Publications/DWA/Science_Timekeeping/index.html http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf


United States Patent 5923618: Leap-second cure for 1999 GPS rollover problem http://www.freepatentsonline.com/5923618.html


Guy Macon http://www.guymacon.com/ Guymacon 15:42, 3 June 2007 (UTC)

Number of satellites needed for positioning.

To get a positional fix in 3D space at least 3 independend parameters are needed. (This could be length, width and higth). Initialy the clock of a GPS receiver can not be used. The GPS receiver does not know the time and therefore can not determine the distance to the first satellite.

1 satellite, no positional information.
2 satellites, 1 delaytime difference (1 parameter). (1D position determinition)
3 satellites, 2 independed delaytime differences (2 parameters) (2D position determinition)
4 satellites, 3 independed delaytime differences (3 parameters) (3D position determinition)

For 3D positioning 4 satellites are needed.
A difference in delaytime of two satellites does not determine a phere where we are but a hyperboloid (in 3D) or a hyperbole (in 2D). The intersection of 2 hyperboloids determines an arc in 3D space where we are. The intersection of 3 hyperboloids determine a point in 3D space. (Exactly: could determine 2 points) See "en:Multilateration also known as hyperbolic positioning".

For 2D positioning 3 satellites are needed.
With the informaton of 3 satellites an arc of the intersection of twee hyperboloids can be determinded. This arc will probably intersect with two points on the surface of the earth. One of those points will be very unstable, so it's easy to determine the correct point on the surface of teh earth.

De clock.
Only after the GPS receiver has determined it's position it is possible the set or correct the time on the clock of the GPS receiver. Once the clock is corrected it can be used (for a short interval) als one of the parameters to determine the time, so a satellite less is needed. But a clockdrift of 1 meter for each second is not exceptional, so after a minute the clockdrift can be as much as 60 meters.

Small note: this assumes an imprecise clock, such as a crystal, which I will grant is completely normal today. However a more stable clock, such as a miniature atomic clock, invalidates much of this concern; and miniaturized atomic clocks for GPS receivers are something that is being pursued in the research community where they have working prototypes right now. - Davandron | Talk 12:44, 6 June 2007 (UTC)

Position with a single satellite.
With a stationaire GPS receiver containing an extreem accurate clock, given the time, the GPS receiver could calculate it's postion with the information of only one satellite. Because satellites are moving all the time, four different positions of one satellite will be enough to determine a position. The four positions of the one satellite should be fairly different though.

The usual explanation of first determining the distance from a set of satellites and than calculate the postition of intersecting spheres is not correct. Although spheres can be used in the calculating process, the position of the GPS receiver is determined by the intersecting hyperboloids. Crazy Software Productions 13:31, 6 June 2007 (UTC)

Any refs for this? At a first glance it seems to contradict the usual (published) explanations of GPS positioning. Especially that a position is needed before clock correction is made (they are usually described as being done at the same time). That 4 satellites is needed: in theory yes, but for the practical matter of recievers on earth the ambiguity can be resolved, and only 3 satellites are typically needed. The GPS system is typically not described as using multilateration, but instead as using absolute TOA measurements. Mossig 12:42, 6 June 2007
The signal of the gps satellite itself is a time stable signal; you don't need to have position to determine relative time (meaning, a signal that pulses at 1 pulse per second or whatever), however, since you don't know how far the signal traveled you can't know absolute time until you know your position. You can know the current time to within 6 seconds (one frame) without locking onto more than one satellite, but anything more accurate requires a position fix. - Davandron | Talk 12:49, 6 June 2007 (UTC)
Exactly. I can make an guess of my position based on my unaccurate clock. The signals from multiple satellites (4) will then only coincide with a position on the earth surface (or near it) when I have adjusted my clock to the correct time. And at the same time I get my position. This method does still not use multilateration. Mossig 13:16, 6 June 2007 (UTC)
If you are earthbound, just saying you are on earth is more accurate than a accurate time within 6 seconds, 6 seconds is about 1.8E9 meters of. Question if not earthbound how do you know that you are only 6 seconds off. If earthbound you are far less of than 6 seconds off with reception of one satelite.
I concure with "you can't know absolute time until you know your position". So position has te be calculated first. Mathematically if two points send a signal at a same time, and you receive them on different times, this same difference in time would occure on any position on a virtual hyperboloide in 3d Space. As a receiver of the signals without extra information you can determine that you are at a point somewhere on the hyperboloide, but you cannot determine where.
With 3 satelites you can determine where you are on a intersection of two hyperboloides (an arc) but can not determine where you are on the arc. (If you are on the earth you can determine where you are on the arc in two points, one point being so unstable that you can easily determine which point is correct).
Calculations can be done with spheres, but your position is still on one, two or three (intersecting) hyperboloides. This is not because of design, but because of mathematics.
Guesses based on an unaccurate clock, will place you within an sphere. But the satelite signals will place you in a point on the hyperboloide intersects.Crazy Software Productions 13:47, 6 June 2007 (UTC)
With 4 satellites you will get an unique point if you have an accurate clock. Of not, you will get a discrepancy, as you get surfaces that do not intersect in one point. By adjusting your clock you can get the spherical surfaces to inteserct in one point, and at this instant you will both have an accurate position and an accurate clock. Mossig 21:36, 6 June 2007 (UTC)
Additional, just did a small test with a GPS receiver, I had not used the GPS receiver for a few days. The clock was off something between .7 en 1.3 seconds. This was the time difference between switching on indoors and after getting an accurate fix. 1 second off is about 3E8 meters, so getting a position based on this would me put just within moon distance of the earth. So for positioning this doesn't give me an accurate position. The clock is therefore completely useless untill you get a more accurate position to get the clock corrected.Crazy Software Productions 14:13, 6 June 2007 (UTC)
Stepping back a second, Crazy I'm having a hard time understanding what exactly are you looking to change in the article? - Davandron | Talk 23:33, 6 June 2007 (UTC)
Hyperboloids?
As far as I know, calculating a position using a GPS receiver uses only spheres. A GPSr starts off with a rough estimate of the current time-of-day, and calculates the distance to each satellite using the transmitted time and a comparison of the received PRN codes with the internally-generated PRN codes. Then, when it has acquired four satellites, it can get a much better estimate of the time-of-day, and thus a more accurate fix, barring the usual atmospheric, geometrical and multipath sources of error.
How can you apply hyperboloids to GPS, and can you bring up a verifiable source explaining it? --  Denelson83  06:34, 1 September 2007 (UTC)
This can be verified using mathematics. Fairly easy to do in 2 D. Use a piece of paper draw two points. For example 12 cm appart, choose a point (for example 4 cm from one point, 8 from the other) now draw all points with a difference in distance of 4 centimeters. These points will result in the shape of a hyperbole. Getting signals from your points and if the signals are four cm appart, you should be on one of the points which make the hyperboole. If this difference in signals (4 cm) is the only information you have got you know you are on the hyperboole, but can not distinguise where. Rotating the paper on the axes through the two points will give you a hyperboloïd in 3 D space. Giving all the points where the diffence to the two points is exactly 4 cm.
Mathematecally proof: If you want to be able to give any position in 3 D space you need 3 independend parameters. This can be x,y,z or longitude, latitude plus hight, or even the distance to 3 fixed points in space. If four satelites ping at the same time, one ping only does not provide any information (except for signal strength, but we are not using that). Two pings give one time difference and therefore one parameter. To be able to describe any position in 3 D space we neede 3 independend parameters, 4 pings give 3 independend timedifferences, so 4 pings is the minimum needed to determine a position in 3D space. With 3 pings we only have 2 independed timedifferences, giving 2 parameters this can never give all the points in 3D space.
Application, with an implementation of pseudoranging using the signal of 4 satellietes (3 time differences) and without a time we can calculate a 3D position. This also determines the distances to the satellites. So if the satellites send more than a ping, such as a send time, we can even get the correct clock time. (Pseudoranging calculates the x, y, z and time difference at the same time. Not precise but to an arbitrary accuracy.)
Remember this is all based on the fact that a GPS receiver does not have an enough accurate clock to determine the distances to the satellites. If there is such a clock available, then there is extra information and other methods can be used. Of course with the reception of four satellites the times can be guessed and guessing the right time will determine the right location. But how to guess the time in a practical way. Actually there are ways in which this can be done, with actual calculation of the pheres based on a guessed time, but compared to Pseudoranging mentioned elsewhere that method is not very practical.
Crazy Software Productions 19:47, 8 September 2007 (UTC)
The thing is, the satellites do send the correct time as part of their Navigation Messages. A timestamp is sent roughly every half a minute. Also, the GPS receiver's not-as-accurate clock assumes it's correct, until the receiver discovers that its pseudoranges from four satellites don't intersect.
Yes satellites do send the correct time. But by the time the signal arrives at the GPS receiver, some time has passed. The GPS system is based on the time pased and only on the time passed, this can not be derived from the signal.
Well if the GPS assumes the clock in the GPS to be correct, Why does my GPS (and all the others that I know off) revert to 2D calculation when only 3 satellites are available (and not use the time for this calculation)? And Why does my GPS (and all the others that I know off) stop positioning when only 2 satellites are available? 3D calculation only needs a good clock and the reception of 3 satellites. 2D calculation only needs a good clock and 2 satellites. (To be fair the display 3D, 2D and not calculating only shows that 60 seconds after losing the fourth, the third, or all sats.) Crazy Software Productions 19:47, 11 September 2007 (UTC)
BTW, You've been maintaining a position that a GPS receiver calculates its position using TDOAs. But that's how LORAN-C works, not GPS. --  Denelson83  03:20, 10 September 2007 (UTC)
My assumption is that consumer models do not have an accurate enough clock for absolute timing. All the quartz clocks I have seen up to date have a rather large timedrift. A timedrift of 1 second a year will make an error of at least 10 meters a second, but for most satellite configurations 15 meters a second. A more realistic timedrift of 1 second a week will give 500 meters a second (minimum), but for most configurations 750 meters a second (that's 120 miles / hour). If the error increases with this speed, positioning will become totally usesless within seconds. Based on this assumption and knowing how GPS models switch from 3D to 2D and to no position calculation I come to the conclusion that the absolute time is not used at all.
Pseudo calculation (See :[1][2]), does not use the absolute time, any time can be used, as long as the differences are the same, a timing error of a large number of seconds. So absolute time does not play a role in this pseudo calculation (as neither the national American Budget :-). So if time does not play a role in this calculation then it must be based on the time differences, as it is. Because the calculation is only base on time differences, I used the term TDOA's. So my only assumption is about the clock, from this follows that the calculation can only be based on time differences, this is totally consistend with the behavior of the GPS receivers I have used and fits the mathmatical model for the pseudorange calculation in the references.
The assumption that with not enough satellites the absolute time of the quartz clock is used is not consistend with the behavior of the GPS receivers I have used. On the internet I was not able to locate an algoritm for this situation, nor an approcimation for this situation. The time difference of arrival of satellite signals do define a hyperboloïd, but calculating with hyperboloïds is far more cumbersome than the referenced pseudo calculation, more difficult to implement en takes far more time to come to a result with the same accuracy. So pseudo calculation beat calculation with hyperboloïds on all points except the in principle the hyperboloïd is actually the 'correct' method. Using spheres and iterating with the time (or guessing the time) is far more cumbersome than the referenced pseudorange calculation. Using the absolute time of the gps receiver and 2 satellites (for 2D) or 3 satellites (for 3D) would lead to very large errors, this is probably why GPS receivers have not implemented this, that is the GPS receivers I have worked with so far.
Conclusion: A GPS with a quartz clock, uses the time differences of arrival of the satellite signals to calculate it's position.
I think that the first assumption (about the quartz clock) is correct.
I think that the reasoning is correct.
I think that this fits with the behavior of GPS receivers.
I think that this fits exactly with the number of satellites needed (minimum=4) for 3D calculation.
I think that this fits exactly with the number of satellites needed 3 for 2D calculation.
In the published pseudo calculation algoritms GPS clock time does not play a role. (The end result is not influenced by this).
The suggested use of the clocktime of the quartz clock of the GPS receiver does not fit the behavior of the GPS receivers. (At least the ones I have seen, and seen documentation about).
The National American Budget (NAB) was not only inserted to be funny, it's not that funny. But more that if the outcome of a solution is not based on one of the parameters (e.g. NAB) used, that parameter (NAB) is not actually used. So although time is used in the calculation, but the actual time does not influence the outcome of the calculation it actually is not considered used. (Although it's plainly visible in the calculating steps, in the same way I could make a calculation where in steps of calculation the NAB is used, but not influencing the endresult).
Crazy Software Productions 19:47, 11 September 2007 (UTC) (Sorry for the length, but I needed this to explain the steps as carefull as possible, sorry.)

New archive: January 2007

I've moved all the content last commented on in January to a new archive. - Davandron | Talk 23:50, 6 June 2007 (UTC)


Four satellites needed?

Don't know if this is worth an edit, but concerning "When four satellites are measured simultaneously, the intersection of the four imaginary spheres reveals the location of the receiver.", three satellites give two possible locations, and if one is in outer space or is moving faster than the fastest known aircraft while the other is in a fixed position on the surface of the earth, it's pretty clear which is the real location. I suspect that there is no possible combination of three satellite positions that puts both points on the surface of the earth and all three satellites above the surface of the earth. Guymacon 15:24, 7 June 2007 (UTC)

You're correct; if you assume you're on or near the surface of the Earth, that surface becomes your fourth sphere. With a poor geometry your two solutions could also be just above and just below the Earth's surface so you'd need the 4th to correctly know your altitude. Also, there is talk about needing a fourth signal to significantly improve error detection and clock correction. - Davandron | Talk 04:11, 8 June 2007 (UTC)


With TOA (a good clock in the GPSreceiver) and a reception of three satelites, the three satellites will all three be above the horizon and forming a plane. There are then two solutions for your location, your actual location and the your actual location mirrored in the plane of the three satellites. Because that plane is always (far) above you, the other solution is also very far above you, beyond the plane of the satelites. This solution is far removed from the eath surface and can for normal usage be eliminated as a correct point. So a fourth satellite is not needed to eliminate this point. (The point is also traveling very fast and can therefore be eliminated as well). A poor geometry would require that you are within a few miles from the plane of the three satellites, even in a high altitude plane or on a high mountain, the likelyhood of receiving three signals of satellites on the horizon and no other signals is so small that you do not have to account for these situations. (All three satelites have to be on the horizon for such a geometry)
The fourth satellite is needed because the GPS receiver initially does not have a usable clock for the positioning. For positioning three intersecting surfaces are needed. Without a good clock one satellite does not provide such a surface. But a pair of satellites does provide a surface. This surface is defined by the TDOA. All points in the surface have the same Time Difference Off Arrival of the two satellite signals they do not form a sphere but a hyperboloid. Three independed surfaces are needed, 4 satellites will give three independend surfaces which makes it possible to calculate a position. If the GPS receiver has enough information to calculate its position it can set its clock knowing the distance from one of the satellites and the TOA of the signal. After the clock is set the GPS receiver can use TOA with three satellites to calculate its position. Depending on the clocks accuracy, it needs the signal of the fourth satellite to get the clock set correctly once in a while. Clock drift of 1 second a year will give a shift of 10 meters a second. The not corrected clock drift on my GPS is more than a second each week. The GPS receiver will not be able to distinguise between clock drift and movement of the receiver. So it needs to adjust the clock quite often and therefore the signal of four satellites.Crazy Software Productions 16:36, 9 June 2007 (UTC)
A pair of satellites gives you a ring of possible positions. --  Denelson83  06:36, 1 September 2007 (UTC)
A: A pair of satellites and NO extra information gives you a hyperboloïd, this hyperboloïd defines all the points with the same timedifference of two satellites. The GPS receiver is located somewhere on this hyperboloïd.
B: A pair of satellites AND the known exact absolute time at the location of the GPS receiver gives you a ring of possible positions. The GPS receiver is located somewhere on this ring.
C: A pair or satellites AND the time of a quartz clock gives you a a ring of possible positions. The distance of this ring to the actual location of the GPS receiver depends on the 'timedrift' of the clock. A reasonable timedrift of 1 second a week would increase the error with 750 meters a second for a typical satellite configuration. A very small (not realistic) timedrift of 1 second a year, would increase the error with 15 meters a second for a typical satellite configuration. At the moment it's not feasable to build cheap enough clocks with an error of less then 10 seconds a year. The error of most GPS receivers is larger. 10 seconds a year would still give you an error increasing with 150 meters a second. Both Garmin GPS receivers I have tested on this had a clockdrift of more than a second a week.
With all the above situations this includes the information send from the GPS satellites (including the send timestamp).
The situation B is not feasable for mobile GPS receivers.
Crazy Software Productions 20:15, 11 September 2007 (UTC)

Ambiguous statement

The article states: "The onboard clocks are extremely accurate, but they do suffer from some clock drift." Is that referring to relativistic clock drift or the wristwatch kind? Is there a source? —Pengo 04:55, 8 June 2007 (UTC)

SHould refer to the wristwatch kind. Relativistic corrections are handled separatly. Mossig 09:29, 8 June 2007 (UTC)

Calculating pseudorange

The C/A code has a period of 1ms, so the time delay of C/A code can only give distance up to 300km. How do you find the ambiguity, the number of times the wavelength the signal covered when traveling to the receiver? According to navstar site, page 9: "The detection of the signal only tells you time with a 1 ms ambiguity (ie., the receiver does not the specific ms segment it is tracking). To resolve the ms ambiguity usually requires decoding the data message and looking at the pattern of bits to make sense of the data message. Since the data message is transmitted at 50 bits/sec this can take a number of seconds to get enough bits to know what they represent." but how is it done? What does the receiver read in the message, the Hour of Week (HOW), the Time of Week (TOW)? Besides, a bit from the data message lasts 20 ms, which makes me think it's impossible to know which of those twenty C/A codes emitted the receiver is tracking. 89.156.246.108 20:11, 12 June 2007 (UTC)


It's actually quite simple to resolve the ambiguity since ALL the C/A codes and navigation messages are synchronized to launch at the same time from all the satellites in the constellation (to within the accuracy of the satellite clock). What matters for the navigation solution is _not_ the absolute distance to satellite, but rather than absolute relative distances from each satellite currently being tracked. This is determined by comparing the relative arrival times of the signals within the receiver, which in turn is determined by keeping track of the number of C/A codes that have gone by since some reference point in the bit sequence. The reference point can be the beginning of the subframe, or any other bit within the nav message so long as the measurement is made from the same bit from all the satellites. 98.195.24.109 19:22, 5 October 2007 (UTC)Alessandro Cerruti, Cornell University

UK jamming

I have reinstated the modest reference to UK MOD jamming. User Dual Freq's commented/asked: "US military performs jamming ... exercises on a routine basis, why is this test important?". Simply, because such jamming is not at all routine in the UK, hence it is notable. Springnuts 07:02, 14 June 2007 (UTC)

Error in article regarding RAIM predictions

"Receiver Autonomous Integrity Monitoring (RAIM) is a feature now included in some receivers, which is designed to provide a warning to the user if jamming or another problem is detected." This is an incorrect statement. ______________________________________________________________________________

The receiver autonomous integrity monitor has nothing to do with jamming or spoofing, and it will not provide any warning or indication of such an occurrence. It is a prediction of satellite availability at a certain place and time to ensure that you have sufficient satellites in view and geometry to provide a suitable navigation solution. You can go to "RAIMPREDICTIOIN.NET" and view a RAIM prediction tool. This tool indicates possible outages and time spans where there may not be sufficient satellites in view to give a good solution. It considers known satellite outages, which are indicated in the satellite message that is transmitted to the GPS receiver. (Satellites are sometimes taken "out of service" for maintenance such as a software change.)

The ability to do a RAIM prediction is one reason that GPS receivers built to FAA Technical Standards Orders (TSO-C129, 145 or 146) are far more expensive than a hand-held aviation unit or GPS units for your car. 165.254.210.3 17:55, 18 June 2007 (UTC)

I think you are mistaken. "RAIM Prediction" and "RAIM" are two different things. RAIM is a feature that detects problems, but it requires yet another satellite in the fix to do it. In some cases it can correct problems by throwing out a bad signal, in which case it needs TWO extra satellites. "RAIM Prediction" is a way of determining whether or not you'll have that extra satellite or two throughout your journey. Reswobslc 18:25, 18 June 2007 (UTC)

GPS time "synchronized... with TAI"

From the article: "The lack of corrections means that GPS time remains synchronized with the International Atomic Time (TAI)". This is rather misleading, since to me at least, "synchronized" implies that there is no offset (e.g. 'synchronize watches' means to set the offset to 0), which is not true -- GPS is 19 seconds ahead of TAI. What is true is that this offset does not change, unlike the GPS/UTC offset. I've made the change to clarify this. -- simxp (talk) 08:27, 20 June 2007 (UTC)

Supportative external links

Links to external sites that are are supportative of the article and do not have undue commercial content are appropriate IMHO. One such link is GPS and GLONASS Simulation, which is intended to help illustrate the geometry and motion of GPS space vehicles. It is totally devoid of any commercial content and is not link spam. Roesser 20:16, 6 July 2007 (UTC)

Hyperboloids

The way of computing positions described in the article seems to imply that the receiver also has a high-precision clock (to be able to compute pseudoranges), which is not realistic.

If the receive is not capable of obtaining absolute time, but only time differences between signals, a pair of 2 satellites define a hyperboloid. At least 4 satellites (3 if at sea level) are necessary for location. David.Monniaux 20:10, 7 July 2007 (UTC)

Pseudoranges and Hyperboloids are two equivalent ways of describing the calculation of the recievers position and time. Ref: "An alternative closed-form solution to the GPS pseudo-rangeequations", Leva, J.L. ,Mitre Corp., Bedford, MA; IEEE Transactions on Aerospace and Electronic Systems, Volume: 32, Issue: 4 Mossig 23:14, 8 July 2007 (UTC)

Two references to Pseudorange calculations :


[3]
[4]
In second page click on : Pseudo-Range Navigation Solution Example

Both use the same mathematical principle, both use the term pseudoranging. This method itterates very fast towards the pricise location where you should be at. The principle is starting off with an estimate where you are, calculate from 4 signals (3 time differences) your actual location and the time offset. Actually the vectors used represent planes (or a set of parallel planes) these are shifted by the same amount until they intersect in one point. This point is then used as the next estimate, this process is repeated until an accurate enough solution is found. With four satellites and using for example the centre of the earth as the first estimate only a few itterations are needed to get a very close approximation to the theoretical precise point. In the matrix calculation the errors for x,y,z and time are calculated at the same time. But for the (x,y,z) it is neccesary to use them in the next calculation, for time this is hardly neccesary.
So the actual calculation (in the examples) is done with planes. And actually with four satellites there are still two (x,y,z,t) solutions which both are in agreement with the signals of the four satellites. But the second point is very difficult to find, because it's very hard to make a first estimate close to that point. It's probably far from the earth and moving fast, while the 'actual' point is very near to the earth surface and reasonable close to the centre of the earth.

The location on hyperboloids and the intersection of hyperboloids are correct, but to calculate with hyperboloids is far more complex than the show pseudorange calculations and because we are looking for one point only this show pseudorange calculation is far more suetable than calculating with the hyperboloids which in the end will get you the same (two) solutions.
My guess is that this is called pseudoranging, because if the calculations where done with infinite accuracy, each itteraton would get closer to the solution, but the solution would never be reached precisely. The second show page is fairly easy to implement in a spreadsheat. Doing so you can check that with a good estimate only two itterations are needed to get to a very addequate solution. Using the centre off the earth as a starting point, three iterations brings you really close. The solution shown does use a modulo the size of 1 millisecond, so using a estimate which is a bit more off, the implementation of the calculation has to be altered to provide for this. The pseudorange calculation toward a point and hyperboloid calculation will lead to the same point, this type of calculation is definitely not the same as trilateration.

At the moment I am working on a text explaning spheres, hyperboloids and planes in context of GPS. (And TOA,TDOA,trilateration,multilateration)
Summery

1 Spheres represent the position of the signals in real time.
2 Hyperboloids represent the location of the receiver when actual time is not know. Intersecting hyperboloids boloids define the actual position. Three are needed for this four satelites are needed.
3 Planes are used in the calculation to iterate towards the correct position. With pseudoranging the time can be corrected as well, but this is not a neccesary part of the process. The iteration to the correct position only needs a few iterations if a reasonable estimate is choosen. (A timing device is needed for determining the Time Differences Off Arival, but a clock is not needed in this calculation).


Crazy Software Productions 18:15, 16 July 2007 (UTC) The text above is not stable yet, it is a bit prone to alterations. (Sorry for this).
Crazy Software Productions 18:15, 16 July 2007 (UTC)

speed of light

There were two revisions over the statement "Since GPS signals propagate [nearly] at the speed of light, ...". The presence of "nearly" seems to be the point of contention.

I submit that each signal travels at the speed of light. It's just that c has a lower numerical velocity in air and decreases as air density increases. —EncMstr 23:40, 16 July 2007 (UTC)

No, due to dispersion, the two GPS signals travel at slightly different speeds in the atmosphere, and this must be accounted for in high precision GPS applications, like surveying. Also, the "speed of light" often refers to the constant speed of light in a vacuum, not the speed of a particular wavelength in a medium. Dhaluza 00:15, 17 July 2007 (UTC)
I think that you mean that c is the "speed of light in a vacuum", and is a constant (no matter the frame of reference). Electromagnetic radiation (including light and radio signals) travels slower in other mediums. It can even be slowed down enough that matter can be accelerated to faster than the speed of light in that medium, creating the analog of a sonic boom with light, Cherenkov radiation. — Val42 06:28, 17 July 2007 (UTC)
This is why Prof. Richard Muller (UC Berkeley) suggests that we refer to it as the 'Einstein constant' instead of the speed of light.--Shinglor 15:44, 3 August 2007 (UTC)

Number of fully-functional GNSS'

I believe that GLONASS is not currently fully functional, since its article states that 24 operational satellites are needed to make it fully functional. Therefore GPS is indeed the only currently fully functional GNSS, contrary to a recent edit of the first sentence. Roesser 15:32, 6 August 2007 (UTC)

Adding the Template:Systems link

As requisted I hereby first propose to add the following Template:Systems under the listing of links:

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Systems and systems science
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