Talk:Global Positioning System

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)

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

...

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

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

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

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

You should be aware that a GPS receiver does not directly calculate latitude and longitude. The position is always first calculated using a three-dimensional rectangular coordinate system, and then converted into whatever coordinate format is selected using whatever geodetic datum is selected. --  Denelson83  07:42, 25 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)