GPS question

[Troy_from _Oz]

Re: GPS question

JB - im glad you dont disagree on too much of it. Your comps seem reasonable.<br /><br />just out of interest the 3 feet that you mentioned represents the error in the pseudorange (timing error * speed of light). If all of your satellites are clustered in one portion of the sky (say if a large part of the sky was masked out), then you might have more error in one component of direction than another (your lats might be really good but your longs might be poor). there are also other errors aswell - the largest being errors in the predicted orbit of the satellites -which will degrade the position, then atmospheric refraction and some other smaller ones.<br /><br />Poolshark - JB's hit the nail on the head there<br /><br />cheers, Troy

 

[Drowned Rat]

Re: GPS question

LubeDude, they will all give you a MPH reading.

 

[sloopy]

Re: GPS question

any gps will give you MPH<br /><br />I only disagree with one thing troy, I am researching it right now and will debate when done Although one of my books has some differences on what J.B has said, But I am not sure and do not want to shoot the only milk cow.<br /><br />this fun, I may turn it into my science fair project! But I would have to think of a project to do.<br /><br />Does anyone know how to state something you read on a forum on a bibliography?

 

[sloopy]

Re: GPS question

And I never meant to say that GPS devises have atomic clocks! The satellites do! This GPS concept revolves around time (and the earth)

 

[JB]

Re: GPS question

Troy.<br /><br />We agree that lack of divergence in the directions from Sats to RX reduces the theoretical accuracy.<br /><br />Though celestial navigation uses direction to stars and GPS uses distance to satellites, each produces a "line" or "arc" that must intersect others to make a "fix". Ideally the lines used to make a fix cover a range of angles of 180 degrees (the classic "triangulation" of celestial and other electronic navigation methods). <br /><br />If you can only read satellites in a small arc of the sky theoretical accuracy is degraded, though I have not experienced this phenomenon in actual use.<br /><br />Got that, Dock Boy? If your book disagrees with the Ancient Mariner, believe the book. I am working with instant recall and old experience as a navigator.

 

[sloopy]

Re: GPS question

Don't worry it only has to do with your numbers.

 

[mattttt25]

Re: GPS question

4 nanoseconds, jb? that's about the best anyone in the world can do. and they get there at the naval observatory. gps clocks do not.<br /><br /> http://tycho.usno.navy.mil/gpsinfo.html

 

[Troy_from _Oz]

Re: GPS question

Sloopy - you were correct in saying that GPS uses atomic clocks. Unfortunately i fear I may have offended more people with my list of myths than i meant to ... - They wernt just derived from what was said on this board, but from the things that I have heard people say about GPS over the years - so when I read what you said before - it reminded me of a number of people that have said that very precise clock (atomic) are used in the receiver. Ive actually seen that written in a 4wd'ing magazine before - which really erkd me - someone writing a tech article should have gotten their facts straight.<br /><br />What reference book are you using Sloopy?<br /><br />Are you wanting to state something from this forum in a bibliography for a school project? or something on another forum here?

 

[sloopy]

Re: GPS question

If I used something a read in a forum in a research paper, How would I write it in a bibliography?

 

[sloopy]

Re: GPS question

And J.B was 100% correct, four nanoseconds... wow

 

[sloopy]

Re: GPS question

One of the books is,<br /><br />Understanding GPS: Principals and Applications.<br />-Elliot Kaplan<br /><br />It is not in print anymore and is an old book. I found it at yard sale.<br /> <br />I also used info from the JPL and NASA web site.

 

[Troy_from _Oz]

Re: GPS question

Sloopy - as this is a bit off topic, if youve got an email address you can post, I can send you the referencing style for citing a reference from a bulletin board.<br /><br />Kaplan is a good book. I'd say you got a good buy if you picked it up from a yard sale.<br />Cheers T

 

[ThomWV]

Re: GPS question

Troy,<br /><br />Good to see you again. Its been a while.<br /><br />Sloopy, when you give a web address as a citation you simply give the URL, the date (very important because web pages are updated from time to time), and the author's name (or pseudo name) if known.<br /><br />Nice discussion. <br /><br />Speed, you can figure that your speed, as indicated by the GPS. is accurate to within 0.1 mph, just in case you wondered. I like the pseudo this and the pseudo that. I'd like Troy to give an explaination of how we actually find the GPS signal, as weak as it is, among all that radio frequency noise that is out there. As he well knows the key lies in what is called Pseudo random code that is broadcast by the GPS satellites at the beginning of their information string. I never understood how encryption machines work until it was explained to me and then a while bunch of applications came immediately to mind. Interesting stuff if you ask me.

 

[Spidybot]

Re: GPS question

Interesting topic.<br /><br />Thank God we don't have to utilize all that info to use the darn thing <br /><br />Great that your thing gets you there even if you don't know how it does it..

 

[Troy_from _Oz]

Re: GPS question

G'Day there Thom. Hope you are well.<br /><br />Sheesh - you had to ask a difficult question didnt ya! <br /><br />The electronics of how we latch onto the signal is not really something im a full bottle on - but i'll give the best description that my understanding allows...<br /><br />There are two issues of how the GPS receiver locks onto the signal, one is knowing what the signal is, and the other is knowing how to see it in all the noise... I'll start with the first one first (naturally).<br /><br />Besides the term pseudorange, there is also term called pseudo-random noise (PRN) code . PRN codes are called this because they are meant to look like random noise, but because they are generated by an algorithm, and arent really random at all (hence the pseudo bit). This is effectively the timing code that the receiver uses to determine the distance between it and the satellite. All it is a binary sequence (ones and zeros). What the receiver tries to do is to align the incoming PRN code (one for each satellite) with one that it determines itself. (knowing the algorithm, it can replicate the code). Incidentally, this is how the military could degrade the signal to us (using selective availability measures) while still being able to use the receivers themselves. There is one ranging code called the C/A code which is transmitted on one frequency, and another code (p-code) transmitted on another frequency (sort of). the military allows us plebs to have access to the C/a ranging code, but not the p-code - hence they could degrade one and keep the other to themselves.. anyways thats off topic. There is a part of the receiver that is known as the code tracking loop that tries to match or correlate the internal PRN code with the received one. When is matches up - your receiver is locked on to that satellite. <br /><br />The second issue (which i probably should have mentioned first in hindsight) is to be able to extract the signal from all of the background noise. As Thom mentioned - it is a very weak signal. I once heard it likened to being the sound of a bees fart (very quiet i would guess!). The signal that is transmitted from the satellite is transmitted as what is known as a spread spectrum signal. What does this mean?? well the nominal frequency for the C/A code is 1575.42MHz. If this were to be transmitted as a clean signal you would have a spike at 1575.42 representing the signal and nothing either side. The spread spectrum signal has some of the signal power either side of the spike- This sort of helps you narrow down where the main part of the signal will be. like tuning in the radio the old fashioned way (with a knob) - you hear the station start to come in then it gets loud, then goes off station again. in the middle of where the station starts to come in and where it starts to go out should be the strongest signal... Why not just tune the receiver to 1575.42 and be done with it?? i think that the signal wanders about a bit as it is being propagated through the atmosphere etc. I would also guess that Einsteins theory of relativity might also play a part there too?? (i just thought it would be cool to quote Einstein!!). Anyways... inside the receiver is also something called a carrier tracking loop.. Its job is to lock on to the base carrier signal (at about 1575 MHz).<br /><br />What actually happens is the carrier tracking loop and the code tracking loop work together (or at least iteratively) to help each other do their job. One by-product of this process is that velocity can be determined in trying to correlate the incoming signals!!! Isnt this where all this started??<br /><br />That about exhausts my grey matter on how the receiver latches onto the signal - and im not sure that i have exactly answered your question Thom. (i think that there are other techniques called cross correlation and squaring that help amplify the signal above the background noise, but ill have to check some books out when i get home). As i mentioned at the start - im not 100% confident of how all this works - so forgive me if ive gone wrong somewhere <br /><br />I hope someone is keeping a tab on how far my 2 cents worth is going with all this.......... <br />Cheers - Troy

 

[sloopy]

Re: GPS question

OZ, can you explain the military thing? Have they ever done it?

 

[Troy_from _Oz]

Re: GPS question

Sloopy - You will probably remember the days of selective availability. What this did was to reduce the accuracy that could be attained on the L1 signal (the one we use at 1575MHz). They also have the p-code though, which is another (more accurate) code that is also modulated onto both the L1 and the 1227 frequency (L2). does that make sense? its a bit confusing<br /><br />the military have p-code receivers ---which aswell as having the ability to generate an internal C/A PRN code, could also generate the p-code signal using a similar algorithm that is not available to the rest of us (classified). Easy! - they just degrade the C/A code using selective availability and keep on using the p-code themselves. For something that was developed in the early 70s it is all pretty well thought out... The russians also have/had their own system that they started at about the same time, but it didnt have some of the cool stuff like this..<br /><br />Cheers T

 

[TwoBallScrewBall]

Re: GPS question

Thanks guys, my head just exploded.