Anyone want to know how GPS works?

[piscator]

so how does your GPS unit measure the legs of the triangles?

simple! all it has to do is time a radio signal from an SV to your unit and multiply by the speed of light, and how hard could that be? am i right?

[Gawdzilla Sama]

so how does your GPS unit measure the legs of the triangles?

simple! all it has to do is time a radio signal from an SV to your unit and multiply by the speed of light, and how hard could that be? am i right?

The legs of the triangles are measured by the time lag, of course.

[piscator]

If you put the satellite on a pole, there are concentric circles on the ground where the signal strikes, each circle larger than the next the signal's lag time. So each satellite creates a "circle of probabilities" for a given distance from it. And just as it takes two lines intersecting to define a point, it takes two circles intersecting to define the two points of probability for a pair of satellites. The third satellite narrows the choice of points down to one spot.

now i'm sure there's a high probability you learned about LORAN, probably in the military, probably back in the 50s or something...




"Circles of probabilities" PFFFT

i can just see some buck sergeant or PO at a chalkboard in front of a class full of guys fighting to stay awake, trying to draw 3 circles intersecting at 1 point
when he couldn't do it just right, he'd take his chalk and scribble in kind of a blob to represent where these 3 circles are supposed to magically intersect

that was the diagram in th book
that was the diagram he was supposed to teach
that was the diagram he drew

[Gawdzilla Sama]

System I learned was a bit better than the civilian models.

[piscator]

so how does your GPS unit measure the legs of the triangles?

simple! all it has to do is time a radio signal from an SV to your unit and multiply by the speed of light, and how hard could that be? am i right?

The legs of the triangles are measured by the time lag, of course.

of course

and how do we measure the time lag?

and is that measured time lag subject to any errors?

[stripes4]

I thought pixies did it

[piscator]

System I learned was a bit better than the civilian models.

yeah, me too

but then again NIMA and DMA were paying for me to get my M.Sc at Ohio State, from the guys who invented the system
i had a TS clearance as an undergrad


and the Israelis cracked the P code in the mid-late 80s, so their knuckledraggers were scratching their heads over hardware without SA and AS just like you

[piscator]

I thought pixies did it

magnets

[piscator]

BTW, maiforpeace - Every Trail is one very cool site!

i had an idea, and actually put a lot of time into it, for a photo share/SN site something like that
my implementation was a lot different, and the marketing was more based on a photoshare like Flickr, but there are lots of similarities


i see they do it with Flash, which is a lot different from my approach
i guess that's how they got around some of the issues i ran headlong into

i'd have had it up in 2008, except i came to the conclusion that i'd be working for the CDNs and server farms, or find myself in the advertising business

I still called friends in Palo Alto to see who's behind GlobalMotion
maybe they kyped some of my ideas when i was shopping my POC around, and i can sue the bastards?

[piscator]

alright, back to measuring TRIANGLES from the satellites to your iPhone...


the speed of light (and all other massless particles, like radio signals from satellites) is 299,792,458 m/s in a vacuum

but we don't live in a vacuum, we live on Mother Gaia, in an atmosphere of non isotropic density (which is gonna slow the signal), on a planet with mass/gravity (which is gonna bend it and make it travel farther than a straight line distance)

shit

so how can we measure the time a radio signal takes to get from a GPS SV to your eTrex, a measuring tape??
yep, except the measuring tape is the radio signal
which is actually the whole point of the satellite in the first place: to broadcast a time signal (or if you are 'Zilla, "radiating circles of probability" like ripples in still water, where there is no pebble tossed, nor wind to blow....)

GPS SVs carry atomic clocks, shiteverybodyknow that

the sats use these clocks to sync up with each other and time stamp their broadcasts to about a billionth of a second
your receiver uses these time stamps to get a rough idea where it is, but a billionth of a second time stamp only works so well when you are dealing with a signal that is traveling close to C and you want to find your position as close as you can (like to land a plane, or find the front door of the medical marijuana dispensary, or fly a cruise missile into a cave in Tora Bora, or something)

so GPS sats broadcast a pseudo random code along with a time stamp, and they broadcast an ephemeris (a table of times and positions) along with that

so your unit receives a signal from an SV, takes the time stamp, plugs it into the ephemeris data it just received, comps a position in space for the SV, sets it's own little clock, takes a roundhouse guess at where it is, and then waits for the delta from when it expects the next time stamp to when it actually gets it
(this, kids, is called a processing algorithm)

what the point of the pseudo random code? if the timestamp + ephemeris= a yardstick, the phase relationship

between the same point on the pseudo random code between what the receiver thinks it should be and what it actually is, is the micrometer caliper

it works exactly like a vernier scale


do this a few iterations, and now we know the position of the SV and almost exactly how long it took the signal to get to us

"So fkn wat?", you say, "Wat the fuck good does that do? All the fuck I know is that it took a fkn radio signal X amount of fkn time to get from fkn there to fkn here! Gadammit!"


back to triangulation...

your HTC Desire doesn't do this with a single SV, it does it with every one it can "see", and makes TRIANGLES between your Garmin and every pair of SVs it can hear

a quick spiffy little calculation of the time stamps/ephemerides (that's plural of ephemeris) gives a very tight position of 2 Space Vehicles, which forms the base of a triangle, and the previous process gives 2 very exact distances to the 2 sats from you

2 of these triangles in 2 different planes (4 sats) is gonna fix your position on ellipsoid Earth fairly tightly
the more SVs, the tighter it gets, Baby

[Ian]

GPS should not be confused with PGS...
http://en.wikipedia.org/wiki/Prompt_Global_Strike
:twisted:

[Rum]

GPS should not be confused with PGS...
http://en.wikipedia.org/wiki/Prompt_Global_Strike
:twisted:

Or GPs. Who are the mainstay of our National Health service here!

[piscator]

or for that matter, STDs...


so get with the PeeGee:

"Free yo mine, an yo ass will follow!"

[piscator]

which brings up the question: WTF is 3G?

3G (4G, WAAS, etc) is Differential GPS (DGPS)

your receiver (iPhone, Android phone, etc) gets Differential Corrections from the cell towers

IOW, there is a reference GPS receiver on the tower or someplace close that knows where it is very precisely based on long occupations of a single point, with post-processing and statistical adjustments of the data to further refine the position down to a centimeter or less

what this reference receiver does is say, "I know I'm here, but the GPS signals comped through a pathetic little standard on-the-fly receiver say I'm 2m North of where I know I am. So, all you little punk cellphones and bitchass handheld Garmins within the sound of my voice need to correct your indicated position by 2m to the South [the differential] and standby for further instructions. I R Badass you are my bitchez."


and so now you all know all about RTCM, 3G/4G, RTK, and the rest of the Differential GPS systems

[Meekychuppet]

Someone showed me a 3d model of how it works, it's really quite interesting. I have a Garmin ETrex and lately I've been doing a bit of mapping of hikes and uploading them to Every Trail, it's great to see what I am walking on a contour map.

That brings back memories. I had one years ago, but phones are way too good to bother with that now.