Apple issues statement on iOS location controversy, says fix is coming - Page 6

You are correct that geostationary refers to the equatorial geosynchronous orbit, but geosynchronous still requires the orbital period to be one day. The GPS constellation is in a lower orbit, and not geosynchronous.

The debate about how many timing signals are required to determine position uniquely is pretty lively. In Euclidean space (a good approximation here), each satellite constrains the GPS unit to the surface of a sphere of radius equal to the distance computed by the time offset required to match the GPS signal.

Data from two satellites therefore constrains to the intersection of two spheres (if we don't include the earth's mean sea level as an additional reference sphere), which will be a circle with a diameter that should not exceed that of the earth itself (that nearly occurs in the case of two satellites nearly opposite each other and close to the horizon).

Data from three satellites should constrain to the intersection of three spheres, which may not exist (we disregard that solution as unphysical), occur at one point (the common unique solution) or at two points (possible but unlikely in this geometry).

However, that does not address timing error considerations, which, as have been pointed out, typically degrade the accuracy of the 3-satellite solution. Adding either the earth mean sea level sphere (approximate due to your unknown elevation), or a fourth GPS satellite sphere constrains to enough accuracy to give you a lock.

UPDATE: in case anyone is still reading - simple geometric considerations rule out the unique solution also - that would require at least one of the satellites to be below the horizon. That leaves just the problem of isolating the physically possible solution from the two points where the third timing sphere intersects the circle from the first two.

Thank you, and thanks for your contributions too. Actually I can't really take much credit there as I'm already familiar with most of this stuff - hence my attempt to contribute.

That exceeds the scope of my knowledge. Thanks for the clear explanation.

Exactly I belive that the orbital period of the satelites in the GPS constellation is around 12 hours

Yes - approximately. They are up around 20000 km.

Then it's quite possible we've run into each other before at some other site or venue. I tend to get around.

Me too, though GPS per se is not my field except in a somewhat broad sense.

I don't have a useful way of performing the test, and I didn't see any links you say you provided. A link is exactly that. It's not a general statement about some site or other.

Now, you've provided a link, which I will read as soon as I finish this reply.

Ok. I would have, and did expect a more professional site, but it was ok.

So I don't see a difference between what we're saying. You can locate with three satellites, as we've been saying, but need four for more accurate location. Nothing major there. More satellites can be used for even more accurate location, as I said.

I don't see anything new there. Most of the rest I'm already familiar with.

You're premature, the fix hasn't been pushed out the the public yet. ;-)

Ok.... they say this and that...Fine.
I want to know is:
Who's paying for the data transmission?
Me, AT&T, Apple or some other.

This link, paragraph 5.
http://www.gpsinformation.org/dale/theory.htm

And from the University of Colorado
http://www.colorado.edu/geography/gc...gps/gps_f.html
http://www.colorado.edu/geography/gc...gps/gps_f.html

And here, the first sentence under GPS Receiver Acquisition Time. Probably the exact match for the only sentence that will apparently appease you. Quote: All GPS units require a certain amount of time to acquire 4 satellites, the minimum number needed for fixing a position (3 for space, 1 for time).

http://firebug.sourceforge.net/gps_tests.htm

Of course there is a whole group out there that won't believe any explanation Apple puts out, to them I wonder how they live with knowing their cell company does track their movement & at times turn that info over to government. This was such a bunch of hullabaloo.

One cool thing that came out of all this, now I understand how my iPhone is giving me such accurate GPS coordinates when I appear to have no clear view of the sky! Very cool!! Hope all the complainers don't ruin this for me.

That's a very good question! I want my $1.50 refund for last months data use!!

Thanks,

Looks like the only person here who fully understood this is Muppetry. The forth satellite apparently does nothing about elevation but is used to correct any timing issues. So what I understand is that 3 satellites gets you all of the location info including the elevation but is too inaccurate to be useful without the fourth satellite to correct the timing errors. Interesting...

You're welcome.

Now as long as Mel is satisfied too then we should be done with the basics of the gps discussion, correct?

Thanks. I guarantee that I never fully understand anything, but at a basic level, GPS is not that complicated. One further observation regarding elevation is that due to the geometry of the earth and the GPS constellation, not all spatial dimensions are equally resolved. The best reception at any given time will be from satellites that are well above the horizon, which makes elevation the least well resolved. That is why, even with more satellites locked, horizontal accuracy typically exceeds vertical.

Very interesting. I have to note that there is disagreement between the first two regarding the maximum number of satellites visible at once. The first link says up to 12, but the second said up to 8, which as far as I know, is the correct number. I was going to work it out mathematically, but honestly, I decided it wasn't important enough to bother with. Possibly the difference comes from the 24 that were first used, and the 32 or so that are supposed to be in place now?

Both articles do say that three satellites can be used for position data in a 2D space, which, again is expected. The forth, which gives greater accuracy by computing the time data errors, also gives the Z position. What we're talking about as to our disagreement, is whether modern receivers are satisfied with using three, or will only work with four or more. I would imagine that now, they will only use four, unless there is some extreme reason not to do so. But for older models, three was common.

Right again. Vertical position errors (elevation) are commonly several times that of horizontal errors.

Well, except that the Garmin manual is apparently incorrect in light of the 4th being used for timing not elevation.

Edit: After reading the last two posts about vertical errors exceeding horizontal, maybe that is what Garmin is referring to.

I believe that for most of the discussion, we do agree. There are subtle differences to what we are saying, and most of that has distracted us from where we do agree. The only thing I really objected to was when you said that modern receivers don't use more than four satellites.

The timing is used for elevation, as well as more accurate positioning on the 2D surface.

This is a cool little animation of position and visibility from a surface point at 45 N, showing a visibility count as a function of time. As I mentioned in an earlier post, it is designed to be at least 8, but is often more.

http://en.wikipedia.org/wiki/File:ConstellationGPS.gif

MStone, I think you still might be missing the relationship between elevation and timing, but it doesn't really matter in the big scheme of things. At least we pretty much agree now.

An Air Force video we posted several months ago. Simple but straightforward.
http://www.gpsreview.net/air-force-s...how-gps-works/

And one from NASA, who technically has little to do with GPS, but obviously understands it.
http://www.youtube.com/watch?v=DsmvTzw3GP4

Correct. And how's this site any different ?

Where was that Mel? I think you misread as that is never a statement I would make.

Water under the bridge in any case and I'll accept your post as an apology and acknowledgement of my correctness.

Kidding Mel. I'd never expect an apology. It was a lively discussion

Actually, no. Initially, Selective Availability was used to limit non-military systems to ~100 meters but this was turned off in 2000.
Military grade GPS has better receivers and antennas, can use Selective Availability and are more robust than off-the-shelf or even standard commercial aviation grade. You can get receivers that are just as good. They will cost a LOT and won't have SA but they are just as accurate.
Selective Availability can be re-enabled in the event of war or national emergency.
If you want

GPS satellites do not respond to any GPS receivers (military or civilian) because nothing is transmitted from a GPS receiver.

Receivers that operate above 18Km or faster than 515 meters/second are considered munitions under federal law and are export controlled.

Even though SA was switched off in 2000, I was unaware that any civilian units are able to use the P code, and thus the L2 frequency, which allows for ionospheric corrections and greater accuracy.

Sounds like you'd better use iPhoneGeotag with iPhoto and Aperture before the software update breaks it!

Block IIR-M (currently 7 satellites) introduced the L2C (civilian) signal.

Also, Wide Area Augmentation System (WAAS) sends correction data.

I think the P code changed over to the Y code several years ago, but no matter either way as it needs an encryption key at the ground based receiver.

I've never seen any mention of the keyfile being made available for civilian use but can't swear that it hasn't for some specific and highly restricted project.

Well it looks like Trimble may have a couple of high-end commercial navigation and surveying solutions that are able to use the P code with L1/L2 receivers.

Nothing I'd paid all that much attention to, usually focused on consumer-grade devices. Gives me something to read up on now.

Here's a good article
http://www.smdc-armyforces.army.mil/...9_NO_3_013.pdf

Thanks. Very informative.

Our posts are crossing each other. I'm not here all the time, which you can see from the times I post. I respond to a post, and sometimes that response comes after another post you made, responding to an earlier post of mine. So you've provided links, in response to my request (finally), and that got posted well after another post you made that didn't have any links that I read, where I requested that you post one.

I hope that made sense out of it.

Kinda makes sense I guess, but you responded to the links yesterday in post 220. I'm just glad we put that little misunderstanding of the facts behind us.