How does GPS determine your Altitude?

Ok Guys,
I'm bored and up for another round.
Let's say the Earth is Flat, and although GPS works,
there's no machinery in space, so it must be ground based.

The relatively poor GPS hardware in an iPhone can provide an Altitude with estimated accuracy of 5 mtrs without waiting long.

It is my position that in order for GPS to determine your Altitude,
the transmitting antennas must be above the receiver at all times.
So how does the GPS determine Altitude?

What do you mean exactly?

I think he was asking how a land based GPS determines altitude.

That much is obvious.
In my first post "My position" means: "The stance I am taking".

That is how I'm opening a debate.

I agree that a 2D ground based positioning system could work if there were enough invisible towers around to cover the Earth.
I do not agree that such a system could be used to determine your Altitude with the accuracy of the current GPS that actually exists.

I'm posting this to say I edited the post above very close to when you posted,
So you don't look silly.

I tried to explain my first post better.

If I remember correctly, the question was simply "How does GPS determine your Altitude ?" (I'm assuming this was in reference to land based GPS rather than satellites, which of course are superflous to the FES Forum and can be disregarded.) I shall await an answer with the rest of you.

Yes, but I gave an example, relating some degree of accuracy,
so it's not open to a system that can only tell if you're on the moon, or on Earth for example,
because we are using one that actually has a known degree of accuracy, even if it's far from perfect.

If I am at Berthoud Pass in Colorado (over 11,000 feet in elevation) how does the GPS transmitter (let's assume there is one in Denver)  determine my altitude ? Maybe it would work if I was in Denver and there was a GPS transmitter atop Berthoud Pass ?

So if I correctly understand you there, it's working only when there is a transmitter higher than you?
I agree.

Not really. In that case, the transmitting antennas are all above your altitude.
I thought this was the debating section.

Quote from: Rama Set on April 19, 2013, 09:31:36 AM

Quote from: sceptimatic on April 19, 2013, 09:22:26 AM

What do you mean exactly?

I think he was asking how a land based GPS determines altitude.

I'm still none the wiser.

This statement is redundant, as we all can see you are never the wiser in any forum thread you have ever posted in. Look up GPS altitude calculations and come back, please. Oh, wait, you won't. Hoo boy.

I looked up an article for you anyway. Do yourself a favor and read this, if you would like to continue any useful conversation on this thread.

http://edu-observatory.org/gps/height.html

To the OP, it cannot. Simple as that. GPS, according to FE, cannot calculate altitude, as far as I have read. You need 3 --sometimes 4-- different satellites in different places orbiting around Earth to calculate it. If all of the stations were on the ground, we can't calculate elevation.

Not to mention how GPS works in the middle of the ocean, where there are no "ground stations"...why are we even discussing this?

This might be a simple explanation. :
All of the land base stations in the U.S.A. for the GPS have antennas on towers about 20,000 feet high to give altitude readings on GPS receivers since the transmissions must be higher than the receptions for the altitude readings to be accurate  ?

Sir Edmund Hillary used GPS when climbing Mt Everest.

Quote

This might be a simple explanation. :
All of the land base stations in the U.S.A. for the GPS have antennas on towers about 20,000 feet high to give altitude readings on GPS receivers since the transmissions must be higher than the receptions for the altitude readings to be accurate  ?

Sir Edmund Hillary used GPS when climbing Mt Everest.

Don't post low content posts in the upper forums.

Quote from: Art on April 19, 2013, 07:47:14 PM

Quote

This might be a simple explanation. :
All of the land base stations in the U.S.A. for the GPS have antennas on towers about 20,000 feet high to give altitude readings on GPS receivers since the transmissions must be higher than the receptions for the altitude readings to be accurate  ?

Sir Edmund Hillary used GPS when climbing Mt Everest.

Don't post low content posts in the upper forums.

With all due respect, it seems like you are mistaking a succinct point (Sir Edmund Hillary used a GPS to navigate at altitudes higher than 20,000 ft ergo 20,000 ft high antennas can be used to validate a view of ground based GPS) with a low content post.

Quote from: Pongo on April 19, 2013, 09:58:24 PM

Quote from: Art on April 19, 2013, 07:47:14 PM

Quote

This might be a simple explanation. :
All of the land base stations in the U.S.A. for the GPS have antennas on towers about 20,000 feet high to give altitude readings on GPS receivers since the transmissions must be higher than the receptions for the altitude readings to be accurate  ?

Sir Edmund Hillary used GPS when climbing Mt Everest.

Don't post low content posts in the upper forums.

With all due respect, it seems like you are mistaking a succinct point (Sir Edmund Hillary used a GPS to navigate at altitudes higher than 20,000 ft ergo 20,000 ft high antennas can be used to validate a view of ground based GPS) with a low content post.

Since Sir Edmund Hillary climbed Mt. Everest in 1953, I hardly think that he used GPS.  Hence the low content warning.

GPS functions with reasonable accuracy at altitudes up to and above 30,000ft (been there, done that, and commercial airlines do it all the time), so land based towers would have to be incredibly tall to give accurate altitude readings. Just a reminder: GPS would be useless for altitude measurements if the transmitters were close to the same height as the receiver.

Quote from: Rama Set on April 19, 2013, 10:19:16 PM

Quote from: Pongo on April 19, 2013, 09:58:24 PM

Quote from: Art on April 19, 2013, 07:47:14 PM

Quote

This might be a simple explanation. :
All of the land base stations in the U.S.A. for the GPS have antennas on towers about 20,000 feet high to give altitude readings on GPS receivers since the transmissions must be higher than the receptions for the altitude readings to be accurate  ?

Sir Edmund Hillary used GPS when climbing Mt Everest.

Don't post low content posts in the upper forums.

With all due respect, it seems like you are mistaking a succinct point (Sir Edmund Hillary used a GPS to navigate at altitudes higher than 20,000 ft ergo 20,000 ft high antennas can be used to validate a view of ground based GPS) with a low content post.

Since Sir Edmund Hillary climbed Mt. Everest in 1953, I hardly think that he used GPS.  Hence the low content warning.

Well then Jroa is just stupid isn't he

If GPS antennas above your head can provide your position in three dimensions (xy movement across the earth + z altitude), then why can't GPS antennas on the same plane as you provide your position in three dimensions?

If GPS antennas above your head can provide your position in three dimensions (xy movement across the earth + z altitude), then why can't GPS antennas on the same plane as you provide your position in three dimensions?

Because they need to be overhead (by a good distance too) to have enough resolution to determine altitude. If the antenna were on (or near) the same plane as you, there would not be sufficient resolution to give altitude within 1000ft, let alone 5ft. If you don't understand why this is the case, you may need to do trig again.

Quote from: Tom Bishop on April 20, 2013, 10:16:28 AM

If GPS antennas above your head can provide your position in three dimensions (xy movement across the earth + z altitude), then why can't GPS antennas on the same plane as you provide your position in three dimensions?

Because they need to be overhead (by a good distance too) to have enough resolution to determine altitude. If the antenna were on (or near) the same plane as you, there would not be sufficient resolution to give altitude within 1000ft, let alone 5ft. If you don't understand why this is the case, you may need to do trig again.

Why wouldn't there be enough resolution?

Why wouldn't there be enough resolution?

I'm with Tom on this I think. As far as I can tell if the planet were flat and assuming bendy light was not responsible for the effects of curvature (in which case GPS signals would conceivably not reach you if far enough away) then it should be possible to discern altitude accurately with a horizontal set of towers.

Vertical movement would create a difference in signal travel distance proportional to the arcsin of the height. Given that GPS satellites are at extremely high altitudes (over 20,000 km I think, is this correct??) I doubt that they would be able to discern higher resolutions than towers on a flat earth.

Of course all of this runs counter to bendy light theory, which would inhibit the functioning of GPS unless we assume that "bendyness" does not affect GPS signal.

According to this website, GPS satellites orbit between 6,000 and 12,000 miles above the earth's surface.

If we assume 20,000ft tall towers on earth (make them any height you like, I'm just saying 20,000ft for this example), then if you have one plane flying at 10, 000ft, and another directly above it at 30,000ft, they would not be able to tell their altitudes apart, because the time it took for the GPS signals to reach them would have been identical.

On the resolution, if we just have one tower (to make the example simple) and a plane flying 20km from it at 10,000ft (about 3,000m), it's line of sight distance from the top of the tower would be about 20,223.75m.
The signal from the GPS tower would take about 0.0000674 seconds to travel that distance.

If the plane now climbs to 12,000ft (remaining at 20km, let's say it's doing a turn with the tower at the cetre), it's line of sight distance would be about 20,143.49m.
The signal from the GPS tower would take about 0.0000671 seconds to travel this distance.

What is the timing resolution of a cheap GPS receiver? Would it be good enough to measure a difference this small?

EDIT: Thinking about other devices that use EMR to measure distance, even a cheap unit would probably have the resolution to measure this difference, but I did make it quite large. If I had used something like a 20ft change at 100km, it would have been a much smaller difference that needed to be measured. Having satellites practically overhead definitely makes it heaps easier to get an  accurate measure of altitude, no matter how far away from the receiver the satellite is.

EDIT 2: According to this wikipedia page the accuracy of GPS receivers is about 100 nanoseconds, or 0.0000001 of a second.

Quote from: Tom Bishop on April 20, 2013, 07:04:11 PM

Why wouldn't there be enough resolution?

I'm with Tom on this I think. As far as I can tell if the planet were flat and assuming bendy light was not responsible for the effects of curvature (in which case GPS signals would conceivably not reach you if far enough away) then it should be possible to discern altitude accurately with a horizontal set of towers.

Vertical movement would create a difference in signal travel distance proportional to the arcsin of the height. Given that GPS satellites are at extremely high altitudes (over 20,000 km I think, is this correct??) I doubt that they would be able to discern higher resolutions than towers on a flat earth.

Of course all of this runs counter to bendy light theory, which would inhibit the functioning of GPS unless we assume that "bendyness" does not affect GPS signal.

Sorry, but you backed the wrong horse. Because of the limitations of the GPS receiver, it is impossible to determine the distance to the transmitter with a precision better than some 2 meters. Divide that by the sine of, say, 0.1 degrees, and you have 2/0.0017 = 1146 meters.

You could calculate the height of the receiver, but the error would be on the order of kilometers. The error would be so bad that, in effect, there is no use for the result at all.

You need at least one of the GPS satellites to be above 30 degrees or so above the horizon to get a good reading of your altitude. And as you say this is not even getting into the quagmire of bendy everything. The towers would be easy to see with your unaided eyes.

So how does the GPS determine Altitude?

Massively oversimplifying: You have three points (towers, satellites, pumpkins, whatever). You find out the distances between yourself and each point. In a two-dimensional space, there will be only one point that matches the given co-ordinates. If you add a third dimension, you get up to two possible points - one above the plane defined by the three points, and one below. You can either introduce a fourth point to disambiguate it, or make a good guess.

https://en.wikipedia.org/wiki/Trilateration

Sorry, but you backed the wrong horse. Because of the limitations of the GPS receiver, it is impossible to determine the distance to the transmitter with a precision better than some 2 meters. Divide that by the sine of, say, 0.1 degrees, and you have 2/0.0017 = 1146 meters.

You're going to have to clarify how you arrived at those calculations. GPS calculations are carried out via triangulation of coordinates between a large set of satellites, I mean towers; often 5 or more. I also don't see how sensitivity to height would be any less of an issue with satellites, remember that we don't have to assume that all towers are at the same height, so sensitivity to low angles isn't necessary.

Massively oversimplifying: You have three points (towers, satellites, pumpkins, whatever). You find out the distances between yourself and each point. In a two-dimensional space, there will be only one point that matches the given co-ordinates. If you add a third dimension, you get up to two possible points - one above the plane defined by the three points, and one below. You can either introduce a fourth point to disambiguate it, or make a good guess.

Disambiguate... Good word.

Quote from: RealScientist on April 20, 2013, 09:25:39 PM

Sorry, but you backed the wrong horse. Because of the limitations of the GPS receiver, it is impossible to determine the distance to the transmitter with a precision better than some 2 meters. Divide that by the sine of, say, 0.1 degrees, and you have 2/0.0017 = 1146 meters.

You're going to have to clarify how you arrived at those calculations. GPS calculations are carried out via triangulation of coordinates between a large set of satellites, I mean towers; often 5 or more. I also don't see how sensitivity to height would be any less of an issue with satellites, remember that we don't have to assume that all towers are at the same height, so sensitivity to low angles isn't necessary.

The distance measurements required for triangulation involve errors too.  If the distance was calculated from a satellite from above, then you'd have something like this:



Where the red dot is your actual position, and the black bars represent the limits in the error. The height h is clearly minimal, and is approximately that of the gap in the error in the distance. Now, if we instead tried to find the altitude from a horizontal distance measurement, then we'd have something like this:



Which would be even more exaggerated in a real world scenario where the radius of of the circular error bars would be even larger and would look seemingly like parallel horizontal and vertical lines respectively in each of the pictures.

Quote from: jason_85 on April 20, 2013, 10:29:43 PM

Quote from: RealScientist on April 20, 2013, 09:25:39 PM

Sorry, but you backed the wrong horse. Because of the limitations of the GPS receiver, it is impossible to determine the distance to the transmitter with a precision better than some 2 meters. Divide that by the sine of, say, 0.1 degrees, and you have 2/0.0017 = 1146 meters.

You're going to have to clarify how you arrived at those calculations. GPS calculations are carried out via triangulation of coordinates between a large set of satellites, I mean towers; often 5 or more. I also don't see how sensitivity to height would be any less of an issue with satellites, remember that we don't have to assume that all towers are at the same height, so sensitivity to low angles isn't necessary.

The distance measurements required for triangulation involve errors too.  If the distance was calculated from a satellite from above, then you'd have something like this:



Where the red dot is your actual position, and the black bars represent the limits in the error. The height h is clearly minimal, and is approximately that of the gap in the error in the distance. Now, if we instead tried to find the altitude from a horizontal distance measurement, then we'd have something like this:



Which would be even more exaggerated in a real world scenario where the radius of of the circular error bars would be even larger and would look seemingly like parallel horizontal and vertical lines respectively in each of the pictures.

Aren't you making the assumption that a tower must then have a low angle of elevation compared to the receiver? If you remove that assumption then the "region of error" rotates, such that the error reduces (in the limit approaching the horizontal).

This is sort of a non-issue anyway because the tower theory requires that the GPS data itself is rigged, such that the receiver processes data which is ostensibly from satellites but actually from towers. Any resulting precision is that which is applicable to the FE tower arrangement under such a theory, and comparing it to the "actual" RE model is moot.

Aren't you making the assumption that a tower must then have a low angle of elevation compared to the receiver? If you remove that assumption then the "region of error" rotates, such that the error reduces (in the limit approaching the horizontal).

This is sort of a non-issue anyway because the tower theory requires that the GPS data itself is rigged, such that the receiver processes data which is ostensibly from satellites but actually from towers. Any resulting precision is that which is applicable to the FE tower arrangement under such a theory, and comparing it to the "actual" RE model is moot.

Yes I am, and RealScientist has already addressed that point, I just thought a pretty picture might clarify it  And wasn't that the main point of the OP anyway? Land/tower based GPS has a tougher time determining altitude than satellite would because of small elevation differences between the tower itself and many receivers.

So what exactly stands in the way of tower based GPS functioning seemingly identically to satellites and thus toppling this theory?

Yes I am, and RealScientist has already addressed that point, I just thought a pretty picture might clarify it  And wasn't that the main point of the OP anyway? Land/tower based GPS has a tougher time determining altitude than satellite would because of small elevation differences between the tower itself and many receivers.

So what exactly stands in the way of tower based GPS functioning seemingly identically to satellites and thus toppling this theory?

I dunno, I think it's one of the few FE theories that is actually theoretically possible. Occum's razor still kills it though, as with every FE theory...

I dunno, I think it's one of the few FE theories that is actually theoretically possible. Occum's razor still kills it though, as with every FE theory...

Of course, which is why they hang onto the conspiracy so dearly because it's their only way to rationalize why Occam's razor works against them in every regard.

Since Sir Edmund Hillary climbed Mt. Everest in 1953, I hardly think that he used GPS.  Hence the low content warning.

Sorry, my mistake:

Bradford Washburn 1999
Mounted a GPS atop Mt Everest.
" A rock head elevation of 8,850 m (29,035 ft), and a snow/ice elevation 1 m (3 ft) higher, were obtained via this device".
Wikipedia.

If GPS antennas above your head can provide your position in three dimensions (xy movement across the earth + z altitude), then why can't GPS antennas on the same plane as you provide your position in three dimensions?

Because the time it would take the signals from each tower to reach the receiver wouldn't
be very different, and commercial GPS receivers have very slow processors so their
batteries can last all day.

You're going to have to clarify how you arrived at those calculations. GPS calculations are carried out via triangulation of coordinates between a large set of satellites, I mean towers; often 5 or more. I also don't see how sensitivity to height would be any less of an issue with satellites, remember that we don't have to assume that all towers are at the same height, so sensitivity to low angles isn't necessary.

Same as above. The further away the transmitters, the longer it takes a signal to get to the receiver,
then, the more accurate the result of the program in the receiver (on a strict time budget)
in timing the distance covered by the signal from each transmitter.
GPS uses triliteration, not triangulation. It looks at the intersection of spheres
of the radius of the calculated distance travelled from each transmitter.
Many of the bonus results can be discarded because they are below sea level where GPS doesn't work.

EDIT: Thinking about other devices that use EMR to measure distance, even a cheap unit would probably have the resolution to measure this difference, but I did make it quite large. If I had used something like a 20ft change at 100km, it would have been a much smaller difference that needed to be measured. Having satellites practically overhead definitely makes it heaps easier to get an  accurate measure of altitude, no matter how far away from the receiver the satellite is.

No matter whether or not a ground based positioning system has been demonstrated
to work on Earth, resolution is always improved if the transmitter is further away.

The transmitter has a peripheral atomic clock so it can send the time and it's location
to the receiver. The receiver has a temperature compensated clock (the best we can do
at the other end, to sell it to the masses).
The receiver measures the distance travelled by comparing the transmission time from
each transmitter's clock to the receive time from the receiver's clock.

In your close to Earth GPS theory, the points of intersection in those spheres are close together,
the spheres are smaller, the erroneous results don't sound as stupid as the erroneous
results from much larger spheres, and you have several plausible locations to choose from.
This is alleviated if you don't have to think about altitude, and just use all information to calculate a 2D horizontal position.

It seems that some flat earthers believe in bendy radio wave. How do we model triangulation using bendy radio wave?

I haven't seen any talk about bendy radio wave.
I think if you could predict it's path it would make no practical difference by the sound of it.

Maybe they say it bends upward to explain why it doesn't arc with the Earth?
and to explain ionosphere skip?
I hope not, it's a bit desperate.