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A Gps In Space


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Okay ... silly question, but I saw a users icon of the galaxy with the "you are here" and "cache" on the other side. At first I laughed, but then I started wondering.

 

What would happen if you tried to use a GPS just outside of the earth's atmosphere? Either at the same level as the satellites, or just above them? Given the signals point at earth, you would have to be in their topmost range.

 

I'm thinking they would still recieve signal, but would then give wacky results as they tried to fix. Maybe it would work, but give you a strange altitude.

 

I know this is a stupid question.... there are few rocks to hide caches under up in space where the satellites are (the moon might be cool -- which I think would not work because the satelittes signals are pointed at earth). I just thought someone with extensive GPSr knowledge might have a clue of the process and be able to give some insight.

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Well, planes use GPS, so it will work to a point. As you go higher, I imagine, you would have fewer and fewer sats to compute your location, thereby giving you increasingly bad accuracy. Eventually, as you go up, you'll get to a point that you can lock on to too few sats to give you a position.

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I would think the higher you would go the more satellites you'd have to compute your accuracy. Also they would be more accurate as there would be less atmosphere interference.

 

Just a though but I could be wrong.

Another question though I guess is how far out past the satelites would they work? The MOON?

Edited by top pin
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What would happen if you tried to use a GPS just outside of the earth's atmosphere? Either at the same level as the satellites, or just above them?

GPS satellites orbit far far higher than what we consider the limit of the atmosphere.

 

Off the top of my head, I recall their orbit being in the 12,000mi range. The atmosphere pretty much runs out of gas around 50-100 miles. Typical spaceflight takes place in the 200-300 mile altitude. Wow, I just said "typical" spaceflight.

 

So even going into earth orbit on the ISS, you're barely any closer to the GPS satellites as you are on the surface of earth. I'd wager that GPS still works perfectly fine there.

 

Jamie

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I would think the higher you would go the more satellites you'd have to compute your accuracy. ...

My thought was that the unit will only be able to track them to the limits of its horizon. I doubt that it will be able to compute a location using data from a sat that it reads as lower than the GPSr is.

 

If this is correct, the GPSr will lose accuracy as you approach the height of the sats and then lose it altogether as the sats fall below its artificial horizon, or worse, it would assume that it is beneath the sats and compute a completely incorrect solution, but I think this is very unlikely.

Edited by sbell111
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any eccentric billionaires scheduled to head to the ISS any time soon?

 

Perhaps we could introduce them to geocaching and get em to bring a GPS.. perhaps place a hide somewhere on the ISS?

 

I'm thinkin it'd be a perfect spot for an altoids tin sized cache. Ruleswise, I believe it's public land, owned by governments, well shoot, it'd be a moving cache though... perhaps special dispensation, since you can track the ISS on some websites... anyone know if you can up the terrain rating to 6 stars?

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any eccentric billionaires scheduled to head to the ISS any time soon?

 

You talking about Richard Branson? :anibad:

 

I think the closer to the satellites you get the GPSr would just go tilt & lock up due to some out-of-range internal calculation error. And the moon is 250000 miles away so, even the mightiest AA Duracells or NiMh batteries probably wouldn't get a good enough signal from earth-based sats :anibad:

 

I suppose if we ever get enough people living on the moon (or Mars, etc), it wouldn't be long before the scientists deployed a few satellites around it.

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Well I wasn't sure how to answer this one, as my Greek teacher use to say, well I cant say that on a family orintated forum. But after Jamie Z comment and quick checking to see if he was correct on satellite altitude, he is. The next thing would be the angle of the beam the sats send out. So depending up on where you are calling space, mosty definde above 150 mies, you might have some reception. But higher up the less likely it would be to work.

cheers

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This is a very cool link that someone posted here months ago (sorry, I forget who it was). It's a Nasa Java applet that shows the current position of hundreds of satellites. When the small window pops up, you can click on the "sateliite" menu option and scroll down to the ones that start with "GPS". Clicking on them shows their orbits. They are way farther out than I would have guessed (nowhere near the cluster of ones that appear to be just outside the atmosphere).

 

Nasa J-Track 3D

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There are government technology export rules that specify that commercial GPS receivers should only work when either lower than 60,000' or at a speed less than 1000 knots, presumably to make them unsuitable for ballistic missile guidance. Most (all?) manufacturers have implemented this rule more strictly than required and make their receivers stop working when they are either above 60,000' *or* over 1000 knots, so this would limit their use at high altitudes. However, special waivers have been granted for GPS receivers used in high altitude balloon flights that exceed 60 kft, and military receivers can also operate above that height.

 

Technically there's no problem with using a GPS receiver in Low Earth Orbit (LEO) and GPS is routinely used on board the Space Shuttle.

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Peter is correct.

 

However, if the receiver were not delimited, it would work fine in space.

 

The reading would still be "at" a lat/long coordinate, which would represent the spot you were directly over. But the altitude reading would be a doozy!

 

(Yes, I used to work for NASA, and I helped launch the very first GPS satellite.)

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Peter is correct.

 

However, if the receiver were not delimited, it would work fine in space.

 

The reading would still be "at" a lat/long coordinate, which would represent the spot you were directly over. But the altitude reading would be a doozy!

 

(Yes, I used to work for NASA, and I helped launch the very first GPS satellite.)

That's what I was thinking though I didn't know about the built in limitation (a non issue for me). The only thing I'm wondering is the math model. If you are above the satalites in space does the model know enough to calculate your postion and give you a LL based on your position over the earth? Does the mode fizzle out at a certain point or is that a function of the GPS maker?

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If you are above the satalites in space does the model know enough to calculate your postion and give you a LL based on your position over the earth? Does the mode fizzle out at a certain point or is that a function of the GPS maker?

Thinking about this more, I would think that after a certain distance the accuracy would start dropping off rapidly. If we can get 3 meter accuracy with the satelites at 125,000 miles away wouldnt ya think that at 200,000 miles away the chance for errors would increase dramatically? Sooner or later you'd get to the distance that smallest error would be giving huge errors in the LL? Given that you were 200,000 miles above a spot on the earth even the smallest error would place you many miles away on a round surface, where being on the surface small errors would only move you so many feet? You'd might assume that the accuracy would actually be less at the higher altitude.???

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Very cool! I was a little concerned that folks would think the question foolish, but I have a much better understanding than I thought. I never knew about the 60K and 1000 speed limit. I also wasn't aware that GPS units are used on the Shuttle.

 

Does anyone know if satelittes themselves have GPS units on them?

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If you are above the satalites in space does the model know enough to calculate your postion and give you a LL based on your position over the earth?  Does the mode fizzle out at a certain point or is that a function of the GPS maker?

Thinking about this more, I would think that after a certain distance the accuracy would start dropping off rapidly. If we can get 3 meter accuracy with the satelites at 125,000 miles away wouldnt ya think that at 200,000 miles away the chance for errors would increase dramatically? Sooner or later you'd get to the distance that smallest error would be giving huge errors in the LL? Given that you were 200,000 miles above a spot on the earth even the smallest error would place you many miles away on a round surface, where being on the surface small errors would only move you so many feet? You'd might assume that the accuracy would actually be less at the higher altitude.???

Consider on the earth side you have atmospheric distortions etc. In space it's clear skies and smooth sailing. You should have a more accurate reading. The only limit is the distance since the signal weakens exponentially with distance.

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Consider on the earth side you have atmospheric distortions etc. In space it's clear skies and smooth sailing. You should have a more accurate reading. The only limit is the distance since the signal weakens exponentially with distance.

True

 

Though right now your gpsr is trying to calculate your positoin on a bubble of x diameter. If you increase that diameter by 10 fold a 1 foot error becomes a 10 foot error. Wouldnt ther further away you are from the surface of the bubble increase the area of your error.

 

 

IE: say your gps's are trying to calculate where you are on a 20 foot bubble. Its accuracy gets your location with a 2inch square somewhere on the surface. Now you take that same measuring devices but increase your bubble size to 100ft. Now the accuracy stays the same since the measuring devices have not changed. Now your 2" square area it has calculated for your location has just increased to 10" square.

 

Does this make sense? I'm not sure what I'm trying to show here without a drawing but I would think the higher you are the larger the bubble the gps'r is trying to calculate for wich would add to the size of the error.

 

I could be way off on this my mind is just imagining it this way.

 

B)<_<

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There are two kinds of error in mesurments. I forget their names. Systematic and Random.

 

A systematic error is as you describe. It's built into the measurment. If your measuring tape reads an extra inch for every foot you can actually account for the error in your measurment and be accurate.

 

Random error is the error from not being able to read a measurment with true precision (I think you can get it other ways as well) in the long run they tend to cancel out. You read the tape to the nearest 1/16" sometimes you read long sometimes you read it short.

 

The GPS comes down to two things. Accuratly reading the signal and measuring time. On earth the air and other things tweak the signal and introduce error (which is why we have WAAS) that lowers accuracy. In space there is less interference. Leaving you with the ability to read the signal accuratly and that closer to reading the measuring tape to the nearest 1/16" than anything else.

 

So long as you can recieve a signal you can calculate it accuratly, more so in space. If they had a systematic error they could account for it.

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The reading would still be "at" a lat/long coordinate, which would represent the spot you were directly over.  But the altitude reading would be a doozy!

Ok so I tried a test on my iFinder Pro..

 

I saved my current (ground) location as a waypoint. Then I edited that waypoint, changing it's altitude from current(740 feet ) to 50000 feet, and told the GPS to go to that waypoint..

 

Sadly it claims I'm within 10 feet of my destination, NOT the nearly 9.5 miles (straight up) my waypoint was set at.. so if anyone's planning any high-flying caches I guess I'm SOL :(

 

How do Magellan & Garmin units behave doing this experiment?

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I'm not sure that it would be clear sailing once you were in space. Our atmosphere filters out lots of radiation, and other bad stuff we can't handle here on earth. How would a GPSr do under increased radiation in space, not to mention space dust which makes tiny holes in sats, and the ISS every day? Wouldn't these play a factor in accuracy?

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There are government technology export rules that specify that commercial GPS receivers should only work when either lower than 60,000' or at a speed less than 1000 knots, presumably to make them unsuitable for ballistic missile guidance. Most (all?) manufacturers have implemented this rule more strictly than required and make their receivers stop working when they are either above 60,000' *or* over 1000 knots, so this would limit their use at high altitudes. However, special waivers have been granted for GPS receivers used in high altitude balloon flights that exceed 60 kft, and military receivers can also operate above that height.

 

Technically there's no problem with using a GPS receiver in Low Earth Orbit (LEO) and GPS is routinely used on board the Space Shuttle.

 

He hit the nail on the head with that. well done :(

Edited by Stony2008
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It really doesn't matter if you are on the ISS, in orbit, on the moon, heading for Mars, or whatever.

 

Remember, the GPSr is designed to determine a position on Earth based upon reference points provided by the satellites. The GPSr will attempt to fix it's position in relation to a point on Earth by lat/long, mgrs, whatever, system of geographical fix reference.

 

So, let's assume we all manage somehow get the billionaire to get a GPSr into orbit, and let's assume the GPSr is receiving signals from, oh, 11 satellites just fine.

 

See where I'm going with this? Go to your "speed" function, and prepare for a shock, since you'll probably be traveling 1000s of miles per hour in relation to the earth (which you are!). Your current "position" will be updating quicker than you can read it. If you match the speed of the earth's rotation, then you will get the location on the face of the earth that you are directly over. Your relative speed will be zero. The only good data you could probably get is your altitude, assuming your GPSr can provide a readout of that many digits.

 

If I remember correctly, the ISS is not in a stationary orbit. So, hiding a cache on the ISS won't do you any good when push the mark button, any more than it would to do you any good to "map" a cruise ship while it is under steam.

 

All assumptions aside, I'm not sure my trusty Garmin could compute that quickly, and would probably shut itself off until I demonstrated a little more common sense. :(

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In low earth orbit, satellites (like the Space Shuttle and ISS) are moving at about five miles a second. If your GPSr has a "refresh" rate of about once a second (as mine seems to have), it's gonna go nuts trying to give you a fix.

 

The satellites are indeed directional, in order to maximize the power of the signal. If they simply radiated in all directions, the signal would be many times weaker than it is. That factor aside, a GPSr would work just fine at all altitudes less than the 12,000-mile orbital altitude of the satellites.

 

The error of the reading is not angular, so the analogy of the bubble is incorrect. Since the fix is computed by time alone, if you were closer to the signal source (the satelllites), the error would be smaller as the time of travel for the signal decreased. Not that we're talking about a lot of time, at the speed of light.

 

Now, to eliminate the orbital velocity factor, let's imagine placing a GPSr in an artillery shell and firing it straight up. The coordinate "fix" would remain almost the same as the shell rose, because it would still be above the same point on the planet, but the altitude reading would change.

 

The reason IVxIV's waypoint didn't change is because he was still at the same lat/long coords. Changing the altitude readout doesn't really change the waypint, because the system reads, essentially, in two-dimensional space. It does give a vertical readout, but to set a THREE-dimensional waypoint, you'd have to enter three coordinates. We only enter two for lat and long.

 

In other words, the system may tell you what your altitude is, but it doesn't compute that factor as a coordinate point. It's an "info only" kind of thing.

 

Clear as mud?

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IVxIV, Garmin and Magellan units do the same. Altitude is irrelevant, since the position is only calculated horizontally. You see the same effect when flying. If you fly over a position, such as a navigation radio transmitter (VOR) you show zero distance when you fly directly over it, but if you navigate using DME (distance measuring equipment), which is transmitted from the VOR, you never see zero, but always the distance you are above it. It takes some getting used to if you're used to DME, but you can see the differences if you have both working at the same time. At 6000 feet altitude, the DME never goes below 1 mile, while the GPS goes to (more or less) zero as you fly over the transmitter. The GPS has no clue whether you're on the ground or in the air, because it doesn't have the elevation of every (or even any) position on earth. All it knows is mean sea level, derived from the datum you're using, and the current altitude. That altitude could be in the air, on the earth, or underneath a mountain. The GPS has no way of knowing.

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Thanks for the info NightPilot :ph34r:

 

Is it then a bit curious though that most consumer GPSr's sold have a satellite status display that specifically indicates whether or not your getting a 2D or a 3D satellite lock? I guess that info is helpful for people travelling using TOPO maps etc, although I've heard the altitude reading on GPSr's is very crude and unreliable, I mean for those units that use triangulation to derive altitude, not the models that have barometric abilities.

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When I was shopping for a GPSr, my main concern was Geocaching. I did, however notice a number of units that were specifically designed for flight (a little big to carry on the trail :ph34r: As we seem to have some experts here, can anyone tell me if these specifically have an upper-limit of altitude to which they claim accuracy?

 

This thread has proven useful and I am starting to get a clearer idea of the process invoved in acquiring GPS signals. Of course, if anyone has a quick, handy link to a site expaining signal acquisition I'm always grateful -- I'll try to google it, but don't just want to get a bunch of information on "special internet deals".

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GPS Information. This is the standard site for all your questions.

 

Because of the way GPS works, vertical position is less accurate than horizontal position. That's why the FAA came up with WAAS. Its purpose is to provide more accurate vertical position information for aircraft flying GPS precision approaches, not for helping geocachers find caches. That's why it's difficult to receive it on the ground, but easy in an aircraft - they don't care if WAAS is available on the ground, so geosynchronous satellites are cheaper, because fewer are needed and they're already up there. The FAA doesn't care at all about consumer GPSrs receiving WAAS. Using WAAS, vertical position should be within about 50', and somewhat better most of the time. For simple aircraft navigation, without altitude information, WAAS isn't at all necessary. We were using GPS for non-precision approaches (no altitude guidance) long before SA was turned off.

Edited by NightPilot
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Is it then a bit curious though that most consumer GPSr's sold have a satellite status display that specifically indicates whether or not your getting a 2D or a 3D satellite lock? I guess that info is helpful for people travelling using TOPO maps etc

The 2D vs. 3D indicator can also be a valuable guide as to whether the horizontal position given by the GPS receiver should be trusted.

 

The receiver is trying to find four variables: your x, y, z coordinates (e.g. lat/long/altitude), and the exact time. To do that properly it needs to be getting signals from at least 4 satellites in a reasonable geometry (i.e. not all together in the sky or in a straight line). But sometimes it's hard to get signals from the necessary four satellites so the receiver uses the assumption that your altitude hasn't changed much since the last time it had a good position fix. Then it only has to solve for three variables and can get by with three satellite signals. This is when it indicates that it has a 2D lock.

 

But there will be a problem if the assumed altitude value is wrong. In that case, the lat/long values calculated by the receiver can end up quite far off as well. For an extreme example, I have sometimes turned on my GPS rcvr. after an airplane trip where it last had a lock at 40000'. Then when it first gets just a 2D lock while on the ground, the position shown can be off by a few miles. As soon as the receiver gets an additional satellite signal and calculates a 3D solution, both the altitude and the horizontal position quickly shift to the correct values.

 

So if your receiver is indicating just a 2D solution, you should be a little more suspicious of the reported positions, especially if you know that your altitude has changed since the last 3D fix.

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J-Track was mentioned in this thread a while back. Here is the link for that web site. http://science.nasa.gov/Realtime/JTrack/Spacecraft.html

The 3D J-Track shows the satelites and the earth, zoom in and out and click on a satelite and get information about it. I checked a few information pages about the GPS satelites and it looks like they are at an ellevation on orbit of 20,200 kms

Edited by spud67
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If you are above the satalites in space does the model know enough to calculate your postion and give you a LL based on your position over the earth?  Does the mode fizzle out at a certain point or is that a function of the GPS maker?

Thinking about this more, I would think that after a certain distance the accuracy would start dropping off rapidly. If we can get 3 meter accuracy with the satelites at 125,000 miles away wouldnt ya think that at 200,000 miles away the chance for errors would increase dramatically? Sooner or later you'd get to the distance that smallest error would be giving huge errors in the LL? Given that you were 200,000 miles above a spot on the earth even the smallest error would place you many miles away on a round surface, where being on the surface small errors would only move you so many feet? You'd might assume that the accuracy would actually be less at the higher altitude.???

I think the error is so many feet, not so many degrees

I don't think that just because that is how it's represented on your error reading either.

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Well, planes use GPS, so it will work to a point.  As you go higher, I imagine, you would have fewer and fewer sats to compute your location, thereby giving you increasingly bad accuracy.  Eventually, as you go up, you'll get to a point that you can lock on to too few sats to give you a position.

I'll give the first person to accurately describe a point in space using longitude and latitude, $5.00 and a six pack of Bud.

Edited by Team cotati697
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How about EGNOS (WAAS) in space? Copied this from a space related newsgroup

 

On 15 April, the ENEIDE mission with the Italian astronaut Roberto Vittori

on board will depart from Baikonur in Kazakhstan.

 

ENEIDE is one of the most important experiments that will be carried out on

board Soyuz during the mission, with the objective of assessing, for the

first time on board an inhabited space vehicle, the reliability under

extreme conditions of the navigation signal provided by the GPS-Egnos

satellite navigation system.

 

Alenia Spazio has produced the multi-standard receiver for ENEIDE, which

will be managed directly by the astronaut through a dedicated interface

installed on a portable PC.

 

Approximately 4 hours after launching, Roberto Vittori will activate the

experiment, which will remain operational for the next 36 hours, and it is

estimated that it will continue to function for the duration of the mission,

estimated at approximately one week.

Edited by PDOP's
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How about EGNOS (WAAS) in space? Copied this from a space related newsgroup

 

On 15 April, the ENEIDE mission with the Italian astronaut Roberto Vittori

on board will depart from Baikonur in Kazakhstan.

 

ENEIDE is one of the most important experiments that will be carried out on

board Soyuz during the mission, with the objective of assessing, for the

first time on board an inhabited space vehicle, the reliability under

extreme conditions of the navigation signal provided by the GPS-Egnos

satellite navigation system.

 

Alenia Spazio has produced the multi-standard receiver for ENEIDE, which

will be managed directly by the astronaut through a dedicated interface

installed on a portable PC.

 

Approximately 4 hours after launching, Roberto Vittori will activate the

experiment, which will remain operational for the next 36 hours, and it is

estimated that it will continue to function for the duration of the mission,

estimated at approximately one week.

How about that. Cool.

How many WAAS in space satellites are there any way?

Edited by Team cotati697
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There are government technology export rules that specify that commercial GPS receivers should only work when either lower than 60,000' or at a speed less than 1000 knots, presumably to make them unsuitable for ballistic missile guidance. Most (all?) manufacturers have implemented this rule more strictly than required and make their receivers stop working when they are either above 60,000' *or* over 1000 knots, so this would limit their use at high altitudes. However, special waivers have been granted for GPS receivers used in high altitude balloon flights that exceed 60 kft, and military receivers can also operate above that height.

 

Technically there's no problem with using a GPS receiver in Low Earth Orbit (LEO) and GPS is routinely used on board the Space Shuttle.

What 'government' would that be?

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Well, planes use GPS, so it will work to a point. As you go higher, I imagine, you would have fewer and fewer sats to compute your location, thereby giving you increasingly bad accuracy. Eventually, as you go up, you'll get to a point that you can lock on to too few sats to give you a position.

My thoughts are if you are in a plane the higher you go the more satelites will come into view as you horizon is getting further below you so the area of view is less restricted by the earth (this is hard to explain without diagrams) thus potentually exposing the unit to more satelite signals, not like celestial nav where in an aircraft you need to work with an artifical horizon. So in theory you will be given a postion fix in relation to a point on earth seeing as the GPS will be set to an earth based datum, not so good if you are trying to fix a position on the Moon.

 

Another thing the position given in relation to sea level is a predetirmined datum as well, because the Earth bulges in the middle sea levels are different around the world, even the two entrances to the Panama Canal are at different levels, the problem of height above sea level in this instance comes from trying to apply spherical geometry the an object that isn't a perfect sphere.

 

Triangleation works well for most GPS fixes but the minute angle differences between the GPS unit and the transmitting satelite that we concider acceptable to give us a GPS fix within a few meters transfered to height above sea level as well can make the fix seem alarmingly inaccurate.

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Triangleation works well for most GPS fixes but the minute angle differences between the GPS unit and the transmitting satelite that we concider acceptable to give us a GPS fix within a few meters transfered to height above sea level as well can make the fix seem alarmingly inaccurate.

Actually GPS uses trilateration (distances) not triangulation (angles)

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Triangleation works well for most GPS fixes but the minute angle differences between the GPS unit and the transmitting satelite that we concider acceptable to give us a GPS fix within a few meters transfered to height above sea level as well can make the fix seem alarmingly inaccurate.

Actually GPS uses trilateration (distances) not triangulation (angles)

Thanks for that, trilateration is the word I actually meant... it was very early in the morning when I wrote that. But basically it still supports the message I was trying to get across.

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