What happens if GPS satellites don't receive ground updates?

[SpitfireIX]

Suppose there's an apocalypse, and all the GPS satellites are still in orbit and working fine, but they stop receiving time corrections from ground stations. My question is, what happens to the accuracy and precision of any GPS units still operating on Earth? As a follow-up, will the signals eventually become so out of kilter that all GPS units simply display error messages, rather than just highly imprecise location information?

I thought I knew the answer to the first question, which was that without updates the system would lose about one meter of precision per day, but when researching the second question, I found so many contradictory answers to the first question that I'm no longer certain of that, either.

 

[Mikemcc]

GPS satellites are in low-earth orbit, which means that their orbits gradually decay. they have onboard fuel to keep them boosted to extend their lives.

They operate by keeping extremely accurate atomic clocks in each satellite and very precise satellite positioning, The time signal drift is one of the biggest errors associated with them. Th ground stations are primarily concerned with general management and correcting for the tiny measurements in General Relativity due to the relative velocities between the ground and the surface.

Any position on earth can be calculated with only three signals, four for altitude, but most receivers expect to see many more signals (10-30 satellites)

Over time the orbits will not be able to update due to Relativity and the satellites will slowly deorbit and burn up.

For ground users the effects will gradually build and become worse over time

 

[Roboramma]

GPS satellites are in low-earth orbit, which means that their orbits gradually decay. they have onboard fuel to keep them boosted to extend their lives.

They operate by keeping extremely accurate atomic clocks in each satellite and very precise satellite positioning, The time signal drift is one of the biggest errors associated with them. Th ground stations are primarily concerned with general management and correcting for the tiny measurements in General Relativity due to the relative velocities between the ground and the surface.

Any position on earth can be calculated with only three signals, four for altitude, but most receivers expect to see many more signals (10-30 satellites)

Over time the orbits will not be able to update due to Relativity and the satellites will slowly deorbit and burn up.

For ground users the effects will gradually build and become worse over time

None of this answers the specific question in the OP. I suspect he already knows everything you wrote in the above post.

 

[The Great Zaganza]

Technically, GPS satellites don't need corrections - that's kinda the whole point.
Their internal clocks will continue to be more accurate than anything on a post apocalypse world.

A terrestrial receiver doesn't need to know where exactly the satellite is, just triangulate the distance by comparing signals from multiple satellites. Neither does the satellite need to stay in a specific orbit except for maximizing coverage.

As long as there are enough GPS or analog satellites in orbit to get a decent signal from 3 or more at once, a terrestrial GPS device will get very accurate location data.

 

[Jack by the hedge]

A terrestrial receiver doesn't need to know where exactly the satellite is, just triangulate the distance by comparing signals from multiple satellites. Neither does the satellite need to stay in a specific orbit except for maximizing coverage...

Wait: how do you triangulate position from time of arrival differences if you don't know the location of anything?

 

[rjh01]

Wait: how do you triangulate position from time of arrival differences if you don't know the location of anything?

Part of the information the satellites provide is their position. It will also tell the receiver the time of the broadcast. The receiver will then know it is a certain distance from the satellite but not the direction. A second satellite will give the same information. This will allow the receiver to calculate a circle in which it is on the circle's perimeter. A third satellite will then be used to say where in that perimeter the receiver is. The receiver also needs to know the time. It can do this by using a fourth satellite.

More information here https://www.faa.gov/about/office_or...units/techops/navservices/gnss/gps/howitworks

This raises the question of how the satellite knows where IT is? GPS of course! But then the entire system could drift out of reality for several reasons. So that is why we need ground stations. And that is the answer to the OP.

 

[The Great Zaganza]

Wait: how do you triangulate position from time of arrival differences if you don't know the location of anything?

you don't need to - the time difference gives you the spatial distances, assuming that all the clocks started out at the same time and have deviated very little - that is reasonable on human timescales, as they are incredibly accurate.

So your receiver knows how far away the satellites are from it, as well as how far the satellites are from each other. This gives you two solutions on where you could be - either close to the surface of the earth, or somewhere in high orbit.
You will have to figure out the last bit yourself.

 

[Jack by the hedge]

Part of the information the satellites provide is their position.

Thanks. That makes sense.

you don't need to - the time difference gives you the spatial distances, assuming that all the clocks started out at the same time and have deviated very little - that is reasonable on human timescales, as they are incredibly accurate.

So your receiver knows how far away the satellites are from it, as well as how far the satellites are from each other. This gives you two solutions on where you could be - either close to the surface of the earth, or somewhere in high orbit.
You will have to figure out the last bit yourself.

Uh. No, that still doesn't make sense.

Triangulation would tell you the exact relationship between the satellites and your receiver, but it doesn't tell you anything about where the planet is, unless the satellites tell you where they are in relation to it.

 

[The Great Zaganza]

Thanks. That makes sense.



Uh. No, that still doesn't make sense.

Triangulation would tell you the exact relationship between the satellites and your receiver, but it doesn't tell you anything about where the planet is, unless the satellites tell you where they are in relation to it.

I think I wasn't clear.

From the distance Data of 3 satellites and one receiver, there will be 2 locations that will fulfill the distance requirement from the Data - one above and one below the plane set up by the satellites.

 

[Jack by the hedge]

I think I wasn't clear.

From the distance Data of 3 satellites and one receiver, there will be 2 locations that will fulfill the distance requirement from the Data - one above and one below the plane set up by the satellites.

Yes, but that doesn't tell you where the planet is, or which bit of the planet is beneath you.

 

[SpitfireIX]

Wow, I'd given up on anyone's taking an interest in my question. I also posted it on Cosmoquest, where I haven't posted in ages, and I did get one somewhat helpful response.

Based on that, and some things I've found online, the answer seems to be, the systems will degrade (by between a tenth of a meter to one meter per day) for 60 to 90 days, but then the satellites' position data will be so inaccurate that the GPS receivers won't even be able to lock on. These times could be extended somewhat if ground-based differential GPS transmitters are still operating.

 

[jrhowell]

There are technical issues involved that I doubt anyone here knows the details about. For example I believe that each satellite transmits approximate position information about the entire constellation of satellites to aid in signal acquisition. I assume that this information must come from the ground stations. There could be some sort of failsafe in the satellite software that would cause them to stop sending GPS data if they loses contact with ground stations for a significant period of time.

Even if the satellites continue to transmit there may be issues with the GPS receiver software on various platforms when confronted with increasingly inaccurate information from the satellites.

 

[SpitfireIX]

you don't need to - the time difference gives you the spatial distances, assuming that all the clocks started out at the same time and have deviated very little - that is reasonable on human timescales, as they are incredibly accurate.

The problem is, the satellites' clocks do deviate, due to relativity. The [ETA: relativity] deviation due to gross orbital movement is corrected for in the system design, (the satellites' clocks actually run slower than atomic clocks on Earth) but other relativistic effects will also cause deviations. For example, a satellite passing over Mount Everest will be subject to different gravity from a satellite passing over the Mariana Trench, which will cause their clocks to become slightly out of synch. Over time, these deviations will accumulate, requiring periodic corrections from the ground control stations.

So your receiver knows how far away the satellites are from it, as well as how far the satellites are from each other. This gives you two solutions on where you could be - either close to the surface of the earth, or somewhere in high orbit.
You will have to figure out the last bit yourself.

That's the other problem with lack of ground updates; the satellites only "know" where they are based on information from tracking stations, so the position information they transmit to GPS receivers will gradually become out of date. Orbits can only be predicted with so much accuracy; eventually, as I mentioned, the receivers won't even be able to find the satellites.

One thing that I read that was very interesting is that there have been proposals to make the GPS system able to correct itself by having the satellites reference each other, without the need for ground control, but this has never been implemented due to cost.

 

[The Great Zaganza]

Yes, but that doesn't tell you where the planet is, or which bit of the planet is beneath you.

I never said anything like that - I joked that you have to figure out if you are above or below the satellites as seen from the perspective of the planet that satellites orbit. You don't need satellites for that, a simple " can I breath" test would suffice.

Which is not relevant in a post- apocalypse scenario anyway.

 

[Jack by the hedge]

I never said anything like that - I joked that you have to figure out if you are above or below the satellites as seen from the perspective of the planet that satellites orbit. You don't need satellites for that, a simple " can I breath" test would suffice.

Which is not relevant in a post- apocalypse scenario anyway.

I understand the triangulation resolves to points above and below the plane of the satellites. We may be at cross purposes. When I said "how do you triagulate position..." I meant position on the earth, not position relative to the satellites.

Knowing where you are in relation to the satellites doesn't tell you where you are on earth unless you know where the satellites are, but you began by saying the receiver doesn't need to know that.