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One of the aspects of most sci fi time travel representations that I find most challenging for my suspension of disbelief is this idea of people travelling through time and not travelling through space. You see the person in the time machine while the world transforms around them; mountains rising, seas forming, dinosaurs roaming past them, etc.

This is implausible for many reasons. The earth rotates, it also orbits around the sun, and the sun orbits the centre of the Milky Way, etc. Even if the existing 3D momentum was preserved and the time machine was at rest relative to its position on the earth, you have problems with continental drift, mountains and oceans changing the relative altitude of the land, etc. Ultimately, you'd end up either embedded in earth, drowning, or plummeting to an untimely (no pun intended) death. In point of fact, I often wonder if that's the reason that the conundrum of time travel (if it's possible, where are all the time travellers in our past?) is satisfied - they're all dead because they landed unsafely. But, I digress.

It struck me that for short term time travel hops (hours, possibly days) preservation of 3D momentum (especially angular momentum) is probably fine. Anything more than that and you risk getting your shoes stuck in the pavement because you left in winter and forgot to incorporate the expansion due to heat for when you landed in summer.

On the other hand, for longer hops, we have a ready-built navigation solution that's already in regular use around the planet every single day; GPS!

If your time machine had some limited 3D movement available to it and could access the GPS signals from throughout the lifespan of the system, then the GPS signals could be used (firstly) to pinpoint the time you want to arrive, and (secondly) to pinpoint the location in space you want to arrive. Marry it with Google Earth and altitude data across the planet for the same time, and you arrive safely every time, more or less.

So; perhaps the reason we haven't seen too many time travellers up until now is that they couldn't navigate to their location because now is when GPS is more reliable and mainstream as a technology. They should be popping up like rabbits any day now (although if they're adept at hiding in our the modern world, this could account for the massive proliferation of new technology in this current age - think about it).

But; is that really the problem?

So here's the question - Is it possible to use GPS signal data from now back through the time of its inception to extrapolate landing sites for time travel machines back past the point where GPS was available?

If so, how would you do it, and what margin of error could you expect as you went back further and further?

For the purposes of this question, let's assume that you have a time signal receiver that can receive the signals from GPS satellites during any and all points of their operation from the past, present and future and that this temporal 'map' of signals is what you use to pinpoint your landing location.

As I see it, the problem with extrapolating a location from GPS into the past is that GPS works via 'relative' signal reception, meaning that the GPS satellites know where they are relative to each other and the Earth, and that the GPS device on the ground uses that information to pinpoint its own location based on the timing of the signals from each satellite in a form of triangulation. In other words, the satellites aren't aware of their 'absolute' location in space, and therefore you can't extrapolate back past their existence to form a map of the earth's journey through space, at least not in a conventional manner.

Hence the question; is there an (un)conventional way to get back past the GPS era safely in my time machine?

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    $\begingroup$ Very related, in fact so closely it's almost a duplicate (but is more general). The top answer is mine, and I remember some interesting comments on the nature of the GPS network. $\endgroup$ – Joe Bloggs Aug 1 '18 at 8:31
  • $\begingroup$ I think you're also forgetting that you need four, not three, satellites to get a decent GPS location, where the fourth is used as a clock to measure distance (because the atomic clock on the satellite is quite more precise than your smartphone). BUT, you could (and should) have an atomic clock on the time machine itself, and use that. Or, maybe even better, have more time machines that move in time relative to each other, just like satellites do in space. $\endgroup$ – ChatterOne Aug 1 '18 at 10:31
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I don't think you can ever make it perfect as Jarred Allen points out. But you could limit risks a bit by making your time machine a space ship, that gives you lots of room for mistakes, or make it a submarine with a soft rule that it needs a lot of empty space around you to use it and has to be at a depth lower than the minimum tide so it can't appear hovering above the water and fall in.

This could be used in a narrative for risky time jumps.

Alternatively, don't think of it as popping in and out of time. But just changing the flow of time. So if you roll forwards or backwards, your time machine moves with the ground up and down and drifts. Big risk though if something like an Earthquake happens, if your moving in time at 10x speed, the Earthquake is 10x faster. Scary...

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There are problems with your idea of using GPS and maps to figure out your location that manifest themselves while you still have a GPS network.

The problem with using GPS to figure those things out is the accuracy of your GPS constellation. The current civilian GPS network operated by the US government promises accuracy to within 6 meters (page 32 of the pdf). This would be very limiting on your ability to precisely place your time machine (6 meters underground and you're never coming back out again, 6 meters above ground and you'll likely hit the ground hard enough to break parts of both your machine and you.

An even bigger problem, though, is imprecision in knowing exactly where you want to go. Maps are limited in their accuracy. The US Geological Survey published a report about their map accuracy. In the report, they detail that their maps are very accurate and provide specific details, but the best estimates I've seen still leave you with several meters of uncertainty, which is more than enough to cause your time machine to appear in the middle of the ground or floating in mid-air. Note that you should use US Geological Survey and not Google Earth because Google Earth does not provide any guarantees to accuracy

To make things worse, elevations change over time. Even if you have perfectly accurate maps of the Earth tomorrow, plate tectonics, erosion, etc. will cause uncertainties to build up at a rate of an inch or two a year in most cases. If you're going back more than a few decades, you won't be able to safely put your time machine on the surface of the Earth, even if you manage to bypass the uncertainty of your GPS unit.

And, even if you manage to solve all of those problems, you still might have to deal with a couple hundred millimeters of uncertainty in your wanted elevation caused by Earth tides. As the moon passes overhead, the Earth moves up and down noticeably from the tidal forces.

In all, you'd have to hand-wave something for your time machine to work, even just going to contemporary times. There's just too much uncertainty involved for your time machine to make a safe landing on terra firma.

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