"Earth spinning faster will break GPS and atomic clocks".. Umm, how?

Question

Since it's just come out that Earth is apparently spinning about a milliscecond and a half faster.. I see a bunch of "articles" claiming this will.. somehow.. break atomic clocks and make GPS useless.. (Google "earth spinning faster break atomic clocks" and go to crazy-town..)

Umm, how and what exactly do they think will happen? A couple things I saw (I read two articles, then quit trying to play whack-a-mole to find one that might actually explain the basis of this claim) did admit that we already have to correct for relativity.. And we already have positive leap seconds added every so often, and my understanding is that various ephemeris data has to be updated every so often.. So would this be be really any different?

I can see that if the speed kept increasing by something like.. say two or three seconds per day, or the Earth started wobbling around that could cause problems.. But we're not talking about that (at least until next year)

Answer 1

None of the articles (e.g. this one) I came across say anything about breaking atomic clocks. The problem is simply a logistical one - if we have to implement a negative leap-second, then we will be doing something which most of the software which implements ordinary leap-seconds has not been explicitly designed to do. And if you've ever written any large-scale computer code, you are probably aware that a tiny change can wreak massive havoc if that change hasn't been thoroughly tested.

Answer 2

I would like to see a real expert discuss, but I think from what I can tell that these clickbait articles started from Forbes magazine

If Earth spins faster then it gets to the same position a little earlier. A half-a-millisecond equates to 10-inches or 26 centimetres at the equator. In short, GPS satellites—which already have to be corrected for the effect of Einstein’s general relativity theory (the curve of space and time)—are quickly going to become useless.

However my understanding is that gps satellite orbit data and clock data are constantly being updated and corrected for and that the IGS has a series of products that update regularly with these orbits and clock corrections.

IGS is the International Global Navigation Satellite Service and has a network of about 500 ground stations that is gathering the orbit and clock data and probably some ionosphere data as well.

The reason the orbit data is so important is that orbits constantly change and without updates would be kilometers out of position in a relatively short time, while for GPS, I think satellite positions are kept within a few centimeters and clock times are kept to the order of a few 10s of nanoseconds for real time positioning applications. The gps position error on the ground is a different much larger number.

So while you can certainly see the difference in clock due to general relativity, and the earth spun faster, I don’t think it translates to any unsolvable or dangerous issues.

Answer 3

Atomic clocks will keep running forever at a speed of exactly one second per second. Earth rotation might slow down from say 86,400 seconds a day to 86,399.999 seconds a day. GPS will perfectly happily find the exact point in space and time where you are, but for example your home might not be where it is expected to be found anymore. That's because your home is rotating at a speed of (I think) 1,700 km per hour around earth.

So GPS has to take into account the earth rotation, and when it has found your absolute 3D coordinates in space, it has to find out where earth exactly is, how far it has rotated through the day, and from that it calculates where it's 3D coordinates are on a map of the earth.

(GPS systems do some tricks to get better results once they have your position on a map of the earth. If GPS finds only three satellites, it cannot find your 3D location and the time simultaneously. It will then goes that most likely you are somewhere on the ground. Say if you are roughly in an are 2,000 feet above sea level, then it guesses that your 3D location is 2,000 feet above sea level - if you are on top of a high building, that guess may be wrong. And if you are driving, it guesses that you are on some street. If there is no street on the map where you are, but there is a street 10 feet to the east, it may guess that you are ten feet to the east).

In practice, you put GPS receivers say in 10 places on the earth. Then you compare what the GPS receivers think where they are, with the unchanged actual location of the receivers (unchanged unless there is a huge earthquake). With this information, you calculate how far an uncorrected GPS receiver would be wrong, and then you fix its calculation. That is actually one way how you measure the speed of earths rotation.

Answer 4

Atomic clocks

Atomic clocks do not deal with leap seconds and the length of a day. All atomic clocks internally just keep a counter that indicates the number of seconds (or rather picoseonds) that have past since some universally agreed point in time in the past. Mid-night on January 1st, 1970 in Greenwich is such an universally used common time point in the past. Time synchronisation between atomic clocks is also based on such a standard.

Computers

EDIT: the below description for computers is correct for leap days, time zones, and such. But apparently I assumed too hastily that it applied to leap seconds as well. Leap seconds are actually incorporated into the running count on some computers. Thanks for Ilmari for pointing that out.

All clocks in computers also just keep a running counter of number of seconds since a fixed time point. They then synchronise that counter over the Network Time Protocol with the internet to keep the clock in sync with other clocks.

It is only when translating those counters to human readable times, that leap seconds (and leap days, time zones, and summer/winter time) come into play. We humans like to have the hours, minutes, and seconds that we use to be correlated to the spinning of the Earth, such that the sun is always directly above at 12:00. For that reasons, computers translate the internal clock counter to a human readable time, according to mapping tables in the operating system. Those tables also take care of all the different time zones in the present and both the past and future.

Those translation tables need to adapted when the Earth spins slower. But that is not a dramatic change. If it is agreed in advance when a second is going to be skipped instead of added (a negative leap second), then those tables can just be adjusted in advance without major consequences.

Sattelite navigation systems

For satellite navigation systems, they are not affected by this mapping at all. The clocks in navigation satellite don’t deal with human times either. Only two things are important for the satellites:

1. that each satellite knows precisely above which geographic location it is located at any point in time.
2. that the clocks in the satellites are synchronised with each other, such that it is guaranteed that they all show the same time from an earth surface point of view.

Due to general relativity, clocks on satellites run faster than on Earth, and need to be compensated for that. Satellite orbits can also not be predicted with perfect accuracy. For both these effects, there are already adjustment mechanisms in place. Both clock speed and the orbit information of the satellites are regularly synced with ground stations in order to make sure the information remains accurate. A milli-second more or less is not going to upset those adjustment mechanisms.