Submitted by gwplayer1 t3_11boje4 in askscience
GPS satellites have to regularly reset their clocks to stay accurate to earth surface time due to the relativistic time difference between the satellite and the earth surface.
How much younger than its Earth surface age is the ISS due to the fact it’s been spinning around since 1998? Do they have to reset their clocks too?
Yes I know different parts went up at different times some as late as 2011. 1998 was when the first part went up.
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Orbital velocity: ~8km/sec
Seconds since original launch date: ~765,849,600
Seconds at observer: ~765,849,600.27268
So to answer your question, the original section of the ISS is about a quarter of a second younger than it would be if the parts had stayed on Earth.
Thank you for answering the question.
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Wouldn’t this make it a quarter of a second older than it would have been? Or is the observer someone on earth?
Also, do you know the calculation for general relativity? Is that effect (from being farther from earth) near the same order of magnitude, or much smaller?
>Wouldn’t this make it a quarter of a second older than it would have been? Or is the observer someone on earth?
Time passes normally in the reference frame of the ISS, while Earth time goes faster. In the reference frame of Earth, the ISS ages slower. It doesn't matter which frame of reference you use.
>Also, do you know the calculation for general relativity? Is that effect (from being farther from earth) near the same order of magnitude, or much smaller?
What effect? "general relativity" is vague.
"General relativity" would refer to time dilation due to gravity; the ISS is higher up in Earth's gravity well, so will age faster than on the Earth's surface.
I can never remember which of the two effects is larger.
The time dilation is not purely due to orbital velocity, there's a gravitational time dilation to take into account as well.
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GPS clocks are not reset. They run at adjusted frequency. In general in a satellite mission you want to avoid resets and be prepared to do adjustments not only for time dilation. In a satellite mission I worked we had an onboard clock that was the source of mission time in "milliseconds" since the satellite was powered on soon before the launch. The clock was never adjusted or reset. Time dilation and other errors accumulated. The satellite periodically transmitted a GPS time stamp along with the mission time to the mission control. Based on that the mission control uploaded a schedule of actions in mission time.
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That’s unusual. Usually the bird has a GPS receiver. In your case, the satellite clock usually has an epoch from which it counts time (time zero, set to something like 1958). This time is then adjusted by a parameter called deltaUT1, which is a tenth of a second resolution correction based on Earths rotation. The time dilation due to relativistic effects is minor, but an unsynced onboard clock that is not corrected will drift due to the clock a noticeable amount within just a few months. I would think Mission Control would have constant issues commanding it to take data at certain times (if it were an imaging bird for example) unless they synced the clock periodically.
Pardon my ignorance, but how come they don't use TAI for precision?
I was not familiar with TAI until your question, but I googled it. It looks to me like TAI is the most accurate GPS time. The problem with GPS time is that it is not “earth time”. By that I mean GPS time is independent of the position of the earth. Since the earth rotation is slowing over time, GPS time deviates from it, running ahead of it as the earth slows. UTC time takes this into account. Leap seconds slow GPS time to align with “earth time”, or the time it takes the earth to rotate one time. Since satellites are often concerned with observing earth, or communicating with earth, it’s important for them to stay aligned with the actual earth rotation, so UTC time is more useful. One of my first assignments was making it easy for ground control to upload deltaUT1 and leap seconds to our satellite (Calipso) so that it’s science data was accurately tied to the ECEF reference frame.
As a matter of technical fact, it's not GPS time that gets adjusted with leap seconds, it's UTC. From a user perspective in most cases the difference isn't particularly meaningful because you probably want to convert between GPS time and UTC and for that use case it doesn't matter whether you add or subtract the offset to one parameter or the other. But the satellites don't update the time they broadcast every so often to align with UTC. They've been counting seconds as accurately as they can since they started broadcasting. Instead, they broadcast, in the GPS navigation message, the offset, in integer seconds, from UTC. If you are reading time directly from a GPS message, you never have to worry about it repeating or skipping an increment. UTC technically could do either one of those.
E: to be clear, the GPS control segment routinely updates the clocks on the satellites to maintain synchronization tight enough to meet the GPS specified error budget, but these adjustments are transparent to users and never anywhere close to entire seconds
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I don't think the satellites periodically reset their clocks. Rather, I think they are just programmed with formulas that constantly counter the relativistic effects. Otherwise the resets would have to happen quite frequently (literally every two minutes). They need to compensate for ~8000 nanoseconds per day, but need to stay within ~10 nanoseconds to work.
So, is this the answer to OP's question?
(roughly) 8000 nanoseconds * 365 = 2.92 ms per year?
So the oldest part of the ISS is like 0.075 seconds younger?
I read that if relativity wasn't corrected for, GPS would accumulate an error of 10km per day. Seeing those nanoseconds translated into the functional accuracy we depend upon, really his this home for me.
GPS satellites run on a slightly different clock frequency to compensate the average time dilation. They do smaller adjustments once in a while because clocks on Earth are more precise.
The ISS has GPS satellites in view all the time so it can simply get the time from there.
EDIT: did not realize you were only correcting the misconception the first part.
This is not the time the clocks show. This is about the age of the things in the ISS compared to earth
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What will really bake your noodle is when you understand the question, "how old is the universe?"
Seriously, if there is no uniform time, how old is anything really? Are there some parts of the universe that are trillions of years old, or others that aren't even a second old?
Same with absolute space. If we measure speed relative to something, maybe we should be asking the same for age: "How old relative to whom?"
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That's a valid point. Supposedly the "Big Bang" was an almost instantaneous expansion but instantaneous relative to what? It's kind of opposite the effect at the event horizon of a black hole where, from the outside perspective (earth), something near the event horizon is slowing down but for the particles perspective, everything appears normal.
>GPS satellites have to regularly reset their clocks to stay accurate to earth surface time due to the relativistic time difference between the satellite and the earth surface.
This is not accurate, they use clocks that run ever so slightly slower
Don't know the exact number but roughly the scale. We are talking about a millionths of time going faster, so over 25 years.. maybe a couple seconds or so younger than we give it credit for.
The satellites do get their internal clocks updated by the control segment fairly regularly, but you're right that the reason isn't the accumulation of the relativistic error.
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As the satellites are following roughly circular orbits, would not any time dilation be averaged out to nothing over time, instead of incrementing a larger positive/negative value? If a satellite, over the course of it's orbit, has a segment where it's moving away from an earth based observer at a fast enough speed to incur relatavistic effects, would not that effect be zeroed out by a corresponding segment of the orbit when it is moving back towards the earth based observer at the same relatavistic speed? What part of orbital mechanics/relativity even allows for the incrementing of a positive/negative time value for a circular orbit?
>would not that effect be zeroed out by a corresponding segment of the orbit when it is moving back towards the earth based observer at the same relatavistic speed?
Direction is irrelevant for time dilation. High speeds always means slower time, never faster.
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There are two relativistic effects that have opposite signs. Time dilation due to movement, and dilation due to being in a gravity well. The gravity well effect dominates.
[deleted] t1_j9z3k30 wrote
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