How old is the ISS REALLY?

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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mfb- t1_j9zhfoe wrote

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.

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EmergencySwitch t1_ja02vfx wrote

Replying to mfb- (#2,000,852)

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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ziptested t1_ja0dw1j wrote

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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8NAL_LOVER t1_ja13g95 wrote

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.

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Psychocumbandit t1_ja1kkra wrote

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?

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macisaint t1_ja1t8g2 wrote

Replying to [deleted] (#2,015,297)

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.

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2xOPisANidiot t1_ja2hji7 wrote

Replying to Psychocumbandit (#2,014,340)

>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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wosmo t1_ja2sr20 wrote

Replying to 8NAL_LOVER (#2,011,026)

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.

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mdogm t1_ja2vbll wrote

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?

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Taxoro t1_ja2x3ql wrote

>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.

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SportsCommercials t1_ja2zmyf wrote

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.

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Sammy81 t1_ja33zcs wrote

Replying to ziptested (#2,006,353)

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.

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Darkeyescry22 t1_ja348ma wrote

Replying to SportsCommercials (#2,022,859)

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?

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C47man t1_ja3aq3e wrote

Replying to Darkeyescry22 (#2,023,791)

>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.

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Lashb1ade t1_ja3c41j wrote

Replying to C47man (#2,025,094)

"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.

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gwplayer1 OP t1_ja4a4we wrote

Replying to mdogm (#2,021,990)

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.

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Sammy81 t1_ja6ej2y wrote

Replying to parikuma (#2,027,593)

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.

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Coomb t1_jadvbz3 wrote

Replying to Sammy81 (#2,046,101)

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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Coomb t1_jadwjsm wrote

Replying to Taxoro (#2,022,362)

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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