The following graph shows the Tiangong-1 radius vector at the perigee, semi-major axis and apogee (scaled to 6371 km just to show an approximate altitude) and its orbit eccentricity:

enter image description here

I used the SGP4 propagator to do all the calculations and 1 dot is 1 TLE.

There is a big discontinuity on 2018-01-27 (706 m on the apogee). It can also be clearly seen a big change in the apogee decay rate. Finally, there are no TLEs for the day 30 (as if the tracking data were uncorrelated).

I can only say that all the three “strangeness” are generated by the Tiangong-1 RCS engines or vernier engines (the acceleration is very small). Is that possible? Can anyone give me another explanation?

EDIT: here is another suspect change:

enter image description here

The graph shows the orbit inclination (the min and max is because of the earth's oblateness). It’s clearly showed a sudden change exactly were the eccentricity changes and the new inclination is confirmed by (at least) 4 TLEs. I’m not able to give any explanation for that other than engines firing.

  • $\begingroup$ Somewhat similar (but different) question; PSLV-C39/IRNSS-1H TLE accuracy. $\endgroup$ – uhoh Feb 1 '18 at 13:27
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    $\begingroup$ I've responded as an edit at the beginning of the answer. $\endgroup$ – uhoh Feb 3 '18 at 0:29
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    $\begingroup$ I read the edit, but it's the first time that I see those strange shapes for those parameters. I need to collect some more TLE to be reasonably sure that the graphs are just showing blips and the fact that JSpOC is publishing just 1 TLE/day is strange as well. $\endgroup$ – Cristiano Feb 3 '18 at 0:40
  • $\begingroup$ Sounds good. Is 1 TLE/day actually strange when a spacecraft is still this high up? Also, altitudes are usually referenced to 6378.xxx km. $\endgroup$ – uhoh Feb 3 '18 at 0:46
  • $\begingroup$ There are 2 to 4 TLE/day for the Tiangong-1 before day 30. No TLE for day 30, then 1 TLE/day: uncorrelated tracking data? Why? If they fired the engines, surely the data are uncorrelated. 6371 is the mean Earth's radius. $\endgroup$ – Cristiano Feb 3 '18 at 0:51

(Posting as an answer so I can insert images) Lending credence to the idea that this is "sensitive response to measurement noise", consider these two plots from SatFlare.com (arrows added). They show just one datum that dropped in perigee and went up in apogee; the next measurement was back on the smooth track.

enter image description here Perigee: enter image description here

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    $\begingroup$ I like your graphics better - they really make the outlier obvious. $\endgroup$ – uhoh Apr 1 '18 at 18:24

Edit: The question has been updated and the plot showing a "blip" in inclination which has the same shape as the "blib" in eccentricity. I've incorporated my comments here:

The only phenomenon I'm really proposing is that when limited measure data is fitted and parameters are extracted, there are sources of noise in the extracted parameters, and the noise can be enhanced when there are statistical correlations between parameters. Put simply, a simultaneous change in two (or more) parameters in a correlated way can result in a nearly identical goodness of fit, and this can amplify the ever-present noise in the initial measurements.

You've now flagged two parameters that changed at the same time (eccentricity and inclination on 28-01-2018) and in a beautifully correlated way. The "blip" in inclination matches the shape of the blip in eccentricity. This is a textbook example of correlation in fitted parameters! Applying Occam's razor to the choice between a well-understood mathematical behavior when fitting imperfect, under-sampled, noisy data, and conspiracy theories of "secret spacecraft reactivations", and what's going to be the most likely explanation?

The way to proceed in a systematic and scientific way would be for you to look at all parameters (rather than "revealing them" one at a time), plotted over much longer periods of time, for many spacecraft in similar decaying orbits. I am guessing you will find blips and wiggles are the rule, rather than the exception. Public TLEs are approximate. They are provided as guidelines for judging roughly where spacecraft are likely to be with many km uncertainty at epoch, and more when propagating to other times. They have never been more than that, and this has been stated time and again.

Probably not. This kind of noise, at the level of 700 meters in SPG4 propagated orbits, probably happens all the time.

See the discussion in this answer; it seems to be the same exact phenomenon. There is a "blipping" of the eccentricity, but not a change in the semimajor axis, so the periapsis goes one way, and the apoapsis goes the opposite way, then everything goes back to the way it was before the blip (except of course for the steady decay in this case). The explanation is probably the same as well; a small error in position can always happen, especially if at a point in the orbit where there are statistical correlations between fitted parameters. As you point out, the positional error is only several hundred meters.

An error that large would be much less likely in the semimajor axis, because this is very strongly linked to mean motion. As your data the data for the object shown below both show, that doesn't happen. A mis-measurement of a few hundred meters than change the fitted eccentricity easily, but since satellites are observed over and over, timing and phasing but a much stronger lock on the semimajor axis.

I'll bet if one were to start looking, noise and blips in eccentricity is probably fairly common. It's likely a non-event. EDIT: I started looking, and sure enough, it's quite noisy!

I'll add some more about ellipticity noise in a few minutes...

below: This plot from here shows a much larger blip in eccentricity for another spacecraft In this case it's maybe 20x larger than that shown in the question. By coincidence, in this plot Tiangong-1 was used as a reference, and does not show significant noise at this scale.

This is data from about 01-oct-2017 to 31-dec-2017:

enter image description here

below: This is only the Tiangong-1 data, showing the change in peri, apo, semi (top, scale = km) and eccentricity (bottom) from one TLE to the next, over the same three months.

This shows that noise at the 1km scale for Tiangong-1 is commonplace.

enter image description here

  • $\begingroup$ @PeterTaylor Corrected, thank you for bringing that to my attention! $\endgroup$ – uhoh Apr 1 '18 at 0:51

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