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Before Feb 2018, the meteosat 11 (MSG-4) were periodically reboosted approximately every 85 days:

enter image description here

EDIT: added the orbital inclination plot to show the clearly visible variation to both semi-major axis and inclination with a single burn.

(TLEs downloaded from www.space-track.org and calculations done with the JSpOC SGP4 library downloaded from the same site).

After the big deboost and reboost that took place on Feb 11/14:

enter image description here

the reboost period increased to about 114 days (but only 1 reboost is currently available).

Question 1: why that big change in the radius vector?
Question 2: why the reboosts are now bigger?

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    $\begingroup$ Can you take a look at the sub-satellite longitude in Earth-fixed coordinates? I"m wondering if this could be related to station-keeping, and the altitude changes are just a "side-effect". Then maybe something was "coming through" in mid-February and they had to get out of it's way perhaps? $\endgroup$ – uhoh Sep 22 '18 at 15:42
  • $\begingroup$ ya in GEO spacecraft don't "loose altitude" this fast naturally, do they? I wonder if it is using constant electric propulsion to hold a specific sub-satellite longitude, and the side-effect of that is a small loss of altitude? $\endgroup$ – uhoh Sep 22 '18 at 15:46
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    $\begingroup$ EUMETSAT give very good explanations here: eumetsat.int/website/home/Satellites/LaunchesandOrbits/… but I found nothing about the single manoeuvres. $\endgroup$ – Cristiano Sep 22 '18 at 16:25
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    $\begingroup$ What you say explains the smaller reboosts, but surely doesn't explain the big manoeuvre (question1) and the recent (bigger) reboost (question 2). $\endgroup$ – Cristiano Sep 22 '18 at 21:03
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    $\begingroup$ If someone is interested in this topic, I uploaded some additional graphs to my lean and mean site: cristianopi.altervista.org/as $\endgroup$ – Cristiano Sep 27 '18 at 15:42
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After another search, I found this link: https://www.eumetsat.int/website/home/Satellites/CurrentSatellites/Meteosat/index.html with the enlightening phrase: “[Meteosat-11] Replaced Meteosat-10 at 0° on 20 February 2018.” and now, with the help of another graph, we can easily understand what happened:

enter image description here

The graph shows the Meteosat-11 longitude and the mean radius vector.

Before February, the satellite longitude range was 3.2 $\div$ 3.6 deg west.
When EUMETSAT needed to replace the Meteosat-10, they deboosted the Meteosat-11 ($\Delta V \simeq 1.3 m/s$) to increase the orbital speed and to start moving it eastward.
When the satellite reached the new intended longitude, they reboosted the satellite ($\Delta V \simeq 1.5 m/s$) to decrease the orbital speed, stopping the eastward motion.

EUMETSAT is now doing the normal station keeping manoeuvres to keep the longitude in the range -0.2 $\div$ 0.8 deg, with a similar range used for the Meteosat-10: https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/SatelliteOrbits/Satellitemanoeuvres/index.html.

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