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I'm following the slot swaps of the Iridium satellites in plane 6 and cannot make sense of the following: The TLEs of three of the old satellites very often show a marked difference in the mean anomaly to the previous day, while the mean anomaly of the other 7 satellites barely changes. Why could this be, what effect does it have?

As an example, I attach the TLEs of three consecutive days. Iridium 80, 77 and 98 show the big leaps in mean anomaly while the other satellites seem fairly unchanged. As far as I know, there was no manoeuvering going on with any of them at that time. The IridiumNEXT sats had already been inserted and were being tested.

Exhibit 1 TLE_1

Exhibit 2 TLE_2

Exhibit 3 TLE_3

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  • $\begingroup$ It would be really nice to give an example of the TLEs or at least mention a date/time and the names of the satellites in plane 6 that you are watching. The more information you give, sometimes the better answer you can get. Of course in this case it looks like you have a helpful answer, but in general it's a good practice. $\endgroup$ – uhoh Apr 16 '17 at 0:52
  • $\begingroup$ @uhuh Absolutely, there's a lot left unsaid and there may be a phasing manoeuvre or two happening if either this plane has one of the new arrivals drifting to it or if simply in response to a co-incidental failure in the first generation. $\endgroup$ – Puffin Apr 16 '17 at 10:16
  • $\begingroup$ @Puffin Thanks for guiding me along here. I have edited my question and added the information you asked for. Hope it's specific enough now. $\endgroup$ – eerie Apr 16 '17 at 16:26
  • $\begingroup$ I suggest as a next simple audit that you take the revs per day (i.e. the mean motion, second line columns 53-63) and then see how many revs fit between one epoch and the next. Lets say its 15.15 (made up for example); if you then add 15.15 * 360 deg per rev to the mean anomaly in the first TLE, does that bring you anywhere close to the mean anomaly in the second TLE? It should, though note that there are other small changes in the orbit, particularly the Argument of Perigee, so its not going to work out exactly. $\endgroup$ – Puffin Apr 16 '17 at 16:41
  • $\begingroup$ @Puffin Thanks - interesting idea, didn't get me anywhere, though. And does not show consistency with the data of the other satellites. $\endgroup$ – eerie Apr 16 '17 at 17:48
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Recall that the mean anomaly indicates the position within the orbit of the satellite/object. In the short term, whilst the other five orbital elements are only very slowly changing, the mean anomaly rotates by 360 degree for every revolution completed.

The significance of the mean anomaly in a TLE then is that it is telling us both:

  • (trivial answer) that the object was estimated to have that combination of mean anomaly at that given epoch. From this you can calculate forwards or backwards a little way and find the position of the satellite at other times.

  • (slightly more meaningful answer) that in the orbit determination effort data was gathered from a number of parts of the orbit, or consecutive orbits, and was decided to publish the TLE with that epoch. You can probably see that this is only slightly more meaningful in the sense that it could be interpreted as the centroid of that data collection effort though it wouldn't surprise me if the epoch is chosen to obscure any hints about the data collection sensors involved.

TLDR

In isolation, without the whole orbit information, there is little that can be learned from the mean anomaly alone. The implication, assuming that the Iridium satellites you refer to aren't being wildly manoeuvred, is that the habitual choice of publication epoch chosen for each satellite varies: for some the mean anomaly changes more than for others as a result.

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