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Here is a screen shot from NY2O of the ground track from GSAT-6A (43241, 2018-027A). What kind of orbit can make this wave shaped ground track?

enter image description here

uhoh
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    Quite similar to Mars's retrograde motion. – ugoren May 14 '18 at 12:27
  • I want to ask the question "is it possible to get a 'straight line' orbit from this elliptical orbit" but have no idea how to begin phrasing it. – Magic Octopus Urn Jun 21 '18 at 16:28
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    @MagicOctopusUrn if the orbital plane is also the Earth's equatorial plane, I think pretty-much yes. If it's not, I think pretty-much no, but why not ask? And in addition to a 'straight line' you could ask about any great circle orbit or an orbit parallel to lines of longitude or lines of latitude. Ask about all of them, it should be answerable in one post. – uhoh Jun 21 '18 at 16:53
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    @uhoh that would've been my guess; I think I'll keep my vague question answered by a vague comment :P. Thanks again good sir! – Magic Octopus Urn Jun 21 '18 at 17:59

1 Answers1

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An elliptical one. The Wikipedia page you link gives a signfiicantly different apogee and perigee and a period of 20.8 hours. So, on average, it moves West to East a bit faster than the Earth does, but at apogee it's moving more slowly and the Earth overtakes it a bit (which are the "S-bends" in the track). It swings a little North of the equatorial plane while approaching the Earth, and a bit South while receding. At the moment of the above screenshot it is close to perigee.

Each full cycle of the wave pattern is an orbit, so after roughly six orbits (120 hours) when the Earth has turned five times beneath it, it will get back to the same longitude. This fits with the apparent size of the wave pattern on the map, which is credibly about 4 time zones.

Steve Linton
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  • Great detective work, wow! Anything about inclination? – uhoh May 13 '18 at 09:15
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    Wikipedia says 3.29 degrees. – Steve Linton May 13 '18 at 09:20
  • I mean the argument of periapsis. Is it close to 0, 90, 180, 270? You did such great detective work, now I'm curious what else can be gleaned from this squiggle. You can probably get the inclination from the map too, the amplitude is almost as big as Borneo. – uhoh May 13 '18 at 09:30
  • So looking at Google maps and clicking around a bit, yes the ground track does seem to get as far as between 3 and 4 North and similary South, which is consistent with wikipedia. Finding the argument of periapsis basically involves comparing the equatorial cross with the mid-point of the S-bend (apogee). By eye they seem to line up pretty well which suggests something close to 180 (ascending node at apogee). – Steve Linton May 13 '18 at 10:13
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    Where the icon is shown in the screen shot looks like it's close to one of the two equator-crossings. Since the orbit is below GEO, it's moving east to west faster than the Earth turns, and at that point it's moving even faster than average so closer to periapsis. I get closer to 0 degrees than 180. – uhoh May 13 '18 at 10:22
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    The Earth turns west to east! So that's the descending node you've found. – Steve Linton May 13 '18 at 10:24
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    Ha! Indeed it does. I think that it used to turn the other way, but I guess they changed it in the early 1960's to make launching from Florida easier. ;-) – uhoh May 13 '18 at 10:26
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    Terrific, concise answer. If I could, I'd give, 2, possibly 2.7 upvotes. – Russell Borogove May 13 '18 at 15:18
  • Could it be that they wanted this spacecraft to reach GEO and the rocket didn't quite make it? Usually the rocket will bring down most of the inclination but the spacecraft has to clean that inclination back down to near zero (at most 0.5 degrees so it can drift up to 1.0 or 1.5 degrees at end of life). But 3.29 degrees is quite high for a GEO. So did the final burn of the upper stage perform below expectations, or did the apogee kick motor fail halfway through the burn? Or neither of these? – ChrisR May 14 '18 at 04:30
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    @ChrisR See the wikipedia page linked in the question -- they lost contact before the final orbit-lifting burn. – Steve Linton May 14 '18 at 07:37
  • Here's the orbit in real time and 3D http://stuffin.space/?intldes=2018-027A&search=2018-027A – bookman B. May 28 '18 at 09:13