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From my elementary school understanding of Satellite orbits, I know that GEO satellites are placed in equatorial plane, MEO satellites are placed in an smaller degree inclined plane while LEO satellites are placed in larger degree inclined plane near the poles.

During my research, I found that only O3b MEO satellite constellation is in the equatorial plane. However, I didn't find any LEO satellites constellations that are placed in the equatorial plane. Why is that?

Why are LEO satellites never place in the equatorial place with near 0 degree inclination?

The one reason I get is that since LEO is close to earth, the coverage cone is very small & if placed in the equatorial plane, it will cover only a small percentage of the earth surface & hence will be wasteful expenditure. Aside from money wasting, what other orbital factors prevent LEO satellite placement in equatorial plane?

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  • $\begingroup$ "Aside from money wasting" is the operative phrase. Satellites go into the best orbit for their mission. Some just want to be above the atmosphere, and take whatever inclination the rocket gives them. Others want a specific inclination for coverage or sun angle reasons. What would make a LEO satellite want to spend the fuel to get into an equatorial orbit? The advantage in GEO is you can use a fixed antenna, but that does not apply in LEO. $\endgroup$ – Ross Millikan Nov 4 '18 at 3:01
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Uhoh touches one side of the problem: "Why" - the lower the orbit, the less of Earth is covered in a single pass, and the closer to equator the orbit, the less do the passes vary further narrowing the area. MEO equatorial satellites make sense. The lower the orbit though, the less useful they become.

Still, there's a slew of tasks that wouldn't be hurt by that inclination - plenty of satellites just need to be in orbit, any orbit, to do their job. Why not the simple equatorial LEO?

Because it's expensive. Orbital plane change in LEO is very expensive maneuver and any straightforward launch followed by insertion (without the dogleg maneuver) will result in orbit of inclination no less that latitude of the spaceport. The French Guiana spaceport, with best location for equatorial launches at 5°14′14″N is best equipped for Arianne which is a very heavy launch platform, and one of the most expensive - and you still need to perform a dogleg of (AFAIR; verify that) ~1km/s to make the orbit equatorial. Plane changes from any other spaceport to equatorial LEO will be of order of 3-5km/s extra.

That way the equatorial LEO is not only of little use, it's also so expensive mostly everything you may need it for, is better achieved by more inclined orbits.

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Nothing physically prevents equatorial orbits at any altitude above the Kármán line. The question is really; what would be the point in flying over exactly the same equatorial band roughly every 90 minutes, when a higher-inclination orbit would let you cover much more of the planet (all of it, eventually, for polar orbits) or a higher orbit would let you both see a wider swath and not be constantly coming into and out of range of stuff on the ground.

Even a cheap orbital launch currently costs millions of dollars. So the suggestion that you offered in your answer is likely the correct answer; it would be a wasteful expenditure.

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    $\begingroup$ I toned down the condescension a bit, and the rhetorical questions. I hope you don't mind. I think this way the answer is the same but it's more comfortable for the OP and future readers when the answer is not quite so passionate. I do that from time to time as well, and get reminded as well. $\endgroup$ – uhoh Nov 3 '18 at 11:57
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    $\begingroup$ A satellite at say ~1000 km in LEO can see a sizable chunk of the Earth's surface every ~90 minutes, over ~15% with elevations above 45 degrees, and it's quite a populated 15% of the Earth at that! Still, Even when there are constellations of thousands, all orbits tend to be inclined and equatorial orbits not included. See SpaceX's 4,425 satellite constellation - what's the method to the madness? $\endgroup$ – uhoh Nov 3 '18 at 12:02
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While there is a currently accepted answer explaining why there are "no LEO satellites in the earth's equatorial plane", I found two of them with inclinations currently less than 3 degrees!

Both are science missions and detect gamma rays. I don't yet know why equatorial orbits were chosen, but I can forward two theories until such time that I find out.

  1. repeatable orbit. Since it is parallel to the Earth's equator, the equatorial bulge does not modify the orbit substantially (as long as inclination stays low). By that I mean the orbit pretty much stays in the same place. Mathematical nodes and apses can move, but nearly circular, near-zero inclination orbit stays roughly the same shape and orientation under this precession. Coverage of the celestial sphere for astronomical observation (and the part blocked by Earth) is more regular and repeatable than for an inclined and precessing orbit.

  2. repeatable access to equatorial ground stations. Since the data may be timely (a gamma ray event may signify an event that Earth observations need to follow up quickly), the systematic downlink schedule is easier to maintain than an inclined, precessing orbit.

    The importance of low-latency in gamma ray burst detection can be understood in answers to a somewhat related question “Who saw” the binary neutron star merger first? What was the sequence of events?


HETE 2

Current TLE:

1 26561U 00061A   18309.58823202  .00001352  00000-0  28876-4 0  9994
2 26561   1.9483 201.9211 0017895  22.8247 337.2589 15.07775517991403

The High Energy Transient Explorer (abbreviated HETE; also known as Explorer 79) was an American astronomical satellite with international participation (mainly Japan and France). The prime objective of HETE was to carry out the first multiwavelength study of gamma-ray bursts with UV, X-ray, and gamma-ray instruments mounted on a single, compact spacecraft. A unique feature of the HETE mission was its capability to localize GRBs with ~10 arc second accuracy in near real time aboard the spacecraft, and to transmit these positions directly to a network of receivers at existing ground-based observatories enabling rapid, sensitive follow-up studies in the radio, IR, and optical bands. The satellite bus for the first HETE-1 was designed and built by AeroAstro, Inc. of Herndon, VA; the replacement satellite, HETE-2, was built by MIT based on the original HETE design. Source


AGILE

Current TLE:

1 31135U 07013A   18309.50114465  .00002183  00000-0  33798-4 0  9999
2 31135   2.4671 121.8649 0012503 326.3083  33.6177 15.31125853641420

AGILE is an Italian gamma ray observatory launched aboard an Indian rocket today, beginning a three-year mission to survey the sky in a search for faraway sources of the Universe's most energetic form of light. The AGILE satellite flew into orbit on top of India's Polar Satellite Launch Vehicle. Liftoff was at 1000 GMT (6:00 a.m. EDT) from the Satish Dhawan Space Center at Sriharikota on India's eastern coast. Source

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