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So, question on the geostationary satellites. A lot of them have relatively high inclinations, like ~14 degrees. At first I thought this was because the count

by peeters 11y ago

So, question on the geostationary satellites. A lot of them have relatively high inclinations, like ~14 degrees. At first I thought this was because the countries that use them are at corresponding latitudes, but then I realized they'd be opposite the equator half the time. Are these orbits to optimize directness during the day, at the expense of the night? If so I guess that would explain why they tend to favor the southern hemisphere over the dark side of the Earth and the northern hemisphere over the light side (because that's where population is concentrated).

MPSimmons 11y ago
There are a couple of ways to do "geosynchronous". The most obvious way is an circular equatorial orbit at 35,786km. As I'm sure you know, but others may not, the orbital period at that distance means that the satellite completes one full orbit in 24 hours, thus it completes a circle in the same time that the earth does, so it's always over the same spot. There is also the concept of a Molniya orbit (https://en.wikipedia.org/wiki/Molniya_orbit https://en.wikipedia.org/wiki/Molniya_orbit), where you have a small cloud of satellites (at least 3) with extremely elliptical orbits that together can provide coverage for non-equatorial areas. If you come up with the satellite's name or designation you found, it's probably relatively easy to determine why it is at the inclination it has. Do you remember what it was you found?
- peeters 11y ago
  I wasn't concentrating on any specific satellite, just noticing a very noticeable trend of 24 h period orbits with inclinations between 0 and 15 degrees. They are not eccentric, so I think it rules out Molniya orbits. The trend is readily apparent if you zoom out and view the Earth in profile (see http://imgur.com/FQpTkLR http://imgur.com/FQpTkLR). There is the obvious trend at 0 degrees and then a noticeable high density from 0-15 degrees before then trailing off. For arbitrary examples, see Palapa 1, Insat 1A, Raduga 6.
  - restalis 11y ago
    The 0 inclination orbit must be the most sought one, hence the highest density there. Then in cases where a small compromise from 0-inclination orbit was tolerable, and considering that the second most important thing is fuel consumption, I presume that the rest of orbits may have something to do with the launching point (a lot of satellites were launched from Baikonur Cosmodrome).
crisf 11y ago
The preferred geosynch orbit has an inclination of 0deg, as that minimizes it's apparent motion in the sky. But this requires a lot of fuel for N/S stationkeeping. The reason the orbits get inclined is because they're being pulled from the side by the sun and moon, and just like any other rotating object pulled by a lateral force, their axis precesses (just like a top). Left to itself, the orbit cycles between 0 and 14 deg inclination.
- peeters 11y ago
  Interesting. Wouldn't the effects tend to cancel each other out because the inclination of the Earth spends half the year with the poles facing each to and from the Sun? Also since the Earth's inclination is 7 degrees, why do the satellites get to 14 degrees inclination? Edit: And furthermore, wouldn't the spread be somewhat random with respect to the longitude of the ascending nodes? This seems to be anything but random: http://imgur.com/FQpTkLR http://imgur.com/FQpTkLR
  - crisf 11y ago
    Some do cancel out. The inclination effect isn't driven by the sun's being above or below the equator - it's basic rotational mechanics. The net effect of the moon and sun's gravity pulling on the satellite causes the rotational axis to precess. The precession shows up in the orbit as the change in inclination (and ascending node). Regarding the non-random spread.... all the orbits start at the same place (0 deg inclination - because that's where people want them). And it's the same sun and moon affecting all of them them in the same way, so you wouldn't expect it to be random. It is interesting that they all appear in that band, I don't have a good explanation for that : )

by peeters 11y ago

MPSimmons 11y ago

There are a couple of ways to do "geosynchronous". The most obvious way is an circular equatorial orbit at 35,786km. As I'm sure you know, but others may not, the orbital period at that distance means that the satellite completes one full orbit in 24 hours, thus it completes a circle in the same time that the earth does, so it's always over the same spot. There is also the concept of a Molniya orbit (https://en.wikipedia.org/wiki/Molniya_orbit https://en.wikipedia.org/wiki/Molniya_orbit), where you have a small cloud of satellites (at least 3) with extremely elliptical orbits that together can provide coverage for non-equatorial areas. If you come up with the satellite's name or designation you found, it's probably relatively easy to determine why it is at the inclination it has. Do you remember what it was you found?

peeters 11y ago
I wasn't concentrating on any specific satellite, just noticing a very noticeable trend of 24 h period orbits with inclinations between 0 and 15 degrees. They are not eccentric, so I think it rules out Molniya orbits. The trend is readily apparent if you zoom out and view the Earth in profile (see http://imgur.com/FQpTkLR http://imgur.com/FQpTkLR). There is the obvious trend at 0 degrees and then a noticeable high density from 0-15 degrees before then trailing off. For arbitrary examples, see Palapa 1, Insat 1A, Raduga 6.
- restalis 11y ago
  The 0 inclination orbit must be the most sought one, hence the highest density there. Then in cases where a small compromise from 0-inclination orbit was tolerable, and considering that the second most important thing is fuel consumption, I presume that the rest of orbits may have something to do with the launching point (a lot of satellites were launched from Baikonur Cosmodrome).

crisf 11y ago

The preferred geosynch orbit has an inclination of 0deg, as that minimizes it's apparent motion in the sky. But this requires a lot of fuel for N/S stationkeeping. The reason the orbits get inclined is because they're being pulled from the side by the sun and moon, and just like any other rotating object pulled by a lateral force, their axis precesses (just like a top). Left to itself, the orbit cycles between 0 and 14 deg inclination.

peeters 11y ago
Interesting. Wouldn't the effects tend to cancel each other out because the inclination of the Earth spends half the year with the poles facing each to and from the Sun? Also since the Earth's inclination is 7 degrees, why do the satellites get to 14 degrees inclination? Edit: And furthermore, wouldn't the spread be somewhat random with respect to the longitude of the ascending nodes? This seems to be anything but random: http://imgur.com/FQpTkLR http://imgur.com/FQpTkLR
- crisf 11y ago
  Some do cancel out. The inclination effect isn't driven by the sun's being above or below the equator - it's basic rotational mechanics. The net effect of the moon and sun's gravity pulling on the satellite causes the rotational axis to precess. The precession shows up in the orbit as the change in inclination (and ascending node). Regarding the non-random spread.... all the orbits start at the same place (0 deg inclination - because that's where people want them). And it's the same sun and moon affecting all of them them in the same way, so you wouldn't expect it to be random. It is interesting that they all appear in that band, I don't have a good explanation for that : )