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To the best of my knowledge the key idea of the Tundra and Molniya orbits is that they are elliptical orbits at non-zero inclination that remain in the same ground track. Obviously, for practical reasons, the orbital period of those orbits are then chosen to synchronise to the solar day rather than the siderial day as is mentioned in this article. Please, re-check and modify articles.
--IndigoMind 10:41, 18 May 2006 (UTC)
- The article is correct. Since the earth rotates once per sidereal day relative to inertial space, that is the necessary orbital period to create a repeating ground track. Kemperb 00:01, 17 May 2007 (UTC)
Spamhog (talk · contribs) changed the lead paragraph on 16 March 2013 with this note: SOLAR not SIDEREAL. Idea is to hover on approx same area at same hours in SOLAR day. If sidereal hover times would drift. I can understand that consistent hours might be desirable; but not as desirable as constant longitude, which would be lost with a solar day orbit. I'll change it back. —Tamfang (talk) 08:32, 26 August 2014 (UTC)
Tamfang, I see your point but I suspect there is no contradiction between solar sync orbit and stable longitude. A geo-stationary orbit is both longitude constant and solar synchronized. Also a Molnya / Tundra orbit can be both. However, here it's a matter of fact, not physics vs. policy. I'll try to check if orbital parameters are physically compatible. I already tried to talk to Lavochkin to see if they published something somewhere but no luck so far. Here is a Molnya with period of exactly 12h. https://www.youtube.com/watch?v=O_Iykeouj3g Spamhog (talk) 09:38, 15 September 2015 (UTC)
- In what way is a geostationary orbit "solar synchronized"? — Let's say the new Spamhog Satellite is in a 24 hour orbit that reaches apogee at noon Greenwich time. Today, let's say, its apogee is over 0° longitude. Tomorrow, at noon Greenwich time, its apogee is over (almost) 1° west, because Earth has turned more than 360° in that time while the orbit stays in the same plane. —Tamfang (talk) 20:17, 23 September 2015 (UTC)
A geostationary satellite hovers above the same point of the Earth at the same time of the day every day of the year because it hovers above that point at every moment of the day. So it's not really sun synchronised.
If a satellite in a tundra orbit would have an orbital period of a solar day, its ground track would be a figure 8 loop drifting to the west at a rate of about one revolution per year. The Earth's rotational period is one sidereal day, so that must be the orbital period of a satellite that has to keep a fixed (average) longitude. This satellite must also have a prograde (to the east) orbit.
To have a sun synchronous orbit (the satellite passing over the same part of the Earth at the same time of the solar day throughout the year), the orbital plane must be (more or less) fixed relative to the line from the Earth to the Sun. For this to happen, the nodal precession of the orbit must be prograde (to the east) and have a period of one year. This is possible by choosing a special inclination and a retrograde orbit (to the west). So a satellite in a geosynchronous (tundra) orbit or a semi-geosynchronous (molniya) orbit can never be in a sun synchronous orbit, as it is already in a prograde orbit.
Instead, a satellite in a tundra orbit will hover over the same part or the Earth every day a few minutes earlier. Without nodal precession, it would be about 4 minutes earlier every day, but because of the retrograde nodal precession it's more than 4 minutes. — PiusImpavidus (talk) 13:50, 17 December 2015 (UTC)
what's special about that angle?
- Could it just be a coincidence that Plesetsk Cosmodrome is at 63° North, from where due East launches reach a 63° orbital inclination with no pricey orbital inclination change maneuver required? (sdsds - talk) 18:57, 26 June 2009 (UTC)
This inclination has a special property: perturbations from the earth's equatorial bulge don't cause any long-term changes in the argument of perigee. That's the angle measured within the orbit plane from the S->N equator crossing to the perigee. At any other inclination the argument of perigee slowly changes and with it the apogee latitude. The Molniya and Tundra orbits are designed to serve high latitudes so you want the argument of perigee to remain at 270 degrees (for the northern hemisphere) or 90 degrees (for the southern hemisphere).
Sirius chose the tundra orbit so that users in the continental US, especially the north central states, could see higher elevation angles than are possible from geostationary orbits over the equator. This reduces the number of terrestrial repeaters needed to provide good coverage in urban canyons. Because they're not stationary (that's possible only over the equator), continuous service requires multiple tundra satellites. They also move in the user's sky, but this isn't a problem because satellite radio uses small, hemispherical antennas that don't have to be pointed. Karn (talk) 10:52, 22 September 2009 (UTC)
Geosynchronicity and Earth asymmetry
I can't find reference to Molnya orbit taking advantage of an axial asymmetry of the planet. http://en.cnki.com.cn/Article_en/CJFDTOTAL-JBXG199001013.htm I remember reading that such asymmetry was a big advantage in keeping synchrony at smaller energy cost. I can barely find some reference to the Pacific - Atlantic asymmetry! — Preceding unsigned comment added by Spamhog (talk • contribs) 13:56, 10 January 2013 (UTC)