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Molniya orbits are always associated with Russian satellites. What advantage do they have over other orbits?

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    $\begingroup$ Downvoted because of no evidence of prior research. $\endgroup$ – Organic Marble Aug 18 '15 at 11:48
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Most countries use geostationary satellites for communications. They have the advantage that the satellite appears to be stationary in the sky, so you can use a simple fixed dish to communicate with it.
Russia/the former USSR has a lot of land at high latitudes. A geostationary satellite would need a lot of transmitter power to be usable there. A Molniya orbit fixes that by having the satellite at a high inclination so it has a good view of a most of Russian territory at the same time. The orbit is also highly elliptical so it spends most of its time in a good position. The drawbacks are that you need a tracking dish for communications, and you need 3 satellites to have continuous coverage.

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  • $\begingroup$ The molniya is also good for ground observation. You can time the "dwell" at apogee to the 8 hours in the day when you expect activity you would like to observe. Or, you can have 24/7 coverage with three satellites, as described. $\endgroup$ – Douglas Held Mar 15 at 20:40
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This wiki article on the molniya orbit provides this helpful starting explanation from which one can begin to tie together both previous answers by Hobbes and Mike:

Much of the area of the former Soviet Union, and Russia in particular, is located at high latitudes. To broadcast to these latitudes from a geostationary orbit (above the Earth's equator) would require considerable power due to the low elevation angles. A satellite in a Molniya orbit is better suited to communications in these regions because it looks directly down on them.

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An additional advantage is that considerably less launch energy is needed to place a spacecraft into a Molniya orbit than into a geostationary orbit. Disadvantages are that, as opposed to a spacecraft in a geostationary orbit, the ground station needs a steerable antenna to track the spacecraft and that the spacecraft will pass the Van Allen radiation belt four times per day.

So there are two immediate advantageous features that suited the former Soviet Union, namely improved elevation angles for communications and the launch energy.

To explain the latter a little more: the propellant mass required to achieve a given orbit depends in part on the difference in degrees between the latitude of the launch site and the inclination of the target orbit. For example, the lowest latitude launch site used by the former Sovient Union was at Baikonur with a latitude of 46 degrees, hence it would take least energy to go into a 46 degree orbit, more to get to 63.4 degrees and much more to get to GEO at 0 degrees.

The next point is "why choose 63.4 degrees specifically?". The same wiki article goes on to say:

In general, the oblateness of the Earth perturbs the argument of perigee, so that even if the apogee started near the north pole, it would gradually move unless constantly corrected with station-keeping thruster burns. To avoid this expenditure of fuel, the Molniya orbit uses an inclination of 63.4°, for which these perturbations are zero.

Put simply, the orbit is most stable there. To determine which feature amongst those mentioned was the driving feature when the former Soviet Union planned the Molniya communications satellite programme would be a big historical pursuit in its own right though it appears that dealing with the perturbation through the satellite life was a significant challenge as they chose 63.4 degrees. I haven't done the sums to compare the launch mass under the three scenarios for propellant and transmitter power by the way.

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    $\begingroup$ This is a great additional answer, i'm glad you are interested in improving existing content. $\endgroup$ – kim holder Nov 17 '15 at 21:39
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This orbit allowed you to carry 1600 kg of payload, instead of the 100 kg into geostationary orbit possible under the same launch conditions, i.e. Baikonur in the 1960s with rocket 8К78.

Russian source (Google Translate this URL).

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    $\begingroup$ Is it in Russian? First off, despite this being a space-related site, not everybody reads Russian, and second, please post the full text of your answer here - links to external resources are welcome as supporting references but not as main content. $\endgroup$ – Deer Hunter Aug 18 '15 at 12:29
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    $\begingroup$ Interesting advantage in addition of the illuminated area, but this deserves an explanation: why a larger payload is possible? $\endgroup$ – mins Aug 19 '15 at 5:51
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    $\begingroup$ @mins: The Russian launch sites are at high latitudes, so even if you launch due East the orbit has high inclination. Removing that inclination to get to GEO is expensive in propellant. The Molniya orbits are much closer to the launch orbit in inclination. $\endgroup$ – Ross Millikan Nov 18 '15 at 4:58
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    $\begingroup$ @mins I think Mike means that less delta-v is required to achieve the orbit. Fun fact, it is harder (on delta-v) to get to geosync, than it is to get into a lunar centric orbit. $\endgroup$ – Aron Nov 18 '15 at 6:25

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