Molniya orbit. Sounds like the name of a Russian ice hockey team, doesn’t it? It’s actually a clever Russian solution to the problem of providing satellite communication coverage to far northern latitudes.
Do you have a satellite dish for your television? If you do, it is aimed neither straight up nor level with the horizon.
https://www.crushpixel.com/big-static11/preview4/satellite-dish-706735.jpg
For a given latitude, it’s always at roughly the same vertical angle to the south even though it may be pointed a bit to the east or the west. That’s because it is aimed at a geostationary communication satellite that is stationed over the equator. In the northern hemisphere, and at Lynchburg, Virginia’s northern latitude of 37.5°, it must be pointed in a southerly direction at an elevation of around 52.5°.
This video is about five minutes long, but it is an excellent explanation of how geostationary satellites work.
For those who just want the bullet points:
- Anything orbiting the Earth will move more slowly the farther it is from the Earth.
- Anything farther out and moving more slowly will take longer to complete one orbit.
- If the satellite’s orbit is equatorial, it will not drift north or south.
- There is a distance (22,236 miles, 35,786 kilometers) at which a satellite will take 24 hours to complete one orbit.
- Since someone standing beneath the satellite on the Earth’s surface will complete one revolution in the same amount of time…
- The satellite will not appear to move in the sky.
For someone directly beneath the satellite, their dish antenna would point straight up. But as one moved farther and farther north, the dish would have to point lower and lower. Eventually, the satellite would appear so low on the horizon, with the signal so attenuated by passing through more and more atmosphere, as to make an equatorial geostationary communication satellite useless.
Most of the land mass of the Earth is in the northern hemisphere. And much of that land mass, especially in Russia and the former Soviet Union, is at high latitudes, where the communication satellites so convenient for countries closer to the equator are less useful.
Enter the clever Soviets! The orbits we’ve been describing so far have all been circular, with constant orbital speeds. If one is in an elliptical orbit, however, with a high point and a low point, a satellite will be moving slowly at the higher altitude and faster at the lower altitude.
https://upload.wikimedia.org/wikipedia/commons/e/ed/Classical_Kepler_orbit_120frames_e0.6.gif
Now angle (incline) your orbit so that it passes over regions that are far north. Of course if it does so, it will also pass over regions that are equally far south.
https://www.duluthnewstribune.com/incoming/7112023-ph86jj-ISS-vs-Hubble-inclination-orbit-Gary-Meader-S.jpg/alternates/BASE_LANDSCAPE/ISS%20vs%20Hubble%20inclination%20orbit%20Gary%20Meader%20S.jpg
But time the orbit so that when it is far from the Earth and moving more slowly, it is over those northern latitudes. When it is over areas in the southern hemisphere, it is moving rapidly and not spending much time there.
https://upload.wikimedia.org/wikipedia/commons/1/12/NASA_molniya_oblique.png
You need more than one satellite to provide continuous coverage. And you need to be able to track the satellite’s path, as it will not appear stationary. Still, this was the basis for the Soviet Union’s national network of satellite television, the world’s first.
You could of course modify this to provide coverage in the southern hemisphere by having the high point in the orbit be over the south. The only unserved area there, however, is Antarctica. Not much of a market!
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