During the last lesson, you might have been enlightened with the information that even though the Soviets have used, and Russians do use, geosynchronous (GEO) orbits for satellites, they seem to have a special place in their vodka-filled hearts for the highly elliptical orbit (HEO). And that last lesson enumerates the reasons why they like it. For one particular instance and for one particular HEO, they liked it so much, they gave the orbit a name.
Soviet satellite scientists were very clever when they first started using the HEO. They already knew the Earth’s squashed pumpkin shape (its oblateness) normally “spins” (or perturbs) a satellite’s orbit slowly about the Earth’s axis. But, they somehow figured out that a HEO satellite with a 12-hour orbital period using a specific inclination of 63.4 degrees nearly nullified that perturbation. That particular period and inclination is a special kind of HEO the Soviets named “Molniya.” By the way, they named their communications satellites in this orbit AND a rocket (a modified R-7), Molniya, too (so no potential for confusion there, I guess).
The Molniya orbit. Image from Wikipedia–click on it–you know you want to…
Let’s translate all that scientific “hoo-haw” into English a little bit. The angle between the path of the satellite’s orbit, as it passes from south to north over the Earth’s equator, and the Earth’s equator itself is 63.4 degrees. The satellite’s particular HEO is a 12-hour long trip around the Earth, which means the Molniya satellite orbits the Earth twice a day. And the inclination was specifically chosen by Soviet scientists because it’s a very stable orbit. The satellite seems to be rising, flying over the Earth, and setting at about the same spots over the globe every single orbit (there’s a slight shift backwards around the Earth of about -.07 degrees per orbit). These characteristics are very specific to a Molniya orbit.
But the best thing about the Molniya is when a satellite in that orbit is near and at the Molniya’s apogee. The satellite appears to “hover” over the Earth (no mystery why—read this lesson for a refresher). Of a 12-hour period, there’s nearly 8-9 hours of time when the satellite can “see” most of a particular part of the northern hemisphere. This characteristic means that for about 8-9 hours, the HEO/Molniya satellite can maintain contact with a ground station, broadcasting messages from that ground station to all receivers within that satellite’s Field Of Regard (FOR), which includes areas above 70 degrees north latitude. Below are a few Wikipedia-cribbed pictures showing a satellite’s FOR in a Molniya orbit four hours before apogee, at apogee, and four hours after apogee. As you can see, there’s a lot of the Earth in view during all that time (Russia, China, India, Koreas, etc.).
View of Earth four hours before apogee in a Molniya orbit. Image from Wikipedia–click to embiggen.
View at apogee in a Molniya orbt. Image from Wikipedia–click to embiggen.
View of Earth four hours after apogee in Molniya orbit. Image from Wikipedia–click to embiggen.
There’s one obvious problem for people wanting to broadcast 24 hours a day with a Molniya satellite: communications only lasts about 8-9 hours from a Molniya satellite’s “rise” to its “set” on the opposite side of the Earth. This means there is “dead air” for their broadcast area for nearly 15-16 hours each day. Yes, 15-16 hours—a broadcaster’s nightmare (unless they’re union). The gap is because the Earth has continued rotating, and by the time the Molniya satellite rises again for the second orbit of the day, the other side of the planet is now in the satellite’s FOR.
There is an answer to this problem, though. If 1 or 2 more satellites are put into different Molniya orbits, ones in opposing orbits (so the orbits create an “open scissors” if you traced their paths—see below), then the broadcast coverage is 24 hours. And this is what the Soviets did, successfully starting the Molniya constellation by launching the Molniya-1 satellite in 1965 (after two satellites had been destroyed in previous launch attempts). And even as the Soviet empire fell, the Russians used and continue to use the Molniya orbit today.
The upshot of the Molniya orbit is that it’s an orbit perfectly suited to help a communications satellite (or satellites with other missions) keep sight of countries at very high latitudes. Using multiple satellites in a Molniya orbit provides a very good, very big, FOR and can be extremely useful for communications, especially with stations and vehicles working within the Arctic Circle.
But what are some of the other things being done with satellites in HEO? Maybe Part 5 of this HEO Series will answer that question.