Satellite highly elliptical orbits (HEOs) are interesting orbits for space operators in any kind of mission, especially considering the challenges inherent in using such orbits. OK–so that statement’s a little nerdy, but if you’re reading this blog, there’s an inner nerd in you just waiting to be outed. In the last lesson, you were, perhaps unknowingly, subjected to learning one of Kepler’s Laws of Planetary Motion to help explain the shape of an orbit. It turns out that even the most circular orbits are ellipses. But satellites in HEO aren’t following a circular path. And while Kepler’s Laws were originally applied to the planets, they also apply to man-made satellites. HEO satellites are an excellent example of Kepler’s laws at work. But what are the considerations and challenges facing space operators of satellites in HEO?
HEO satellites must face a few challenges geosynchronous (GEO) and low earth orbit (LEO) satellites don’t. HEO satellites move around the Earth. So do GEO satellites, but GEOs move at the speed of the Earth’s rotation, so the GEO satellites appear to “hover” over one particular spot of the Earth 24 hours a day. GEO satellite ground antennas barely have to move in order to communicate with GEO satellites. But HEO satellite ground terminals are more like LEO satellite ground communication terminals: in order to communicate effectively with satellites of both HEO and LEO orbits, the ground terminal antennas have to move.
As discussed in this LEO lesson, the antenna on the ground “tracks” the LEO satellite above it to maintain communications contact. Remember, because LEO satellites are orbiting the Earth so closely, they are also moving fairly quickly over their ground antennas. This means the time for communications from the satellite’s rise above the antenna’s horizon through its traversing over the antenna and to the satellite’s setting below the horizon, is only about as long as 15 minutes or so.
But this shortcoming in time is one of the advantages of the HEO satellite orbit, depending on a few factors (to be talked about in a future lesson). A HEO satellite at its orbital apogee (remember—that’s the part of the orbit furthest away from the Earth–pictured below) takes on some GEO characteristics. It almost appears to hover over particular parts of the Earth.
The closer the satellite gets to apogee, the slower it appears to be going. But the closer the HEO satellite gets to perigee (the part of the orbit closest to the Earth), the faster the satellite is moving. This “speeding up” and “slowing down” of the satellite is the second of Kepler’s Laws of Planetary Motion (seen in action below). This post will not go into the law’s details–you can go to the wiki for that.
So, the satellite ground antenna moves, always pointing along the satellite’s orbit path at where the satellite should be, tracking the HEO satellite to maintain a constant communications link. The HEO satellite ground antenna can keep in contact with the HEO satellite for hours, vs. the minutes of communications time with a LEO. This means the HEO satellite, if augmented with two or more other HEO satellites, could be a very good communications satellite—especially if the country interested in communicating has a big land mass in the very northern latitudes of the Earth (this country will be talked about some more in the next post).
The drawback to having more than one HEO satellite is the need for more than one satellite ground antenna–since every single satellite will be in different part of the HEO path (or a different HEO altogether) a different antenna is required to track those satellites as well. This could also mean that while a HEO’s ground antennas are contact for a long time, there’s also a period when the satellite is not in contact with the ground antenna. Both lengths of time depend on the orbit’s period.
The other challenge a HEO satellite faces which a GEO satellite normally doesn’t, is the HEO satellite’s orbit transits the Earth’s Van Allen belts four times a day. The Van Allen belts are layers around the Earth full of charged particles—very energetic electrons and protons—which the Earth’s magnetic field has captured. The charged particles can do some very bad things to a satellite’s electronics like the solar cells, sensors, and circuits.
Satellites anticipated to transit the Van Allen belts are designed with shielding to minimize the odds of a stray electron or proton causing problems. Like electrical power requirements (talked about here), shielding is also a balance of risk versus cost versus weight. And weight can equal cost in the amount of fuel a rocket needs to lift the satellite into orbit. If the satellite is too heavy, the rocket might not be able to lift it into the necessary orbit. So HEO satellites are playing the odds, with lots of smart people figuring out a balance between risk and reward during the satellite design phase. While the odds are lowered through design, there’s still the chance of an electrical problem occurring because a very energized particle happens to hit a circuit or sensor “just so,” with odds of such an event happening increasing with every subsequent transit.
But why on Earth should someone even want a satellite to go into HEO? Why would someone want a satellite that has to transit the Van Allen belts?
That discussion will be in the next lesson.