Category Archives: HEO

Highly Elliptical Orbit

Why Space Matters: HEO Satellite Operations, Part 5–Spysats, IR, and MMS

The Molniya orbit, a type of highly elliptical orbit (HEO), was the focus of the last HEO post.  And it was noted that the Molniya is an orbit perfectly suited to communication.  But what other missions and satellites use the HEO?

Believe it or not, the US has organizations that use the HEO for a few missions.  The most recent launch of the NROL-33 “spy satellite” is kind of related to the HEO, in that it’s part of a possible satellite communications and data constellation using a combination of HEO and geosynchronous (GEO) satellites called the Space Data Segment (SDS).

SDS-3 Constellation. Image on

In the illustration above, they call this particular combination of HEO and GEO satellites the “SDS-3 Constellation.”  Does anyone really know what’s on these satellites, aside from the people working with them?  One observation:  realize the government likes to maximize on their investments with additional payloads on satellites.  Just look at what NASA has done with their satellites and payloads to get an idea.  There’s nothing to prevent the installation of some kind of extra monitoring payloads on any of these satellites.  As discussed before regarding why the Russians use this kind of orbit, the view of areas that might be interesting to the US is pretty good.  Only the people working with this constellation really know what it’s all for, though.

Image from Wikipedia. SBIRS constellation. Old picture, as SBIRS LEO is now STSS.

A few more HEO birds owned by the US are the overpriced Space Based InfraRed System (SBIRS) HEO satellites.  There are currently two in orbit, with a third theoretically on the way.  This system is a little more straightforward in purpose, as it’s part of the early warning system for missile launch detection.  Why would the US want two to three missile launch detection satellites in highly elliptical orbit?  The Russians/Soviets also did and still do use satellites in HEO for early warning.  But the SDS, SBIRS, and OKO (Russian) satellites have to do with terrestrial activities and likely are trying to give their corresponding militaries an edge on terrestrial battlefields.

The most interesting mission using the HEO is one that’s coming up and has nothing to do with human activities on Earth:  NASA’s Magnetospheric MultiScale mission (MMS).  The focus of this two-phase mission isn’t necessarily the Earth, but the Earth’s magnetic field.  And not just the Earth’s magnetic field, but the Sun’s, too, and observing the interplay (NASA say the connecting and disconnecting of the fields) between the two fields.  The mission uses four satellites that will be “flying” in formation and placed in a “low-inclination” elliptical orbit of 28 degrees.  This means the orbit’s tilt is at a 28 degree angle to the Earth’s Equator.

Different phases, different elliptical orbital periods. Image on MMS site.

During phase 1, the satellites will have a perigee (point of orbit closest to the Earth) of 7,645 kilometers (4,750 miles) and an apogee (point of orbit furthest away from the Earth) of 76,452 kilometers (47,505 miles–at least I think NASA means apogee—on their website they say perigee twice).  During Phase 2, the perigee’s the same, but apogee increases to 159,274 kilometers (a little over 98,968 miles).  Seems like that’s pretty far, but believe it or not, the Moon is still further away, at over 238,857 miles average distance from Earth.

The two different orbits are designed to pass through the reconnection points of the magnetic fields of the Earth and the Sun.  So they are necessarily big.  But such an orbit also allows the satellites to “sample” different portions of the magnetic fields.  And they fly in a pyramid formation to help map out the reconnection events in 3-D.  Why go through all of this just to observe these reconnection events?  The reconnection events tend to release energy, and the energy can impact the electronics here on Earth and on the satellites orbiting around it.  There’s more to the science of these satellites on the MMS website.  As a side note, the MMS satellites will be controlled out of the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado (same place the Kepler mission is run out of).  Worth visiting, if only just to walk around the campus in Spring.

And while you’re in Boulder, enjoy the offerings of the Avery Brewing Company, which has a terrific Belgian Wheat (White Rascal) beer and a tasty Imperial Oktoberfest (The Kaiser) beer.  Sadly, there are no beers for HEO—which means we are now at the empty bottle part of the lesson.

Why Space Matters: HEO Satellite Operations, Part 4–Moley, Moley, Moley, Molniya!

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.

Moley 1

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.


Why Space Matters: HEO Satellite Operations, Part 3–What’s the Frequency, Komrade(th)?

The previous lesson left you hanging, maybe even perturbed, with the question of why anyone would want to use a satellite in a highly elliptical orbit (HEO).   After all, a HEO satellite requires a ground station or two with a moveable (steerable) antenna to speak to it (or the satellite’s antenna has to be gimbaled to allow continuous aiming at the ground station).  The satellite goes through the Van Allen belts four times a day.  And there are times in its orbit when it appears to hover, but other times when it’s moving very quickly around the Earth.

But it’s a very useful orbit if a country or organization uses it well.  The Russians, in particular, seem very fond of the HEO for their satellites.  Some of their satellites still use HEOs today.  Consider the location of Russia on the world’s face.  The country is primarily at very northern latitudes, with the bulk of the country between 50 and 70 degrees north latitude.  The country also spans two continents.


Cross-country communications were important to the Soviets–especially to its military (as it is with the US). One way to lower communications infrastructure costs and still deliver country-wide/global communications capability is through using communications satellites (satellites that could receive and transmit radio signals from and to the Earth).  Another factor for consideration is the Soviet Union’s huge coastline within the Arctic Circle.  The majority of the coastline is above 70 degrees north of the equator.  Why is this factor important in a satellite communications discussion?

During the Cold War, the Soviets had built up a tremendous submarine fleet, with a few ports well within the Arctic Circle.  The Soviet submarines routinely prowled those waters, but they would have been a less effective force with no communications link from headquarters.  There were also bomber bases in the Arctic “garden spot.”  Satellites were an obvious solution to the communications problem, but geosynchronous (GEO) communications satellites can’t see very well above 70 degrees latitude (read the “Curves and Angles” lesson for a refresher).  Radio signals also have a tough time going through all that atmosphere to and from GEO satellites—a very powerful radio transmitter would need to be used, meaning more weight would be added for batteries, solar panels, etc.

Could the Soviets have used GEO communications satellites?  Yes.  They have used, and the Russians still use, satellites out in that orbit.  But for the Soviets, there weren’t too many launch location options below 50 degrees latitude.  There are reasons why the United States uses the Kennedy Space Center to launch satellites into GEO.  There are reasons why the European Space Agency launches from French Guiana.  Both launch complexes are relatively close to the Earth’s equator.  Kennedy is a little over 28 degrees north of the equator, and Kourou, French Guiana is an even closer 5 degrees north of the equator.  These are great locations to launch a satellite into GEO, with Kourou the better of the two options.

Launch sites

The reason both are excellent launch sites has primarily to do with fuel and payload when trying to get a satellite out to GEO.  And of course there are barely any populated areas directly east of the sites, meaning no on gets hurt if something goes wrong during a launch.  The further a launch site is to the north or south of the equator, the more fuel it needs to get a satellite into GEO.  More fuel means more weight, affecting satellite payload weight requirements for launching to GEO.  Another option is building newer, more powerful rockets to put capable satellites into GEO.  But either option meant more money for getting satellites into GEO for the Soviets.

The HEO, though, was a relatively inexpensive orbit, fuel-wise, to launch Soviet communications satellites into.  The Soviets wouldn’t need as much fuel to get the satellite into a HEO inclination when compared to the fuel requirements for a GEO.  And the orbit’s Field of Regard (FOR) from apogee could also easily encompass the Earth’s North Pole.  Especially if they were using an orbit at a particular angle relative to the Earth’s equator (the orbit’s inclination) with a particular orbital period.

There’s a name for that orbit, to be talked about next lesson.

Why Space Matters: HEO Satellite Operations, Part 2–Those Pesky Overcharges

Charged particles

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.

Sometimes they're exposed to the elements...

Example of a satellite ground antenna.

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.

Football 4

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.

Kepler’s Second Law of Planetary Motion. Image from Wikimedia.


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.

This is an older picture. NASA now knows there’s a third, “transitional” belt between the inner and outer belts. Image from Wikimedia.

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.

Oh!  And to answer the question posed to you from this post:  the MMS satellites will be in a HEO.

DIY Space: Build a LEGO NASA Satellite

If you buy enough Legos, you can build all four of these! Image from NASA.

The stacked “hockey pucks” in the picture above have something in common with the site’s latest HEO lesson.  However, they are also LEGO models you can build.  In this case, the model is of a NASA satellite that hasn’t been launched yet.  The Magnetospheric Multiscale (MMS) spacecraft model shown on NASA’s very own website looks a little like a blue hockey puck, with antennas sticking out.  Students put this video together to show some of the detail and effort they put into the MMS design.

The PDFs and Excel files of parts lists and directions are available on the NASA site.  If you have the money, you can always buy the kit.  But if you’re more choosy (and still have money), you can always go to LEGO’s “Pick A Brick” site.  Or maybe you have all the parts already.  You can then reference the numbers on the supplied PDF/Excel files and select the bricks you need.  To build the stack of MMS spacecraft in the picture, you’ll have to shell out the coin thrice more.

LEGO has sure come a long way since I was a kid.  I remember trying to build star destroyers with my meager collection of bricks as a child.  It didn’t look as nice as the ones in kits now, but in my mind, it was AWESOME.

Then, after you’ve read through the NASA MMS site, maybe you can tell me what the MMS spacecraft has in common with the HEO lesson?