Category Archives: GEO

Geosynchronous/Geostationary–another acronym, rarely seen but sometimes used is GSO

Gravity Check: Thousands of Satellites Orbit Earth

Counting Satellites

Quick–just how many satellites, operational or not, are orbiting Earth?  Pretend you’re trying to impress your fellow engineers.  Even better, pretend you’re trying to impress people in a bar (although that strategy might backfire).  Have you guessed?  Do you really want to know if you’re correct or are you satisfied with impressing the folks in the pool hall?  If it’s the former, then this Talking Points Memo post helpfully gives several numbers regarding satellites orbiting the Earth.  So next time, you’ll be very accurate and the biker in the leather jacket will buy you that beer for your numerical diligence.  Well, it might helpful, at least, for those who love minute details and numbers.  Maybe you should bring it up at an accountants meeting instead?

But before you go over to the post, did you guess a number?  You’d be closer if the number were in the thousands.  Do you know who owns all of them?  What countries do they belong to? Remember, you’re going to have to include cubesats, small sats, GEOs, LEOs, MEOs, and HEOs.  It might help during your counting if you have some excellent optics and a pad and pen.  Or you could just go to Talking Points Memo’s post and find out.  That would certainly be easier, and take less time.  But if you’re like me, maybe you’re not so busy…

Why Space Matters: GEO Satellite operations, Part 7–Curves and Angles

Geosynchronous (GEO) satellites are wonderful.  They can see a lot of the Earth from 22,236 miles in space.  It’s why they make great observation and communications satellites.  Previous chapters explained some of the problems facing GEO satellites, too, such as solar influence and eclipses.  But now’s the time for a few lesser known problems GEO satellites deal with have to do with:  angles and curves.

When talking about points on the Earth’s surface, I’ll be referencing latitude (the points north or south of the Equator), and longitude (the points east or west of the Prime Meridian).  If you aren’t acquainted with those topics, please go ahead and read my Zulu lesson series, here.

Not quite accurate, but you get the idea, right?  The 70 latitudes are represented by the red lines.

Not quite accurate, but you get the idea, right? The 70 degree latitudes are represented by the red lines.

Pole Position

If you live anywhere above the Antarctic or below the Arctic circles, specifically past 70 degrees South or North latitude, you’ve probably not had to deal with one particular angular issue.  Theoretically, a GEO satellite can see most anything on the Earth’s surface—to a point.  That point is the 70 degree latitude mark.  Remember how the Earth is shaped like a squashed pumpkin and therefore curved?  The Earth curves away from the satellite the further north or south from the equator you get.  Same deal with longitude.  The further away east or west from the area of longitude the satellite is stationed over the more the Earth curves.

This is a two-way problem, since a GEO satellite not seeing a spot on the Earth means that particular spot probably can’t get line-of-sight of the GEO satellite.  Once past 70 degrees latitude, you can’t see the satellite anymore from the Earth.  If you attempted to do so, the Earth would be getting in the way (your antenna would be aimed at the ground).  If you can’t see a satellite, you can’t communicate with it (unless you’re relaying communications–another story).  There is currently no “radiowave-passing-through-the-ground-through-the-atmosphere-into-space” communications technology, yet.  So anyone wanting a DISHTV or DirecTV receiver in those latitudes is out of luck.  If you don’t believe me, you can just use this convenient Geostationary Satellite Azimuth and Elevation Calculator to see what numbers you get past 70 degrees.

Polar Gap

High latitudes are a problem for the NOAA/NASA GEO imaging satellites, too.  It’s just one of the reasons why those organizations use Low Earth Orbiting (LEO) satellites for their imagery to augment their capability to get imagery of the poles.  There is another particular orbit, which will be discussed later, that also helps cover the polar region of the Earth.

For East and West longitudes, the answer is simpler—just place another satellite in GEO further east or west and continue in that manner until you get global coverage.  It only takes about three GEO satellites to get rudimentary global coverage.  The reason why the poles can’t be covered in the same way is because satellites are moving, orbiting the Earth–it’s just that in GEO the satellites are moving with the Earth’s spin.  There are orbits that do help compensate, but those aren’t GEO.  Again, that’s a topic to be discussed later.

But why do we care about communications at the poles?  Why do we want pictures of the poles?  Well, believe it or not, aircraft do tend to fly over, or pretty close to, the North Pole.  It can be the shortest way to get from one side of the planet to the other.  Wouldn’t it be good if a passenger aircraft had good communications with a satellite?  Coast Guard cutters breaking up the ice probably wouldn’t mind consistent satellite communications, either (although they might be using the Iridium satellites to help compensate for that).  And climatewise, there’s always something interesting happening at the poles.

Snell!! Snell!!

It’s a simple problem, then.  The Earth gets in the way of GEO satellites and the Polar Regions are difficult, if not impossible, to see from GEO.  But, the curving Earth is causing another, more complicated issue for the GEO satellites, too.  Remember that air that you breathe?  Yup, that is part of the Earth’s atmosphere, which, thankfully, hugs the Earth and creates that nice environment conducive to life.  The Earth’s atmosphere, while wonderful, is denser than the vacuum of space.

The atmosphere is not just uniformly dense, but changes density the closer to the Earth’s surface the atmosphere is.  This is where Snell’s Law comes in.  I won’t get into the specifics of Snell’s Law in this post—the wonderful folks at the Khan Academy have a great lesson series, with one particular lesson explaining Snell’s Law below.

The Bends

The upshot is that satellites deal with refraction, the bending of light as it passes from one medium, the vacuum of space, to another medium, the Earth’s atmosphere.  GEO satellites deal with refraction.  At the point above the Equator, the refraction isn’t obvious, because there’s barely any bending.  But the further away from that point, North, South, East, or West, the light bends more and more.

Not only is there bending, but the more the atmosphere curves away from the satellite, the more atmosphere there is for the light to have to go through when looking at it from GEO.  This can affect accuracy for determining where clouds are with weather satellites.  It might make a lake appear further north or south than it actually is.  It can wreak havoc with radio signals.  It’s one of the reasons why multiple satellites are better for looking at the Earth—two or three satellites looking at the same cloud can give a better “geo-location” coordinate for that cloud.

Now you know of some of the major problems of, as well as the great reasons for, using GEO satellites.  I did say there was another orbital solution to pick up the slack of coverage in the Polar Regions of the Earth, right?  I wonder if there’s anyone with a good inclination to read about that ;-)?

http://www.ashburysatcom.co.za/coverage/inmarsat-network-coverage/

LATE ADDITION:  I found the above link on Ashbury Satcom’s site.  It leads to a picture that shows the footprint of the three GEO INMARSAT satellites.  Note how the poles are not covered? 

Why Space Matters: GEO Satellite operations, Part 6–Eclipse

Eclipse

All that you touch, all that you see…

Yep, you guessed it!  We’re going to talk about another issue common to geostationary (GEO) satellites:  the eclipse.  This issue is almost opposite from the problem discussed in Part 5 of the GEO lesson series.  Instead of being overpowered by the sun’s energy, the satellite can’t function because of a lack of it.

Here’s how that works.

Twice a year, the Earth gets in the way of the GEO satellite’s view of the sun.  Those two periods occur around the vernal and autumnal equinoxes.  The eclipse period starts in very small time increments at first, gradually increasing over a period of days.  Eventually the eclipse peaks for nearly 70 minutes, then starts decreasing over the next few days.  Each eclipse period during each equinox lasts a total of 45 days.  For some satellites, though, these periods can be too long.

Forever’s gonna start tonight…

You see, the satellite’s systems are powered by electricity.  There’s no extension cord long enough to plug the satellite in, but the satellite must be powered somehow.  The common engineering answer has been to use the sun’s energy, absorbed and changed to electricity through a satellite’s solar panels.  The power is then saved in the satellite’s batteries.  The batteries then power the rest of the satellite, sub-systems, payload, etc.  But what happens when there is no sun as a source of energy for the satellite solar panels to convert?  Well there’s nothing I can say other than it’s a total eclipse of the part (of the satellite).

Again, you may be thinking, “No biggie.  I have a cell phone in my pocket that has more capability than a satellite, and it can last for days.  Surely satellite weighing tons can last longer on batteries than my tiny phone.  Right?  Right?”  The answer to this is:  It’s complicated.

You would think the designers of these very expensive satellites would put enough batteries on board a satellite to compensate for an eclipse.  They try, but here’s a question for you:  Have you ever lifted a car battery?  Now pretend the satellite has same car battery weighing at the low end of 15lbs—and multiply that weight by 103.  That’s about what the Nickel Hydrogen batteries of a modern GEO satellite weigh.

…Everything under the sun is in tune…

What this heavy fact means is there are only so many batteries that can be put on a satellite before it becomes too heavy, and therefore, too expensive to launch.  So batteries are part of a price/weight/required power/risk management balancing act.  For most satellites, the eventual design is just fine.  When eclipses come around, the satellites can rely on the battery to eventually get them through the full 70-minute eclipse.  Operations can continue without a hiccup.

But what can be done if an operational satellite’s batteries start going south?  Or if some other part of the power sub-system starts degrading or just stops working?  Suddenly the capacity to power a satellite is greatly diminished.  Suddenly the satellite may not have enough power to not even turn itself on again (at least until the sun charges them up again—but totally depleting a battery is sometimes not a good thing either).  There are also certain preventative maintenance functions a satellite operator uses to help with satellite battery life.  But all of this is what makes the battery balancing act tricky.  Now, here’s another question for you:  How long do batteries last in your household?  Especially the rechargeable ones?

I don’t know what to do, I’m always in the dark…

There’s no Walmart satellite servicing garage in space just now.  Satellite servicing and battery change capability aren’t available to satellites (yet—see this interesting tidbit).  The batteries that are designed and eventually loaded onto a satellite’s bus must be robust, long-lasting, and reliable.  If any of those criteria are violated, the satellite just becomes a very expensive man-made star.  Since satellites are so pricey to get into orbit to begin with, quite a few owners expect the satellites to be designed to last a long time–some as long as 15 years.  So guess what also needs to last that long?  Would it amaze you to know the batteries do last this long on some satellites?  They do, but eclipses still cause power issues.  At least they are predictable.

Of course, this sort of problem isn’t only the province of the GEO satellite.  The other satellites also have some power issues to deal with, including eclipse.  But this problem is very obviously seen on GEO satellites, especially since a lot of the public relies on communications and weather from those sources.

There is another problem GEO satellites do face, but that’s a lesson for another day.

HOLD ON!!!  I JUST ADDED THIS:  It seems appropriate for my readers, and I like DIY things.  Those of you from a different age might recognize Patrick Norton from ZDTV.  He’s been for the longest time a podcast host on Revision3.  The particular episode (below) posted on 27 Jan 2014 is all about making your own batteries to charge a cell phone–without lemons.

Why Space Matters: GEO Satellite operations, Part 5.5–Indian GEO Flight Plan

Just a short blurb for you about a really great blog post from Planetary.org.  It’s definitely related to The Mad Spaceball’s GEO lesson series on this site.  I think it’s better written.  I wanted to provide, at the very least, the link for this post, titled How to get a satellite to geostationary orbit.

As you might imagine, a lot of things need to happen to get a satellite into a geosynchronous/geostationary orbit.  Planetary.org’s post does a really great job of describing the steps for attaining GEO, using India’s first successful launch of a GEO satellite to help explain the process.

For those who really don’t want to think too hard on this topic, let me warn you the author uses terms like “transfer orbit” and “apogee.”  But if those readers want to see some excellent explanations and pictures, then go there.  You’ll be rewarded and maybe even have learned a thing or two.

Why Space Matters: GEO Satellite operations, Part 5–Lights & Music

During the last few lessons, the great advantages of satellites in a geostationary orbit (GEO) were espoused about ad nauseum.  The characteristics of persistence in communications and observations are the direct benefits of using a satellite in GEO.  Include the huge field of regard and simplified ground system requirements, and it’s really a no-brainer to use GEO satellites for communications, remote sensing, observations, etc.

But, there are also a few disadvantages affecting satellite operations.  Problems with names such as “solar interference,” “eclipses,” “latitudinal limitations,” and “space weather.”  And while being at such a high altitude (35,786 km (22,236 mi)) from the Earth’s surface is good for seeing the whole world, a GEO satellite won’t “see” the details (technology is getting better, however).  What exactly, then, do these problems pose to satellite operations?  And how do some satellite operators deal with them?

Solar influence (or sun fade/solar transit) sounds somewhat benign.  It is also fairly easy to explain, and we’ll explain it with music.  Specifically, loud music.  If you’ve ever been to a rock concert, rave, etc., you understand the meaning of “loud music.”  You and your friends stand in front of the stage while your ears are sonically overpowered.  One of your friends tries to talk with you during this sonic assault.  You can’t hear the conversation.  So your friend tries yelling.  You still can’t hear the conversation.  Your friend’s voice simply can’t compete with the band’s amplified speakers.  Attempts at conversation stop until a lull.

This is essentially what occurs during solar interference, except it’s happening with radio waves.  There are times when the sun is directly behind the GEO satellite and above the satellite’s ground station.  The sun emits a lot of noisy radio wave energy.  It’s the equivalent of the amplified speaker overpowering your friend, the satellite.  During this alignment of the sun, GEO satellite and ground station, the ground station is unable to sort out the satellite’s communications signal from the sun’s noisy radio wave assault.  The video below shows the alignment occurring.  During that time of alignment, no signal could be received from the satellite.

This communication “outage” (no satellite communications), is predictable and typically starts gradually—a few minutes at first.  Slightly longer when it’s in direct alignment.  It’s the reason why certain agencies issue notifications like the one below.

As the Earth and satellite rotate, the sun will slowly come into alignment, with the time of the communication outage peaking at the exact time of the alignment of the sun, satellite, and ground station.  Then the communication outage shortens as the sun slowly moves away from the alignment, until it’s not even close to the alignment anymore.  Contact between the ground station and GEO satellite means satellite operators will be able to conduct 24/7 operations once again.  For more detailed information about this, please go to this Intelsat page and this Celestrak page.

Now you satellite television subscribers know why your signal goes out about twice a year.  You’re under the influence–solar influence, that is.  It’s only for a few minutes and hopefully you’re watching nothing terribly important.

More about the other GEO satellite disadvantages next week.