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.

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?