A SEA OF TELEVISION AERIALS
Back in the 1940s and 50s, television
was being introduced into homes all over North America.
In order to receive the television stations' signals, you
required a television antenna. So, up they went, all over the country, normally
on the roofs of the houses. They were big metal structures that had to be placed
high up to pick up the television (VHF and UHF) signals that had a very limited
range. A problem with the aerials is that they had to be oriented in a specific
way, depending on the originating TV transmitter location, the signal direction
and the strength. There was also that very disturbing picture of the father
struggling (sometimes violently) with the "rabbit ears" desperately trying to
get the picture to stop wrinkling up and snowing over.
Then along came cable TV in the 1960's. Suddenly,
households no longer required those straggly, intrusive metal aerials. Many were
taken down in favor of the better reception cable TV provided. However, the
problem with cable was that it required, well, cable! It had to be strung for
miles along telephone poles, and later underground, and amplified every few
miles to counteract the inevitable line losses. With all these problems, cable
TV has endured extremely well for the past 40 years or so. Cable TV is still a
problem for those living in very remote areas. The cost of all that cable is not
very economical, nor practical. So, until the mid 1980's many remote dwellings
had to rely on the good old aerial on the roof.
THE FIRST PROPOSAL OF WORLDWIDE SATELLITE
One constant that has prevailed in
history is the requirement for faster communications. The
so-called "need to know" is one driving force behind any communication empire, oh
yes, the money too! From Morse to Marconi, communications became more reliable, faster, and most importantly, timely (and
of course profitable). So, it was only a
matter of time until a theory of using orbiting communication relays was put
There is some controversy over who was the first to propose a satellite
communications infrastructure, but the credit is normally given to Sir Arthur C.
Clarke, author of "2001, a Space Odyssey". In October 1945, he published an
article entitled "Extra-terrestrial
Relays" in which he proposed that three satellites, placed 120 degrees
apart, within a specific orbit radius over the Earth's equator would be able to
provide reliable worldwide radio communications. He argued that there was a specific
orbit radius such that the satellite will orbit the Earth in exactly the same
time as the Earth rotates on its axis, thus keeping the three satellites over
the same locations on Earth at all times. It is worth noting that his article
was published 12 years before Sputnik, the first artificial satellite, was
launched. What a pioneer indeed!
Figure 1: Sir Arthur C. Clarke's original drawing,
illustrating the proposal of placing three
satellites in orbit to relay radio transmissions to potential customers all over
One way to explain his unique
orbit proposal is to use Kepler's Third Law of orbital mechanics. It basically
states that the square of the period of orbit is proportional to the cube of the
average distance of the object from the parent body. Check it out! If you
compare the ratio of the square of the period vs. the cube of the average
distance of the planets from the Sun, you should get nearly the same number for
all the nine (or ten) planets! We can use the Earth's only natural satellite,
the Moon, to determine the approximate orbit radius the hypothetical satellite
would need to orbit in synch with the Earth's rotation period (about 24 hours).
amoon = average
distance from the Moon to the center of the Earth = 384,400 km (Observer's
asat = average distance from the satellite to the center of
the Earth = ??????
Tmoon = period of the Moon's orbit about Earth = 27.322
days (Observer's Handbook)
Tsat = period of the satellite's orbit about Earth = 1 day
Using Kepler's Third Law, the following is
/ T2sat = a3moon
asat = (a3moon
/ T2moon x T2sat)1/3
If you use the values above in the
equation derived, you will find that the approximate ideal orbit radius of the
Clarke satellite is about 42,330km. If you subtract the Earth's equatorial
radius (6378km), you will find that the satellites would be about 36,000km above
the Earth's surface. If the satellite was placed outside this orbit radius, it
would orbit too slowly, and lag behind the Earth's rotation. Conversely, if it
was closer than that, it would be orbiting too fast. Thus began the "Clarke
THE CLARKE BELT AND YOU
So, what exactly does the Clarke Belt
have to do with your satellite dish? By now you might have deduced that if the
satellite appears stationary with respect to the Earth's surface, you will not
have to move your satellite dish to receive signals. You would be right! If
you could see a Clarke Belt satellite with your naked eye, you would see that
the satellite would seem to hang in the same location in the sky at all times.
Remember that the satellite orbits with the same period as the Earth's rotation,
so it would appear to be stationary in the sky. Today, the Clarke Belt is called
the Geostationary Belt, which is a much more descriptive name.
Every single satellite dish you see in your
neighbourhood is oriented at a satellite orbiting in the Geostationary (Clarke)
Belt. Despite the modern claims of "high-tech", the original proposal for our
modern satellite communications infrastructure was published at the end of World War II.
Not only does the geostationary satellite orbit make your life easier, but it also
simplifies the satellite controller's life by knowing just where the satellite
is at all times in order to communicate with it reliably. In other words, it
does not appear to be a moving target, at least not in the short term anyway.
THE 23.934 HOUR DAY
We were taught in school that one day
is exactly 24 hours, and we still believe that today. For most, that
will do just fine, however, there is a problem with geostationary satellites
orbiting at a period of exactly 24 hours. They would fall behind, and therefore
the satellite would drift slowly westward from day to day. This would cause
customers to require to change the positions of their satellite dishes
constantly throughout the year, and endure massive blind spots as the satellite
passes below their local horizon for over half the year.
Why, do you ask, is this true? Earth rotates on its
axis in 24 hours, does it not? This depends on what you are measuring the
rotation against. The 24-hour day is based
on the Sun's apparent position in the sky. As the Earth rotates on its axis, it
is also orbiting the Sun. In other words, the Sun's apparent position with respect to the sky
background is dependent on the Earth's rotation AND its motion in its orbit.
Therefore, we cannot rely on the Sun's apparent position as a reference point
for its true rotation period.
So, what can
we use as an ideal Earth rotation period, and therefore geostationary satellite orbit period?
Astronomers have observed for centuries that the stars
rise (and therefore set) about 4 minutes earlier every passing day. What clock
are we basing this on? We use timepieces that are set to solar time, in which
exactly 24 hours per day is used as the standard. What if we had used the
stellar background as the
benchmark for our standard day? The great thing about using the stellar
background is that the Earth's orbital motion is a negligible component. Why?
Because the stars are too far away for this motion to affect their observed
positions. In other words, the parallax is much to small to matter. What time
span is necessary for the stars to be in exactly the same positions in the sky
from night to night? The answer is exactly
23 hours, 56 minutes and 4 seconds if measured in solar time (about 4 minutes
less!). So 24 hours of this specific time standard (called sidereal time, by the
way) is 23 hours, 56 minutes and 4 seconds of solar time.
Figure 2: The Solar Day vs. the the Sidereal Day. Two
observers, denoted by the green and red dots are on the opposite sides of the
world and viewing the Sun and a distant star, respectively. At t=0, the observers take a note of the Sun's position and the distant star's position,
respectively. In 24 hours, in which the Earth has both rotated and moved in its
orbit, Observer Green will see the Sun at the same location in the sky as he/she
saw it 24 hours earlier. Observer Red, however, will not see the distant star in
the same position. He/she will see the star shifted slightly westward from the
day before. Observer Red will actually observe the distant star reaching the
same position 4 minutes before Observer Green sees the Sun reach its last
observed position. This is because the Earth has to spin a little more to
compensate for its new position in its orbit. Observer Green will have seen the
Solar Day, while Observer Red will have seen the Sidereal Day.
So, the ideal geostationary orbit period must be close
to 23 hours, 56 minutes and 4 seconds long (with respect to the solar clock) in order for the satellites to remain
in their positions with respect to the fixed satellite dishes on Earth. How does
this affect the geostationary orbit altitude calculated above? It should have to
be slightly closer for it to orbit in a sidereal day, and indeed it is; 35,875km to be exact,
which is about 125km less than the 36,000km calculated above.
A SEA OF SATELLITE DISHES - THE "GREAT" DISH
In order to truly appreciate just how much we depend on satellites
in our daily lives, I took a 2-hour stroll around my neighbourhood and had a look
at the various households to see which ones had satellite dishes installed. I
brought my camera along to get a few images of them, while trying to avoid
getting unwanted attention from the police. What I saw was astounding. In just
two hours of walking, I saw approximately 50 satellite dishes. The two biggest
names were Bell ExpressVu and Star Choice, both of which are receiving their
signals from several Canadian-owned satellites.
Figure 3: A typical Bell ExpressVu and Star Choice
satellite dish in Ottawa. Most
of the satellite dishes that were observed were from one of these two companies.
Notice the different shapes of the dishes.
I know what you're thinking. Walking around taking
images of satellite dishes is a very strange thing for someone to do with
his/her time. My excuse is that the persistent clouds made me do it. What else
can an astronomer do? However, the Sun did come out for a few hours, and I am
very glad it did, as it gave me an idea for a unique experiment involving
How do you prove that specific satellite dishes from a
specific satellite dish provider is pointing at the same satellite? I was very
fortunate to see that the Sun was in the right position such that the shadow of
the dish amplifier (that little arm in front of the dish) was cast onto the dish
itself. How is this important? If specific dishes were pointing at a specific
satellite located at a specific azimuth and altitude, then wouldn't all the
shadows be cast on the same area of the dishes at one specific time? Take a look
at Figures 4 and 5.
Figure 4: Take a look at these three different Bell
ExpressVu satellite dishes. Each are located in a different area of Orleans
(Ottawa). Take a look at the shadow cast by their signal amplifiers. In each
case, the shadow lies in the lower right-hand corner of the dish. This indicates
that these three dishes are pointing at the same azimuth and altitude, and
therefore the same satellite.
Figure 5: Take a look at these two different Star Choice
satellite dishes. They are located in two different area of Orleans (Ottawa).
Take a look at the shadow cast by their signal amplifiers. In each case, the
shadow lies near the bottom of the dish. This indicates that these dishes are
pointing at the same azimuth and altitude, and therefore the same satellite, but
at a different satellite than the Bell ExpressVu dishes.
Figure 6: Can you spot the seven satellite dishes in the
image? This image is a composite of two images of a single block of houses near
the northern end of Orleans Boulevard in Ottawa (near the Ottawa River). This
single image illustrates our dependence on the geostationary satellite
population, more specifically, on five of them: Nimiq 1, Nimiq 2, Nimiq 3, Anik F1-R
and Anik F2. All the satellite dishes shown are pointing at one of these
five satellites, as are most of those in the Ottawa area, and all of southern
you look at satellite dishes in the Ottawa area, you might have noticed that
they are all pointing to a rather low altitude above the horizon. You will also
notice that none of them are pointing at the zenith (those that are working
anyway). What exactly is going on? Remember that all the geostationary
satellites are orbiting about the Earth's equator at about 35,875km from the
surface, and this is a clue as to why you have to point your dish to a low
altitude. The dish's orientation is also dependent on your latitude on Earth. In
the northern hemisphere, the further North you are, the lower your dish will
need to be pointed to access a specific geostationary satellite. There is a
point at which you are so far North that the altitude of the satellite is too
low to be useful because of atmospheric disturbances as well as natural and
man-made obstructions. In this case, a different orbit strategy is utilized (see
the Satellite Radio section).
Figure 7: Why satellite dishes in the Ottawa area are
pointed at such a low altitude. An observer on Earth (depicted by the
green dot) has a local horizon (the green line). The geostationary satellite
(the red dot) is orbiting the Earth's equator (the red line). The satellite dish
sees the geostationary satellite along the purple line of sight. The altitude is
measured from the horizon to that line of sight. If the dish had been oriented
to the observer's zenith, there would be no way the dish could pick up the
satellite's signal. Only those living in the equatorial region would need to
point their satellite dishes at their local zenith, and this is only if the
satellite is positioned over their specific longitude. If you imagine this
observer going further and further North, you will find that there is a point at
which the satellite lies near or on that observer's horizon. At that point, the
dish cannot see or access the satellite.
WHAT DOES YOUR DISH SEE?
enjoy the services that your satellite TV provides, do you ever wonder what your
satellite dish is seeing? Both Bell
ExpressVu and Star Choice satellite services are relayed by satellites
owned and operated by Telesat Canada. If you use Bell ExpressVu, your satellite dish is
pointing at either the Nimiq 1 (and Nimiq 3, which is co-located)
satellite, or the
Nimiq 2 satellite. If you are using Star
Choice, you are pointing your dish at either the new
(English-Canadian service) or Anik F2 (mainly French-Canadian service).
See the reference table at the end of this article for specific angular location
data for these particular geostationary satellites.
Figure 8: This is what your satellite dish sees. The stars
streak by, while the two
geostationary satellites (the two dots) appear motionless because of their orbit
period is the same as the Earth's sidereal rotation period (23h 56m 4s). The two geostationary satellites
shown are Anik F1 (bottom) and Anik F1-R (top). Anik F1 mainly services South America,
while Anik F1-R services Canada for the Star Choice direct-to-home satellite
service. Since these two satellites are positioned over the Earth's equator,
each can service a different continent.
TODAY'S GEOSTATIONARY SATELLITE POPULATION
At the present time, in 2006, there are
approximately 250 active geostationary satellites orbiting the Earth. This is a
far cry from the 3 satellites that Clarke had originally envisioned.
Competition, innovation and engineering can greatly diversify a pioneering idea.
They serve the entire world, from ABC TV to Zookeepers. Notice in Figure
9 that the satellites are clustered in three main areas: one over North and
South America, another over Europe and Africa, and the third over Asia. If you
check the suggested positions of the three geostationary satellites in
Sir Arthur C.
Clarke's proposal, they roughly correspond to these three areas.
Figure 9: Today's geostationary satellite population.
There are approximately 250 individual satellites in this image, all serving
specific regions of the world. Nimiq 1, 2 and 3, and Anik F1-R and F2 are
located near the 90 degrees W portion of the belt, which gives them access to
Canada. Sir Arthur C. Clarke's proposed positions for the geostationary satellites
is labeled on this image. With the exception of the Asian region, the satellites
cluster over the postulated zones very nicely. Note the large gap over the
Pacific Ocean, and the thinner population over the Atlantic Ocean. It appears
there is no purpose to have many geostationary satellites over such vast
you thought you would never read in an astronomy article. Howard Stern. Yes,
Howard Stern. Why? Recently, Mr. Stern has announced that he will be
broadcasting exclusively over Sirius Satellite Radio. Now, remember Sir Arthur C.
Clarke's proposal. We have come full circle in the sense that the proposal to
broadcast radio from geostationary satellites is now a reality. Mr. Stern's
views about commercial radio aside, satellite radio services offer CD quality
music anywhere in North America. This is different from commercial radio in the
sense that you can listen to the same station wherever you are in the continent
(even in your car!), as long as you have a satellite radio receiver (aha,
there's the catch!).
Sirius Satellite Radio uses (wait for it!) 3
satellites, just as Clarke had proposed, except that instead of orbiting
the Earth's equator, they orbit at an inclination of 63.5 degrees from the
Earth's equator. This introduces a new concept called the "geosynchronous" orbit. What
is the difference between a geostationary and a geosynchronous orbit? They both
orbit in the same time as the Earth's rotation: 23h 56m 4s. The geostationary
orbit is exclusively over the Earth's equator. The geosynchronous satellite can
orbit within any plane (including equatorial). The geostationary orbit is circular, or a zero
eccentricity. The geosynchronous orbit can have any eccentricity. In effect, all geostationary satellites
are also geosynchronous, but not vice-versa.
You can count on the geosynchronous orbiting satellite
to appear in the same point in its orbit every 23 hours, 56 minutes and 4
seconds, but you cannot count on it being in the same point in the sky at all
times, like geostationary satellites.
Sirius' only major competitor, XM Satellite Radio, utilizes
two co-located genuine geostationary satellites. Since it only services North
America, there is no need for three or more, which might keep this particular
service's costs low. Figure 10 and 11 illustrates the orbits of these two competing
satellite radio providers.
Disconcerting as it is to see Sir Arthur C. Clarke and
Howard Stern mentioned in the same paragraph, they now have something in common.
What interesting times we live in! Satellite radio has only increased our
dependence on artificial satellites for our communications and entertainment.
Figure 10: The Sirius and XM satellite radio constellation orbits.
The two co-located XM
satellites are geostationary (equatorial, circular), while Sirius satellites are
geosynchronous (63.5 degree plane, eccentricity of 0.27. The Sirius satellite
constellation is better placed for northern region reception (such as the
Arctic). This figure was generated by AGI's Satellite Tool Kit version 6.0.
Figure 11: The ground tracks of the XM and Sirius
satellite radio constellations. The XM constellation consists of geostationary
satellites, and as a consequence, they appear as small dots on the surface of
the Earth. The arrow points to their location on the equator. The Sirius
constellation consists of geosynchronous satellites favoring the North American
continent. Their orbit eccentricity places them over North America for longer
periods of time than they are over South America. This figure was generated by AGI's Satellite Tool Kit version 6.0.
In short, just by looking at the sea of
satellite dishes in your neighborhood, you can appreciate just how dependent
everyday people are on geostationary satellites. Satellite radio will only
deepen this dependence, if it catches on. The geostationary orbit was
specifically designed for the convenience of both the satellite controller and
By the way, you might be wondering if I
have a satellite dish or subscribe to satellite radio. My answer is no and no.
Although I greatly appreciate the satellite community that makes direct-to-home
satellite television broadcasting possible, I have already seen Gilligan's
Island, although the reception was not as good at the time! As for music, I get
that from somewhere else.
So, when you are taking a stroll around your
neighborhood and see a satellite dish, just remember how dependent it is on
concepts in astronomy, such as the solar and sidereal days and all of Keplerian
orbital mechanics. Hopefully, it will deepen your appreciation for our vast sea
This article is in no way meant as an
advertisement for any of the following: Bell ExpressVu, Star Choice, Telesat,
the satellite industry in general, AGI, the cable industry in general, Sirius,
XM, Arthur C. Clarke, Howard Stern, or Gilligan's Island (sitcom or reality TV
show). These were all merely used as examples and descriptive tools. There, I
think that covers everything!
REFERENCE TABLE FOR MENTIONED GEOSTATIONARY SATELLITES
JANUARY 9, 2006)
||OTTAWA AZ (o)
||OTTAWA ALT (o)
|Nimiq 3 (DBS 3)
||Bell Express Vu
||Star Choice (English)
||Star Choice (French)
||XM Satellite Radio
||XM Satellite Radio