Two practical advantages derive from this. First, a single satellite at that distance,
equal to six times the Earth’s radius, can observe a third of the entire planet. This
means that with only three satellites at 120° apart from each other, you can monitor
the whole Earth from that orbit. The second great advantage is that ground stations
can transmit to those satellites without moving the antennas, which simplifies the
entire connection and hardware. This is why weather and telecommunications satellites fly at that altitude; from there, you can constantly monitor the weather over an
entire continent or you can transmit TV signals from one point to another on the
planet without using any ground repeater. For this reason, geostationary orbit is also
highly strategic for the armed forces. From there, satellites provide crucial support
for command and control centres and for all military assets on air, land and sea.
However vast space is, geostationary orbit is very crowded because all the satellites are concentrated on a precise strip above the Earth’s equator. If they were
elsewhere, their trajectory relative to the surface, named downrange, would not be
a motionless point but instead would be moving up and down, making the pointing
of antennas much more difficult. This means that hundreds of satellites from different countries are clustered in space above the most strategic areas of the planet,
sometimes interfering with each other. There are international rules to manage
these orbital slots, just like for airplane take-offs and landings, but often the rule
is determined by the law of the strongest, and those who can occupy the best positions in space.
Satellites in geostationary orbit take photographs, but the resolution is very low
due to the huge distance. With a space telescope with a ten-foot diameter, you can
get images with a blur up to 26 feet, meaning that two objects at that distance are
not distinguishable from each other. Practically, one can photograph the cloudy
perturbations on the continents, check the state of cultivated fields, or track the
infrared signature of a ballistic missile taking off from Siberia – that is the job of
the US SBIRS satellites – but terrorist groups in the desert cannot be detected. For
this kind of intelligence, i.e. espionage, it is necessary to use satellites in LEO. A
space-based camera 6.5 feet in diameter flying at 168 miles can take pictures with
a blur under 2 feet, and in this way, even people can be identified.
This may make it seem like we are being constantly observed by a space-based
Big Brother, but the reality is a bit different. As said, a spy satellite flying over us
has less than two minutes to take photographs before it will leave our field of view.
After an hour and a half, it will have made a complete tour of the Earth but will
not pass over our heads again because in the meantime the Earth’s rotation will
have moved us by 1,500 miles. We will see it transit over us again only after 16
more orbits, i.e. a whole day.
Contrary to popular belief, spying from space is common strategic practise for
the few nations that have the technology to do it, but it is not done constantly and
globally. If we wanted to observe the entire planet 24 hours a day, we would have
to put into orbit more than 5,000 satellites with 6.5-foot-diameter telescopes.
Fundamentals of Astronautics 5
equal to six times the Earth’s radius, can observe a third of the entire planet. This
means that with only three satellites at 120° apart from each other, you can monitor
the whole Earth from that orbit. The second great advantage is that ground stations
can transmit to those satellites without moving the antennas, which simplifies the
entire connection and hardware. This is why weather and telecommunications satellites fly at that altitude; from there, you can constantly monitor the weather over an
entire continent or you can transmit TV signals from one point to another on the
planet without using any ground repeater. For this reason, geostationary orbit is also
highly strategic for the armed forces. From there, satellites provide crucial support
for command and control centres and for all military assets on air, land and sea.
However vast space is, geostationary orbit is very crowded because all the satellites are concentrated on a precise strip above the Earth’s equator. If they were
elsewhere, their trajectory relative to the surface, named downrange, would not be
a motionless point but instead would be moving up and down, making the pointing
of antennas much more difficult. This means that hundreds of satellites from different countries are clustered in space above the most strategic areas of the planet,
sometimes interfering with each other. There are international rules to manage
these orbital slots, just like for airplane take-offs and landings, but often the rule
is determined by the law of the strongest, and those who can occupy the best positions in space.
Satellites in geostationary orbit take photographs, but the resolution is very low
due to the huge distance. With a space telescope with a ten-foot diameter, you can
get images with a blur up to 26 feet, meaning that two objects at that distance are
not distinguishable from each other. Practically, one can photograph the cloudy
perturbations on the continents, check the state of cultivated fields, or track the
infrared signature of a ballistic missile taking off from Siberia – that is the job of
the US SBIRS satellites – but terrorist groups in the desert cannot be detected. For
this kind of intelligence, i.e. espionage, it is necessary to use satellites in LEO. A
space-based camera 6.5 feet in diameter flying at 168 miles can take pictures with
a blur under 2 feet, and in this way, even people can be identified.
This may make it seem like we are being constantly observed by a space-based
Big Brother, but the reality is a bit different. As said, a spy satellite flying over us
has less than two minutes to take photographs before it will leave our field of view.
After an hour and a half, it will have made a complete tour of the Earth but will
not pass over our heads again because in the meantime the Earth’s rotation will
have moved us by 1,500 miles. We will see it transit over us again only after 16
more orbits, i.e. a whole day.
Contrary to popular belief, spying from space is common strategic practise for
the few nations that have the technology to do it, but it is not done constantly and
globally. If we wanted to observe the entire planet 24 hours a day, we would have
to put into orbit more than 5,000 satellites with 6.5-foot-diameter telescopes.
Fundamentals of Astronautics 5