Communication Using Groundbased Cables and Satellites in Space

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CommunicationUsing Ground-based Cablesand
Satellites in Space
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• We send information down cables as electric
  currents.

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• We send information down cables as electric
  currents.
• What are these currents and how fast do these
  electrical signals travel?

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• We send information down cables as electric
  currents.
• What are these currents and how fast do these
  electrical signals travel?
• Electric currents consist of moving electrons
  (tiny, very light, negative particles).

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• We send information down cables as electric
  currents.
• What are these currents and how fast do these
  electrical signals travel?
• Electric currents consist of moving electrons
  (tiny, very light, negative particles).
• These electrons travel quite slowly about 6mm
  (¼inch) per second in a normal wire to a bedside
  lamp,

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• We send information down cables as electric
  currents.
• What are these currents and how fast do these
  electrical signals travel?
• Electric currents consist of moving electrons
  (tiny, very light, negative particles).
• These electrons travel quite slowly about 6mm
  (¼inch) per second in a normal wire to a bedside
  lamp, but the pulse, which tells each electron to
  move, travels at the speed of light. This is
  300,000km/sec (186,000miles/sec) in a vacuum.

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• If the wire is not surrounded by a vacuum but by
  a plastic insulator then the pulse travels at the
  speed of light in that insulator, somewhat less
  than the speed in a vacuum.

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• If the wire is not surrounded by a vacuum but by
  a plastic insulator then the pulse travels at the
  speed of light in that insulator, somewhat less
  than the speed in a vacuum.
• We can measure this speed in a piece of co-axial
  cable, the sort you use to send the signal to a
  TV set.

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• If the wire is not surrounded by a vacuum but by
  a plastic insulator then the pulse travels at the
  speed of light in that insulator, somewhat less
  than the speed in a vacuum.
• We can measure this speed in a piece of co-axial
  cable, the sort you use to send the signal to a
  TV set.
• To do this we need to measure two things the
  length of the cable and the time taken for the
  pulse to travel that distance since speed
  distance/time.

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• Lets decide how much cable and what sort of
  clock (timer) we might need.

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• Lets decide how much cable and what sort of
  clock (timer) we might need.
• Cables commonly come in 200m rolls. Will this do?

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• Lets decide how much cable and what sort of
  clock (timer) we might need.
• Cables commonly come in 200m rolls. Will this do?
• If the speed is 300million metres/second then a
  pulse would take 2/3 of a millionth of a second
  to go the whole length.

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• Lets decide how much cable and what sort of
  clock (timer) we might need.
• Cables commonly come in 200m rolls. Will this do?
• If the speed is 300million metres/second then a
  pulse would take 2/3 of a millionth of a second
  to go the whole length. The question then is,
  Do we have a fast enough timer?.

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• Lets decide how much cable and what sort of
  clock (timer) we might need.
• Cables commonly come in 200m rolls. Will this do?
• If the speed is 300million metres/second then a
  pulse would take 2/3 of a millionth of a second
  to go the whole length. The question then is,
  Do we have a fast enough timer?.
• Luckily, common laboratory devices called
  oscilloscopes will time this accurately.

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• This is what well do

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• This is what well do

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• The pulse generator works a bit like an
  electronic keyboard and produces a stream of
  short pulses at a rate of 250 thousand per second
  (250kHz).

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• The pulse generator works a bit like an
  electronic keyboard and produces a stream of
  short pulses at a rate of 250 thousand per second
  (250kHz). For sound, this would be about 4
  octaves above the top of a normal piano keyboard.

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• The pulse generator works a bit like an
  electronic keyboard and produces a stream of
  short pulses at a rate of 250 thousand per second
  (250kHz). For sound, this would be about 4
  octaves above the top of a normal piano keyboard.
• These pulses occur with a 1/250,000 second gap
  between them.

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• The pulse generator works a bit like an
  electronic keyboard and produces a stream of
  short pulses at a rate of 250 thousand per second
  (250kHz). For sound, this would be about 4
  octaves above the top of a normal piano keyboard.
• These pulses occur with a 1/250,000 second gap
  between them.
• (1/250,000 second is four millionths of a
  second, 4 microseconds, 4µs)

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• We can set the oscilloscope so that it just
  displays two pulses from the pulse generator,

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• We can set the oscilloscope so that it just
  displays two pulses from the pulse generator,
  with a time delay between them of 4millionths of
  a second.

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• If we now connect the 200m of cable, these pulses
  will travel to the far, joined end, get
  reflected, and return back to be shown on the
  oscilloscope before the next pulse starts.

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• If we now connect the 200m of cable, these pulses
  will travel to the far, joined end, get
  reflected, and return back to be shown on the
  oscilloscope before the next pulse starts.
• As you can see, they arrive a bit distorted.

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• If we now connect the 200m of cable, these pulses
  will travel to the far, joined end, get
  reflected, and return back to be shown on the
  oscilloscope before the next pulse starts.
• As you can see, they arrive a bit distorted.

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• How do we know theres a reflection?

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• How do we know theres a reflection? Undoing the
  ends of the cable produces a different type of
  reflection the wrong way up!

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• We can even fool the pulse into thinking that the
  cable does not end by adding a suitable resistor

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• We can even fool the pulse into thinking that the
  cable does not end by adding a suitable resistor
  we have now created an effective radio antenna
  and the pulse reflection disappears.

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• Now for the calculation

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• Now for the calculation
• Distance travelled by pulse on return journey
  down the cable

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• Now for the calculation
• Distance travelled by pulse on return journey
  down the cable
• 2 x length of cable 2 x 200m 400m

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• Now for the calculation
• Distance travelled by pulse on return journey
  down the cable
• 2 x length of cable 2 x 200m 400m
• Time from start of pulse to start of reflected
  pulse

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• Now for the calculation
• Distance travelled by pulse on return journey
  down the cable
• 2 x length of cable 2 x 200m 400m
• Time from start of pulse to start of reflected
  pulse
• 1½microseconds (1.5µs)

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• Speed of pulse
• distance/time

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• Speed of pulse
• distance/time 400m/1½x 10-6sec

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• Speed of pulse
• distance/time 400m/1½x 10-6sec
• 270million metres/sec

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• Speed of pulse
• distance/time 400m/1½x 10-6sec
• 270million metres/sec
• Comparing this with the speed of light in a
  vacuum we get

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• Speed of pulse
• distance/time 400m/1½x 10-6sec
• 270million metres/sec
• Comparing this with the speed of light in a
  vacuum we get
• speed of light in a vacuum 300million m/s
• speed of pulse along wire 270million m/s

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• Speed of pulse
• distance/time 400m/1½x 10-6sec
• 270million metres/sec
• Comparing this with the speed of light in a
  vacuum we get
• speed of light in a vacuum 300million m/s
• speed of pulse along wire 270million m/s
• 1.12

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• Speed of pulse
• distance/time 400m/1½x 10-6sec
• 270million metres/sec
• Comparing this with the speed of light in a
  vacuum we get
• speed of light in a vacuum 300million m/s
• speed of pulse along wire 270million m/s
• 1.12
• Refractive Index of the plastic insulation
  around the wire.

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• If we measure the speed of light pulses along an
  optical fibre, we get a very similar result.

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• If we measure the speed of light pulses along an
  optical fibre, we get a very similar result.
• In order to telephone by cable from Los Angeles
  to London, a distance of about 8750km,

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• If we measure the speed of light pulses along an
  optical fibre, we get a very similar result.
• In order to telephone by cable from Los Angeles
  to London, a distance of about 8750km, the signal
  takes 8.75 x 106m/2.7x108m/s, which is about 3
  hundredths of a second.

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• This small time delay is not noticed as we hold a
  conversation, so why is there sometimes a time
  delay on this type of telephone call or on a TV
  signal?

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• Both TV and phone signals may travel by a
  satellite link, rather than going via the optical
  fibre link we have assumed. Why does this make a
  difference?

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• Both TV and phone signals may travel by a
  satellite link, rather than going via the optical
  fibre link we have assumed. Why does this make a
  difference?
• To answer that we have to know where
  communications satellites are, and that requires
  some different Physics.

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• What holds satellites in place?

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• What holds satellites in place? Nothing!

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• What holds satellites in place? Nothing!
• Satellites are in free-fall towards the Earth

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• What holds satellites in place? Nothing!
• Satellites are in free-fall towards the Earth and
  also moving around the Earth with a speed which
  allows them to follow the curvature of their
  orbit.

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• What holds satellites in place? Nothing!
• Satellites are in free-fall towards the Earth and
  also moving around the Earth with a speed which
  allows them to follow the curvature of their
  orbit.
• As a shell is fired
• from a gun, it will
• travel further as
• its launching speed
• is increased

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• In all these trajectories (neglecting any
  air-resistance) the only force after launch will
  be gravity.

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• In all these trajectories (neglecting any
  air-resistance) the only force after launch will
  be gravity.
• If the shell makes a circular orbit then we need
  to consider that everything moving in a circle
  has to have a force on it of mv2/r towards the
  centre of the circle.

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• In all these trajectories (neglecting any
  air-resistance) the only force after launch will
  be gravity.
• If the shell makes a circular orbit then we need
  to consider that everything moving in a circle
  has to have a force on it of mv2/r towards the
  centre of the circle.
• Going round a curve in a car is more exciting if
  the speed (v) is greater or the corner is sharper
  (its radius (r) is smaller).

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• In any circular orbit where gravity is the only
  force, we have
• Gravitational force centripetal force

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• In any circular orbit where gravity is the only
  force, we have
• Gravitational force centripetal force
• mg mv2/r

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• In any circular orbit where gravity is the only
  force, we have
• Gravitational force centripetal force
• mg mv2/r
• or g v2/r

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• In any circular orbit where gravity is the only
  force, we have
• Gravitational force centripetal force
• mg mv2/r
• or g v2/r
• For a very low Earth orbit, the speed needed
  (often called the escape velocity) is given by
• v vgr

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• In any circular orbit where gravity is the only
  force, we have
• Gravitational force centripetal force
• mg mv2/r
• or g v2/r
• For a very low Earth orbit, the speed needed
  (often called the escape velocity) is given by
• v vgr
• where g acceleration due to gravity at the
  Earths surface (10m/s2) and r radius of orbit
  (radius of Earth) 6400km

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• v v(10m/s2 x 6.4 x 106)m

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• v v(10m/s2 x 6.4 x 106)m
• v(64 x 106)m2/s2

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• v v(10m/s2 x 6.4 x 106)m
• v(64 x 106)m2/s2
• 8000m/s (18,000mph)

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• v v(10m/s2 x 6.4 x 106)m
• v(64 x 106)m2/s2
• 8000m/s (18,000mph)
• We cant have such low orbiting satellites
  because of air resistance,

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• v v(10m/s2 x 6.4 x 106)m
• v(64 x 106)m2/s2
• 8000m/s (18,000mph)
• We cant have such low orbiting satellites
  because of air resistance, but at 300km altitude
  (giving a radius of 6700km) we can.

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• v v(10m/s2 x 6.4 x 106)m
• v(64 x 106)m2/s2
• 8000m/s (18,000mph)
• We cant have such low orbiting satellites
  because of air resistance, but at 300km altitude
  (giving a radius of 6700km) we can.
• This gives a speed of 8185m/s (18420mph) and this
  means that orbiting takes a time
• t circumference of orbit / speed

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• t circumference of orbit / speed 2pro/v

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• t circumference of orbit / speed 2pro/v
• 2p x 6.7 x 106m / 8185m/s

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• t circumference of orbit / speed 2pro/v
• 2p x 6.7 x 106m / 8185m/s
• 5143sec 1hr 26min

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• t circumference of orbit / speed 2pro/v
• 2p x 6.7 x 106m / 8185m/s
• 5143sec 1hr 26min, which is the minimum time
  for an orbiting Earth satellite.

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• t circumference of orbit / speed 2pro/v
• 2p x 6.7 x 106m / 8185m/s
• 5143sec 1hr 26min, which is the minimum time
  for an orbiting Earth satellite.
• When we consider higher altitude satellites, we
  also have to consider that the gravitational
  force gets less according to Newtons
  relationship g a 1/r2.

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• t circumference of orbit / speed 2pro/v
• 2p x 6.7 x 106m / 8185m/s
• 5143sec 1hr 26min, which is the minimum time
  for an orbiting Earth satellite.
• When we consider higher altitude satellites, we
  also have to consider that the gravitational
  force gets less according to Newtons
  relationship g a 1/r2. What we want for a
  communications satellite is that it appears to
  hover over the same spot on the Earths surface,
  so that our antennas can always point to the same
  place.

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• This condition means that communications
  satellites need to be above the equator

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.
• We have found that gorbit v2/rorbit for any
  orbit from the equation for circular motion

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.
• We have found that gorbit v2/rorbit for any
  orbit from the equation for circular motion
• and gorbit gsurface rEarth2 / rorbit2 from
  Newtons law of gravitation

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.
• We have found that gorbit v2/rorbit for any
  orbit from the equation for circular motion
• and gorbit gsurface rEarth2 / rorbit2 from
  Newtons law of gravitation
• and so gsrE2/ ro2 v2/ro

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.
• We have found that gorbit v2/rorbit for any
  orbit from the equation for circular motion
• and gorbit gsurface rEarth2 / rorbit2 from
  Newtons law of gravitation
• and so gsrE2/ ro2 v2/ro
• v2 gsrE2/ro

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• This condition means that communications
  satellites need to be above the equator and to
  orbit the Earth in the same time as the Earth
  rotates, i.e. 24 hours.
• We have found that gorbit v2/rorbit for any
  orbit from the equation for circular motion
• and gorbit gsurface rEarth2 / rorbit2 from
  Newtons law of gravitation
• and so gsrE2/ ro2 v2/ro
• v2 gsrE2/ro which relates the velocity and
  radius of this orbit at this point we know
  neither.

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• But, we do know that the time of orbit
• t 2pro/v 24 hours
• giving v2 (2pro/t)2
• which also relates v and ro

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• Equating these two expressions for v2 we get
• gsrE2/ ro (2pro/t)2

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• Equating these two expressions for v2 we get
• gsrE2/ ro (2pro/t)2 where now we know all the
  terms except ro.

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• Equating these two expressions for v2 we get
• gsrE2/ ro (2pro/t)2 where now we know all the
  terms except ro.
• Expanding the bracket gives gsrE2 4p2ro2
• ro t2

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• Equating these two expressions for v2 we get
• gsrE2/ ro (2pro/t)2 where now we know all the
  terms except ro.
• Expanding the bracket gives gsrE2 4p2ro2
• ro t2
• and rearranging gives ro3 gsrE2 t2 / 4p2

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• Equating these two expressions for v2 we get
• gsrE2/ ro (2pro/t)2 where now we know all the
  terms except ro.
• Expanding the bracket gives gsrE2 4p2ro2
• ro t2
• and rearranging gives ro3 gsrE2 t2 / 4p2
• Now we can enter the numbers we know

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• ro3 gsrE2 t2 / 4p2
• 10m/s2

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5
• 7.745 x 1022

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5
• 7.745 x 1022
• and so ro 3v 7.745 x 1022

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5
• 7.745 x 1022
• and so ro 3v 7.745 x 1022
• 42,630,000m

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5
• 7.745 x 1022
• and so ro 3v 7.745 x 1022
• 42,630,000m
• 42,630km ( 26,600miles)

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• ro3 gsrE2 t2 / 4p2
• 10m/s2 (6.4 x 106m)2 (24 x 3600s)2 / 39.5
• 7.745 x 1022
• and so ro 3v 7.745 x 1022
• 42,630,000m
• 42,630km ( 26,600miles)
• which is an altitude of 42630-6400
• 36,230km or 22,640miles

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Earth and Moon
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• After all that hard work, what does it tell us
  about the time needed for signals to get from Los
  Angeles to London via satellite?

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• After all that hard work, what does it tell us
  about the time needed for signals to get from Los
  Angeles to London via satellite?

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• After all that hard work, what does it tell us
  about the time needed for signals to get from Los
  Angeles to London via satellite?
• To make this trip, we need to use two satellites,
  with the signal travelling up and down to each.

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• After all that hard work, what does it tell us
  about the time needed for signals to get from Los
  Angeles to London via satellite?
• To make this trip, we need to use two satellites,
  with the signal travelling up and down to each.
  This means going at least 36,230km four times,
  i.e. about 145,000km.

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• After all that hard work, what does it tell us
  about the time needed for signals to get from Los
  Angeles to London via satellite?
• To make this trip, we need to use two satellites,
  with the signal travelling up and down to each.
  This means going at least 36,230km four times,
  i.e. about 145,000km.
• At the speed of light this takes just about half
  a second, a very noticeable delay when holding a
  conversation. We normally reply just before our
  speaking partner finishes.

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• This delay is often noticeable on news reports
  from across the world as they use satellites to
  link. If you are able to receive radio or TV
  signals via both cable or normal antenna and
  also by satellite dish, there is a noticeable
  time-lag on the satellite signal. This causes
  some problems when giving accurate times via the
  airwaves.