Communicating with distant space probes

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• Communicating with distant space probes

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• Communicating with distant space probes
• what are the problems?

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• Communicating with distant space probes
• what are the problems?
• 1 We need to able to send enough power to operate
  our receiver here on Earth

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• Communicating with distant space probes
• what are the problems?
• 1 We need to able to send enough power to operate
  our receiver here on Earth
• 2 We need to have the satellite to know where the
  Earth is.

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• Im going to concentrate on the first of these.

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• Im going to concentrate on the first of these.
• For satellites and probes a long distance from
  the Earth, we need to get the signal to be sent
  in a beam towards the Earth,

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• Im going to concentrate on the first of these.
• For satellites and probes a long distance from
  the Earth, we need to get the signal to be sent
  in a beam towards the Earth, otherwise its energy
  will be spread out over a huge area.

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• Im going to concentrate on the first of these.
• For satellites and probes a long distance from
  the Earth, we need to get the signal to be sent
  in a beam towards the Earth, otherwise its energy
  will be spread out over a huge area.
• To focus signals we use a parabolic antenna, just
  like your TV dish,

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• Im going to concentrate on the first of these.
• For satellites and probes a long distance from
  the Earth, we need to get the signal to be sent
  in a beam towards the Earth, otherwise its energy
  will be spread out over a huge area.
• To focus signals we use a parabolic antenna, just
  like your TV dish, all the satellite antennas
  around the world and all large telescopes.

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• On a satellite or probe we might be able to make
  this antenna 10m in diameter.

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• On a satellite or probe we might be able to make
  this antenna 10m in diameter. For satellite and
  probe communication we use a frequency of about
  10GHz, similar to that used for radar.

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• On a satellite or probe we might be able to make
  this antenna 10m in diameter. For satellite and
  probe communication we use a frequency of about
  10GHz, similar to that used for radar.
• There is a fundamental property of all waves
  called diffraction

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• On a satellite or probe we might be able to make
  this antenna 10m in diameter. For satellite and
  probe communication we use a frequency of about
  10GHz, similar to that used for radar.
• There is a fundamental property of all waves
  called diffraction and this limits the quality of
  focusing by an antenna.

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• On a satellite or probe we might be able to make
  this antenna 10m in diameter. For satellite and
  probe communication we use a frequency of about
  10GHz, similar to that used for radar.
• There is a fundamental property of all waves
  called diffraction and this limits the quality of
  focusing by an antenna.
• It turns out that antennas radiate into a cone,
  whose angle is dependent on the size of the
  antenna and the wavelength of the signal.

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• The angle of the cone is given by the fraction
  wavelength/aperture (size) of antenna-

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• The angle of the cone is given by the fraction
  wavelength/aperture (size) of antenna-
• tan angle ? ? /a

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• The angle of the cone is given by the fraction
  wavelength/aperture (size) of antenna-
• tan angle ? ? /a

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• For 10GHz microwaves the wavelength is, from
  wavelength speed/frequency, ? c/?,

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• For 10GHz microwaves the wavelength is, from
  wavelength speed/frequency, ? c/?,
• 3x108m/s / 10x109Hz 0.03m (about 1 inch)

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• For 10GHz microwaves the wavelength is, from
  wavelength speed/frequency, ? c/?,
• 3x108m/s / 10x109Hz 0.03m (about 1 inch)
• And so for our 10m antenna
• tan? 0.03m/10m 0.003

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• For 10GHz microwaves the wavelength is, from
  wavelength speed/frequency, ? c/?,
• 3x108m/s / 10x109Hz 0.03m (about 1 inch)
• And so for our 10m antenna
• tan? 0.03m/10m 0.003
• radius of cone / distance to probe

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• For 10GHz microwaves the wavelength is, from
  wavelength speed/frequency, ? c/?,
• 3x108m/s / 10x109Hz 0.03m (about 1 inch)
• And so for our 10m antenna
• tan? 0.03m/10m 0.003
• radius of cone / distance to probe
• i.e. radius of cone 0.003 x distance to probe

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• radius of cone 0.003 x distance to probe

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• radius of cone 0.003 x distance to probe
• Area of end of cone (area over which the signal
  is spread)
• pr2

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• radius of cone 0.003 x distance to probe
• Area of end of cone (area over which the signal
  is spread)
• pr2 p x (0.003 x distance to probe)2

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• radius of cone 0.003 x distance to probe
• Area of end of cone (area over which the signal
  is spread)
• pr2 p x (0.003 x distance to probe)2
• 0.00003 x (distance to probe)2

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• radius of cone 0.003 x distance to probe
• Area of end of cone (area over which the signal
  is spread)
• pr2 p x (0.003 x distance to probe)2
• 0.00003 x (distance to probe)2
• area
• distance to probe

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voyager.jpl.nasa.gov/mission/weekly-reports/index.
htm
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• Voyager 1 is now the most distant man-made
  object.

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• Voyager 1 is now the most distant man-made
  object. Launched in 1977 it is now 16 trillion
  (16 x 1012) metres from Earth.

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• Voyager 1 is now the most distant man-made
  object. Launched in 1977 it is now 16 trillion
  (16 x 1012) metres from Earth. Its transmitter
  produces about 200watts of power.

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• Voyager 1 is now the most distant man-made
  object. Launched in 1977 it is now 16 trillion
  (16 x 1012) metres from Earth. Its transmitter
  produces about 200watts of power.
• At this distance, the area into which its signals
  will spread is, using our formula, 0.00003 x (16
  trillion)2

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• Voyager 1 is now the most distant man-made
  object. Launched in 1977 it is now 16 trillion
  (16 x 1012) metres from Earth. Its transmitter
  produces about 200watts of power.
• At this distance, the area into which its signals
  will spread is, using our formula, 0.00003 x (16
  trillion)2
• 8 x 1021 m2
• Its signals are received by an antenna on Earth
  with a radius of 55m (area 10,000m2)

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• Fraction of power received by antenna
• area of antenna / area of cone

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• Fraction of power received by antenna
• area of antenna / area of cone
• 10,000m2 / 8 x1021m2

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• Fraction of power received by antenna
• area of antenna / area of cone
• 10,000m2 / 8 x1021m2
• 1 x 10-18

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• Fraction of power received by antenna
• area of antenna / area of cone
• 10,000m2 / 8 x1021m2
• 1 x 10-18(one quintillionth)
• So power received 200 x 1 x 10-18watts

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• Fraction of power received by antenna
• area of antenna / area of cone
• 10,000m2 / 8 x1021m2
• 1 x 10-18(one quintillionth)
• So power received 200 x 1 x 10-18watts
• 2 x 10-16 watts

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• Fraction of power received by antenna
• area of antenna / area of cone
• 10,000m2 / 8 x1021m2
• 1 x 10-18(one quintillionth)
• So power received 200 x 1 x 10-18watts
• 2 x 10-16 watts
• This is approaching the smallest signal we can
  detect. A factor of ten less is about the lower
  limit.

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive.

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive. This is 53000seconds or 15hours.

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive. This is 53000seconds or 15hours.
• Conversations are slow!

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive. This is 53000seconds or 15hours.
• Conversations are slow!
• Two things have happened to make communication
  difficult.

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive. This is 53000seconds or 15hours.
• Conversations are slow!
• Two things have happened to make communication
  difficult.
• This time delay

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• From this distance of 16 x 1012 metres, it
• takes 16 x 1012/3 x 108 seconds (time
• distance/speed, t d/c) for this signal
• to arrive. This is 53000seconds or 15hours.
• Conversations are slow!
• Two things have happened to make communication
  difficult.
• This time delay
• The low power received

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• Now imagine we would like to visit the nearest
  star to the solar system.

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• Now imagine we would like to visit the nearest
  star to the solar system. Setting aside the time
  of the journey, we still have these twin problems
  of signal strength and signal round-trip time.

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• Now imagine we would like to visit the nearest
  star to the solar system. Setting aside the time
  of the journey, we still have these twin problems
  of signal strength and signal round-trip time.
• Assessing the second of these is easy

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• Now imagine we would like to visit the nearest
  star to the solar system. Setting aside the time
  of the journey, we still have these twin problems
  of signal strength and signal round-trip time.
• Assessing the second of these is easy proxima
  centauri is 4.4 light-years away, so a round trip
  time for the signals would be 8.8years not very
  convenient.

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• 4.4 light years is 4 x 1016 metres.

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• 4.4 light years is 4 x 1016 metres. This is 2500
  times further away than Voyager 1.

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• 4.4 light years is 4 x 1016 metres. This is 2500
  times further away than Voyager 1.
• From a comparable power source this would mean a
  signal 6 million times weaker (2500)2.

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• 4.4 light years is 4 x 1016 metres. This is 2500
  times further away than Voyager 1.
• From a comparable power source this would mean a
  signal 6 million times weaker (2500)2.
• Even if we could make an antenna 100m in diameter
  and have a ten-fold gain in power, it would still
  be several thousand times too small on arrival at
  Earth.

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• 4.4 light years is 4 x 1016 metres. This is 2500
  times further away than Voyager 1.
• From a comparable power source this would mean a
  signal 6 million times weaker (2500)2.
• Even if we could make an antenna 100m in diameter
  and have a ten-fold gain in power, it would still
  be several thousand times too small on arrival at
  Earth.
• The journey time (even assuming a speed ten times
  higher than Voyager 1) would be about 6500 years
  not very practical.

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