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

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slide4
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

slide5
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.

slide7
I’m 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,

slide8
I’m 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.

slide9
I’m 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,

slide10
I’m 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.

slide15
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.
slide16
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

slide17
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.

slide18
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.

slide19
The angle of the cone is given by the fraction wavelength/aperture (size) of antenna:-
slide20
The angle of the cone is given by the fraction wavelength/aperture (size) of antenna:-

tan angle θ = λ/a

slide21
The angle of the cone is given by the fraction wavelength/aperture (size) of antenna:-

tan angle θ = λ/a

slide23
For 10GHz microwaves the wavelength is, from wavelength = speed/frequency, λ = c/υ,

= 3x108m/s / 10x109Hz= 0.03m (about 1 inch)

slide24
For 10GHz microwaves the wavelength is, from wavelength = speed/frequency, λ = c/υ,

= 3x108m/s / 10x109Hz= 0.03m (about 1 inch)

And so for our 10m antenna

tanθ = = 0.03m/10m = 0.003

slide25
For 10GHz microwaves the wavelength is, from wavelength = speed/frequency, λ = c/υ,

= 3x108m/s / 10x109Hz= 0.03m (about 1 inch)

And so for our 10m antenna

tanθ = = 0.03m/10m = 0.003

= radius of cone / distance to probe

slide26
For 10GHz microwaves the wavelength is, from wavelength = speed/frequency, λ = c/υ,

= 3x108m/s / 10x109Hz= 0.03m (about 1 inch)

And so for our 10m antenna

tanθ = = 0.03m/10m = 0.003

= radius of cone / distance to probe

i.e. radius of cone = 0.003 x distance to probe

slide28
radius of cone = 0.003 x distance to probe

Area of end of cone (area over which the signal is spread)

= πr2

slide29
radius of cone = 0.003 x distance to probe

Area of end of cone (area over which the signal is spread)

= πr2 = π x (0.003 x distance to probe)2

slide30
radius of cone = 0.003 x distance to probe

Area of end of cone (area over which the signal is spread)

= πr2 = π x (0.003 x distance to probe)2

~ 0.00003 x (distance to probe)2

slide31
radius of cone = 0.003 x distance to probe

Area of end of cone (area over which the signal is spread)

= πr2 = π x (0.003 x distance to probe)2

~ 0.00003 x (distance to probe)2

area

distance to probe

slide34
Voyager 1 is now the most distant man-made object. Launched in 1977 it is now 16 trillion (16 x 1012) metres from Earth.
slide35
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.
slide36
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

slide37
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)

slide39
Fraction of power received by antenna

= area of antenna / area of cone

slide40
Fraction of power received by antenna

= area of antenna / area of cone

= 10,000m2 / 8 x1021m2

slide41
Fraction of power received by antenna

= area of antenna / area of cone

= 10,000m2 / 8 x1021m2

~ 1 x 10-18

slide42
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

slide43
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

slide44
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.

slide45
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.

slide46
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.

slide47
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!

slide48
From this distance of 16 x 1012metres, 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.

slide49
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
slide50
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
slide52
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.
slide53
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:

slide54
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: proximacentauri is 4.4 light-years away, so a round trip time for the signals would be 8.8years – not very convenient.

slide57
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.

slide58
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.

slide59
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.