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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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## Communicating with distant space probes

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**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**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.**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,**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.**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,**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.**On a satellite or probe we might be able to make this**antenna 10m in diameter.**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.**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**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.**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.**The angle of the cone is given by the fraction**wavelength/aperture (size) of antenna:-**The angle of the cone is given by the fraction**wavelength/aperture (size) of antenna:- tan angle θ = λ/a**The angle of the cone is given by the fraction**wavelength/aperture (size) of antenna:- tan angle θ = λ/a**For 10GHz microwaves the wavelength is, from wavelength =**speed/frequency, λ = c/υ,**For 10GHz microwaves the wavelength is, from wavelength =**speed/frequency, λ = c/υ, = 3x108m/s / 10x109Hz= 0.03m (about 1 inch)**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**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**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**radius of cone = 0.003 x distance to probe**Area of end of cone (area over which the signal is spread) = πr2**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**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**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**Voyager 1 is now the most distant man-made object. Launched**in 1977 it is now 16 trillion (16 x 1012) metres from Earth.**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.**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**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)**Fraction of power received by antenna**= area of antenna / area of cone**Fraction of power received by antenna**= area of antenna / area of cone = 10,000m2 / 8 x1021m2**Fraction of power received by antenna**= area of antenna / area of cone = 10,000m2 / 8 x1021m2 ~ 1 x 10-18**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**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**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.**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.**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.**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!**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.**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**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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