Communicating with distant space probes

2. Communicating with distant space probes

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

4. 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

5. 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.

6. I’m going to concentrate on the first of these.

7. 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,

8. 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.

9. 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,

10. 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.

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

15. 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.

16. 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

17. 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.

18. 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.

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

20. The angle of the cone is given by the fraction wavelength/aperture (size) of antenna:- tan angle θ = λ/a

21. The angle of the cone is given by the fraction wavelength/aperture (size) of antenna:- tan angle θ = λ/a

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

23. For 10GHz microwaves the wavelength is, from wavelength = speed/frequency, λ = c/υ, = 3x108m/s / 10x109Hz= 0.03m (about 1 inch)

24. 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

25. 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

26. 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

27. radius of cone = 0.003 x distance to probe

28. radius of cone = 0.003 x distance to probe Area of end of cone (area over which the signal is spread) = πr2

29. 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

30. 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

31. 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

32. voyager.jpl.nasa.gov/mission/weekly-reports/index.htm

33. Voyager 1 is now the most distant man-made object.

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

35. 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.

36. 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

37. 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)

39. Fraction of power received by antenna = area of antenna / area of cone

40. Fraction of power received by antenna = area of antenna / area of cone = 10,000m2 / 8 x1021m2

41. Fraction of power received by antenna = area of antenna / area of cone = 10,000m2 / 8 x1021m2 ~ 1 x 10-18

42. 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

43. 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

44. 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.

45. 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.

46. 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.

47. 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!

48. 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.

49. 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

50. 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