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Communication Using Ground-based Cables and Satellites in Space

Communication Using Ground-based Cables and Satellites in Space We send information down cables as electric currents. We send information down cables as electric currents. What are these currents and how fast do these electrical signals travel?

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Communication Using Ground-based Cables and Satellites in Space

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  1. CommunicationUsing Ground-based Cablesand Satellites in Space

  2. We send information down cables as electric currents.

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

  4. 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).

  5. 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,

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. Let’s decide how much cable and what sort of clock (timer) we might need.

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

  12. Let’s 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.

  13. Let’s 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?”.

  14. Let’s 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.

  15. This is what we’ll do:

  16. This is what we’ll do:

  17. 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).

  18. 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.

  19. 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.

  20. 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)

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

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. How do we know there’s a reflection?

  27. How do we know there’s a reflection? Undoing the ends of the cable produces a different type of reflection – the wrong way up!

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

  29. 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.

  30. Now for the calculation:

  31. Now for the calculation: Distance travelled by pulse on return journey down the cable

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

  33. 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

  34. 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)

  35. Speed of pulse = distance/time

  36. Speed of pulse = distance/time = 400m/1½x 10-6sec

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

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

  39. 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

  40. 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

  41. 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.

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

  43. 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,

  44. 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.

  45. 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?

  46. 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?

  47. 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.

  48. What holds satellites in place?

  49. What holds satellites in place? Nothing!

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