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Functions of satellite TV and satellite

Discussion in 'Radio and Communications' started by Willen, Jun 10, 2012.

  1. Les Jones

    Les Jones Well-Known Member

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    They cannot pause a satellite. A satellite must always be rotating about another object. If you are referring to geostationary satellites then they are not really stationary. The just have an angular velocity the same as the rotation of the earth and have the same axis of rotation as the earth so they stay above the same point on the surface of the earth. This can only occur when the radius of the orbit is about 42164 Km.

    Les
     
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  2. Willen

    Willen Well-Known Member

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    Hi, how simple! If they fixed the rotation of the satellite same as earth, the satellite looks like paused to us but it's rotating the earth.

    In every night, we get many running stars (not falling) in the black sky. What are they?
     
  3. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    (My emphasis).
    Can you describe further what you mean by running stars?
     
  4. dave

    Dave New Member

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  5. Willen

    Willen Well-Known Member

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    Um...they look like an ordinary star. They are shining and moving straight ahead sometime from west of the sky to east, sometime from south-west to north-east, sometime from south to north etc. I look at them continually. Some of them stops to shining after few second, maybe they entered within shadow (night) sky of the earth so. If I carefully observed evening sky, these moving lights are pretty common.

    Maybe they are not Boeing crafts because they NEVER blink and never produce sound.
     
    Last edited: Aug 23, 2016
  6. JimB

    JimB Super Moderator Most Helpful Member

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    Or maybe they are, look here:
    http://www.boeing.com/space/boeing-satellite-family/

    What you are seeing is almost certainly one of the many thousands of satellites which are orbiting the earth.
    For you to be able to see them, they have to be in sunlight.
    Sometimes as the bright shiny satellite moves across the sky it will disappear, almost instantly, this is due to the satellite moving into the earths shadow where there is no sunlight.

    JimB
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  8. crutschow

    crutschow Well-Known Member Most Helpful Member

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    To fine-tune a satellite's position or rotation in space they use small reaction thrusters on the satellite.
    In space, the only way to affect a spacecraft's movement is by momentum transfer.
    Thus you propel a small mass at high speed away from the vehicle with a given momentum which moves the spacecraft's much larger mass at a slow speed in the opposite direction with the same momentum.
    The faster you can propel the small mass, giving it more momentum, the less mass you need to affect the vehicle's position or rotation a given amount.
    Thus at present, the most efficient thrusters are ion types which can propel a very small mass at 20-50 km/s.
    You want an efficient thruster since, when you run out of mass to throw, you can no longer control the spacecraft.
    Thus you need to have enough propellent on board to perform the expected corrections needed to its position and rotation for the life of the satellite.
     
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  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Exactly, the life of a satellite is determined by the amount of fuel it has on board, and how efficiently it's used.

    The satellites run by SES Astra in Europe generally well exceed their design life, as they are VERY careful about fuel usage - as I recall they keep them within a 25 mile cube, back in the analogue days you could actually see the changes in signal as they slowly drift out of position, and the fairly rapid return when correction was applied.
     
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  10. nsaspook

    nsaspook Well-Known Member

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    It was good luck that INMARSAT was using a old bird with limited spot keeping fuel during the search for MH370.

    https://www.physicsforums.com/threads/inmarsat-analysis-for-missing-flight-mh370.745000/

    Zoom to see the N/S station movement from the satellite's geostationary position.
    http://www.n2yo.com/?s=23839
     
  11. Dr_Doggy

    Dr_Doggy Well-Known Member

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  12. Willen

    Willen Well-Known Member

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    Hi,
    I cannot download and run the app just now but it seems awesome!!! When I get moving star around my head, I can point cellphone there to scratch their details. Wow!

    I heard that in the space, if we kicked an object once, this will go far always and always ahead. Then how age of satellite depends on its fuel? I guessed when one starts to rotate to the earth, then it rotates always without any force (no more fuel needed after that).
     
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Yes, there's no significant friction in deep space to slow down a synchronous satellite, but the satellite will slowly drift and/or rotate out of position relative to a fixed spot on earth due to various factors, such as the small perturbing force of the gravity of the moon and sun (which causes the tides on earth), so you need a way to periodically tweak it back into position.
    That's why you periodically need to use the reaction rockets.
     
  14. Willen

    Willen Well-Known Member

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    Does gyroscope and acelerometer work at space for navigation? (again thinking about reference point)
     
  15. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Yes.
    They use the inertia of their internal components for sensing.
    The only reference point they have is the point they are in space.
    They do not use gravity, (although an accelerometer can sense gravitational acceleration) so the presence or absence of gravity has no significant effect on their operation otherwise.
     
  16. nsaspook

    nsaspook Well-Known Member

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    For deep space probes radio based navigation is used but a cool system of using X-Ray pulsars to decouple space navigation from direct earth contact is being tested.
    It's much like traditional sea navigation.
    http://link.springer.com/article/10.1007/s10686-016-9496-z
     

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