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  1. #1 science satellites 
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    scientific satellites that orbit the earth send electromagnetic signals to the surface and they get a reading off what bounces back.

    Can they send any frequency?

    Do they have to put more 'power' into the wave (I think it would be amplitude) to get enough of the wave to bounce back so they can decipher it?


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  3. #2 Re: science satellites 
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    Quote Originally Posted by fatman57
    scientific satellites that orbit the earth send electromagnetic signals to the surface and they get a reading off what bounces back.

    Can they send any frequency?
    Of course not.

    Quote Originally Posted by fatman57
    Do they have to put more 'power' into the wave (I think it would be amplitude) to get enough of the wave to bounce back so they can decipher it?
    More than what ?


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    i appreciate that certain parts of the light spectrum cannot get through different parts of the atmosphere so i guess that answers my question.

    as to the 'power' - more than they would have otherwise i guess! If i send a signal which is worth 5 watts of electrical energy at the source it won't get as far as say 500 watts..............excuse my terminology but i'm trying to get a grasp of these things.

    I am presuming that to get a reading it has to bounce back like radar does - so my question is at a given frequency how much 'power' is needed to get a wave to bounce off the eath's surface and get back to the satellite?

    I appreciate that you may need the frequency to answer this question in which case i can provide one unless someone is able to offer some math for this so i can work it out myself?
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  5. #4  
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    Quote Originally Posted by fatman57
    i appreciate that certain parts of the light spectrum cannot get through different parts of the atmosphere so i guess that answers my question.

    as to the 'power' - more than they would have otherwise i guess! If i send a signal which is worth 5 watts of electrical energy at the source it won't get as far as say 500 watts..............excuse my terminology but i'm trying to get a grasp of these things.

    I am presuming that to get a reading it has to bounce back like radar does - so my question is at a given frequency how much 'power' is needed to get a wave to bounce off the eath's surface and get back to the satellite?

    I appreciate that you may need the frequency to answer this question in which case i can provide one unless someone is able to offer some math for this so i can work it out myself?
    You need to go to a book on classical electrodynamics in order understand the physics of reflection of electromagnetic waves. Yes, it can be calculated. But to do so you need to know the geometry of the target, its conductivity and the frequency of the incident wave. Some of the wave is transmitted, some absorbed and some reflected. A classic text is Jackson's Classical Electrodynamics. You might also take a look at texts on radar engineeering.
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    thanks - that look slike quite a book though - couple of hundred pages long.

    would be a useful reference & learning guide too - it is rather expensive though and its more than 10 years old - anyone know of any more recent books at all?

    much obliged.
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    Why do you need to know the exact math?

    Can they send any frequency?
    Anything up to terahertz radiation really, but oscillators cannot generate visible light yet or anything above it. For that, you need a special device...which are not like oscillators in that its very hard or impossible to specifically tune the frequency.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
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  8. #7  
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    Quote Originally Posted by fatman57
    thanks - that look slike quite a book though - couple of hundred pages long.

    would be a useful reference & learning guide too - it is rather expensive though and its more than 10 years old - anyone know of any more recent books at all?

    much obliged.
    That is the standard text for the subject. You don't have to worry about its age. The older editions are still quite valid, and the new one is also. It is the best available text on electrodynamics. The only significant rival is Landau and Lefshitz and it is older and more difficult to find.

    Maxwell's equations are pretty old too.
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    I would not suggest reading about Classical Electrodynamics to study RF propagation. A more relevant book to read would be:

    "Antennas and Propagation for Wireless Communications Systems" By Saunders and Aragon-Zavala, a Wiley book.

    Page 139 starts an entire chapter on propagation losses between Satellite and Earth. It is dependent on a lot of factors, but if you understand how to do basic math, that book will show you how to calculate the power required to push a signal down and back again. The book assumes you know the fundamentals of electrodynamics, and builds a set of approximations and models based on real data.

    If you want a very quick and very dirty approximation that will get you in the ball park, but about 40 yards left of the target, you can use the following:

    PL = 40Log(d) - 20Log(Ht) - 20Log(Hr) - Gt - Gr

    where PL is the path loss (subtract this from your signal power in dB), d is the distance in a base unit (ft or meters), Ht is the height of your sat, Hr is the height of the rx antenna, Gt is the transmitter antenna gain, Gr is the rx antenna gain. All logs are Base 10.

    This assumes a flat Earth, and a direct line of sight, ignoring atmospheric effects, scattering, and all other bad stuff you might actually encounter. It is not accurate for this type of problem, but it can provide you with a "ball park" for figures you should expect when you actually go find the information you need to solve your problem.

    I will, however, reiterate, that if you do not know the basics, the likelihood of you actually understanding why they do what they do to develop models is very small. So you should try to understand electrodynamics before starting on this little quest.
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    -Benjamin Franklin, An Historical Review of Pennsilvanya, 1759
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