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satellite uplink and downlink

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zio_nneo

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As we all of know that uplink frequency of satellit is greater than its downlink frequency. Can anybody explain me why this is so.
 

there are many reasons for this.....

1. as frequency directly proportional to energy there is a energy constraint as the satellite cannot expend more of its stored power

2. antenna size directly proportional to frequency and hence the placing of a huge antenna on a satellite is hard.......


these r the two reasons that i know........


regards

Anand srinivasan
 

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    zio_nneo

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2. antenna size directly proportional to frequency and hence the placing of a huge antenna on a satellite is hard.......

I can't understand the above point.Since the satellite has to recieve higher frequency signal and transmit lower frequency signal, thus it requires two antennae
of different sizes.
 

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    zio_nneo

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receiving is not a problem with the antenna but when it comes to transmitting there must be a certain level of efficiency for the signal to reach earth station and hence there is a constraint on the antenna length...... the reception part depends on the earth station antenna which is transmitting.......
 

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    zio_nneo

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Do mean to say about the directivity. I think for uplink there should be more directive while at the downlink the directivity is not more concern as compared with the uplink. Does this one reason.?

Added after 1 minutes:

Then what about the GSM system. It is just opposite as compared with the satellite link. why ?
 

consider the number of receivers in GSM system from the point of view that the antenna tower is the transmitter then it would make clear the difference between the satellite situation and the GSM situation..........
 

Do you think only the large number of receiver will affect the frequency of the uplink and downlink frequency. Then what about the FM and AM radio system. It has more receiver then the GSM system then why its frequency is low as compared with this then? Would you plz give me specific answer on this.
 

With frequency the distance the signal travels increases and GSM towers have more range than FM and AM.........

I'm not absolutely sure of this but i think the GSM towers r closer because the handsets are too compatible and cant transmit longer due to power consideration.... i think the power used by handsets for transmitting is 2 Watts, how far do u think u can transmit with that less power....
 

I think with that much of power celluar phone can transmit upto 50km. For your kind information todays communication system are operated with less power and still it is of research topic how to operate the communicatonsystem with less power sothat it will not effect the other system.
 

oh i see.... sorry i was not aware of this information..... i will refer more into this... if u find information about this then please do post it here.........
 

As we all of know that uplink frequency of satellit is greater than its downlink frequency. Can anybody explain me why this is so.
Click to expand...

As I understand the choice of frequency has basis in the Path Loss dependance on the freq.
Higher the freq. higher the losses. Now, the power pack in the Satellite (being in the sky) has limitations so it Tx (down link) with low freq.->low path loss->less Tx power to traverse the Tx-Rx distance. While, Receivers on earth have no limitation for power packs so they Tx (uplink) with higher freq.->higher path loss->Higher power to traverse the Tx-Rx distance which can be manageable at earth.
 

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    zio_nneo

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Do you have more information on this. Actually this is the point that I am searching for. If you have more notes on this plz upload it on edaboard forum.
 

i dont have that much information about it but i heard that the atmospheres attenuation is a very nonlinear function of frequency and is related to the physical property of water vapour, oxygen etc....... and 14GHz and 16GHz are often used for downlink and uplink bec the attenuation is low somewhat at those frequencies....
 

Yes that true but why different frequency why not some?
 

one thing is due to power consideration and other is interference,...... here u can transmit simultaneously.... but if same frequency is used for both then how can u transmit simultaneously.........
 

but if same frequency is used for both then how can u transmit simultaneously...
Click to expand...
I think we are now talking something called duplexing...if we use FDD (freq. division duplexing)we are Transmitting and Receiving at different frequencies but if we use TDD (Time division duplexing) we are Transmitting and receiving at the same frequency BUT at different time slots. Now, I am not sure that at the present state of technology we use TDD in satellite communication.

Earlier Satellite communication and the choice fo frequencies etc. depended on the legacy of Microwace communication as the availability of electronics! in the band was easily available and copuld be directly used for Satcom systems in those bands (say C band). But with the more and more developments in the firld now the earlier restrictions on Satcom and choice of Band etc. have been overcome.....
Try to read some of the basic books on Satcom, the things will be clear...
I will try to find out the name and post
-Sidd
 

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