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FM for uplink ,PM for downlink,why?

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afz23

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I have seen many satellites using FM for uplink, PM for down link?

Mainly geostationary or geosynchronous, why this scheme is preferrd.

I donot get convincing answers for this.

What I know is FM demodulation becomes simple onboard, due to non-coherent
receiver and FM provides more modulation index and BW?

where as PM reduces transmitter power requirement on board,which is really
big gain, as power crunch is main concern onboard.
 

I believe it is simply due to battery power available. PSK uses coherent detection, and as such has a 3 dB detection advantage. So they can use a transmit amplifier in the satellite that is half as big as if they tried to downlink with FSK.

Added after 4 hours 2 minutes:

Also, you can have much better error correcting codes if you have coherent modulation. That means by using psk, you get more system coding gain, and therefore can once again lower the transmit power on the satellite.
 

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    afz23

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thanks biff4

That makes lot of sense to me.

We can always have very big parabolic reflective antennas on ground to improve
SNR further.

Onboard you cnanot carry those.
 

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