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Carrier doppler shift and Signal doppler shift

Started by nbtech 4 weeks ago5 replieslatest reply 4 weeks ago107 views

Hi all,

Whats the difference between a "carrier doppler shift" and "signal doppler shift" in satellite communications?

These parameters are listed on specs of a satellite emulator.

Thanks


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Reply by SlartibartfastAugust 20, 2019

In my experience the terms are used interchangeably, and if whoever is using the terms intends for there to be a distinction between them they need to be clear about it.

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Reply by LKoppAugust 19, 2019

Hello..As I see it:

A Doppler shifted signal is a time compressed version of the original signal: if S(t) is emitted S(at) is received, where a=1-v/c approximately.

Now if S(t)=exp(2ipift)E(t) with carrier at frequency f and modulation E(t), then S(at) corresponds to a shifted carrier at fequency af and compressed modulation E(at).

So Doppler on carrier is one thing and Doppler on signal (i.e. modulation) is another thing..

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Reply by rbjAugust 21, 2019

i do not see why the Doppler shift effect should be any different for the modulation than it is for the carrier.  the whole signal gets scaled in time by a single factor that depends on the velocity of the transmitter relative to the receiver.

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Reply by VariableSpeedAugust 19, 2019

There are 3 radios involved a transmitter, a receiver and the satellite which both receives and transmits while moving with respect to the other 2 somewhat independently.  So there are 2 independent doppler shifts involved.  I can imagine the satellite receives a doppler shifted signal, decodes it leaving the shift untouched and retransmits which already shifted signal on a pristine carrier get shifted again. 

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Reply by SlartibartfastAugust 21, 2019

Most comm satellites don't do demod-remod, they just mix an uplink band by a fixed amount and retransmit it in the downlink band.   Any shift on the uplink will get added to the shift on the downlink.

There are some exceptions (e.g., Iridium), but geosynchronous comm satellites are typically just bent pipes.   The OP didn't distinguish the context.