How to do Doppler compensation for a satellite payload

Hello everyone, I'm new to this forum.

I need help for a project I'm working on: I must develop a satellite payload consisting of a transciever operating in the uhf band, using GMSK modulation, low data-rate.
This is a school project for which I've been given limited information and I'm not supposed to go too much in detail. I'm also not experienced on the subject at all .

The system must compensate up to ±8kHz of Doppler shift, so how do I get this done ? Is it possible to use AFC or PLL to achieve this ? If so, what would be a reasonable implementation ? Or do I need instead to obtain the information from the demod ?

Thanks in advance for any response.

I suppose you must determine whether to set your transmitter for a higher frequency, or lower frequency, depending on what frequency the other is transmitting at.

I think you need to query what frequency the guy at the other end is transmitting at. Then compare that to what frequency you're picking him up at.

Suppose he reports he is transmitting at 300 MHz. You are picking him up on 300.1 MHz.

This means the radio waves are being compressed. So you should transmit on 299.9 MHz. Then he will receive you at 300 MHz.

I could be wrong.

You're right, that's the idea.

The system must be able to correct its Tx frequency based on an estimation of the doppler shift, but I also think should be able to tune itself properly while receiving (or am I wrong ?). In any case there must be a frequency control system that acts on the local oscillators (for either tx or rx purposes or both); this would also take care of any drift in the LO frequency caused by temperature, aging and other physical causes if acting on the reference oscillator.

Many integrated transcievers have some AFC (automatic frequency control) feature. FM receivers used to have analog AFCs based on discriminators and tank circuits.
I know that, to pick an example closer to my case, GSM receivers have some sort of integrated AFC that allows to correct drifts in the carrier frequency LO's.

However I'm still struggling to find any information on how this feature is implemented, any help on this matter would be greatly appreciated.

assuming the frequency drift is larger than the bandwidth of the receiver, then yes you need to do something to track it. Most systems have some sort of carrier tracking loop. You need to turn on the system, let the loop hunt around, and eventually lock up. This may be done digitally or with analog sweep techniques.

It is important to know something about the doppler characteristics, as a simple doppler shift is relatively easy to track, but an accelerating doppler shift needs, as I recall, at least a 2nd order tracking loop.

If you have "bursty" communications, you might need to employ other more complicated means, such as DSP processing a larger bandwidth than you would like to, finding the signal, and then digital filtering the desired portion of the spectrum that the actual signal resides in at that moment. Sometimes known header information in the digital modulation allows the digital locking of the signal in a faster time.

There are all sorts of carrier tracking algorithms, some where the signal is doubled or quadrupled to strip off the modulation and see that bare carrier, etc.