I'm looking into building a QO-100 setup, but am struggling to find good/easy to understand resources for beginners which would outline the most common approaches and their pros/cons. From what I could gather, there seem to be two main approaches:

1. Use an SDR for RX on ~700Mhz coming out of LNB. TX on 70cm and up-convert to 2.4GHz
2. For RX, use down-converter from ~700MHz coming out of LNB to 2m band, fed into the radio. On TX, same as first approach (TX on 70cm, up-converted to 2.4GHz)

After looking at some existing implementations, I found that some also use a GNSS module to supposedly synchronize some frequency.

1. What frequency is GNSS synchronizing?
2. Why is it a GNSS module and not something simpler, like a crystal oscillator? GNSS feels like complete overkill, if the only goal is to provide an external frequency reference.
3. Why do some setups use GNSS and others don't?

I'm looking into building a QO-100 setup, but am struggling to find good/easy to understand resources for beginners which would outline the most common approaches and their pros/cons. From what I could gather, there seem to be two main approaches:

1. Use an SDR for RX on ~700Mhz coming out of LNB. TX on 70cm and up-convert to 2.4GHz
2. For RX, use down-converter from ~700MHz coming out of LNB to 2m band, fed into the radio. On TX, same as first approach (TX on 70cm, up-converted to 2.4GHz)

After looking at some existing implementations, I found that some also use a GNSS module to supposedly synchronize some frequency.

1. What frequency is GNSS synchronizing?
2. Why is it a GNSS module and not something simpler, like a crystal oscillator? GNSS feels like complete overkill, if the only goal is to provide an external frequency reference.
3. Why do some setups use GNSS and others don't?

EDIT: Here are a couple setups I found are using GNSS for the frequency reference:

1. https://dxpatrol.pt/produto/new-dxpatrol-qo-100-groundstation/
2. https://www.dd1us.de/Downloads/Portable%20station%20for%20QO-100%20English%202020-09-26.pdf