Reach RS for GCP - why are two needed?

I want to purchase a Reach RS for establishing a few ground control points for Drone surveying, maybe 5 points at the most. I have been told that I need 2 Reach RS units for this, one as a base and one as a roving station. Is this really necessary?

I can understand why two would ne necessary if surveying numerous locations but surely, the first reference point where the base is established will have its coordinate and altitude accurately determined by the base unit without the use of a roving unit?

Why then can we not just use a single ReachRS to establish the coordinates and altitude of each GCP, even if it takes a little longer using the one unit? Thanks.

You need two receivers for RTK to work, one Reach RS without corrections from base station will not have cm precision, just precision of a normal GPS.

So the accuracy of the base station coordinates/elevation is not cm accuracy? ie the base station doesnt have the correction applied - and you need two units to be able to apply the correction factor?

cm accuracy is relative to the base, so you need to either position base on a known point or post-process it relative to another base station (governmental for example). If you use base autonomous position, rover coordinates still will be precise, but they will be shifted by the inaccuracy of the base.

So if we dont have a known base position we can still get accurate rover coordinates but they will just be relative to the base position - ie we coulld not then accurately locate the GCP’s based on a national grid based system?

You can get absolute coordinates if you correct them with data from official part that provides you with absolute coordinate.
Like in our country, we download satellite data from same periode of time we logged data from rover and postprocess absolute coodinates. Or if you have a base somewhere or ntrip access, start logging base data and collect rover data and process. You can place base over known point and get absolute values, but it doesnt have to be linked live to rover, it can be postprocessed later.
You dont need RTK live feed to rover for absolute or relative coordinate when postprocessing.

So just to be clear, we set up the base station at a known location that has absolute accurate coordinates and complete the survey, and then after the survey we correct the base station survey coordinates with the accurate known coordinates and that then corrects the GCP coordinates so that they are absolute?

Yes, thats one way to do it.

@Luke_Wijnberg does some magic stuff with reach.
More about it here Emlid Reach for placing GCP - Blogs - diydrones
And on this forum
Really good tracklog with Reach and a RPA fixed wing

Would it be possible to take the Reach RS, fix it at a certain location (base), logging data for some hours, than taking this same unit and using it as a hover for collecting a few GCPs, and them using the PPK algorithms to correct the GCPs position based on the base position? This would assume that the error based on meteorological factors didnt changed on that time. Is it possible or the PPK solution is also completely time-dependent? Just a curiosity.

No, you need to collect data from both locations during the same timespan.