STW or SOG?

I've never paid attention to the source chosen on the radar to show the true vectors.
Two basic settings are commonly in use. Speed Through Water(STW) and Speed Over Ground(SOG).
SOLAS requires Radars to have STW feed.
But because I worked on the ship where water Log was not accurate, GPS speed was used.
So I've got used to that preset.
But I had several debates on what exactly should be used for collision avoidance.
So. I went down to the root. What Exactly do we need to calculate Closest Point of Approach(CPA), Bow Crossing Range(BCR) and Time of (CPA)?
Risk of collision exists if distance is decreasing and bearing to the target stays the same.
It means that to determine all the data Radar should get feed from Compass(in that case compass should not be precise, but must be fixed to a certain direction. It original North can be at True 146 degree for example, but it should know that for entire observation). Thnking more and deeper I realize that it shouldn't.
Then Radar can measure distance.
And a clock to have the time.
In old times, it was a performance involving several persons. Captain was getting bearing and distance from radar at fixed intervals. Every 2 or 3 minutes. Duty Officer was plotting the data on paper and was calculating  That was pre-printed circle with grid. Own vessel was in the middle heading Up.
As soon as target's echo was in range calculations began. That was zero point. 3rd minute point would give a clue if there was danger of collision. 6th minute would give easy way to calculate TCPA.
To calculate CPA the line is drawn through continiously plated dots. The shortest distance from own position to the line is the CPA.
In other words. IT DOES NOT MATTER what speed input is used, because none of them is used for CPA/TCPA calculations.