Ok, the radar, I assume, refers to the speed it was going it when it was heading west.
It later turned again to reach the Inmarsat arc and Doppler analysis (plus lack of radar detection) indicated it turned south, sea bound, not north, land bound.
If they've used radar to deduce the speed, they've either assumed something about how the westerly speed related to the southerly speed, or they've picked up a further radar point from some country or another that gives them more information (for example, any radar point south of the last westerly point would give an idea of southerly speed)
Downton, the partial handshake was never explained but IIRC it was only a few minutes after the full handshake so may not have been able to be fully analysed.