Maybe a few areas of misunderstanding about radalt. I was about to try to explain in my own words, but then found the following. Sorry, I have no idea what language this was written in, but it explains some very important priciples about radalt and what it actually measures - notwithstanding East Street points about beamwith.
The transmitted pulse reaches the surface first on a small point. As the pulse advances, the illuminated area grows rapidly from a point to a disk, as does the returned power. The reflecting area depends on the beamwidth of the antenna. The energy from the center of the main beam has to travel the shorter path than the energy from the edges. An annulus is formed and the geometry is such that the annulus area remains constant as the diameter increases. The returned signal strength, which depends on the reflecting area, grows rapidly until the annulus is formed, remains constant until the growing annulus reaches the edge of the radar beam, where it starts to diminish.
It's not just a simple measurement of distance from Tx/Rx to the nearest object.