Just to expand on one point the other two replies touched on: The beam does indeed scan radially outward from the center (though the angle can either be produc

Just to expand on one point the other two replies touched on: The beam does indeed scan radially outward from the center (though the angle can either be produced by physically rotating the deflection plates, or computing sin/cos electronically and applying to the x/y plates), and the amplitude of the returned signal does directly drive the beam current (so a target with a larger return will appear brighter. Additionally, there's also a storage tube effect going on, too--but it works like the variable-persistence mode found in certain oscilloscopes, not the bi-stable mode found in storage-tube x-y vector displays for early computers (some o'scopes had both var persist and bi-stable modes). One consequence of the above is related to early attempts at designing stealth airplanes, like the SR-71 and XB-70: The beam current is set such that the radar tube tends to build up the "blip" over multiple sweeps, and the blip usually moves less than its own diameter between sweeps. But, if the airplane has an intrinsically low radar cross-section and if it is moving so fast that the blimp moves a greater distance between sweeps... the radar operator may just interpret the little ghost blips as noise, especially if he or she is overwhelmed by a large number of targets.