[coredog64]

ATC does have access to radar that locates aircraft via skin paint. TBH, I’d push that back on ATC. If I’m confused about my altitude, it’s not because I suddenly can’t read the numbers on the dial. ATC should understand that the confusion can be resolved by checking with an outside source.

[aaronmdjones]

Primary radar as used by ATC is extremely limited. It is not designed to identify targets at all; nor does it have particularly good vertical resolution. It can tell you where in 2D space a plane is, but without secondary radar and a cooperating transponder, it can't tell you what plane it is, and its uncertainty with respect to altitude can be several thousand feet.

[lxgr]

What do you mean by “skin paint”?

Civil primary radar usually isn’t 3D and does indeed depend on transponder, i.e. secondary radar, supplied altitude data.

They should know what their radar supports in any case, and get suspicious when an aircraft asks them for a readback of airplane-supported altitude data.